cAI/cKit/Memory/c_Buddy.c

#include <c_Buddy.h>
#include <c_Alignment.h>
#include <c_Memory.h>
/* ------------------------------------------------------------------------------------------------------------------ */
/*  */

C_STATIC_FORCE_INLINE
c_bool_t is_power_of_two(c_size_t x) {
    return (x & (x-1)) == 0;
}

C_STATIC_FORCE_INLINE
c_BuddyBlock_t* buddy_block_next(c_BuddyBlock_t* block) {
    return (c_BuddyBlock_t*)((uint8_t*)block + block->size);
}

static c_BuddyBlock_t *buddy_block_split(c_BuddyBlock_t *block, const int size) {
    if (block != NULL && size != 0) {
        // 當目標大小小於當前區塊時，進行對半裂變
        while (size < block->size) {
            int sz = block->size >> 1;
            block->size = sz;

            // 裂變出的右半邊（Right Buddy）設為空閒
            c_BuddyBlock_t* right_buddy = buddy_block_next(block);
            right_buddy->size = sz;
            right_buddy->is_free = C_TRUE;

            // 經典夥伴系統優先分配左半邊（Left Buddy），因此 block 指針保持在左側繼續循環
        }

        if (size <= block->size) {
            return block;
        }
    }
    return NULL;
}

static
c_BuddyBlock_t *buddy_block_find_best(c_BuddyBlock_t *head, c_BuddyBlock_t *tail, int size) {
    c_BuddyBlock_t *best_block = NULL;
    c_BuddyBlock_t *curr = head;

    while (curr < tail) {
        if (curr->is_free && curr->size >= size) {
            // 尋找符合條件且最小的區塊（Best Fit），減少外部碎片
            if (best_block == NULL || curr->size < best_block->size) {
                best_block = curr;
            }
        }
        // 精確前進當前區塊的實際大小，不論其內部被切得多碎，都能地毯式掃描
        curr = buddy_block_next(curr);
    }

    if (best_block != NULL) {
        return buddy_block_split(best_block, size);
    }
    return NULL;
}

C_STATIC_FORCE_INLINE
int buddy_block_size_required(c_Buddy_t *b, int size) {
    int actual_size = b->alignment;
    const int total_needed = size + b->alignment; // 預留足夠空間確保 padding 後依然夠用

    while (actual_size < total_needed) {
        actual_size <<= 1;
    }
    return actual_size;
}

static
void buddy_block_coalescence(c_BuddyBlock_t *head, c_BuddyBlock_t *tail) {
    for (;;) {
        c_BuddyBlock_t *curr = head;
        c_bool_t consolidated_any = C_FALSE;

        while (curr < tail) {
            c_BuddyBlock_t *next = buddy_block_next(curr);

            // 檢查安全邊界
            if (next >= tail) {
                break;
            }

            // 凝聚條件：兩相鄰區塊皆空閒，且大小相等
            // 注意：在夥伴系統中，左夥伴的偏移量必須是其兩倍大小的整數倍（對齊檢查）
            if (curr->is_free && next->is_free && curr->size == next->size) {
                const c_uintptr_t offset = (c_uintptr_t)curr - (c_uintptr_t)head;
                if ((offset % (curr->size << 1)) == 0) {
                    curr->size <<= 1; // 融合！容量翻倍
                    consolidated_any = C_TRUE;
                    // 融合後，下一次推進會自動從融合後的大區塊末端繼續，不用前進 next
                    continue;
                }
            }

            // 若無法融合，正常步進到下一個區塊
            curr = next;
        }

        // 如果整輪掃描下來沒有任何區塊可以再融合，代表凝聚完成，退出循環
        if (!consolidated_any) {
            break;
        }
    }
}


/* ------------------------------------------------------------------------------------------------------------------ */
/*  */

void c_Buddy_Init(c_Buddy_t *b, void *data, int size, int alignment) {
    assert(b != NULL);
    assert(data != NULL);
    assert(is_power_of_two(size) && "size is not a power-of-two");
    assert(is_power_of_two(alignment) && "alignment is not a power-of-two");

    // 確保對齊基準至少能放下一個標頭結構體
    if (alignment < (int)sizeof(c_BuddyBlock_t)) {
        alignment = C_ALIGN_UPB(sizeof(c_BuddyBlock_t), 2); // 向上取 2 的冪次
        while (!is_power_of_two(alignment)) {
            alignment++; // 穩健保險
        }
    }
    assert((c_uintptr_t)data % alignment == 0 && "data is not aligned to minimum alignment");

    b->alignment = alignment;
    b->head      = (c_BuddyBlock_t *)data;
    b->head->size    = size;
    b->head->is_free = C_TRUE;

    // 哨兵結尾
    b->tail = buddy_block_next(b->head);
}

void *c_Buddy_Alloc(c_Buddy_t *b, int size) {
    if (!b || size <= 0) return NULL;

    int actual_size = buddy_block_size_required(b, size);

    // 第一階段：尋找現有最合適的空閒塊
    c_BuddyBlock_t *found = buddy_block_find_best(b->head, b->tail, actual_size);
    if (found == NULL) {
        // 第二階段：若找不到，強制進行全面碎片凝聚（Coalesce），再搜尋一次
        buddy_block_coalescence(b->head, b->tail);
        found = buddy_block_find_best(b->head, b->tail, actual_size);
    }

    if (found != NULL) {
        found->is_free = C_FALSE;
        return (void *)((uint8_t *)found + b->alignment);
    }

    return NULL; // OOM
}

void c_Buddy_Free(c_Buddy_t *b, void *data) {
    if (data != NULL) {
        assert((c_uintptr_t)b->head <= (c_uintptr_t)data);
        assert((c_uintptr_t)data < (c_uintptr_t)b->tail);

        c_BuddyBlock_t *block = (c_BuddyBlock_t *) ((uint8_t *) data - b->alignment);
        block->is_free = C_TRUE;

        // NOTE: Coalescence could be done now but it is optional
        // buddy_block_coalescence(b->head, b->tail);
    }
}