enum {ulid_byte_len=26};

/**
 * Generate a ULID (Universally Unique Lexicographically Sortable Identifier)
 * Returns str given arena.
 *
 * ULID format: 26 characters (Crockford Base32).
 * - 48 bits = milliseconds since Unix epoch (6 bytes, big‑endian)
 * - 80 bits = random data (10 bytes)
 *
 */
static str generate_ulid(arena *A) {
  // Crockford Base32 alphabet (no I, L, O, U)
  static const char ALPHABET[] = "0123456789ABCDEFGHJKMNPQRSTVWXYZ";

  // 1. Get current time in milliseconds
  struct timespec ts;
  if (clock_gettime(CLOCK_REALTIME, &ts) != 0) return (str){0};
  uint64_t ms = (uint64_t)ts.tv_sec * 1000 + ts.tv_nsec / 1000000;

  // 2. Generate 10 random bytes (80 bits)
  uint8_t random_bytes[10];
  #if defined(__APPLE__) || defined(__FreeBSD__)
    arc4random_buf(random_bytes, sizeof(random_bytes));
  #else
    int fd = open("/dev/urandom", O_RDONLY);
    if (fd < 0) return (str){0};
    ssize_t n = read(fd, random_bytes, sizeof(random_bytes));
    close(fd);
    if (n != sizeof(random_bytes)) return (str){0};
  #endif

  // 3. Build the 16‑byte ULID payload (big‑endian timestamp + random)
  uint8_t payload[16];
  // Timestamp (6 bytes, most significant byte first)
  for (int i = 0; i < 6; i++) {
    payload[5 - i] = (uint8_t)(ms & 0xFF);
    ms >>= 8;
  }
  // Random part
  memcpy(payload + 6, random_bytes, 10);

  // 4. Encode to 26 Base32 characters (Crockford)
  int bit_buf = 0;
  int bit_cnt = 0;
  int out_pos = 0;

  uint8_t *encoded = new(A, uint8_t, ulid_byte_len);
  encoded[0]=0;

  for (int i = 0; i < 16; i++) {
    // Feed the payload byte by byte
    bit_buf = (bit_buf << 8) | payload[i];
    bit_cnt += 8;

    while (bit_cnt >= 5 && out_pos < ulid_byte_len) {
      // Extract top 5 bits
      int idx = (bit_buf >> (bit_cnt - 5)) & 0x1F;
      encoded[out_pos++] = ALPHABET[idx];
      bit_cnt -= 5;
    }
  }

  // Flush any remaining bits (should be exactly 3 bits for ULID)
  if (bit_cnt > 0 && out_pos < ulid_byte_len) {
    int idx = (bit_buf << (5 - bit_cnt)) & 0x1F;
    encoded[out_pos++] = ALPHABET[idx];
  }

  return str_from_buf(encoded, ulid_byte_len);
}