#include "freelist.h"

/* Header block used to keep track of all the free blocks */
struct __attribute__((packed)) FreeListBlock {
    uint16_t next_offset; /* May be 8, 16, 32, 64, whatever */
};

/* Align to nearest multiple of align */
static inline size_t align_up_to(size_t n, size_t align) {
    return (n + align - 1) & ~(align - 1);
}

/* Convert pointer -> 1-based offset (0 means NULL) */
static inline uint32_t ptr_to_offset(FreeList *fl, void *ptr) {
    if (!ptr)
        return 0; // NULL maps to 0
    uintptr_t diff = (uintptr_t)ptr - fl->start;
    return (uint32_t)(diff / fl->size) + 1;
}

/* Convert 1-based offset -> pointer (0 means NULL) */
static inline void *offset_to_ptr(FreeList *fl, uint32_t offset) {
    if (offset == 0)
        return NULL; // 0 = invalid/null
    return (void *)(fl->start + (uintptr_t)(offset - 1) * fl->size);
}

/* Initialize the FreeList */
int fl_init(FreeList *fl, uintptr_t start, size_t size_bytes, size_t itemsize) {
    /* Fiddle around according to your need, (void *) alignment seems to be enough,
     * but MAX_ALIGN_T is also an option. Especially for allocator implementation. */
    size_t size = align_up_to(itemsize, sizeof(void *));

    if (size < sizeof(FreeListBlock) || !fl)
        return 0;

    fl->start = start;
    fl->end = start + size_bytes;
    fl->size = size;
    fl->allocated = 0;

    FreeListBlock *block = (FreeListBlock *)start;

    for (size_t offset = 1; offset < fl_capacity(fl) + 1; offset++) {
        block->next_offset = (int32_t)offset;
        block = offset_to_ptr(fl, offset);
    }

    block->next_offset = 0; /* Last block */
    fl->free = (FreeListBlock *)start;

    return 1;
}

/* Allocate some memory from the FreeList */
void *fl_alloc(FreeList *fl) {
    if (!fl->free)
        return NULL;

    FreeListBlock *m = fl->free;

    fl->free = offset_to_ptr(fl, fl->free->next_offset); /* May be null, which is fine */
    fl->allocated++;

    /* Wipe it before sending it out, could use memset
     * here, or even wiping the entire block */
    m->next_offset = 0;

    /* For reference: */
    // memset((void *)m, 0, fl->size); /* Wipe entire block */
    // memset((void *)m, 0, sizeof(FreeListBlock)); /* Wipe only header */

    return ((void *)m);
}

/* Return some memory to the FreeList */
int fl_free(FreeList *fl, void *ptr) {
    if (!fl_is_managed(fl, ptr))
        return 0; /* We cant free memory we do not own */

    FreeListBlock *block = (FreeListBlock *)ptr;

    block->next_offset = ptr_to_offset(fl, fl->free); /* May be null, which is fine */
    fl->free = block;
    fl->allocated--;

    return 1;
}

/* Returns how many slots are occupied */
size_t fl_allocated(FreeList *fl) {
    return fl->allocated;
}

/* Returns how many free slots are available (O(1)) */
size_t fl_available(FreeList *fl) {
    return fl_capacity(fl) - fl->allocated;
}

/* Returns the total amount of items the freelist will hold */
size_t fl_capacity(FreeList *fl) {
    return (fl->end - fl->start) / fl->size;
}

/* Check if a piece of memory is managed by the FreeList */
int fl_is_managed(FreeList *fl, void *ptr) {
    uintptr_t p = (uintptr_t)ptr;

    if (p < fl->start || p >= fl->end) {
        return 0; // outside pool
    }

    return ((p - fl->start) % fl->size) == 0; // aligned to block
}

/* Returns the ratio of metadata versus data as a scalar in range 0..1 */
float fl_utilization(FreeList *fl, size_t itemsize) {
    return (float)itemsize / fl->size;
}

/* Walks the free pages/slots, returns total count (O(n), given no cycles) */
size_t fl_check(FreeList *fl) {
    int            avail = 0;
    FreeListBlock *cursor = fl->free;

    while (cursor) {
        avail++;

        if (!fl_is_managed(fl, cursor) || avail > fl_capacity(fl))
            return 0;

        if (cursor->next_offset == 0)
            break;

        cursor = offset_to_ptr(fl, cursor->next_offset);
    }

    return avail;
}