// smallobject_segment_mid_v3.c
// Phase v11a-2: Multi-class MID v3.5 segment implementation (L2 physical layer)

#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <stdio.h>
#include "box/smallobject_segment_mid_v3_box.h"
#include "box/region_id_v6_box.h"

// ============================================================================
// SmallPageMeta - Page metadata for MID v3
// ============================================================================

typedef struct SmallPageMeta {
    void *ptr;                      // Page base pointer
    uint32_t capacity;              // Slots per page
    uint8_t class_idx;              // Size class (C5-C7)
    uint32_t alloc_count;           // Total allocations on this page
    uint32_t free_count;            // Total frees on this page
    void *segment;                  // Back-pointer to SmallSegment_MID_v3
    struct SmallPageMeta *next;     // For free stack linking
} SmallPageMeta;

// ============================================================================
// Geometry Constants
// ============================================================================

#define SMALL_MID_SEGMENT_SIZE (2 * 1024 * 1024)  // 2 MiB
#define SMALL_MID_PAGE_SIZE (64 * 1024)            // 64 KiB
#define SMALL_MID_PAGES_PER_SEG (SMALL_MID_SEGMENT_SIZE / SMALL_MID_PAGE_SIZE)  // 32

// ============================================================================
// Helper: class_idx to slots mapping
// ============================================================================

static uint32_t class_idx_to_slots(uint32_t class_idx) {
    // MID v3.5 targets C5-C7 (257-1024B)
    // C5: 257-384B  → 64KiB / 384B  ≈ 170 slots
    // C6: 385-640B  → 64KiB / 640B  ≈ 102 slots
    // C7: 641-1024B → 64KiB / 1024B ≈ 64 slots

    switch (class_idx) {
        case 5: return 170;  // C5
        case 6: return 102;  // C6
        case 7: return 64;   // C7
        default: return 0;
    }
}

// ============================================================================
// Segment Lifecycle
// ============================================================================

SmallSegment_MID_v3* small_segment_mid_v3_create(void) {
    // 1. Allocate SmallSegment_MID_v3 structure
    SmallSegment_MID_v3 *seg = malloc(sizeof(*seg));
    if (!seg) return NULL;

    // 2. mmap 2MiB contiguous memory
    seg->start = mmap(NULL, SMALL_MID_SEGMENT_SIZE,
                      PROT_READ | PROT_WRITE,
                      MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
    if (seg->start == MAP_FAILED) {
        free(seg);
        return NULL;
    }

    seg->total_size = SMALL_MID_SEGMENT_SIZE;
    seg->page_size = SMALL_MID_PAGE_SIZE;
    seg->num_pages = SMALL_MID_PAGES_PER_SEG;

    // 3. Initialize per-class free page stacks
    for (int i = 0; i < 8; i++) {
        seg->free_pages[i] = NULL;
        seg->free_count[i] = 0;
        seg->current_page[i] = NULL;
        seg->page_offset[i] = 0;
    }

    // 4. Allocate and initialize page metadata array
    seg->pages = malloc(sizeof(SmallPageMeta*) * SMALL_MID_PAGES_PER_SEG);
    if (!seg->pages) {
        munmap(seg->start, SMALL_MID_SEGMENT_SIZE);
        free(seg);
        return NULL;
    }

    // Allocate individual page metadata structures
    for (uint32_t i = 0; i < SMALL_MID_PAGES_PER_SEG; i++) {
        SmallPageMeta *meta = malloc(sizeof(SmallPageMeta));
        if (!meta) {
            // Cleanup on failure
            for (uint32_t j = 0; j < i; j++) {
                free(seg->pages[j]);
            }
            free(seg->pages);
            munmap(seg->start, SMALL_MID_SEGMENT_SIZE);
            free(seg);
            return NULL;
        }

        meta->ptr = (char*)seg->start + (i * SMALL_MID_PAGE_SIZE);
        meta->capacity = 0;
        meta->class_idx = 0xFF;  // Unassigned
        meta->alloc_count = 0;
        meta->free_count = 0;
        meta->segment = seg;
        meta->next = NULL;

        seg->pages[i] = meta;
    }

    // 5. Register with RegionIdBox
    seg->region_id = region_id_register_v6(seg->start, seg->total_size,
                                           REGION_KIND_MID_V3, seg);

    seg->total_allocations = 0;
    seg->total_frees = 0;
    seg->last_refill_count = 0;

    return seg;
}

void small_segment_mid_v3_destroy(SmallSegment_MID_v3 *seg) {
    if (!seg) return;

    // Unregister from RegionIdBox
    region_id_unregister_v6(seg->region_id);

    // Free page metadata
    if (seg->pages) {
        for (uint32_t i = 0; i < SMALL_MID_PAGES_PER_SEG; i++) {
            if (seg->pages[i]) {
                free(seg->pages[i]);
            }
        }
        free(seg->pages);
    }

    // Unmap segment memory
    if (seg->start && seg->start != MAP_FAILED) {
        munmap(seg->start, seg->total_size);
    }

    free(seg);
}

// ============================================================================
// Page Management
// ============================================================================

