#include <time.h>
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <stdint.h>
#include <sys/random.h>

#define UUID_T_LENGTH (16)
#define UNIX_TS_LENGTH (6)
#define RAND_A_LENGTH (2)
#define RAND_B_LENGTH (8)

typedef uint8_t uuid_t[UUID_T_LENGTH];

static struct {
    uint64_t  unix_ts;
    uint8_t   rand_a[RAND_A_LENGTH];
    uint8_t   rand_b[RAND_B_LENGTH];
} state;

uint64_t get_milliseconds(void)
{
    struct timespec tp;
    clock_gettime(CLOCK_REALTIME, &tp);
    return ((tp.tv_sec * 1000) + (tp.tv_nsec / 1000000));
}

void get_random_bytes(uint8_t buffer[], size_t len)
{
    getentropy(buffer, len);
    if (errno != EXIT_SUCCESS)
    {
        exit(EXIT_FAILURE);
    }
}

void create_uuid7_stateless(uuid_t uuid)
{
    static const int rand_ab_length = RAND_A_LENGTH + RAND_B_LENGTH;

    // get the timestamp bytes
    uint64_t unix_ts = get_milliseconds();
    for(int i = 0; i < UNIX_TS_LENGTH; i++)
    {
        uuid[UNIX_TS_LENGTH - 1 - i] = (uint8_t) (unix_ts >> (8 * i));
    }

    // get the random bytes
    uint8_t rand_ab[rand_ab_length];
    get_random_bytes(rand_ab, rand_ab_length);
    for(int i = 0; i < rand_ab_length; i++)
    {
        uuid[UNIX_TS_LENGTH + i] = rand_ab[i];
    }

    // set version and variant
    uuid[6] = 0x70 | (uuid[6] & 0x0f); // version 7
    uuid[8] = 0x80 | (uuid[8] & 0x3f); // variant 2
}

void create_uuid7_stateful_method1_type1(uuid_t uuid)
{
    // get the timestamp bytes
    uint64_t unix_ts = get_milliseconds();
    for(int i = 0; i < UNIX_TS_LENGTH; i++)
    {
        uuid[UNIX_TS_LENGTH - 1 - i] = (uint8_t) (unix_ts >> (8 * i));
    }

    // get the rand_a bytes
    if (unix_ts > state.unix_ts) {
        get_random_bytes(state.rand_a, RAND_A_LENGTH);
    } else {
		// increment rand_a bytes from right to left
		for (int i = RAND_A_LENGTH - 1; i >= 0; i--) {
			if (++state.rand_a[i] != 0x00) {
				break;
			}
		}
    }
    for(int i = 0; i < RAND_A_LENGTH; i++)
    {
        uuid[UNIX_TS_LENGTH + i] = state.rand_a[i];
    }

    // get the rand_b bytes
    get_random_bytes(state.rand_b, RAND_B_LENGTH);
    for(int i = 0; i < RAND_B_LENGTH; i++)
    {
        uuid[UNIX_TS_LENGTH + RAND_A_LENGTH + i] = state.rand_b[i];
    }

    // save the last timestamp
    state.unix_ts = unix_ts;

    // set version and variant
    uuid[6] = 0x70 | (uuid[6] & 0x0f); // version 7
    uuid[8] = 0x80 | (uuid[8] & 0x3f); // variant 2
}

void create_uuid7_stateful_method1_type2(uuid_t uuid)
{
    // get the timestamp bytes
    uint64_t unix_ts = get_milliseconds();
    for(int i = 0; i < UNIX_TS_LENGTH; i++)
    {
        uuid[UNIX_TS_LENGTH - 1 - i] = (uint8_t) (unix_ts >> (8 * i));
    }

    // get the rand_a bytes
    if (unix_ts > state.unix_ts) {
        get_random_bytes(state.rand_a, RAND_A_LENGTH);
    } else {
        // get random byte
        uint8_t increment[1];
        get_random_bytes(increment, 1);

        // set increment between 1 and 255
        const uint8_t increment_max = 0xff;
        increment[0] = increment[0] % increment_max + 1;

        // increment rand_a bytes from right to left
        if ((uint32_t) state.rand_a[1] + (uint32_t) increment[0] <= increment_max)
        {
            state.rand_a[1] += increment[0];
        } else {
            state.rand_a[1] += increment[0];
            state.rand_a[0] += 1;
        }
    }
    for(int i = 0; i < RAND_A_LENGTH; i++)
    {
        uuid[UNIX_TS_LENGTH + i] = state.rand_a[i];
    }

