#include "game.h"
#include "sys.h"
#include "util.h"
#include "world.h"
#include "sprite.h"
#include "sound.h"
#include "mixer.h"
#include "math.h"
#include "scratch.h"
#include "atomic.h"
#include "app.h"
#include "log.h"
#include "collider.h"

struct contact_lookup_entry {
    u64 hash;
    struct entity_handle contact_ent_handle;
    struct contact_lookup_entry *next;
    struct contact_lookup_entry *prev;
};

struct contact_lookup_bucket {
    struct contact_lookup_entry *first;
    struct contact_lookup_entry *last;
};

struct contact_lookup {
    struct arena arena;
    struct contact_lookup_bucket buckets[4096];
    struct contact_lookup_entry *first_free_entry;
};

#if COLLIDER_DEBUG
/* TODO: Remove this (debugging) */
struct collision_debug_lookup {
    struct arena arena;
    struct fixed_dict dict;
};
#endif

GLOBAL struct {
    struct atomic_i32 game_thread_shutdown;
    struct sys_thread game_thread;

    b32 paused;
    struct sprite_scope *sprite_frame_scope;

    /* For debugging */
    struct v2 user_cursor;

    /* TODO: Remove this (testing) */
    b32 extra_spawn;
    b32 should_reset_level;

    /* Game thread input */
    struct sys_mutex game_cmds_mutex;
    struct arena game_cmds_arena;
    struct entity *root;

    /* Constants */
    struct math_spring_result contact_softness;
    struct math_spring_result mouse_joint_linear_softness;
    struct math_spring_result mouse_joint_angular_softness;
    f32 mouse_joint_max_force;

    /* Bookkeeping structures */
    struct contact_lookup contact_lookup;
#if COLLIDER_DEBUG
    struct collision_debug_lookup collision_debug_lookup;
#endif

    /* Ticks */
    struct sys_mutex prev_tick_mutex;
    struct atomic_u64 prev_tick_id;
    struct atomic_u64 prev_tick_continuity_gen;
    struct world prev_tick;
    struct world tick;
} G = ZI, DEBUG_ALIAS(G, G_game);

/* ========================== *
 * Startup
 * ========================== */

INTERNAL APP_EXIT_CALLBACK_FUNC_DEF(game_shutdown);
INTERNAL SYS_THREAD_ENTRY_POINT_FUNC_DEF(game_thread_entry_point, arg);
INTERNAL void reset_world(void);

struct game_startup_receipt game_startup(struct mixer_startup_receipt *mixer_sr,
                                         struct sprite_startup_receipt *sheet_sr,
                                         struct sound_startup_receipt *sound_sr)
{
    (UNUSED)mixer_sr;
    (UNUSED)sheet_sr;
    (UNUSED)sound_sr;

    /* Initialize constants */
    {
        const f32 substep_dt = (1.f / ((f32)GAME_FPS * (f32)GAME_PHYSICS_SUBSTEPS));

        const f32 contact_frequency = (1.f / substep_dt) / 8.f;
        const f32 contact_damping_ratio = 10;
        G.contact_softness = math_spring(contact_frequency, contact_damping_ratio, substep_dt);

        const f32 mouse_joint_linear_frequency = 5.0;
        const f32 mouse_joint_linear_damping_ratio = 0.7;
        const f32 mouse_joint_angular_frequency = 0.5f;
        const f32 mouse_joint_angular_damping_ratio = 0.1f;
        G.mouse_joint_max_force = 1000;
        G.mouse_joint_linear_softness = math_spring(mouse_joint_linear_frequency, mouse_joint_linear_damping_ratio, substep_dt);
        G.mouse_joint_angular_softness = math_spring(mouse_joint_angular_frequency, mouse_joint_angular_damping_ratio, substep_dt);
    }

    /* Initialize game cmd storage */
    G.game_cmds_mutex = sys_mutex_alloc();
    G.game_cmds_arena = arena_alloc(GIGABYTE(64));

    /* Initialize empty world */
    reset_world();

    /* Initialize prev tick */
    world_alloc(&G.prev_tick);
    /* FIXME: Make the world struct itself readonly as well */
    arena_set_readonly(&G.prev_tick.entity_store->arena);
    G.prev_tick_mutex = sys_mutex_alloc();

    G.game_thread = sys_thread_alloc(&game_thread_entry_point, NULL, STR("[P2] Game thread"));
    app_register_exit_callback(&game_shutdown);

    return (struct game_startup_receipt) { 0 };
}

INTERNAL APP_EXIT_CALLBACK_FUNC_DEF(game_shutdown)
{
    __prof;
    atomic_i32_eval_exchange(&G.game_thread_shutdown, true);
    sys_thread_wait_release(&G.game_thread);
}

/* ========================== *
 * Game cmd
 * ========================== */

INTERNAL void push_cmds(struct game_cmd_array cmd_array)
{
    struct sys_lock lock = sys_mutex_lock_e(&G.game_cmds_mutex);
    struct game_cmd *cmds = arena_push_array(&G.game_cmds_arena, struct game_cmd, cmd_array.count);
    MEMCPY(cmds, cmd_array.cmds, cmd_array.count * sizeof(*cmds));
    sys_mutex_unlock(&lock);
}

INTERNAL struct game_cmd_array pop_cmds(struct arena *arena)
{
    struct game_cmd_array array = ZI;
    if (G.game_cmds_arena.pos > 0) {
        struct sys_lock lock = sys_mutex_lock_e(&G.game_cmds_mutex);
        struct buffer game_cmds_buff = arena_to_buffer(&G.game_cmds_arena);
        arena_align(arena, alignof(struct game_cmd));
        array.cmds = (struct game_cmd *)arena_push_array(arena, u8, game_cmds_buff.size);
        array.count = game_cmds_buff.size / sizeof(struct game_cmd);
        MEMCPY(array.cmds, game_cmds_buff.data, game_cmds_buff.size);
        arena_reset(&G.game_cmds_arena);
        sys_mutex_unlock(&lock);
    }
    return array;
}

/* ========================== *
 * Activate
 * ========================== */

INTERNAL void activate_now(struct entity *ent)
{
    entity_enable_prop(ent, ENTITY_PROP_ACTIVE);
    ent->activation_tick = G.tick.tick_id;
    ++ent->continuity_gen;
}

/* ========================== *
 * Contact lookup
 * ========================== */

INTERNAL void contact_lookup_alloc(struct contact_lookup *l)
{
    MEMZERO_STRUCT(l);
    l->arena = arena_alloc(GIGABYTE(64));
}

INTERNAL void contact_lookup_release(struct contact_lookup *l)
{
    arena_release(&l->arena);
}

INTERNAL u64 contact_lookup_hash_from_entities(struct entity_handle h0, struct entity_handle h1)
{
    struct buffer b0 = BUFFER_FROM_STRUCT(&h0);
    struct buffer b1 = BUFFER_FROM_STRUCT(&h1);
    u64 hash = hash_fnv64(HASH_FNV64_BASIS, b0);
    hash = hash_fnv64(hash, b1);
    return hash;
}

INTERNAL struct contact_lookup_entry *contact_lookup_get(struct contact_lookup *l, u64 hash)
{
    u64 index = hash % ARRAY_COUNT(l->buckets);
    struct contact_lookup_bucket *bucket = &l->buckets[index];
    struct contact_lookup_entry *res = NULL;
    for (struct contact_lookup_entry *e = bucket->first; e; e = e->next) {
        if (e->hash == hash) {
            res = e;
            break;
        }
    }
    return res;
}

INTERNAL void contact_lookup_set(struct contact_lookup *l, u64 hash, struct entity_handle handle)
{
    u64 index = hash % ARRAY_COUNT(l->buckets);
    struct contact_lookup_bucket *bucket = &l->buckets[index];

    struct contact_lookup_entry *prev = NULL;
    struct contact_lookup_entry **slot = &bucket->first;
    while (*slot) {
        if ((*slot)->hash == hash) {
            break;
        }
        prev = *slot;
        slot = &(*slot)->next;
    }

    struct contact_lookup_entry *entry = *slot;
    if (entry) {
        /* Set existing entry */
        entry->contact_ent_handle = handle;
    } else {
        /* Allocate entry */
        if (l->first_free_entry) {
            entry = l->first_free_entry;
            l->first_free_entry->prev = NULL;
            l->first_free_entry = entry->next;
        } else {
            entry = arena_push(&l->arena, struct contact_lookup_entry);
        }
        MEMZERO_STRUCT(entry);

        entry->hash = hash;
        entry->contact_ent_handle = handle;
        if (prev) {
            entry->prev = prev;
            prev->next = entry;
        }

        bucket->last = entry;
        *slot = entry;
    }
}

INTERNAL void contact_lookup_remove(struct contact_lookup *l, struct contact_lookup_entry *entry)
{
    struct contact_lookup_bucket *bucket = &l->buckets[entry->hash % ARRAY_COUNT(l->buckets)];
    struct contact_lookup_entry *prev = entry->prev;
    struct contact_lookup_entry *next = entry->next;

    if (prev) {
        prev->next = next;
    } else {
        bucket->first = next;
    }

    if (next) {
        next->prev = prev;
    } else {
        bucket->last = prev;
    }

    if (l->first_free_entry) {
        l->first_free_entry->prev = entry;
    }
    entry->next = l->first_free_entry;
    entry->prev = NULL;
    l->first_free_entry = entry;
}

/* ========================== *
 * Reset
 * ========================== */

INTERNAL void reset_world(void)
{
    if (G.tick.entity_store) {
        /* Release world */
        world_release(&G.tick);
        /* Release bookkeeping */
#if COLLIDER_DEBUG
        arena_release(&G.collision_debug_lookup.arena);
#endif
        contact_lookup_release(&G.contact_lookup);
    }

    /* Create bookkeeping */
    contact_lookup_alloc(&G.contact_lookup);
#if COLLIDER_DEBUG
    G.collision_debug_lookup.arena = arena_alloc(GIGABYTE(64));
    G.collision_debug_lookup.dict = fixed_dict_init(&G.collision_debug_lookup.arena, 4096);
#endif

    /* Re-create world */
    world_alloc(&G.tick);
    G.tick.continuity_gen = atomic_u64_eval(&G.prev_tick_continuity_gen) + 1;
    G.tick.timescale = GAME_TIMESCALE;
}

/* ========================== *
 * Test
 * ========================== */

/* TODO: Remove this */

INTERNAL void spawn_test_entities(void)
{
    struct entity *root = entity_from_handle(G.tick.entity_store, G.tick.entity_store->root);
    root->mass_unscaled = F32_INFINITY;
    root->inertia_unscaled = F32_INFINITY;

    /* Player */
    struct entity *player_ent = entity_nil();
    //if (!G.extra_spawn) {
    {

        struct entity *e = entity_alloc(root);

        struct v2 pos = V2(1, -1);

        //struct v2 size = V2(0.5, 0.5);
        //struct v2 size = V2(0.5, 0.25);
        struct v2 size = V2(1.0, 1.0);

        //f32 r = PI / 4;
        f32 r = 0;

        if (!G.extra_spawn) {
            entity_enable_prop(e, ENTITY_PROP_PLAYER_CONTROLLED);
            entity_enable_prop(e, ENTITY_PROP_TEST);
            e->sprite = sprite_tag_from_path(STR("res/graphics/tim.ase"));
            e->mass_unscaled = 10;
            e->inertia_unscaled = 5;
        } else {
            entity_enable_prop(e, ENTITY_PROP_TEST);
            e->sprite = sprite_tag_from_path(STR("res/graphics/tim.ase"));
            e->mass_unscaled = 10;
            e->inertia_unscaled = 5;
#if 0
            e->sprite = sprite_tag_from_path(STR("res/graphics/box.ase"));
            e->mass_unscaled = 100;
            e->inertia_unscaled = 100;
#endif
        }

