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 *   F64 — double slots
 *   I32 — int32_t slots with atomic CAS/add
 *   Str — fixed-length string slots (4-byte length prefix + data)
 */

#ifndef POOL_H
#define POOL_H

#include <stdint.h>
#include <stdbool.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <time.h>
#include <limits.h>
#include <signal.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/file.h>
#include <sys/syscall.h>
#include <linux/futex.h>
#include <sys/eventfd.h>

/* ================================================================
 * Constants
 * ================================================================ */

#define POOL_MAGIC        0x504F4C31U  /* "POL1" */
#define POOL_VERSION      1
#define POOL_ERR_BUFLEN   256

#define POOL_VAR_RAW  0
#define POOL_VAR_I64  1
#define POOL_VAR_F64  2
#define POOL_VAR_I32  3
#define POOL_VAR_STR  4

#define POOL_ALIGN8(x) (((x) + 7) & ~(uint64_t)7)

/* ================================================================
 * Header (128 bytes = 2 cache lines)
 * ================================================================ */

typedef struct {
    /* ---- Cache line 0 (0-63): immutable after create ---- */
    uint32_t magic;          /* 0 */
    uint32_t version;        /* 4 */
    uint32_t elem_size;      /* 8 */
    uint32_t variant_id;     /* 12 */
    uint64_t capacity;       /* 16: number of slots */
    uint64_t total_size;     /* 24: total mmap size */
    uint64_t data_off;       /* 32: offset to slot data */
    uint64_t bitmap_off;     /* 40: offset to allocation bitmap */
    uint64_t owners_off;     /* 48: offset to per-slot owner PIDs */
    uint8_t  _pad0[8];       /* 56-63 */

    /* ---- Cache line 1 (64-127): mutable state ---- */
    uint32_t used;           /* 64: allocated count (futex word) */
    uint32_t waiters;        /* 68: blocked on alloc */
    uint8_t  _pad1[8];       /* 72-79 */
    uint64_t stat_allocs;    /* 80 */
    uint64_t stat_frees;     /* 88 */
    uint64_t stat_waits;     /* 96 */
    uint64_t stat_timeouts;  /* 104 */
    uint64_t stat_recoveries;/* 112 */
    uint8_t  _pad2[8];       /* 120-127 */
} PoolHeader;

#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
_Static_assert(sizeof(PoolHeader) == 128, "PoolHeader must be 128 bytes");
#endif

/* ================================================================
 * Process-local handle
 * ================================================================ */

typedef struct {
    PoolHeader *hdr;
    uint64_t   *bitmap;
    uint32_t   *owners;
    uint8_t    *data;
    size_t      mmap_size;
    uint32_t    bitmap_words;
    char       *path;
    int         notify_fd;
    int         backing_fd;
    uint32_t    scan_hint;
} PoolHandle;

/* ================================================================
 * Utility
 * ================================================================ */

static inline int pool_pid_alive(uint32_t pid) {
    if (pid == 0) return 1;
    return !(kill((pid_t)pid, 0) == -1 && errno == ESRCH);
}

static inline void pool_make_deadline(double timeout, struct timespec *deadline) {
    clock_gettime(CLOCK_MONOTONIC, deadline);
    deadline->tv_sec += (time_t)timeout;
    deadline->tv_nsec += (long)((timeout - (double)(time_t)timeout) * 1e9);
    if (deadline->tv_nsec >= 1000000000L) {
        deadline->tv_sec++;
        deadline->tv_nsec -= 1000000000L;
    }
}

static inline int pool_remaining_time(const struct timespec *deadline,
                                       struct timespec *remaining) {
    struct timespec now;
    clock_gettime(CLOCK_MONOTONIC, &now);
    remaining->tv_sec = deadline->tv_sec - now.tv_sec;
    remaining->tv_nsec = deadline->tv_nsec - now.tv_nsec;
    if (remaining->tv_nsec < 0) {
        remaining->tv_sec--;
        remaining->tv_nsec += 1000000000L;
    }
    return remaining->tv_sec >= 0;