    // get the rand_b bytes
    get_random_bytes(state.rand_b, RAND_B_LENGTH);
    for(int i = 0; i < RAND_B_LENGTH; i++)
    {
        uuid[UNIX_TS_LENGTH + RAND_A_LENGTH + i] = state.rand_b[i];
    }

    // save the last timestamp
    state.unix_ts = unix_ts;

    // set version and variant
    uuid[6] = 0x70 | (uuid[6] & 0x0f); // version 7
    uuid[8] = 0x80 | (uuid[8] & 0x3f); // variant 2
}

void create_uuid7_stateful_method2_type1(uuid_t uuid)
{
    // get the timestamp bytes
    uint64_t unix_ts = get_milliseconds();
    for(int i = 0; i < UNIX_TS_LENGTH; i++)
    {
        uuid[UNIX_TS_LENGTH - 1 - i] = (uint8_t) (unix_ts >> (8 * i));
    }

    // get the random bytes
    if (unix_ts > state.unix_ts) {
        get_random_bytes(state.rand_a, RAND_A_LENGTH);
        get_random_bytes(state.rand_b, RAND_B_LENGTH);
    } else {
		// increment rand_b bytes from right to left
		for (int i = RAND_B_LENGTH - 1; i >= 0; i--) {
			if (++state.rand_b[i] != 0x00) {
				break;
			}
		}
    }
    for(int i = 0; i < RAND_A_LENGTH; i++)
    {
        uuid[UNIX_TS_LENGTH + i] = state.rand_a[i];
    }
    for(int i = 0; i < RAND_B_LENGTH; i++)
    {
        uuid[UNIX_TS_LENGTH + RAND_A_LENGTH + i] = state.rand_b[i];
    }

    // save the last timestamp
    state.unix_ts = unix_ts;

    // set version and variant
    uuid[6] = 0x70 | (uuid[6] & 0x0f); // version 7
    uuid[8] = 0x80 | (uuid[8] & 0x3f); // variant 2
}

void create_uuid7_stateful_method2_type2(uuid_t uuid)
{
    // get the timestamp bytes
    uint64_t unix_ts = get_milliseconds();
    for(int i = 0; i < UNIX_TS_LENGTH; i++)
    {
        uuid[UNIX_TS_LENGTH - 1 - i] = (uint8_t) (unix_ts >> (8 * i));
    }

    // get the random bytes
    if (unix_ts > state.unix_ts) {
        get_random_bytes(state.rand_a, RAND_A_LENGTH);
        get_random_bytes(state.rand_b, RAND_B_LENGTH);
    } else {
        // get random byte
        uint8_t increment[1];
        get_random_bytes(increment, 1);

        // set increment between 1 and 255
        const uint8_t increment_max = 0xff;
        increment[0] = increment[0] % increment_max + 1;

        // increment rand_b bytes from right to left
        if ((uint32_t) state.rand_b[7] + (uint32_t) increment[0] <= increment_max) {
            state.rand_b[7] += increment[0];
        } else {
            state.rand_b[7] += increment[0];
            for (int i = RAND_B_LENGTH - 2; i >= 0; i--) {
                if (++state.rand_b[i] != 0x00) {
                    break;
                }
            }
        }
    }
    for(int i = 0; i < RAND_A_LENGTH; i++)
    {
        uuid[UNIX_TS_LENGTH + i] = state.rand_a[i];
    }
    for(int i = 0; i < RAND_B_LENGTH; i++)
    {
        uuid[UNIX_TS_LENGTH + RAND_A_LENGTH + i] = state.rand_b[i];
    }

    // save the last timestamp
    state.unix_ts = unix_ts;

    // set version and variant
    uuid[6] = 0x70 | (uuid[6] & 0x0f); // version 7
    uuid[8] = 0x80 | (uuid[8] & 0x3f); // variant 2
}

void print_uuid(uuid_t uuid)
{
    static const char hex[16] = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};

    int s = 0;
    char str[36 + 1];
    for(int i = 0; i < UUID_T_LENGTH; i++)
    {
        if(i == 4 || i == 6 || i == 8 || i == 10)
        {
            str[s++] = '-';
        }
        str[s++] = hex[uuid[i] >> 4];
        str[s++] = hex[uuid[i] & 0x0f];
    }
    str[s++] ='\0';

    printf("%s\n", str);
}

int main()
{

    printf("\n");
    printf("Stateless UUID v7:\n\n");

    for(int i = 0; i < 10; i++) {
        uuid_t uuid;
        create_uuid7_stateless(uuid);
        print_uuid(uuid);
    }