        //e->sprite = sprite_tag_from_path(STR("res/graphics/box_rounded.ase"));
        //e->sprite_span_name = STR("idle.unarmed");
        //e->sprite_span_name = STR("idle.one_handed");
        e->sprite_span_name = STR("idle.two_handed");
        e->sprite_collider_slice = STR("shape");

        struct xform xf = XFORM_TRS(.t = pos, .r = r, .s = size);
        //xf.bx.y = -1.f;

        entity_set_xform(e, xf);

        e->linear_ground_friction = 250;
        e->angular_ground_friction = 200;

        //e->control_force = 500;
        e->control_force = 500;
        //e->control_force_max_speed = 4;
        e->control_torque = 1000;
        e->control_force_max_speed = 4;

        entity_enable_prop(e, ENTITY_PROP_PHYSICAL_DYNAMIC);

        player_ent = e;
    }

    /* Enemy */
    {
        struct entity *e = entity_alloc(root);

        struct v2 pos = V2(1, -2);
        f32 r = 0;
        struct v2 size = V2(1, 1);
        struct xform xf = XFORM_TRS(.t = pos, .r = r, .s = size);
        entity_set_xform(e, xf);

        e->sprite = sprite_tag_from_path(STR("res/graphics/tim.ase"));
        e->sprite_collider_slice = STR("shape");

        entity_enable_prop(e, ENTITY_PROP_PHYSICAL_DYNAMIC);
        e->mass_unscaled = 10;
        e->inertia_unscaled = 10;
        e->linear_ground_friction = 250;
        e->angular_ground_friction = 200;
    }


    /* Box */
#if 0
    {
        struct entity *e = entity_alloc(root);

        struct v2 pos = V2(1, -0.5);
        f32 r = 0;
        struct v2 size = V2(0.5, 0.25);
        struct xform xf = XFORM_TRS(.t = pos, .r = r, .s = size);
        entity_set_xform(e, xf);

        e->sprite = sprite_tag_from_path(STR("res/graphics/box.ase"));
        e->sprite_collider_slice = STR("shape");

        entity_enable_prop(e, ENTITY_PROP_PHYSICAL_DYNAMIC);
        e->mass_unscaled = 10;
        e->inertia_unscaled = 10;
        e->linear_ground_friction = 250;
        e->angular_ground_friction = 5;
    }
#endif

    /* Player weapon */
    if (player_ent->valid) {
        struct entity *e = entity_alloc(player_ent);
        e->sprite = sprite_tag_from_path(STR("res/graphics/gun.ase"));

        entity_enable_prop(e, ENTITY_PROP_ATTACHED);
        e->attach_slice = STR("attach.wep");

        entity_enable_prop(e, ENTITY_PROP_WEAPON);
        e->trigger_delay = 1.0f / 10.0f;

        player_ent->equipped = e->handle;
    }

    /* Camera */
    if (!G.extra_spawn && player_ent->valid) {
        struct entity *e = entity_alloc(root);
        entity_set_xform(e, XFORM_IDENT);

        entity_enable_prop(e, ENTITY_PROP_CAMERA);
        entity_enable_prop(e, ENTITY_PROP_CAMERA_ACTIVE);
        e->camera_follow = player_ent->handle;

        f32 width = (f32)DEFAULT_CAMERA_WIDTH;
        f32 height = (f32)DEFAULT_CAMERA_HEIGHT;
        e->camera_quad_xform = XFORM_TRS(.s = V2(width, height));
    }

    G.extra_spawn = true;
}





/* ========================== *
 * TESTING CONTACT CONSTRAINT
 * ========================== */

INTERNAL void create_contacts(void)
{
    struct entity_store *store = G.tick.entity_store;
    struct entity *root = G.root;

    for (u64 check0_index = 0; check0_index < store->reserved; ++check0_index) {
        struct entity *check0 = &store->entities[check0_index];
        if (!entity_is_valid_and_active(check0)) continue;
        if (!(entity_has_prop(check0, ENTITY_PROP_PHYSICAL_DYNAMIC) || entity_has_prop(check0, ENTITY_PROP_PHYSICAL_KINEMATIC))) continue;
        if (check0->local_collider.count <= 0) continue;

        struct xform check0_xf = entity_get_xform(check0);
        struct collider_shape check0_collider = check0->local_collider;

        for (u64 check1_index = 0; check1_index < store->reserved; ++check1_index) {
            struct entity *check1 = &store->entities[check1_index];
            if (check1 == check0) continue;
            if (!entity_is_valid_and_active(check1)) continue;
            if (!(entity_has_prop(check1, ENTITY_PROP_PHYSICAL_DYNAMIC) || entity_has_prop(check1, ENTITY_PROP_PHYSICAL_KINEMATIC))) continue;
            if (check1->local_collider.count <= 0) continue;

            /* Deterministic entity order based on index */
            struct entity *e0;
            struct entity *e1;
            struct xform e0_xf;
            struct xform e1_xf;
            struct collider_shape e0_collider;
            struct collider_shape e1_collider;
            if (check0_index < check1_index) {
                e0 = check0;
                e1 = check1;
                e0_xf = check0_xf;
                e1_xf = entity_get_xform(check1);
                e0_collider = check0_collider;
                e1_collider = check1->local_collider;
            } else {
                e0 = check1;
                e1 = check0;
                e0_xf = entity_get_xform(check1);
                e1_xf = check0_xf;
                e0_collider = check1->local_collider;
                e1_collider = check0_collider;
            }

            u64 lookup_hash = contact_lookup_hash_from_entities(e0->handle, e1->handle);
            struct contact_lookup_entry *entry = contact_lookup_get(&G.contact_lookup, lookup_hash);

            struct entity *constraint_ent = entity_nil();
            if (entry) {
                constraint_ent = entity_from_handle(store, entry->contact_ent_handle);
                if (entity_is_valid_and_active(constraint_ent)) {
                    if (constraint_ent->contact_constraint_data.last_iteration >= G.tick.tick_id) {
                        /* Already processed constraint this iteration */
                        continue;
                    } else {
                        ++constraint_ent->contact_constraint_data.last_iteration;
                    }
                } else {
                    /* Constraint ent no longer valid, delete entry */
                    contact_lookup_remove(&G.contact_lookup, entry);
                    entry = NULL;
                }
            }

            /* Calculate collision */
            struct collider_collision_points_result collider_res = collider_collision_points(&e0_collider, &e1_collider, e0_xf, e1_xf);

            /* Parts of algorithm are hard-coded to support 2 contact points */
            CT_ASSERT(ARRAY_COUNT(constraint_ent->contact_constraint_data.points) == 2);
            CT_ASSERT(ARRAY_COUNT(collider_res.points) == 2);

            struct contact_constraint *constraint = NULL;
            if (collider_res.num_points > 0) {
                if (!entity_is_valid_and_active(constraint_ent)) {
                    /* Create hit event */
                    {
                        struct entity *event = entity_alloc(root);
                        entity_enable_prop(event, ENTITY_PROP_HIT_EVENT);
                        event->hit_event.e0 = e0->handle;
                        event->hit_event.e1 = e1->handle;
                        event->hit_event.normal = collider_res.normal;
                        entity_enable_prop(event, ENTITY_PROP_RELEASE_AT_END_OF_FRAME);
                        entity_enable_prop(event, ENTITY_PROP_ACTIVE);

                        /* Calculate point */
                        struct v2 point = collider_res.points[0].point;
                        if (collider_res.num_points > 1) {
                            point = v2_add(point, v2_mul(v2_sub(collider_res.points[1].point, point), 0.5f));
                        }
                        event->hit_event.point = point;

                        /* Calculate relative velocity */
                        struct v2 vrel;
                        {
                            struct v2 v0 = e0->linear_velocity;
                            struct v2 v1 = e1->linear_velocity;
                            f32 w0 = e0->angular_velocity;
                            f32 w1 = e1->angular_velocity;
                            struct v2 vcp0 = v2_sub(point, e0_xf.og);
                            struct v2 vcp1 = v2_sub(point, e1_xf.og);
                            struct v2 vel0 = v2_add(v0, v2_perp_mul(vcp0, w0));
                            struct v2 vel1 = v2_add(v1, v2_perp_mul(vcp1, w1));
                            vrel = v2_sub(vel0, vel1);
                        }
                        event->hit_event.vrel = vrel;
                    }

                    /* Create constraint */
                    {
                        constraint_ent = entity_alloc(root);
                        constraint_ent->contact_constraint_data.e1 = e1->handle;
                        constraint_ent->contact_constraint_data.e0 = e0->handle;
                        constraint_ent->contact_constraint_data.skip_solve = entity_has_prop(e0, ENTITY_PROP_SENSOR) || entity_has_prop(e1, ENTITY_PROP_SENSOR)
                                                                             || !(entity_has_prop(e0, ENTITY_PROP_PHYSICAL_DYNAMIC) || entity_has_prop(e1, ENTITY_PROP_PHYSICAL_DYNAMIC));
                        entity_enable_prop(constraint_ent, ENTITY_PROP_ACTIVE);

                        /* TODO: Should we recalculate normal as more contact points are added? */
                        entity_enable_prop(constraint_ent, ENTITY_PROP_CONTACT_CONSTRAINT);
                        activate_now(constraint_ent);
                        ASSERT(!entry);  /* Existing entry should never be present here */
                        contact_lookup_set(&G.contact_lookup, lookup_hash, constraint_ent->handle);
                    }
                }
                constraint = &constraint_ent->contact_constraint_data;
                constraint->normal = collider_res.normal;
                constraint->friction = math_sqrt(e0->friction * e1->friction);

                /* Delete old contacts that are no longer present */
                for (u32 i = 0; i < constraint->num_points; ++i) {
                    struct contact_point *old = &constraint->points[i];
                    u32 id = old->id;
                    b32 found = false;
                    for (u32 j = 0; j < collider_res.num_points; ++j) {
                        if (collider_res.points[j].id == id) {
                            found = true;
                            break;
                        }
                    }
                    if (!found) {
                        /* Delete contact by replacing with last in array */
                        *old = constraint->points[--constraint->num_points];
                        --i;
                    }
                }

                /* Update / insert returned contacts */
                for (u32 i = 0; i < collider_res.num_points; ++i) {
                    struct collider_collision_point *res_point = &collider_res.points[i];
                    struct v2 point = res_point->point;
                    f32 sep = res_point->separation;
                    u32 id = res_point->id;
                    struct contact_point *contact = NULL;
                    /* Match */
                    for (u32 j = 0; j < constraint->num_points; ++j) {
                        struct contact_point *t = &constraint->points[j];
                        if (t->id == id) {
                            contact = t;
                            break;
                        }
                    }
                    if (!contact) {
                        /* Insert */
                        contact = &constraint->points[constraint->num_points++];
                        MEMZERO_STRUCT(contact);
                        contact->id = id;
                        constraint->softness = G.contact_softness;
                        constraint->pushout_velocity = 3.0f;
                    }

                    /* Update points & separation */
                    contact->point_local_e0 = xform_invert_mul_v2(e0_xf, point);
                    contact->point_local_e1 = xform_invert_mul_v2(e1_xf, point);
                    contact->starting_separation = sep;

#if COLLIDER_DEBUG
                    contact->dbg_pt = point;
#endif
                }
            } else if (constraint_ent->valid) {
                constraint_ent->contact_constraint_data.num_points = 0;
            }

