/* TODO: Elminiate meta arena. Just store levels in first 4096 bytes of buddy arena, and then zone header data at the beginning of each allocation. */

////////////////////////////////
//~ Buddy ctx

BuddyCtx *AllocBuddyCtx(u64 reserve)
{
    /* TODO: Determine meta reserve dynamically */
    Arena *meta_arena = AllocArena(Gibi(64));
    BuddyCtx *ctx = PushStruct(meta_arena, BuddyCtx);
    ctx->meta_arena = meta_arena;
    ctx->data_arena = AllocArena(reserve);

    /* TODO: Minimum block size */
    ctx->levels = PushArray(ctx->meta_arena, BuddyLevel, 64);
    for (u64 i = 0; i < 64; ++i)
    {
        BuddyLevel *level = &ctx->levels[i];
        level->ctx = ctx;
        level->tier = i;
        level->size = (u64)1 << i;
    }

    return ctx;
}

void ReleaseBuddyCtx(BuddyCtx *ctx)
{
    ReleaseArena(ctx->data_arena);
    ReleaseArena(ctx->meta_arena);
}

////////////////////////////////
//~ Block alloc / release

BuddyBlock *AllocBuddyBlock(BuddyCtx *ctx, u64 size)
{
    if (size > 0x00FFFFFFFFFFFFFFULL)
    {
        /* TODO: Error */
        Assert(0);
    }

    /* TODO: Minimum block size */

    /* TODO: Faster MSB calculation */

    u64 desired_block_size = 1;
    u64 desired_level_tier = 0;
    while (desired_block_size < size && desired_level_tier < 64)
    {
        desired_block_size <<= 1;
        ++desired_level_tier;
    }

    BuddyBlock *block = GetUnusedBuddyBlock(ctx, &ctx->levels[desired_level_tier]);

    return block;
}

void ReleaseBuddyBlock(BuddyBlock *block)
{
    block->is_used = 0;
    BuddyLevel *level = block->level;
    BuddyBlock *parent = block->parent;
    BuddyBlock *sibling = block->sibling;
    if (!sibling->is_used && parent != 0)
    {
        /* Merge siblings */
        BuddyCtx *ctx = level->ctx;
        PopBuddyBlock(ctx, level, block);
        PopBuddyBlock(ctx, level, sibling);
        /* Release parent */
        ReleaseBuddyBlock(parent);
    }
    else
    {
        if (level->first_unused_block)
        {
            block->next = level->first_unused_block;
            level->first_unused_block->prev = block;
        }
        level->first_unused_block = block;
    }
}

////////////////////////////////
//~ Block push / pop

//- Push
BuddyBlock *PushBuddyBlock(BuddyCtx *ctx)
{
    BuddyBlock *block;
    if (ctx->first_free_block)
    {
        block = ctx->first_free_block;
        ctx->first_free_block = block->next;
    }
    else
    {
        block = PushStructNoZero(ctx->meta_arena, BuddyBlock);
    }
    MEMZERO_STRUCT(block);
    return block;
}

//- Pop
void PopBuddyBlock(BuddyCtx *ctx, BuddyLevel *level, BuddyBlock *block)
{
    /* Remove from unused list */
    {
        BuddyBlock *prev = block->prev;
        BuddyBlock *next = block->next;
        if (prev)
        {
            prev->next = next;
        }
        else
        {
            level->first_unused_block = next;
        }
        if (next)
        {
            next->prev = prev;
        }
    }
    block->next = ctx->first_free_block;
    ctx->first_free_block = block;
}

////////////////////////////////
//~ Get unused block

BuddyBlock *GetUnusedBuddyBlock(BuddyCtx *ctx, BuddyLevel *level)
{
    BuddyBlock *block = 0;

    /* TODO: Tier oob check */
    if (level->first_unused_block)
    {
        block = level->first_unused_block;
        level->first_unused_block = block->next;
        if (level->first_unused_block)
        {
            level->first_unused_block->prev = 0;
        }
        block->is_used = 1;
        block->next = 0;
    }
    else
    {
        if (level->backed)
        {
            BuddyLevel *parent_level = &ctx->levels[level->tier + 1];
            BuddyBlock *parent_block = GetUnusedBuddyBlock(ctx, parent_level);

            /* Create left (used) block from parent block */
            BuddyBlock *left = PushBuddyBlock(ctx);
            left->is_used = 1;
            left->level = level;
            left->parent = parent_block;
            left->memory = parent_block->memory;

            /* Create right (UNUSED) block from parent block */
            BuddyBlock *right = PushBuddyBlock(ctx);
            right->is_used = 0;
            right->level = level;
            right->parent = parent_block;
            right->memory = left->memory + level->size;
            if (level->first_unused_block)
            {
                right->next = level->first_unused_block;
                level->first_unused_block->prev = right;
            }
            level->first_unused_block = right;

            left->sibling = right;
            right->sibling = left;
            block = left;
        }
        else
        {
            Arena *arena = ctx->data_arena;

            /* Grow arena */
            i64 level_commit_diff = (level->size * 2) - arena->pos;
            if (level_commit_diff > 0)
            {
                PushArrayNoZero(arena, u8, level_commit_diff);
                Assert(arena->pos == (level->size * 2));
            }

            /* Create left (used) block from existing child block memory */
            BuddyBlock *left = PushBuddyBlock(ctx);
            left->is_used = 1;
            left->level = level;
            left->memory = ArenaBase(arena);

            /* Create right (UNUSED) block from new arena memory */
            BuddyBlock *right = PushBuddyBlock(ctx);
            right->is_used = 0;
            right->level = level;
            right->memory = left->memory + level->size;

            left->sibling = right;
            right->sibling = left;
            block = left;

            level->backed = 1;
        }
    }

    return block;
}