////////////////////////////////////////////////////////////
//~ Collision data types

#define PP_ContactSpringHz 25
#define PP_ContactSpringDamp 10

Struct(PP_CollisionData)
{
    PP_EntKey e0;
    PP_EntKey e1;
    Vec2 point;
    Vec2 normal;   /* Normal of the collision from e0 to e1  */
    Vec2 vrel;     /* Relative velocity at point of collision */
    b32 is_start;       /* Did this collision just begin */
    f32 dt;             /* How much time elapsed in the step when this event occurred (this will equal the physics timestep unless an early time of impact occurred) */
};

/* Callback can return 1 to prevent the physics system from resolving */
struct PP_SimStepCtx;
#define PP_CollisionCallbackFuncDef(name, arg_collision_data, arg_sim_step_ctx) b32 name(PP_CollisionData *arg_collision_data, struct PP_SimStepCtx *arg_sim_step_ctx)
typedef PP_CollisionCallbackFuncDef(PP_CollisionCallbackFunc, collision_data, ctx);

////////////////////////////////////////////////////////////
//~ Step ctx

/* Structure containing data used for a single physics step */
Struct(PP_PhysStepCtx)
{
    struct PP_SimStepCtx *sim_step_ctx;
    PP_CollisionCallbackFunc *collision_callback;
};

////////////////////////////////////////////////////////////
//~ Contact types

Struct(PP_ContactPoint)
{
    /* Contact point relative to the center of each entity.
     *
     * NOTE: We use fixed (non-rotated) points relative to the entities
     * rather than points fully in local space because contact manifolds
     * shouldn't really be affected by rotation accross substeps
     * (imagine re-building the manifold of a rotated shape, it would still be
     *  on the same side of the shape that it originally occured on) */
    Vec2 vcp0;
    Vec2 vcp1;

    u32 id;                     /* ID generated during clipping */
    f32 starting_separation;    /* How far are original points along normal */
    f32 normal_impulse;         /* Accumulated impulse along normal */
    f32 tangent_impulse;        /* Accumulated impulse along tangent */

    f32 inv_normal_mass;
    f32 inv_tangent_mass;

    /* Debugging */
#if DeveloperIsEnabled
    Vec2 dbg_pt;
#endif
};

Struct(PP_ContactConstraint)
{
    u64 last_phys_iteration;  /* To avoid checking collisions for the same constraint twice in one tick */
    b32 skip_solve;
    b32 wrong_dir;
    PP_EntKey e0;
    PP_EntKey e1;
    f32 inv_m0;
    f32 inv_m1;
    f32 inv_i0;
    f32 inv_i1;

    Vec2 normal;   /* Normal vector of collision from e0 -> e1 */
    u64 last_iteration;
    PP_ContactPoint points[2];
    u32 num_points;

    f32 friction;

    f32 pushout_velocity;
};

Struct(PP_ContactDebugData)
{
    PP_EntKey e0;
    PP_EntKey e1;
    CLD_CollisionData collision_result;

    PP_ContactPoint points[2];
    u32 num_points;

    Vec2 closest0;
    Vec2 closest1;

    Xform xf0;
    Xform xf1;
};

////////////////////////////////////////////////////////////
//~ Motor joint types

Struct(PP_MotorJointDesc)
{
    PP_EntKey e0;
    PP_EntKey e1;
    f32 correction_rate;
    f32 max_force;
    f32 max_torque;
};

Struct(PP_MotorJoint)
{
    PP_EntKey e0;
    PP_EntKey e1;
    f32 correction_rate;
    f32 max_force;
    f32 max_torque;

    f32 inv_m0;
    f32 inv_m1;
    f32 inv_i0;
    f32 inv_i1;

    Vec2 linear_impulse;
    f32 angular_impulse;

    Vec2 point_local_e0;
    Vec2 point_local_e1;

    Xform linear_mass_xf;
    f32 angular_mass;
};

////////////////////////////////////////////////////////////
//~ Mouse joint types

Struct(PP_MouseJointDesc)
{
    PP_EntKey target;
    Vec2 point_local_start;
    Vec2 point_end;
    f32 linear_spring_hz;
    f32 linear_spring_damp;
    f32 angular_spring_hz;
    f32 angular_spring_damp;
    f32 max_force;
};