            /* TODO: Remove this (debugging) */
#if COLLIDER_DEBUG
            {
                struct string fdkey = STRING_FROM_BUFFER(BUFFER_FROM_STRUCT(&lookup_hash));
                struct entity_handle *dbg_ent_handle = fixed_dict_get(&G.collision_debug_lookup.dict, fdkey);
                if (!dbg_ent_handle) {
                    /* FIXME: Handle never released */
                    dbg_ent_handle = arena_push_zero(&G.collision_debug_lookup.arena, struct entity_handle);
                }

                struct entity *dbg_ent = entity_from_handle(store, *dbg_ent_handle);

                if (!dbg_ent->valid) {
                    /* FIXME: Entity never released */
                    dbg_ent = entity_alloc(root);
                    entity_enable_prop(dbg_ent, ENTITY_PROP_COLLISION_DEBUG);
                    *dbg_ent_handle = dbg_ent->handle;
                    fixed_dict_set(&G.collision_debug_lookup.arena, &G.collision_debug_lookup.dict, fdkey, dbg_ent_handle);
                }

                struct collision_debug *dbg = &dbg_ent->collision_debug_data;

                if (dbg->res.num_points == 0) {
                    if (collider_res.num_points > 0) {
                        ++e0->colliding;
                        ++e1->colliding;
                    }
                } else {
                    if (collider_res.num_points == 0) {
                        --e0->colliding;
                        --e1->colliding;
                    }
                }
                dbg->e0 = e0->handle;
                dbg->e1 = e1->handle;
                dbg->res = collider_res;

                if (constraint) {
                    MEMCPY(dbg->points, constraint->points, sizeof(dbg->points));
                    dbg->num_points = constraint->num_points;
                } else {
                    dbg->num_points = 0;
                }

                dbg->xf0 = e0_xf;
                dbg->xf1 = e1_xf;

                /* Update closest points */
                {
                    struct collider_closest_points_result closest_points_res = collider_closest_points(&e0_collider, &e1_collider, e0_xf, e1_xf);
                    dbg->closest0 = closest_points_res.p0;
                    dbg->closest1 = closest_points_res.p1;
                }
            }
#endif
        }
    }
}

INTERNAL void prepare_contacts(void)
{
    struct entity_store *store = G.tick.entity_store;
    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *constraint_ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(constraint_ent)) continue;
        if (!entity_has_prop(constraint_ent, ENTITY_PROP_CONTACT_CONSTRAINT)) continue;

        struct contact_constraint *constraint = &constraint_ent->contact_constraint_data;

        u32 num_points = constraint->num_points;
        struct entity *e0 = entity_from_handle(store, constraint->e0);
        struct entity *e1 = entity_from_handle(store, constraint->e1);
        if (num_points > 0 && entity_is_valid_and_active(e0) && entity_is_valid_and_active(e1)) {
            struct v2 normal = constraint->normal;
            struct v2 tangent = v2_perp(normal);

            struct xform e0_xf = entity_get_xform(e0);
            struct xform e1_xf = entity_get_xform(e1);

            /* TODO: Cache this */
            /* Calculate masses */
            f32 inv_m0;
            f32 inv_m1;
            f32 inv_i0;
            f32 inv_i1;
            {
                f32 scale0 = math_fabs(xform_get_determinant(e0_xf));
                f32 scale1 = math_fabs(xform_get_determinant(e1_xf));
                inv_m0 = 1.f / (e0->mass_unscaled * scale0);
                inv_m1 = 1.f / (e1->mass_unscaled * scale1);
                inv_i0 = 1.f / (e0->inertia_unscaled * scale0);
                inv_i1 = 1.f / (e1->inertia_unscaled * scale1);
            }
            constraint->inv_m0 = inv_m0;
            constraint->inv_m1 = inv_m1;
            constraint->inv_i0 = inv_i0;
            constraint->inv_i1 = inv_i1;

            if (entity_has_prop(e0, ENTITY_PROP_PHYSICAL_KINEMATIC)) {
                constraint->inv_m0 = 0;
                constraint->inv_i0 = 0;
            }
            if (entity_has_prop(e1, ENTITY_PROP_PHYSICAL_KINEMATIC)) {
                constraint->inv_m1 = 0;
                constraint->inv_i1 = 0;
            }

            /* Update / insert returned contacts */
            for (u32 i = 0; i < num_points; ++i) {
                struct contact_point *contact = &constraint->points[i];

                struct v2 vcp0 = v2_sub(xform_mul_v2(e0_xf, contact->point_local_e0), e0_xf.og);
                struct v2 vcp1 = v2_sub(xform_mul_v2(e1_xf, contact->point_local_e1), e1_xf.og);

                /* Normal mass */
                {
                    f32 vcp0_wedge = v2_wedge(vcp0, normal);
                    f32 vcp1_wedge = v2_wedge(vcp1, normal);
                    f32 k = (inv_m0 + inv_m1) + (inv_i0 * vcp0_wedge * vcp0_wedge) + (inv_i1 * vcp1_wedge * vcp1_wedge);
                    contact->inv_normal_mass = k > 0.0f ? 1.0f / k : 0.0f;
                }

                /* Tangent mass */
                {
                    f32 vcp0_wedge = v2_wedge(vcp0, tangent);
                    f32 vcp1_wedge = v2_wedge(vcp1, tangent);
                    f32 k = (inv_m0 + inv_m1) + (inv_i0 * vcp0_wedge * vcp0_wedge) + (inv_i1 * vcp1_wedge * vcp1_wedge);
                    contact->inv_tangent_mass = k > 0.0f ? 1.0f / k : 0.0f;
                }

#if !GAME_PHYSICS_ENABLE_WARM_STARTING
                contact->normal_impulse = 0;
                contact->tangent_impulse = 0;
#endif
            }
        } else {
            /* Mark constraint for removal */
            entity_disable_prop(constraint_ent, ENTITY_PROP_ACTIVE);
            entity_enable_prop(constraint_ent, ENTITY_PROP_RELEASE_AT_END_OF_FRAME);
            /* Remove from lookup */
            u64 hash = contact_lookup_hash_from_entities(constraint->e0, constraint->e1);
            struct contact_lookup_entry *entry = contact_lookup_get(&G.contact_lookup, hash);
            if (entry) {
                contact_lookup_remove(&G.contact_lookup, entry);
            } else {
                ASSERT(false);  /* This should always exist */
            }
        }
    }
}

INTERNAL void warm_start_contacts(void)
{
    struct entity_store *store = G.tick.entity_store;

    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *constraint_ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(constraint_ent)) continue;
        if (!entity_has_prop(constraint_ent, ENTITY_PROP_CONTACT_CONSTRAINT)) continue;

        struct contact_constraint *constraint = &constraint_ent->contact_constraint_data;

        u32 num_points = constraint->num_points;
        struct entity *e0 = entity_from_handle(store, constraint->e0);
        struct entity *e1 = entity_from_handle(store, constraint->e1);

        if (num_points > 0 && entity_is_valid_and_active(e0) && entity_is_valid_and_active(e1) && !constraint->skip_solve) {
            struct xform e0_xf = entity_get_xform(e0);
            struct xform e1_xf = entity_get_xform(e1);

            f32 inv_m0 = constraint->inv_m0;
            f32 inv_m1 = constraint->inv_m1;
            f32 inv_i0 = constraint->inv_i0;
            f32 inv_i1 = constraint->inv_i1;

            struct v2 v0 = e0->linear_velocity;
            struct v2 v1 = e1->linear_velocity;
            f32 w0 = e0->angular_velocity;
            f32 w1 = e1->angular_velocity;

            /* Warm start */
            struct v2 normal = constraint->normal;
            struct v2 tangent = v2_perp(normal);
            f32 inv_num_points = 1.f / num_points;
            for (u32 i = 0; i < num_points; ++i) {
                struct contact_point *point = &constraint->points[i];
                struct v2 vcp0 = v2_sub(xform_mul_v2(e0_xf, point->point_local_e0), e0_xf.og);
                struct v2 vcp1 = v2_sub(xform_mul_v2(e1_xf, point->point_local_e1), e1_xf.og);

                struct v2 impulse = v2_add(v2_mul(normal, point->normal_impulse), v2_mul(tangent, point->tangent_impulse));
                impulse = v2_mul(impulse, inv_num_points);

                v0 = v2_sub(v0, v2_mul(impulse, inv_m0));
                v1 = v2_add(v1, v2_mul(impulse, inv_m1));
                w0 -= v2_wedge(vcp0, impulse) * inv_i0;
                w1 += v2_wedge(vcp1, impulse) * inv_i1;
            }

            e0->linear_velocity = v0;
            e0->angular_velocity = w0;
            e1->linear_velocity = v1;
            e1->angular_velocity = w1;
        }
    }
}

INTERNAL void solve_contacts(f32 dt, b32 apply_bias)
{
    struct entity_store *store = G.tick.entity_store;
    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *constraint_ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(constraint_ent)) continue;
        if (!entity_has_prop(constraint_ent, ENTITY_PROP_CONTACT_CONSTRAINT)) continue;

        struct contact_constraint *constraint = &constraint_ent->contact_constraint_data;

        struct entity *e0 = entity_from_handle(store, constraint->e0);
        struct entity *e1 = entity_from_handle(store, constraint->e1);

        struct v2 v0 = e0->linear_velocity;
        struct v2 v1 = e1->linear_velocity;
        f32 w0 = e0->angular_velocity;
        f32 w1 = e1->angular_velocity;

        u32 num_points = constraint->num_points;
        if (num_points > 0 && entity_is_valid_and_active(e0) && entity_is_valid_and_active(e1) && !constraint->skip_solve) {
            struct xform e0_xf = entity_get_xform(e0);
            struct xform e1_xf = entity_get_xform(e1);

            f32 inv_m0 = constraint->inv_m0;
            f32 inv_m1 = constraint->inv_m1;
            f32 inv_i0 = constraint->inv_i0;
            f32 inv_i1 = constraint->inv_i1;

            /* Normal impulse */
            struct v2 normal = constraint->normal;
            for (u32 point_index = 0; point_index < num_points; ++point_index) {
                struct contact_point *point = &constraint->points[point_index];
                struct v2 p0 = xform_mul_v2(e0_xf, point->point_local_e0);
                struct v2 p1 = xform_mul_v2(e1_xf, point->point_local_e1);
                struct v2 vcp0 = v2_sub(p0, e0_xf.og);
                struct v2 vcp1 = v2_sub(p1, e1_xf.og);

                f32 separation = v2_dot(v2_sub(p1, p0), normal) + point->starting_separation;

                f32 velocity_bias = 0.0f;
                f32 mass_scale = 1.0f;
                f32 impulse_scale = 0.0f;

                if (separation > 0.0f) {
                    /* Speculative */
                    velocity_bias = separation / dt;
                } else if (apply_bias) {
                    /* Soft constraint */
                    struct math_spring_result softness = constraint->softness;
                    f32 pushout_velocity = constraint->pushout_velocity;
                    mass_scale = softness.mass_scale;
                    impulse_scale = softness.impulse_scale;
                    velocity_bias = max_f32(softness.bias_rate * separation, -pushout_velocity);
                }

                struct v2 vel0 = v2_add(v0, v2_perp_mul(vcp0, w0));
                struct v2 vel1 = v2_add(v1, v2_perp_mul(vcp1, w1));
                struct v2 vrel = v2_sub(vel0, vel1);

                f32 k = point->inv_normal_mass;

                /* (to be applied along n) */
                f32 vn = v2_dot(vrel, normal);
                f32 j = ((k * mass_scale) * (vn - velocity_bias)) - (point->normal_impulse * impulse_scale);

                f32 old_impulse = point->normal_impulse;
                f32 new_impulse = max_f32(old_impulse + j, 0);
                f32 delta = new_impulse - old_impulse;
                point->normal_impulse = new_impulse;

                struct v2 impulse = v2_mul(normal, delta);
                v0 = v2_sub(v0, v2_mul(impulse, inv_m0));
                v1 = v2_add(v1, v2_mul(impulse, inv_m1));
                w0 -= v2_wedge(vcp0, impulse) * inv_i0;
                w1 += v2_wedge(vcp1, impulse) * inv_i1;
            }

