Experimental stateless UUIDv7 implementation as of RFC draft 02

uuidv7.c

/**
 * uuidv7.c - Experimental stateless UUIDv7 implementation as of RFC draft 02
 *
 * Required: `clock_gettime()`, `arc4random_buf()`, and ~15-microseconds or
 * finer system clock resolution.
 *
 * This implementation employs 52-bit timestamp (consisting of 36-bit whole
 * seconds and 16-bit sub-second fraction) and 70-bit cryptographically strong
 * random integer in the following layout. It does not guarantee the monotonic
 * order of the generated IDs within the same timestamp (i.e. within ~15
 * microseconds).
 *
 * ```
 *  0                   1                   2                   3
 *  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 * |                            unixts                             |
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 * |unixts |        subsec         |  ver  |subsec |     rand      |
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 * |var|                         rand                              |
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 * |                             rand                              |
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 * ```
 *
 * Copyright 2022 LiosK
 *
 * Licensed under the Apache License, Version 2.0
 */
#include <assert.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

/**
 * @param out uint8_t id[16]
 */
void uuidv7_generate(uint8_t *out) {
  struct timespec tp;
  clock_gettime(CLOCK_REALTIME, &tp);

  uint64_t timestamp52 =
      ((uint64_t)tp.tv_sec << 16) | (((uint64_t)tp.tv_nsec << 16) / 1000000000);
  out[6] = 0x70 | (timestamp52 & 0x0F); // set version bits
  timestamp52 >>= 4;
  for (int i = 0; i < 6; i++) {
    out[5 - i] = (uint8_t)timestamp52;
    timestamp52 >>= 8;
  }

  arc4random_buf(&out[7], sizeof(uint8_t) * 9);
  out[8] = 0x80 | (out[8] & 0x3F); // set variant bits
}

/**
 * @param id uint8_t id[16]
 * @param out char text[37]
 */
void uuidv7_to_string(const uint8_t *id, char *out) {
  static const char digits[16] = "0123456789abcdef";

  char *p = out;
  for (int i = 0; i < 16; i++) {
    if (i == 4 || i == 6 || i == 8 || i == 10) {
      *p++ = '-';
    }
    *p++ = digits[id[i] >> 4];
    *p++ = digits[id[i] & 0xF];
  }
  *p = '\0';
}

static long get_clock_resolution() {
  struct timespec tp;
  int error = clock_getres(CLOCK_REALTIME, &tp);
  return error == 0 ? tp.tv_nsec : 1000000000;
}

int main() {
  uint8_t id[16];
  char text[37];
  assert(get_clock_resolution() <= (1000000000 >> 16));
  uuidv7_generate(id);
  uuidv7_to_string(id, text);
  puts(text);
  return 0;
}