#include "object_pool.h"

#include "../core/memory.h"

typedef struct BzObjectPool {
    void *objects;
    size_t stride;
    size_t numObjects;
    i32 firstFree;
} BzObjectPool;

static i32 *bzObjectPoolGetObject(const BzObjectPool *pool, size_t idx) {
    return (i32 *) ((u8 *) pool->objects + idx * pool->stride);
}

BzObjectPool *bzObjectPoolCreate(const BzObjectPoolDesc *desc) {
    BZ_ASSERT(desc->objectSize > 0);

    // NOTE: Since object bits are used as a free list
    // when not in use, we must ensure they can hold i32.
    size_t stride = desc->objectSize;
    if (stride < sizeof(i32)) {
        stride = sizeof(i32);
    }

    size_t numBytes = sizeof(BzObjectPool) + desc->numObjects * stride;
    BzObjectPool *pool = bzAlloc(numBytes);
    *pool = (BzObjectPool) {
        .objects=pool + 1,
        .stride=stride,
        .numObjects=desc->numObjects,
        .firstFree=0,
    };

    // Link free list
    for (size_t i = 0; i < pool->numObjects - 1; i++) {
        i32 *object = bzObjectPoolGetObject(pool, i);
        *object = (i32) (i + 1);
    }
    i32 *lastObject = bzObjectPoolGetObject(pool, pool->numObjects - 1);
    *lastObject = -1;

    return pool;
}
void bzObjectPoolDestroy(BzObjectPool *pool) {
    bzFree(pool);
}

void *bzObjectPool(BzObjectPool *pool) {
    if (pool->firstFree == -1)
        return NULL;
    i32 *object = bzObjectPoolGetObject(pool, pool->firstFree);
    pool->firstFree = *object;
    return object;
}
void bzObjectPoolRelease(BzObjectPool *pool, void *object) {
    size_t objectIdx = (size_t) object - (size_t)pool->objects;
    BZ_ASSERT(objectIdx < pool->numObjects);
    *(i32 *) objectIdx = pool->firstFree;
    pool->firstFree = (i32) objectIdx;
}