            /* Tangent impulse */
            struct v2 tangent = v2_perp(normal);
            for (u32 point_index = 0; point_index < num_points; ++point_index) {
                struct contact_point *point = &constraint->points[point_index];
                struct v2 vcp0 = v2_sub(xform_mul_v2(e0_xf, point->point_local_e0), e0_xf.og);
                struct v2 vcp1 = v2_sub(xform_mul_v2(e1_xf, point->point_local_e1), e1_xf.og);

                struct v2 vel0 = v2_add(v0, v2_perp_mul(vcp0, w0));
                struct v2 vel1 = v2_add(v1, v2_perp_mul(vcp1, w1));
                struct v2 vrel = v2_sub(vel0, vel1);

                f32 k = point->inv_tangent_mass;

                /* (to be applied along t) */
                f32 vt = v2_dot(vrel, tangent);
                f32 j = vt * k;

                f32 max_friction = constraint->friction * point->normal_impulse;
                f32 old_impulse = point->tangent_impulse;
                f32 new_impulse = clamp_f32(old_impulse + j, -max_friction, max_friction);
                f32 delta = new_impulse - old_impulse;
                point->tangent_impulse = new_impulse;

                struct v2 impulse = v2_mul(tangent, delta);
                v0 = v2_sub(v0, v2_mul(impulse, inv_m0));
                v1 = v2_add(v1, v2_mul(impulse, inv_m1));
                w0 -= v2_wedge(vcp0, impulse) * inv_i0;
                w1 += v2_wedge(vcp1, impulse) * inv_i1;
            }

            e0->linear_velocity = v0;
            e0->angular_velocity = w0;
            e1->linear_velocity = v1;
            e1->angular_velocity = w1;
        }
    }
}







/* ========================== *
 * TESTING MOTOR JOINT
 * ========================== */

INTERNAL void prepare_motor_joints(void)
{
    struct entity_store *store = G.tick.entity_store;
    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *joint_ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(joint_ent)) continue;
        if (!entity_has_prop(joint_ent, ENTITY_PROP_MOTOR_JOINT)) continue;

        struct motor_joint *joint = &joint_ent->motor_joint_data;

        struct entity *e0 = entity_from_handle(store, joint->e0);
        struct entity *e1 = entity_from_handle(store, joint->e1);

        if (entity_is_valid_and_active(e0) && entity_is_valid_and_active(e1)) {
            struct xform e0_xf = entity_get_xform(e0);
            struct xform e1_xf = entity_get_xform(e1);

            /* TODO: Cache this */
            /* Calculate masses */
            f32 inv_m0;
            f32 inv_m1;
            f32 inv_i0;
            f32 inv_i1;
            {
                f32 scale0 = math_fabs(xform_get_determinant(e0_xf));
                f32 scale1 = math_fabs(xform_get_determinant(e1_xf));
                inv_m0 = 1.f / (e0->mass_unscaled * scale0);
                inv_m1 = 1.f / (e1->mass_unscaled * scale1);
                inv_i0 = 1.f / (e0->inertia_unscaled * scale0);
                inv_i1 = 1.f / (e1->inertia_unscaled * scale1);
            }
            joint->inv_m0 = inv_m0;
            joint->inv_m1 = inv_m1;
            joint->inv_i0 = inv_i0;
            joint->inv_i1 = inv_i1;

            joint->point_local_e0 = V2(0, 0);
            joint->point_local_e1 = V2(0, 0);

            struct v2 vcp0 = v2_sub(xform_mul_v2(e0_xf, joint->point_local_e0), e0_xf.og);
            struct v2 vcp1 = v2_sub(xform_mul_v2(e1_xf, joint->point_local_e1), e1_xf.og);

            struct xform linear_mass_xf;
            linear_mass_xf.bx.x = inv_m0 + inv_m1 + vcp0.y * vcp0.y * inv_i0 + vcp1.y * vcp1.y * inv_i1;
            linear_mass_xf.bx.y = -vcp0.y * vcp0.x * inv_i0 - vcp1.y * vcp1.x * inv_i1;
            linear_mass_xf.by.x = linear_mass_xf.bx.y;
            linear_mass_xf.by.y = inv_m0 + inv_m1 + vcp0.x * vcp0.x * inv_i0 + vcp1.x * vcp1.x * inv_i1;
            joint->linear_mass_xf = xform_invert(linear_mass_xf);

            joint->angular_mass = 1.f / (inv_i0 + inv_i1);

#if !GAME_PHYSICS_ENABLE_WARM_STARTING
            joint->linear_impulse = V2(0, 0);
            joint->angular_impulse = 0;
#endif
        } else {
            /* Mark joint for removal */
            entity_disable_prop(joint_ent, ENTITY_PROP_ACTIVE);
            entity_enable_prop(joint_ent, ENTITY_PROP_RELEASE_AT_END_OF_FRAME);
        }
    }
}

INTERNAL void warm_start_motor_joints(void)
{
    struct entity_store *store = G.tick.entity_store;
    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *joint_ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(joint_ent)) continue;
        if (!entity_has_prop(joint_ent, ENTITY_PROP_MOTOR_JOINT)) continue;

        struct motor_joint *joint = &joint_ent->motor_joint_data;

        struct entity *e0 = entity_from_handle(store, joint->e0);
        struct entity *e1 = entity_from_handle(store, joint->e1);

        struct xform e0_xf = entity_get_xform(e0);
        struct xform e1_xf = entity_get_xform(e1);

        f32 inv_m0 = joint->inv_m0;
        f32 inv_m1 = joint->inv_m1;
        f32 inv_i0 = joint->inv_i0;
        f32 inv_i1 = joint->inv_i1;

        struct v2 vcp0 = v2_sub(xform_mul_v2(e0_xf, joint->point_local_e0), e0_xf.og);
        struct v2 vcp1 = v2_sub(xform_mul_v2(e1_xf, joint->point_local_e1), e1_xf.og);

        e0->linear_velocity = v2_sub(e0->linear_velocity, v2_mul(joint->linear_impulse, inv_m0));
        e1->linear_velocity = v2_add(e1->linear_velocity, v2_mul(joint->linear_impulse, inv_m1));
        e0->angular_velocity -= (v2_wedge(vcp0, joint->linear_impulse) + joint->angular_impulse) * inv_i0;
        e1->angular_velocity += (v2_wedge(vcp1, joint->linear_impulse) + joint->angular_impulse) * inv_i1;
    }
}

INTERNAL void solve_motor_joints(f32 dt)
{
    struct entity_store *store = G.tick.entity_store;
    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *joint_ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(joint_ent)) continue;
        if (!entity_has_prop(joint_ent, ENTITY_PROP_MOTOR_JOINT)) continue;

        struct motor_joint *joint = &joint_ent->motor_joint_data;

        struct entity *e0 = entity_from_handle(store, joint->e0);
        struct entity *e1 = entity_from_handle(store, joint->e1);

        struct xform e0_xf = entity_get_xform(e0);
        struct xform e1_xf = entity_get_xform(e1);

        f32 inv_m0 = joint->inv_m0;
        f32 inv_m1 = joint->inv_m1;
        f32 inv_i0 = joint->inv_i0;
        f32 inv_i1 = joint->inv_i1;

        struct v2 v0 = e0->linear_velocity;
        struct v2 v1 = e1->linear_velocity;
        f32 w0 = e0->angular_velocity;
        f32 w1 = e1->angular_velocity;

        f32 correction_rate = joint->correction_rate / dt;

        /* Angular constraint */
        {
            f32 max_impulse = joint->max_torque * dt;

            f32 angular_separation = math_unwind_angle(xform_get_rotation(e1_xf) - xform_get_rotation(e0_xf));
            f32 angular_bias = angular_separation * correction_rate;

            f32 impulse = -joint->angular_mass * (w1 - w0 + angular_bias);

            f32 old_impulse = joint->angular_impulse;
            joint->angular_impulse = clamp_f32(joint->angular_impulse + impulse, -max_impulse, max_impulse);
            impulse = joint->angular_impulse - old_impulse;

            w0 -= impulse * inv_i0;
            w1 += impulse * inv_i1;
        }

        /* Linear constraint */
        {
            struct v2 vcp0 = v2_sub(xform_mul_v2(e0_xf, joint->point_local_e0), e0_xf.og);
            struct v2 vcp1 = v2_sub(xform_mul_v2(e1_xf, joint->point_local_e1), e1_xf.og);

            f32 max_impulse = joint->max_force * dt;

            struct v2 linear_separation = v2_sub(v2_add(e1_xf.og, vcp1), v2_add(e0_xf.og, vcp0));
            struct v2 linear_bias = v2_mul(linear_separation, correction_rate);

            struct v2 vrel = v2_sub(v2_add(v1, v2_perp_mul(vcp1, w1)), v2_add(v0, v2_perp_mul(vcp0, w0)));
            struct v2 impulse = v2_neg(xform_basis_mul_v2(joint->linear_mass_xf, v2_add(vrel, linear_bias)));

            struct v2 old_impulse = joint->linear_impulse;
            joint->linear_impulse = v2_clamp_len(v2_add(joint->linear_impulse, impulse), max_impulse);
            impulse = v2_sub(joint->linear_impulse, old_impulse);

            v0 = v2_sub(v0, v2_mul(impulse, inv_m0));
            v1 = v2_add(v1, v2_mul(impulse, inv_m1));
            w0 -= v2_wedge(vcp0, impulse) * inv_i0;
            w1 += v2_wedge(vcp1, impulse) * inv_i1;
        }

        e0->linear_velocity = v0;
        e0->angular_velocity = w0;
        e1->linear_velocity = v1;
        e1->angular_velocity = w1;
    }
}

#if 1

/* ========================== *
 * TESTING MOUSE JOINT
 * ========================== */

INTERNAL void create_mouse_joints(struct game_cmd_array game_cmds)
{
    struct entity_store *store = G.tick.entity_store;
    struct entity *root = G.root;

    b32 start_dragging = false;
    b32 stop_dragging = false;
    for (u64 i = 0; i < game_cmds.count; ++i) {
        struct game_cmd cmd = game_cmds.cmds[i];
        b32 start = cmd.state == GAME_CMD_STATE_START;
        b32 stop = cmd.state == GAME_CMD_STATE_STOP;
        switch (cmd.kind) {
            case GAME_CMD_KIND_DRAG_OBJECT:
            {
                if (start) {
                    start_dragging = true;
                } else if (stop) {
                    stop_dragging = true;
                }
            } break;
            default: break;
        }
    }

    struct v2 cursor = G.user_cursor;

    struct entity *joint_ent = entity_find_first_match_one(store, ENTITY_PROP_MOUSE_JOINT);
    struct entity *target_ent = entity_from_handle(store, joint_ent->mouse_joint_data.target);

    if (start_dragging) {
        struct xform mouse_xf = xform_from_pos(cursor);
        struct collider_shape mouse_shape = ZI;
        mouse_shape.points[0] = V2(0, 0);
        mouse_shape.count = 1;

        for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
            struct entity *ent = &store->entities[entity_index];
            if (!entity_is_valid_and_active(ent)) continue;
            if (!entity_has_prop(ent, ENTITY_PROP_PHYSICAL_DYNAMIC)) continue;

            struct collider_shape ent_collider = ent->local_collider;
            if (ent_collider.count > 0) {
                struct xform ent_xf = entity_get_xform(ent);
                /* TODO: Can just use boolean GJK */
                struct collider_collision_points_result res = collider_collision_points(&ent_collider, &mouse_shape, ent_xf, mouse_xf);
                if (res.num_points > 0) {
                    target_ent = ent;
                    break;
                }
            }
        }
    } else if (stop_dragging) {
        target_ent = entity_nil();
    }

    if (entity_is_valid_and_active(target_ent)) {
        if (!entity_is_valid_and_active(joint_ent)) {
            joint_ent = entity_alloc(root);
            joint_ent->mass_unscaled = F32_INFINITY;
            joint_ent->inertia_unscaled = F32_INFINITY;
            entity_enable_prop(joint_ent, ENTITY_PROP_MOUSE_JOINT);
            entity_enable_prop(joint_ent, ENTITY_PROP_ACTIVE);
        }

        struct mouse_joint *joint = &joint_ent->mouse_joint_data;

        struct xform xf = entity_get_xform(target_ent);
        f32 mass = target_ent->mass_unscaled * math_fabs(xform_get_determinant(xf));

        if (!entity_handle_eq(joint->target, target_ent->handle)) {
            joint->point_local_start = xform_invert_mul_v2(xf, cursor);
            joint->target = target_ent->handle;
        }
        joint->point_local_end = xform_invert_mul_v2(xf, cursor);

        joint->linear_softness = G.mouse_joint_linear_softness;
        joint->angular_softness = G.mouse_joint_angular_softness;
        joint->max_force = G.mouse_joint_max_force * mass;
    } else {
        if (entity_is_valid_and_active(joint_ent)) {
            joint_ent->mouse_joint_data.target = entity_nil_handle();
        }
    }
}