// Take a page from the free stack (LIFO)
void* small_segment_mid_v3_take_page(SmallSegment_MID_v3 *seg, uint32_t class_idx) {
    if (!seg || class_idx >= 8) return NULL;

    if (seg->free_count[class_idx] > 0) {
        // Pop from free stack
        void **stack = seg->free_pages[class_idx];
        if (!stack || seg->free_count[class_idx] == 0) return NULL;

        // Get the top page
        void *page = stack[seg->free_count[class_idx] - 1];
        seg->free_count[class_idx]--;

        return page;
    }

    // No free pages available
    return NULL;
}

// Release a page back to the free stack (LIFO)
void small_segment_mid_v3_release_page(SmallSegment_MID_v3 *seg, void *page, uint32_t class_idx) {
    if (!seg || !page || class_idx >= 8) return;

    // Allocate stack if needed (lazy initialization)
    if (!seg->free_pages[class_idx]) {
        seg->free_pages[class_idx] = malloc(sizeof(void*) * SMALL_MID_PAGES_PER_SEG);
        if (!seg->free_pages[class_idx]) return;
    }

    // Push to free stack
    seg->free_pages[class_idx][seg->free_count[class_idx]] = page;
    seg->free_count[class_idx]++;
}

// Get region ID
uint32_t small_segment_mid_v3_region_id(SmallSegment_MID_v3 *seg) {
    return seg ? seg->region_id : 0;
}

// Set region ID (called by RegionIdBox)
void small_segment_mid_v3_set_region_id(SmallSegment_MID_v3 *seg, uint32_t region_id) {
    if (seg) {
        seg->region_id = region_id;
    }
}

// Check if page is within segment bounds
bool small_segment_mid_v3_contains_page(SmallSegment_MID_v3 *seg, void *page) {
    if (!seg || !page) return false;

    uintptr_t page_addr = (uintptr_t)page;
    uintptr_t seg_start = (uintptr_t)seg->start;
    uintptr_t seg_end = seg_start + seg->total_size;

    return (page_addr >= seg_start && page_addr < seg_end);
}

// Get page metadata for pointer
struct SmallPageMeta* small_segment_mid_v3_get_page_meta(
    SmallSegment_MID_v3 *seg,
    void *page
) {
    if (!seg || !page || !seg->pages) return NULL;

    // Calculate page index
    uintptr_t page_addr = (uintptr_t)page;
    uintptr_t seg_start = (uintptr_t)seg->start;

    if (page_addr < seg_start) return NULL;

    uintptr_t offset = page_addr - seg_start;
    uint32_t page_idx = offset / SMALL_MID_PAGE_SIZE;

    if (page_idx >= SMALL_MID_PAGES_PER_SEG) return NULL;

    return (struct SmallPageMeta*)seg->pages[page_idx];
}

// ============================================================================
// Statistics
// ============================================================================

SmallSegmentStatsMID_v3 small_segment_mid_v3_get_stats(SmallSegment_MID_v3 *seg) {
    SmallSegmentStatsMID_v3 stats = {0};

    if (!seg) return stats;

    stats.total_allocations = seg->total_allocations;
    stats.total_frees = seg->total_frees;

    // Count active pages per class
    for (uint32_t i = 0; i < SMALL_MID_PAGES_PER_SEG; i++) {
        SmallPageMeta *meta = seg->pages[i];
        if (meta && meta->class_idx < 8 && meta->class_idx != 0xFF) {
            stats.active_pages[meta->class_idx]++;
        }
    }

    return stats;
}

void small_segment_mid_v3_reset_stats(SmallSegment_MID_v3 *seg) {
    if (!seg) return;

    seg->total_allocations = 0;
    seg->total_frees = 0;
    seg->last_refill_count = 0;
}

// ============================================================================
// Validation & Debugging
// ============================================================================

bool small_segment_mid_v3_validate(SmallSegment_MID_v3 *seg) {
    if (!seg) return false;
    if (!seg->start || seg->start == MAP_FAILED) return false;
    if (seg->total_size != SMALL_MID_SEGMENT_SIZE) return false;
    if (seg->num_pages != SMALL_MID_PAGES_PER_SEG) return false;
    if (!seg->pages) return false;

    return true;
}

void small_segment_mid_v3_debug_print(SmallSegment_MID_v3 *seg) {
    if (!seg) {
        fprintf(stderr, "[MID_v3] Segment: NULL\n");
        return;
    }

    fprintf(stderr, "[MID_v3] Segment %p:\n", (void*)seg);
    fprintf(stderr, "  start=%p size=%zu pages=%u region_id=%u\n",
            seg->start, seg->total_size, seg->num_pages, seg->region_id);
    fprintf(stderr, "  allocs=%lu frees=%lu\n",
            seg->total_allocations, seg->total_frees);

    for (int i = 0; i < 8; i++) {
        if (seg->free_count[i] > 0) {
            fprintf(stderr, "  C%d: free_pages=%u\n", i, seg->free_count[i]);
        }
    }
}