Struct(PP_MouseJoint)
{
    PP_EntKey target;
    Vec2 point_local_start;
    Vec2 point_end;
    f32 linear_spring_hz;
    f32 linear_spring_damp;
    f32 angular_spring_hz;
    f32 angular_spring_damp;
    f32 max_force;

    f32 inv_m;
    f32 inv_i;

    Vec2 linear_impulse;
    f32 angular_impulse;

    Xform linear_mass_xf;
};

////////////////////////////////////////////////////////////
//~ Weld joint types

Struct(PP_WeldJointDesc)
{
    PP_EntKey e0;
    PP_EntKey e1;

    /* The xform that transforms a point in e0's space into the desired e1 space
     * (IE `xf` * VEC2(0, 0) should evaluate to the local point that e1's origin will lie) */
    Xform xf;

    f32 linear_spring_hz;
    f32 linear_spring_damp;
    f32 angular_spring_hz;
    f32 angular_spring_damp;
};

Struct(PP_WeldJoint)
{
    PP_EntKey e0;
    PP_EntKey e1;
    Xform xf0_to_xf1;

    f32 linear_spring_hz;
    f32 linear_spring_damp;
    f32 angular_spring_hz;
    f32 angular_spring_damp;

    f32 inv_m0;
    f32 inv_m1;
    f32 inv_i0;
    f32 inv_i1;

    Vec2 linear_impulse0;
    Vec2 linear_impulse1;
    f32 angular_impulse0;
    f32 angular_impulse1;
};

////////////////////////////////////////////////////////////
//~ Contact operations

b32 PP_CanEntsContact(struct PP_Ent *e0, struct PP_Ent *e1);
void PP_CreateAndUpdateContacts(PP_PhysStepCtx *ctx, f32 elapsed_dt, u64 phys_iteration);
void PP_PrepareContacts(PP_PhysStepCtx *ctx, u64 phys_iteration);
void PP_WarmStartContacts(PP_PhysStepCtx *ctx);
void PP_SolveContacts(PP_PhysStepCtx *ctx, f32 dt, b32 apply_bias);

////////////////////////////////////////////////////////////
//~ Motor joint operations

PP_MotorJointDesc PP_CreateMotorJointDef(void);
PP_MotorJoint PP_MotorJointFromDef(PP_MotorJointDesc def);
void PP_PrepareMotorJoints(PP_PhysStepCtx *ctx);
void PP_WarmStartMotorJoints(PP_PhysStepCtx *ctx);
void PP_SolveMotorJoints(PP_PhysStepCtx *ctx, f32 dt);

////////////////////////////////////////////////////////////
//~ Mouse joint operations

PP_MouseJointDesc PP_CreateMouseJointDef(void);
PP_MouseJoint PP_MouseJointFromDef(PP_MouseJointDesc def);
void PP_PrepareMouseJoints(PP_PhysStepCtx *ctx);
void PP_WarmStartMouseJoints(PP_PhysStepCtx *ctx);
void PP_SolveMouseJoints(PP_PhysStepCtx *ctx, f32 dt);

////////////////////////////////////////////////////////////
//~ Weld joint operations

PP_WeldJointDesc PP_CreateWeldJointDef(void);
PP_WeldJoint PP_WeldJointFromDef(PP_WeldJointDesc def);
void PP_PrepareWeldJoints(PP_PhysStepCtx *ctx);
void PP_WarmStartWeldJoints(PP_PhysStepCtx *ctx);
void PP_SolveWeldJoints(PP_PhysStepCtx *ctx, f32 dt);

////////////////////////////////////////////////////////////
//~ Integration operations

Xform PP_GetDerivedEntXform(struct PP_Ent *ent, f32 dt);
void PP_IntegrateForces(PP_PhysStepCtx *ctx, f32 dt);
void PP_IntegrateVelocities(PP_PhysStepCtx *ctx, f32 dt);

////////////////////////////////////////////////////////////
//~ Time of impact operations

f32 PP_DetermineEarliestToi(PP_PhysStepCtx *ctx, f32 step_dt, f32 tolerance, u32 max_iterations);

////////////////////////////////////////////////////////////
//~ Spatial operations

void PP_UpdateAabbs(PP_PhysStepCtx *ctx);

////////////////////////////////////////////////////////////
//~ Step

void PP_StepPhys(PP_PhysStepCtx *ctx, f32 timestep);