INTERNAL void prepare_mouse_joints(void)
{
    struct entity_store *store = G.tick.entity_store;
    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *joint_ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(joint_ent)) continue;
        if (!entity_has_prop(joint_ent, ENTITY_PROP_MOUSE_JOINT)) continue;

        struct mouse_joint *joint = &joint_ent->mouse_joint_data;
        struct entity *ent = entity_from_handle(store, joint->target);
        if (entity_is_valid_and_active(ent)) {
            struct xform xf = entity_get_xform(ent);

            /* TODO: Cache this */
            /* Calculate masses */
            f32 inv_m;
            f32 inv_i;
            {
                f32 scale = math_fabs(xform_get_determinant(xf));
                inv_m = 1.f / (ent->mass_unscaled * scale);
                inv_i = 1.f / (ent->inertia_unscaled * scale);
            }
            joint->inv_m = inv_m;
            joint->inv_i = inv_i;

            struct v2 vcp = v2_sub(xform_mul_v2(xf, joint->point_local_start), xf.og);

            struct xform linear_mass_xf;
            linear_mass_xf.bx.x = inv_m + inv_i * vcp.y * vcp.y;
            linear_mass_xf.bx.y = -inv_i * vcp.x * vcp.y;
            linear_mass_xf.by.x = linear_mass_xf.bx.y;
            linear_mass_xf.by.y = inv_m + inv_i * vcp.x * vcp.x;
            joint->linear_mass_xf = xform_invert(linear_mass_xf);

#if !GAME_PHYSICS_ENABLE_WARM_STARTING
            joint->linear_impulse = V2(0, 0);
            joint->angular_impulse = 0;
#endif
        } else {
            /* Mark joint for removal */
            entity_disable_prop(joint_ent, ENTITY_PROP_ACTIVE);
            entity_enable_prop(joint_ent, ENTITY_PROP_RELEASE_AT_END_OF_FRAME);
        }
    }
}

INTERNAL void warm_start_mouse_joints(void)
{
    struct entity_store *store = G.tick.entity_store;
    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *joint_ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(joint_ent)) continue;
        if (!entity_has_prop(joint_ent, ENTITY_PROP_MOUSE_JOINT)) continue;

        struct mouse_joint *joint = &joint_ent->mouse_joint_data;
        struct entity *ent = entity_from_handle(store, joint->target);
        if (entity_is_valid_and_active(ent)) {
            f32 inv_m = joint->inv_m;
            f32 inv_i = joint->inv_i;
            struct xform xf = entity_get_xform(ent);
            struct v2 vcp = v2_sub(xform_mul_v2(xf, joint->point_local_start), xf.og);
            ent->linear_velocity = v2_add(ent->linear_velocity, v2_mul(joint->linear_impulse, inv_m));
            ent->angular_velocity += (v2_wedge(vcp, joint->linear_impulse) + joint->angular_impulse) * inv_i;
        }
    }
}

INTERNAL void solve_mouse_joints(f32 dt)
{
    struct entity_store *store = G.tick.entity_store;
    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *joint_ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(joint_ent)) continue;
        if (!entity_has_prop(joint_ent, ENTITY_PROP_MOUSE_JOINT)) continue;

        struct mouse_joint *joint = &joint_ent->mouse_joint_data;
        struct entity *ent = entity_from_handle(store, joint->target);
        if (entity_is_valid_and_active(ent)) {
            struct v2 v = ent->linear_velocity;
            f32 w = ent->angular_velocity;

            f32 inv_m = joint->inv_m;
            f32 inv_i = joint->inv_i;

            /* Angular impulse */
            {
                struct math_spring_result softness = joint->angular_softness;
                f32 mass_scale = softness.mass_scale;
                f32 impulse_scale = softness.impulse_scale;
                f32 impulse = mass_scale * (-w / inv_i) - impulse_scale * joint->angular_impulse;
                joint->angular_impulse += impulse;
                w += impulse * inv_i;
            }

            /* Linear impulse */
            {
                f32 max_impulse = joint->max_force / dt;

                struct xform xf = entity_get_xform(ent);

                struct v2 point_start = xform_mul_v2(xf, joint->point_local_start);
                struct v2 point_end = xform_mul_v2(xf, joint->point_local_end);

                struct v2 vcp = v2_sub(point_start, xf.og);
                struct v2 separation = v2_sub(point_start, point_end);

                struct math_spring_result softness = joint->linear_softness;
                struct v2 bias = v2_mul(separation, softness.bias_rate);
                f32 mass_scale = softness.mass_scale;
                f32 impulse_scale = softness.impulse_scale;

#if 0
                struct v2 vel = v2_add(v, v2_perp_mul(vcp, w));
                struct v2 b = v2_mul(xform_basis_mul_v2(xform_invert(joint->linear_mass_xf), v2_add(vel, bias)), mass_scale);

                struct v2 old_impulse = joint->linear_impulse;
                struct v2 impulse = v2_sub(v2_mul(old_impulse, -impulse_scale), b);
                joint->linear_impulse = v2_clamp_len(v2_add(joint->linear_impulse, impulse), max_impulse);
                impulse = v2_sub(joint->linear_impulse, old_impulse);
#else
                struct v2 vel = v2_add(v, v2_perp_mul(vcp, w));
                struct v2 b = xform_basis_mul_v2(joint->linear_mass_xf, v2_add(vel, bias));

                struct v2 impulse;
                impulse.x = -mass_scale * b.x - impulse_scale * joint->linear_impulse.x;
                impulse.y = -mass_scale * b.y - impulse_scale * joint->linear_impulse.y;

                struct v2 old_impulse = joint->linear_impulse;
                joint->linear_impulse.x += impulse.x;
                joint->linear_impulse.y += impulse.y;

                joint->linear_impulse = v2_clamp_len(joint->linear_impulse, max_impulse);

                impulse.x = joint->linear_impulse.x - old_impulse.x;
                impulse.y = joint->linear_impulse.y - old_impulse.y;
#endif

                v = v2_add(v, v2_mul(impulse, inv_m));
                w += v2_wedge(vcp, impulse) * inv_i;
            }

            ent->linear_velocity = v;
            ent->angular_velocity = w;
        }
    }
}

#else

INTERNAL void prepare_mouse_joints(void)
{
}

INTERNAL void warm_start_mouse_joints(void)
{
}

INTERNAL void solve_mouse_joints(f32 dt)
{
    (UNUSED)dt;
}

#endif





/* ========================== *
 * TESTING PHYSICS INTEGRATION
 * ========================== */

INTERNAL void integrate_velocities_from_forces(f32 dt)
{
    struct entity_store *store = G.tick.entity_store;
    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(ent)) continue;
        if (!entity_has_prop(ent, ENTITY_PROP_PHYSICAL_DYNAMIC) && !entity_has_prop(ent, ENTITY_PROP_PHYSICAL_KINEMATIC)) continue;

        struct xform xf = entity_get_xform(ent);
        f32 det_abs = math_fabs(xform_get_determinant(xf));
        f32 mass = ent->mass_unscaled * det_abs;
        f32 inertia = ent->inertia_unscaled * det_abs;

        /* Determine force & torque acceleration */
        struct v2 force_accel = v2_mul(v2_div(ent->force, mass), dt);
        f32 torque_accel = (ent->torque / inertia) * dt;

        /* Integrate & clamp */
        ent->linear_velocity = v2_clamp_len(v2_add(ent->linear_velocity, force_accel), GAME_MAX_LINEAR_VELOCITY);
        ent->angular_velocity = clamp_f32(ent->angular_velocity + torque_accel, -GAME_MAX_ANGULAR_VELOCITY, GAME_MAX_ANGULAR_VELOCITY);

        /* Reset forces */
        ent->force = V2(0, 0);
        ent->torque = 0;
    }
}

INTERNAL void integrate_positions_from_velocities(f32 dt)
{
    struct entity_store *store = G.tick.entity_store;

    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(ent)) continue;
        if (!entity_has_prop(ent, ENTITY_PROP_PHYSICAL_DYNAMIC) && !entity_has_prop(ent, ENTITY_PROP_PHYSICAL_KINEMATIC)) continue;

        /* Clamp velocities */
        ent->linear_velocity = v2_clamp_len(ent->linear_velocity, GAME_MAX_LINEAR_VELOCITY);
        ent->angular_velocity = clamp_f32(ent->angular_velocity, -GAME_MAX_ANGULAR_VELOCITY, GAME_MAX_ANGULAR_VELOCITY);

        struct v2 tick_linear_velocity = v2_mul(ent->linear_velocity, dt);
        f32 tick_angular_velocity = ent->angular_velocity * dt;

        struct xform xf = entity_get_xform(ent);
        xf.og = v2_add(xf.og, tick_linear_velocity);
        xf = xform_basis_rotated_world(xf, tick_angular_velocity);
        entity_set_xform(ent, xf);
    }
}










/* ========================== *
 * TESTING TOI
 * ========================== */

/* Takes 2 shapes and their xforms at t=0 and t=1.
 * Returns time of impact in range [0, 1]. */
INTERNAL f32 toi(struct collider_shape *c0, struct collider_shape *c1,
                 struct xform xf0_t0, struct xform xf1_t0,
                 struct xform xf0_t1, struct xform xf1_t1,
                 f32 tolerance)
{
    /* Find direction p0 -> p1 at t=0 */
    struct v2 dir;
    struct v2 dir_neg;
    {
        struct collider_closest_points_result closest_points_res = collider_closest_points(c0, c1, xf0_t0, xf1_t0);
        if (closest_points_res.colliding) {
            /* Shapes are penetrating at t=0 */
            return 0;
        }
        dir = v2_norm(v2_sub(closest_points_res.p1, closest_points_res.p0));
        dir_neg = v2_neg(dir);
    }

    /* Safety check that shapes penetrate at t=1 */
    f32 sep;
    {
        struct v2 p0 = collider_support_point(c0, xf0_t1, dir);
        struct v2 p1 = collider_support_point(c1, xf1_t1, dir_neg);
        sep = v2_dot(dir, v2_sub(p1, p0));
        if (sep > tolerance) {
            /* Shapes are not penetrating at t=1 */
            return 1;
        }
    }

    /* Bisect until distance is within tolerance */
    /* TODO: Implement false position method as well? (should speed up more linear cases) */
    f32 t0 = 0.0;
    f32 t1 = 1.0;
    f32 t = 0.5f;
    while (math_fabs(sep) > tolerance) {
        struct xform xf0 = xform_lerp(xf0_t0, xf0_t1, t);
        struct xform xf1 = xform_lerp(xf1_t0, xf1_t1, t);

        struct v2 p0 = collider_support_point(c0, xf0, dir);
        struct v2 p1 = collider_support_point(c1, xf1, dir_neg);

        sep = v2_dot(dir, v2_sub(p1, p0));

        /* Update bracket */
        if (sep > 0) {
            t0 = t;
        } else {
            t1 = t;
        }
        t = (t1 + t0) / 2.0;
    }

    return t;
}

INTERNAL f32 determine_earliest_toi(f32 dt, f32 tolerance)
{
    __prof;
    f32 smallest_t = 1;

    struct entity_store *store = G.tick.entity_store;
    //struct entity *root = G.root;


    for (u64 e0_index = 0; e0_index < store->reserved; ++e0_index) {
        struct entity *e0 = &store->entities[e0_index];
        if (!entity_is_valid_and_active(e0)) continue;
        if (!(entity_has_prop(e0, ENTITY_PROP_PHYSICAL_DYNAMIC) || entity_has_prop(e0, ENTITY_PROP_PHYSICAL_KINEMATIC))) continue;
        if (e0->local_collider.count <= 0) continue;

        struct collider_shape e0_collider = e0->local_collider;
        struct xform e0_xf_t0 = entity_get_xform(e0);
        struct xform e0_xf_t1 = e0_xf_t0;
        {
            /* Calculate xform at t=1 */
            struct v2 linear_velocity = v2_clamp_len(e0->linear_velocity, GAME_MAX_LINEAR_VELOCITY);
            f32 angular_velocity = clamp_f32(e0->angular_velocity, -GAME_MAX_ANGULAR_VELOCITY, GAME_MAX_ANGULAR_VELOCITY);
            struct v2 tick_linear_velocity = v2_mul(linear_velocity, dt);
            f32 tick_angular_velocity = angular_velocity * dt;
            e0_xf_t1.og = v2_add(e0_xf_t1.og, tick_linear_velocity);
            e0_xf_t1 = xform_basis_rotated_world(e0_xf_t1, tick_angular_velocity);
        }


        /* Start e1 index at e0 index + 1 to prevent redundant checks */
        for (u64 e1_index = e0_index + 1; e1_index < store->reserved; ++e1_index) {
            struct entity *e1 = &store->entities[e1_index];
            if (e1 == e0) continue;
            if (!entity_is_valid_and_active(e1)) continue;
            if (!(entity_has_prop(e1, ENTITY_PROP_PHYSICAL_DYNAMIC) || entity_has_prop(e1, ENTITY_PROP_PHYSICAL_KINEMATIC))) continue;
            if (e1->local_collider.count <= 0) continue;

            struct collider_shape e1_collider = e1->local_collider;
            struct xform e1_xf_t0 = entity_get_xform(e1);
            struct xform e1_xf_t1 = e1_xf_t0;
            {
                /* Calculate xform at t=1 */
                struct v2 linear_velocity = v2_clamp_len(e1->linear_velocity, GAME_MAX_LINEAR_VELOCITY);
                f32 angular_velocity = clamp_f32(e1->angular_velocity, -GAME_MAX_ANGULAR_VELOCITY, GAME_MAX_ANGULAR_VELOCITY);
                struct v2 tick_linear_velocity = v2_mul(linear_velocity, dt);
                f32 tick_angular_velocity = angular_velocity * dt;
                e1_xf_t1.og = v2_add(e1_xf_t1.og, tick_linear_velocity);
                e1_xf_t1 = xform_basis_rotated_world(e1_xf_t1, tick_angular_velocity);
            }

            f32 t = toi(&e0_collider, &e1_collider, e0_xf_t0, e1_xf_t0, e0_xf_t1, e1_xf_t1, tolerance);
            if (t != 0 && t < smallest_t) {
                smallest_t = t;
            }

        }
    }

    return smallest_t;
}





















/* ========================== *
 * Update
 * ========================== */

INTERNAL void publish_game_tick(void)
{
    __prof;
    struct sys_lock lock = sys_mutex_lock_e(&G.prev_tick_mutex);
    arena_set_readwrite(&G.prev_tick.entity_store->arena);
    {
        world_copy_replace(&G.prev_tick, &G.tick);
    }
    arena_set_readonly(&G.prev_tick.entity_store->arena);
    atomic_u64_eval_exchange(&G.prev_tick_id, G.prev_tick.tick_id);
    atomic_u64_eval_exchange(&G.prev_tick_continuity_gen, G.prev_tick.continuity_gen);
    sys_mutex_unlock(&lock);
}

INTERNAL void game_update(struct game_cmd_array game_cmds)
{
    __prof;

    struct temp_arena scratch = scratch_begin_no_conflict();

    /* ========================== *
     * Reset level if necessary
     * ========================== */

    if (G.should_reset_level) {
        logf_info("Clearing level");
        G.should_reset_level = false;
        G.extra_spawn = false;
        reset_world();
    }

    /* ========================== *
     * Begin frame
     * ========================== */

    ++G.tick.tick_id;
    G.tick.tick_ts = sys_timestamp();
    G.tick.dt = max_f64(0.0, (1.0 / GAME_FPS) * G.tick.timescale);
    G.tick.time += G.tick.dt;

    f64 dt = G.tick.dt;
    f64 time = G.tick.time;
    G.sprite_frame_scope = sprite_scope_begin();
    G.root = entity_from_handle(G.tick.entity_store, G.tick.entity_store->root);

    struct entity *root = G.root;
    struct entity_store *store = G.tick.entity_store;
    struct sprite_scope *sprite_frame_scope = G.sprite_frame_scope;

    (UNUSED)dt;
    (UNUSED)time;

    /* ========================== *
     * Spawn test entities
     * ========================== */

     /* TODO: remove this (testing) */
     /* Initialize entities */
    {
        static b32 run = 0;
        if (!run) {
            run = 1;
            spawn_test_entities();
        }
    }

    /* ========================== *
     * Process global game cmds
     * ========================== */

    for (u64 cmd_index = 0; cmd_index < game_cmds.count; ++cmd_index) {
        struct game_cmd cmd = game_cmds.cmds[cmd_index];

        switch (cmd.kind) {
            /* Cursor */
            case GAME_CMD_KIND_CURSOR_MOVE:
            {
                G.user_cursor = cmd.cursor_pos;
            } break;

            /* Clear level */
            case GAME_CMD_KIND_CLEAR_ALL:
            {
                G.should_reset_level = true;
            } break;

            /* Spawn test */
            case GAME_CMD_KIND_SPAWN_TEST:
            {
                logf_info("Spawning (test)");
                spawn_test_entities();
            } break;
            default: break;
        };
    }

    /* ========================== *
     * Activate entities
     * ========================== */

    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *ent = &store->entities[entity_index];
        if (!ent->valid) continue;

        if (!entity_has_prop(ent, ENTITY_PROP_ACTIVE)) {
            u64 atick = ent->activation_tick;
            if (atick != 0 || G.tick.tick_id >= atick) {
                activate_now(ent);
            }
        }
    }

    /* ========================== *
     * Reset triggered entities
     * ========================== */

    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(ent)) continue;

        if (entity_has_prop(ent, ENTITY_PROP_TRIGGER_NEXT_TICK)) {
            entity_disable_prop(ent, ENTITY_PROP_TRIGGER_NEXT_TICK);
            entity_enable_prop(ent, ENTITY_PROP_TRIGGERED_THIS_TICK);
        } else if (entity_has_prop(ent, ENTITY_PROP_TRIGGERED_THIS_TICK)) {
            entity_disable_prop(ent, ENTITY_PROP_TRIGGERED_THIS_TICK);
        }
    }

    /* ========================== *
     * Update entity from sprite
     * ========================== */

    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(ent)) continue;
        if (sprite_tag_is_nil(ent->sprite)) continue;

        /* Update animation */

        struct sprite_sheet *sheet = sprite_sheet_from_tag_await(sprite_frame_scope, ent->sprite);
        {
            struct sprite_sheet_span span = sprite_sheet_get_span(sheet, ent->sprite_span_name);

            f64 time_in_frame = ent->animation_time_in_frame + dt;
            u64 frame_index = ent->animation_frame;
            if (frame_index < span.start || frame_index > span.end) {
                frame_index = span.start;
            }

            struct sprite_sheet_frame frame = sprite_sheet_get_frame(sheet, frame_index);
            while (time_in_frame > frame.duration) {
                time_in_frame -= frame.duration;
                ++frame_index;
                if (frame_index > span.end) {
                    /* Loop animation */
                    frame_index = span.start;
                }
                frame = sprite_sheet_get_frame(sheet, frame_index);
            }

            ent->animation_time_in_frame = time_in_frame;
            ent->animation_frame = frame_index;
        }

        /* Update sprite local xform */

        {
            struct sprite_sheet_slice slice = sprite_sheet_get_slice(sheet, STR("pivot"), ent->animation_frame);
            struct v2 sprite_size = v2_div(sheet->frame_size, (f32)PIXELS_PER_UNIT);

            struct v2 dir = v2_mul_v2(sprite_size, slice.dir);
            f32 rot = v2_angle(dir) + PI / 2;

            struct xform xf = XFORM_IDENT;
            xf = xform_rotated(xf, -rot);
            xf = xform_scaled(xf, sprite_size);
            xf = xform_translated(xf, v2_neg(slice.center));
            ent->sprite_local_xform = xf;
        }

        /* Update collider from sprite */
        if (ent->sprite_collider_slice.len > 0) {
            struct xform cxf = ent->sprite_local_xform;

#if 0
            if (entity_has_prop(ent, ENTITY_PROP_TEST)) {
                f32 scale = 0.5;
                cxf = xform_scaled(cxf, V2(scale, scale));
            }
#endif

            struct sprite_sheet_slice slice = sprite_sheet_get_slice(sheet, ent->sprite_collider_slice, ent->animation_frame);
            ent->local_collider = collider_from_quad(xform_mul_quad(cxf, quad_from_rect(slice.rect)));

#if 1
            if (entity_has_prop(ent, ENTITY_PROP_TEST)) {
            //if ((true)) {
#if 0
                ent->local_collider.points[0] = V2(0, 0);
                ent->local_collider.count = 1;
                ent->local_collider.radius = 0.5;
#elif 1
                ent->local_collider.points[0] = v2_with_len(V2(0.08, 0.17), 0.15);
                ent->local_collider.points[1] = v2_with_len(V2(-0.07, -0.2), 0.15);
                ent->local_collider.count = 2;
                ent->local_collider.radius = 0.075;
#elif 1
#if 0
                /* "Bad" winding order */
                ent->local_collider.points[0] = V2(-0.15, 0.15);
                ent->local_collider.points[1] = V2(0.15, 0.15);
                ent->local_collider.points[2] = V2(0, -0.15);
#else
                ent->local_collider.points[0] = V2(0, -0.15);
                ent->local_collider.points[1] = V2(0.15, 0.15);
                ent->local_collider.points[2] = V2(-0.15, 0.15);
#endif
                ent->local_collider.count = 3;
                ent->local_collider.radius = 0.25;
                //ent->local_collider.radius = math_fabs(math_sin(G.tick.time) / 3);
#else
                //ent->local_collider.radius = 0.5;
                ent->local_collider.radius = 0.25;
                //ent->local_collider.radius = 0.;
#endif
            }
#endif
        }
    }

    /* ========================== *
     * Update attachments
     * ========================== */

    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(ent)) continue;
        if (!entity_has_prop(ent, ENTITY_PROP_ATTACHED)) continue;

        struct entity *parent = entity_from_handle(store, ent->parent);
        struct sprite_tag parent_sprite = parent->sprite;
        struct sprite_sheet *parent_sheet = sprite_sheet_from_tag_await(sprite_frame_scope, parent_sprite);

        struct xform parent_sprite_xf = parent->sprite_local_xform;

        struct sprite_sheet_slice attach_slice = sprite_sheet_get_slice(parent_sheet, ent->attach_slice, parent->animation_frame);
        struct v2 attach_pos = xform_mul_v2(parent_sprite_xf, attach_slice.center);
        struct v2 attach_dir = xform_basis_mul_v2(parent_sprite_xf, attach_slice.dir);

        struct xform xf = entity_get_local_xform(ent);
        xf.og = attach_pos;
        xf = xform_basis_with_rotation_world(xf, v2_angle(attach_dir) + PI / 2);
        entity_set_local_xform(ent, xf);
    }

    /* ========================== *
     * Update control from player cmds
     * ========================== */

    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(ent)) continue;

        if (entity_has_prop(ent, ENTITY_PROP_PLAYER_CONTROLLED)) {
            /* Process cmds */
            struct v2 move = ent->control.move;
            struct v2 focus = ent->control.focus;
            b32 firing = entity_has_prop(ent, ENTITY_PROP_TRIGGERING_EQUIPPED);

            for (u64 i = 0; i < game_cmds.count; ++i) {
                struct game_cmd cmd = game_cmds.cmds[i];
                b32 start = cmd.state == GAME_CMD_STATE_START;
                b32 stop = cmd.state == GAME_CMD_STATE_STOP;

                /* TODO: Combine movement from multiple inputs? E.G. a sudden
                 * start and immediate stop cmd should still move the player a
                 * tad. */
                switch (cmd.kind) {
                    case GAME_CMD_KIND_PLAYER_MOVE:
                    {
                        move = cmd.move_dir;
                        focus = cmd.aim_dir;
                    } break;

                    case GAME_CMD_KIND_PLAYER_FIRE:
                    {
                        if (start) {
                            firing = true;
                        } else if (stop) {
                            firing = false;
                        }
                    } break;

                    default: break;
                }
            }

            /* Movement */
            if (v2_len_sq(move) > 1) {
                /* Cap movement vector magnitude at 1 */
                move = v2_norm(move);
            }
            ent->control.move = move;
            ent->control.focus = focus;

            /* Firing */
            if (firing) {
                entity_enable_prop(ent, ENTITY_PROP_TRIGGERING_EQUIPPED);
            } else {
                entity_disable_prop(ent, ENTITY_PROP_TRIGGERING_EQUIPPED);
            }
        }
    }

    /* ========================== *
     * Test
     * ========================== */

#if 0
    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(ent)) continue;
        if (!entity_has_prop(ent, ENTITY_PROP_TEST)) continue;

#if 0
        if (!ent->test_initialized) {
            ent->test_initialized = true;
            ent->test_start_local_xform = entity_get_local_xform(ent);
            ent->test_start_sprite_xform = ent->sprite_local_xform;
        }

        f32 t = (f32)time;
        struct v2 og = v2_mul(V2(math_cos(t), math_sin(t)), 3);
        f32 r = t * 3;
        struct v2 s = V2(1 + (math_fabs(math_sin(t * 5)) * 3), 1);
        (UNUSED)og;
        (UNUSED)r;
        (UNUSED)s;

        og = v2_add(og, ent->test_start_local_xform.og);
        r += xform_get_rotation(ent->test_start_local_xform);
        s = v2_add(s, xform_get_scale(ent->test_start_local_xform));

        struct xform xf = entity_get_local_xform(ent);
        xf.og = og;
        xf = xform_rotated_to(xf, r);
        xf = xform_scaled_to(xf, s);
        entity_set_local_xform(ent, xf);
#else
        f32 t = (f32)time;
        struct v2 og = v2_mul(V2(math_cos(t), math_sin(t)), 3);
        f32 rot = t * PI / 3;
        struct v2 scale = V2(1 + (math_fabs(math_sin(t * 5)) * 3), 1);
        (UNUSED)og;
        (UNUSED)rot;
        (UNUSED)scale;

        struct xform xf = entity_get_local_xform(ent);
        xf = xform_rotated_to(xf, rot);
        xf = xform_scaled_to(xf, scale);
        entity_set_local_xform(ent, xf);
#endif
    }
#endif

    /* ========================== *
     * Trigger equipped
     * ========================== */

    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(ent)) continue;

        if (entity_has_prop(ent, ENTITY_PROP_TRIGGERING_EQUIPPED)) {
            struct entity *eq = entity_from_handle(store, ent->equipped);
            if (entity_is_valid_and_active(eq)) {
                entity_enable_prop(eq, ENTITY_PROP_TRIGGERED_THIS_TICK);
            }
        }
    }

    /* ========================== *
     * Process triggered entities
     * ========================== */

    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(ent)) continue;
        if (!entity_has_prop(ent, ENTITY_PROP_TRIGGERED_THIS_TICK)) continue;
        if ((time - ent->last_triggered < ent->trigger_delay) && ent->last_triggered != 0) continue;

        ent->last_triggered = time;

        /* Fire weapon */
        if (entity_has_prop(ent, ENTITY_PROP_WEAPON)) {
            struct sprite_tag sprite = ent->sprite;
            u32 animation_frame = ent->animation_frame;
            struct sprite_sheet *sheet = sprite_sheet_from_tag_await(sprite_frame_scope, sprite);

            struct xform sprite_local_xform = ent->sprite_local_xform;

            struct sprite_sheet_slice out_slice = sprite_sheet_get_slice(sheet, STR("out"), animation_frame);
            struct v2 rel_pos = xform_mul_v2(sprite_local_xform, out_slice.center);
            struct v2 rel_dir = xform_basis_mul_v2(sprite_local_xform, out_slice.dir);

            {
                /* Spawn bullet */
                struct entity *bullet = entity_alloc(root);
                bullet->sprite = sprite_tag_from_path(STR("res/graphics/bullet.ase"));

                bullet->bullet_src = ent->handle;
                bullet->bullet_src_pos = rel_pos;
                bullet->bullet_src_dir = rel_dir;
                //bullet->bullet_impulse = 0.1f;
                //bullet->bullet_impulse = 0.25f;
                //bullet->bullet_impulse = 5.f;
                bullet->bullet_impulse = 100000.f;
                bullet->mass_unscaled = 0.04f;
                bullet->inertia_unscaled = 0.00001f;
                bullet->sprite_collider_slice = STR("shape");

                entity_enable_prop(bullet, ENTITY_PROP_BULLET);
                entity_enable_prop(bullet, ENTITY_PROP_SENSOR);
            }
        }
    }

    /* ========================== *
     * Create forces from control move
     * ========================== */

#if 0
    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(ent)) continue;

        if (entity_has_prop(ent, ENTITY_PROP_PLAYER_CONTROLLED)) {
            struct v2 move = ent->control.move;
            struct v2 force = v2_mul(move, ent->control_force);
            entity_apply_force_to_center(ent, force);
        }
    }
#else
    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(ent)) continue;

        if (entity_has_prop(ent, ENTITY_PROP_PLAYER_CONTROLLED)) {
            struct entity *joint_ent = entity_from_handle(store, ent->move_joint);
            if (!entity_is_valid_and_active(joint_ent)) {
                joint_ent = entity_alloc(root);
                joint_ent->mass_unscaled = F32_INFINITY;
                joint_ent->inertia_unscaled = F32_INFINITY;
                entity_enable_prop(joint_ent, ENTITY_PROP_MOTOR_JOINT);
                entity_enable_prop(joint_ent, ENTITY_PROP_ACTIVE);
                ent->move_joint = joint_ent->handle;

                struct motor_joint_def def = ZI;
                def.e0 = joint_ent->handle;  /* Re-using joint entity as e0 */
                def.e1 = ent->handle;
                def.correction_rate = 0;
                def.max_force = ent->control_force;
                def.max_torque = 0;
                joint_ent->motor_joint_data = motor_joint_from_def(def);
            }

            entity_set_xform(joint_ent, XFORM_IDENT);  /* Reset joint ent position */
            joint_ent->linear_velocity = v2_mul(v2_clamp_len(ent->control.move, 1), ent->control_force_max_speed);
        }
    }
#endif

    /* ========================== *
     * Create forces from control focus (aim)
     * ========================== */

#if GAME_PLAYER_AIM
    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(ent)) continue;

        if (entity_has_prop(ent, ENTITY_PROP_PLAYER_CONTROLLED)) {
            struct xform xf = entity_get_xform(ent);
            struct xform sprite_xf = xform_mul(xf, ent->sprite_local_xform);

            /* Retrieve / create aim joint */
            struct entity *joint_ent = entity_from_handle(store, ent->aim_joint);
            if (!entity_is_valid_and_active(joint_ent)) {
                joint_ent = entity_alloc(root);
                joint_ent->mass_unscaled = F32_INFINITY;
                joint_ent->inertia_unscaled = F32_INFINITY;
                entity_enable_prop(joint_ent, ENTITY_PROP_PHYSICAL_KINEMATIC);  /* Since we'll be setting velocity */
                entity_enable_prop(joint_ent, ENTITY_PROP_MOTOR_JOINT);
                entity_enable_prop(joint_ent, ENTITY_PROP_ACTIVE);
                ent->aim_joint = joint_ent->handle;

                struct motor_joint_def def = ZI;
                def.e0 = joint_ent->handle;  /* Re-using joint entity as e0 */
                def.e1 = ent->handle;
                def.correction_rate = 0.1;
                def.max_force = 0;
                def.max_torque = ent->control_torque;
                joint_ent->motor_joint_data = motor_joint_from_def(def);
            }

            /* Solve for final angle using law of sines */
            f32 new_angle;
            {
                struct v2 ent_pos = xf.og;
                struct v2 focus_pos = v2_add(ent_pos, ent->control.focus);

                struct v2 sprite_hold_pos;
                struct v2 sprite_hold_dir;
                {
                    struct sprite_sheet *sheet = sprite_sheet_from_tag_await(sprite_frame_scope, ent->sprite);
                    struct sprite_sheet_slice slice = sprite_sheet_get_slice(sheet, STR("attach.wep"), ent->animation_frame);
                    sprite_hold_pos = slice.center;
                    sprite_hold_dir = slice.dir;
                }

                struct v2 hold_dir = xform_basis_mul_v2(sprite_xf, sprite_hold_dir);
                struct v2 hold_pos = xform_mul_v2(sprite_xf, sprite_hold_pos);
                if (v2_eq(hold_pos, ent_pos)) {
                    /* If hold pos is same as origin (E.G if pivot is being used as hold pos), then move hold pos forward a tad to avoid issue */
                    sprite_hold_pos = v2_add(sprite_hold_pos, V2(0, -1));
                    hold_pos = xform_mul_v2(sprite_xf, sprite_hold_pos);
                }

                f32 forward_hold_angle_offset;
                {
                    struct xform xf_unrotated = xform_basis_with_rotation_world(xf, 0);
                    struct v2 hold_pos_unrotated = xform_mul_v2(xf_unrotated, xform_mul_v2(ent->sprite_local_xform, sprite_hold_pos));
                    forward_hold_angle_offset = v2_angle_from_dirs(V2(0, -1), v2_sub(hold_pos_unrotated, xf_unrotated.og));
                }

                struct v2 hold_ent_dir = v2_sub(ent_pos, hold_pos);
                struct v2 focus_ent_dir = v2_sub(ent_pos, focus_pos);

                f32 hold_ent_len = v2_len(hold_ent_dir);
                f32 focus_ent_len = v2_len(focus_ent_dir);

                f32 final_hold_angle_btw_ent_and_focus = v2_angle_from_dirs(hold_ent_dir, hold_dir);
                f32 final_focus_angle_btw_ent_and_hold = math_asin((math_sin(final_hold_angle_btw_ent_and_focus) * hold_ent_len) / focus_ent_len);
                f32 final_ent_angle_btw_focus_and_hold = PI - (final_focus_angle_btw_ent_and_hold + final_hold_angle_btw_ent_and_focus);

                new_angle = math_unwind_angle(v2_angle_from_dirs(V2(0, -1), v2_sub(focus_pos, ent_pos)) + final_ent_angle_btw_focus_and_hold - forward_hold_angle_offset);
            }

            f32 new_vel = 0;
            if (!F32_IS_NAN(new_angle)) {
                const f32 angle_error_allowed = 0.001;
                struct xform joint_xf = entity_get_xform(joint_ent);
                f32 diff = math_unwind_angle(new_angle - xform_get_rotation(joint_xf));
                if (math_fabs(diff) > angle_error_allowed) {
                    new_vel = diff / dt;
                }
            }
            joint_ent->angular_velocity = new_vel;
        }
    }
#endif

    /* ========================== *
     * Create ground friction force (gravity)
     * ========================== */

    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(ent)) continue;
        if (!entity_has_prop(ent, ENTITY_PROP_PHYSICAL_DYNAMIC)) continue;

        struct entity *joint_ent = entity_from_handle(store, ent->ground_friction_joint);

        struct motor_joint_def def = ZI;
        def.e0 = root->handle;
        def.e1 = ent->handle;
        def.correction_rate = 0;
        def.max_force = ent->linear_ground_friction;
        def.max_torque = ent->angular_ground_friction;
        if (joint_ent->motor_joint_data.max_force != def.max_force || joint_ent->motor_joint_data.max_torque != def.max_torque) {
            if (!entity_is_valid_and_active(joint_ent)) {
                joint_ent = entity_alloc(root);
                entity_enable_prop(joint_ent, ENTITY_PROP_MOTOR_JOINT);
                entity_enable_prop(joint_ent, ENTITY_PROP_ACTIVE);
                joint_ent->motor_joint_data = motor_joint_from_def(def);
                ent->ground_friction_joint = joint_ent->handle;
            }
        }
    }

    /* ========================== *
     * Physics
     * ========================== */

    (UNUSED)create_contacts;
    (UNUSED)prepare_contacts;
    (UNUSED)warm_start_contacts;
    (UNUSED)solve_contacts;

    (UNUSED)prepare_motor_joints;
    (UNUSED)warm_start_motor_joints;
    (UNUSED)solve_motor_joints;

    (UNUSED)create_mouse_joints;
    (UNUSED)prepare_mouse_joints;
    (UNUSED)warm_start_mouse_joints;
    (UNUSED)solve_mouse_joints;

    (UNUSED)integrate_velocities_from_forces;
    (UNUSED)integrate_positions_from_velocities;
#if 1
    {
        f32 remaining_dt = dt;
        while (remaining_dt > 0) {
            f32 min_toi = 0.000001f;
            f32 tolerance = 0.001f;
            f32 earliest_toi = max_f32(determine_earliest_toi(remaining_dt, tolerance), min_toi);
            f32 step_dt = remaining_dt * earliest_toi;

            integrate_velocities_from_forces(step_dt);
            create_contacts();
            create_mouse_joints(game_cmds);

            prepare_contacts();
            prepare_motor_joints();
            prepare_mouse_joints();

            f32 substep_dt = step_dt / GAME_PHYSICS_SUBSTEPS;
            for (u32 i = 0; i < GAME_PHYSICS_SUBSTEPS; ++i) {
    #if GAME_PHYSICS_ENABLE_WARM_STARTING
                warm_start_contacts();
                warm_start_motor_joints();
                warm_start_mouse_joints();
    #endif

    #if GAME_PHYSICS_ENABLE_COLLISION
                solve_contacts(substep_dt, true);
    #endif
                solve_motor_joints(substep_dt);
                solve_mouse_joints(substep_dt);

                integrate_positions_from_velocities(substep_dt);

    #if GAME_PHYSICS_ENABLE_COLLISION && GAME_PHYSICS_ENABLE_RELAXATION
                solve_contacts(substep_dt, false);  /* Relaxation */
    #endif
            }
            remaining_dt -= step_dt;
        }
    }
#endif

    /* ========================== *
     * Respond to hit events
     * ========================== */

    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(ent)) continue;
        if (!entity_has_prop(ent, ENTITY_PROP_HIT_EVENT)) continue;

        struct hit_event *event = &ent->hit_event;

        struct entity *e0 = entity_from_handle(store, event->e0);
        struct entity *e1 = entity_from_handle(store, event->e1);

        if (entity_is_valid_and_active(e0) && entity_is_valid_and_active(e1)) {
            /* Bullet hit entity */
            if (entity_has_prop(e0, ENTITY_PROP_BULLET) || entity_has_prop(e1, ENTITY_PROP_BULLET)) {
                struct entity *bullet = entity_has_prop(e0, ENTITY_PROP_BULLET) ? e0 : e1;
                struct entity *target = e0 == bullet ? e1 : e0;

                (UNUSED)bullet;
                (UNUSED)target;
                entity_enable_prop(bullet, ENTITY_PROP_RELEASE_AT_END_OF_FRAME);

                /* Create test blood */
                /* TODO: Remove this */
                {
                    struct xform xf = XFORM_TRS(.t = event->point);
                    struct entity *decal = entity_alloc(root);
                    decal->sprite = sprite_tag_from_path(STR("res/graphics/blood.ase"));
                    entity_set_xform(decal, xf);

                    entity_enable_prop(decal, ENTITY_PROP_PHYSICAL_KINEMATIC);
                    decal->linear_velocity = v2_mul(v2_norm(event->normal), 0.5f);
                    decal->angular_velocity = 1 - (((f32)sys_rand_u32() / (f32)U32_MAX) * 2);
                }
            }
        }
    }

    /* ========================== *
     * Initialize bullet kinematics from sources
     * ========================== */

    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(ent)) continue;

        /* FIXME: Apply src entity velocity to bullet velocity */

        if (entity_has_prop(ent, ENTITY_PROP_BULLET) && ent->activation_tick == G.tick.tick_id) {
            struct entity *src = entity_from_handle(store, ent->bullet_src);
            struct xform src_xf = entity_get_xform(src);

            struct v2 pos = xform_mul_v2(src_xf, ent->bullet_src_pos);
            struct v2 impulse = xform_basis_mul_v2(src_xf, ent->bullet_src_dir);
            impulse = v2_with_len(impulse, ent->bullet_impulse);

            struct xform xf = XFORM_TRS(.t = pos, .r = v2_angle(impulse) + PI / 2);
            entity_set_xform(ent, xf);
            entity_enable_prop(ent, ENTITY_PROP_PHYSICAL_KINEMATIC);

            entity_apply_linear_impulse_to_center(ent, impulse);
        }
    }

    /* ========================== *
     * Update camera position
     * ========================== */

    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(ent)) continue;

        /* Camera follow */
        if (entity_has_prop(ent, ENTITY_PROP_CAMERA)) {
            struct entity *follow = entity_from_handle(store, ent->camera_follow);

            struct xform xf = entity_get_xform(ent);

            f32 aspect_ratio = 1.0;
            {
                struct xform quad_xf = xform_mul(entity_get_xform(ent), ent->camera_quad_xform);
                struct v2 camera_size = xform_get_scale(quad_xf);
                if (!v2_is_zero(camera_size)) {
                    aspect_ratio = camera_size.x / camera_size.y;
                }
            }
            f32 ratio_y = 0.33f;
            f32 ratio_x = ratio_y / aspect_ratio;
            struct v2 camera_focus_dir = v2_mul_v2(follow->control.focus, V2(ratio_x, ratio_y));
            struct v2 camera_focus_pos = v2_add(entity_get_xform(follow).og, camera_focus_dir);
            ent->camera_xform_target = xf;
            ent->camera_xform_target.og = camera_focus_pos;

            /* Lerp camera */
            if (ent->camera_applied_lerp_continuity_gen_plus_one == ent->camera_lerp_continuity_gen + 1) {
                f32 t = 1 - math_pow(2.f, -20.f * (f32)G.tick.dt);
                xf = xform_lerp(xf, ent->camera_xform_target, t);
            } else {
                /* Skip lerp */
                xf = ent->camera_xform_target;
            }
            ent->camera_applied_lerp_continuity_gen_plus_one = ent->camera_lerp_continuity_gen + 1;
            entity_set_xform(ent, xf);
        }
    }

    /* ========================== *
     * Update sound emitters
     * ========================== */

     /* TODO: Sound entities should be created by game thread, but played by the
      * user thread. This is so sounds play at the correct time on the user
      * thread regardless of interp delay. */

    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
        struct entity *ent = &store->entities[entity_index];
        if (!entity_is_valid_and_active(ent)) continue;

        if (entity_has_prop(ent, ENTITY_PROP_TEST_SOUND_EMITTER)) {
            struct mixer_desc desc = ent->sound_desc;
            desc.speed = G.tick.timescale;

            desc.pos = entity_get_xform(ent).og;
            struct sound *sound = sound_load_async(ent->sound_name, 0);
            b32 played = ent->sound_handle.gen != 0;
            if (sound) {
                if (!played) {
                    ent->sound_handle = mixer_play_ex(sound, desc);
                } else {
                    mixer_track_set(ent->sound_handle, desc);
                }
            }
        }
    }

    /* ========================== *
     * Release entities
     * ========================== */

     /* TODO: Breadth first iteration to only release parent entities (since
      * child entities will be released along with parent anyway) */

    {
        struct temp_arena temp = arena_temp_begin(scratch.arena);

        struct entity **ents_to_release = arena_dry_push(temp.arena, struct entity *);
        u64 ents_to_release_count = 0;
        for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
            struct entity *ent = &store->entities[entity_index];
            if (!ent->valid) continue;

            if (entity_has_prop(ent, ENTITY_PROP_RELEASE_AT_END_OF_FRAME)) {
                *arena_push(temp.arena, struct entity *) = ent;
                ++ents_to_release_count;
            }
        }

        for (u64 i = 0; i < ents_to_release_count; ++i) {
            struct entity *ent = ents_to_release[i];
            if (ent->valid) {
                /* Release */
                entity_release(store, ent);
            }
        }

        arena_temp_end(temp);
    }

    /* ========================== *
     * Publish tick
     * ========================== */

    /* Publish tick */
    publish_game_tick();
    __profframe("Game");

    /* ========================== *
     * End frame cache scopes
     * ========================== */

    sprite_scope_end(sprite_frame_scope);

    scratch_end(scratch);
}

/* ========================== *
 * Game thread
 * ========================== */

INTERNAL SYS_THREAD_ENTRY_POINT_FUNC_DEF(game_thread_entry_point, arg)
{
    struct temp_arena scratch = scratch_begin_no_conflict();

    (UNUSED)arg;
    sys_timestamp_t last_frame_ts = 0;
    f64 target_dt = GAME_FPS > (0) ? (1.0 / GAME_FPS) : 0;
    while (!atomic_i32_eval(&G.game_thread_shutdown)) {
        __profscope(game_update_w_sleep);
        struct temp_arena temp = arena_temp_begin(scratch.arena);
        sleep_frame(last_frame_ts, target_dt);
        last_frame_ts = sys_timestamp();
        {
            struct game_cmd_array game_cmds = pop_cmds(temp.arena);
            if (!G.paused) {
                game_update(game_cmds);
            }
            /* Check for pause / next frame cmds */
            for (u64 i = 0; i < game_cmds.count; ++i) {
                struct game_cmd cmd = game_cmds.cmds[i];
                switch (cmd.kind) {
                    case GAME_CMD_KIND_PAUSE: {
                        G.paused = !G.paused;
                    } break;

                    case GAME_CMD_KIND_STEP: {
                        if (G.paused) {
                            G.paused = false;
                            game_update(game_cmds);
                            G.paused = true;
                        }
                    } break;

                    default: break;
                }
            }
        }
        arena_temp_end(temp);
    }

    scratch_end(scratch);
}

/* ========================== *
 * Interface
 * ========================== */

void game_get_latest_tick(struct world *dest)
{
    __prof;
    struct sys_lock lock = sys_mutex_lock_s(&G.prev_tick_mutex);
    world_copy_replace(dest, &G.prev_tick);
    sys_mutex_unlock(&lock);
}

u64 game_get_latest_tick_id(void)
{
    return atomic_u64_eval(&G.prev_tick_id);
}

u64 game_get_latest_tick_continuity_gen(void)
{
    return atomic_u64_eval(&G.prev_tick_continuity_gen);
}

void game_push_cmds(struct game_cmd_array cmd_array)
{
    push_cmds(cmd_array);
}