    printf("              ^^^^ ^^^^ ^^^^^^^^^^^^ <- always random\n\n");

    printf("Stateful UUID v7 using Method 1 and Type A:\n\n");

    for(int i = 0; i < 10; i++) {
        uuid_t uuid;
        create_uuid7_stateful_method1_type1(uuid);
        print_uuid(uuid);
    }

    printf("                   ^^^^ ^^^^^^^^^^^^ <- always random\n");
    printf("              ^^^^ <- plus 1\n\n");

    printf("Stateful UUID v7 using Method 1 and Type B:\n\n");

    for(int i = 0; i < 10; i++) {
        uuid_t uuid;
        create_uuid7_stateful_method1_type2(uuid);
        print_uuid(uuid);
    }

    printf("                   ^^^^ ^^^^^^^^^^^^ <- always random\n");
    printf("              ^^^^ <- plus n, where 1 <= n <= 255\n\n");

    printf("Stateful UUID v7 using Method 2 and Type A:\n\n");

    for(int i = 0; i < 10; i++) {
        uuid_t uuid;
        create_uuid7_stateful_method2_type1(uuid);
        print_uuid(uuid);
    }

    printf("              ^^^^ ^^^^ ^^^^^^^^^^^^ <- plus 1\n\n");

    printf("Stateful UUID v7 using Method 2 and Type B:\n\n");

    for(int i = 0; i < 10; i++) {
        uuid_t uuid;
        create_uuid7_stateful_method2_type2(uuid);
        print_uuid(uuid);
    }

    printf("              ^^^^ ^^^^ ^^^^^^^^^^^^ <- plus n, where 1 <= n <= 255\n\n");
}

/*

## OUTPUT:

Stateless UUID v7:

017fb8c8-040e-747d-b7b0-4484f54bdcf3
017fb8c8-040e-767f-9e95-260c79ad415d
017fb8c8-040e-7a67-a735-b2a6c1a226e3
017fb8c8-040e-7d89-b4da-de1d91276d32
017fb8c8-040e-7337-9317-4b8acf8113a6
017fb8c8-040e-72f7-bc9b-bbcfbf73782b
017fb8c8-040e-7e00-b5ae-3518d2bcb3fa
017fb8c8-040e-7c7a-8cd2-04264d191231
017fb8c8-040e-72d7-9117-e155c3fe2ed1
017fb8c8-040e-7b47-beff-3ac66dbfe08a
              ^^^^ ^^^^ ^^^^^^^^^^^^ <- always random

Stateful UUID v7 using Method 1 and Type A:

017fb8c8-040e-7635-b82d-6c1a912cc929
017fb8c8-040e-7636-81bf-2beb1b820814
017fb8c8-040e-7637-965f-a62f7582fcb5
017fb8c8-040e-7638-ad73-b13aec618618
017fb8c8-040e-7639-9406-e0a03537c37a
017fb8c8-040e-763a-a2e3-5413d99b84d4
017fb8c8-040e-763b-b362-2fe9d7946abe
017fb8c8-040e-763c-9f77-efd19f534e6e
017fb8c8-040f-7a47-b67c-2f449248fbf3
017fb8c8-040f-7a48-a8db-8aa3594b1edc
                   ^^^^ ^^^^^^^^^^^^ <- always random
              ^^^^ <- plus 1

Stateful UUID v7 using Method 1 and Type B:

017fb8c8-040f-7aa8-b5e3-5dc7dc6e4c2a
017fb8c8-040f-7b30-96f8-f5c225eef8ba
017fb8c8-040f-7b93-8743-940cbf19a388
017fb8c8-040f-7c5e-b21d-f0d079cbc05d
017fb8c8-040f-7ca1-bc08-7c8d4e43e8b4
017fb8c8-040f-7cb0-974d-49932443f329
017fb8c8-040f-7cc9-865f-9d2262b4eee3
017fb8c8-040f-7dae-b084-b5e5a03289a5
017fb8c8-040f-7e77-a839-a3f4c3802a07
017fb8c8-040f-7f4d-913c-8ce5c548298b
                   ^^^^ ^^^^^^^^^^^^ <- always random
              ^^^^ <- plus n, where 1 <= n <= 255

Stateful UUID v7 using Method 2 and Type A:

017fb8c8-040f-7f4d-913c-8ce5c548298c
017fb8c8-040f-7f4d-913c-8ce5c548298d
017fb8c8-040f-7f4d-913c-8ce5c548298e
017fb8c8-040f-7f4d-913c-8ce5c548298f
017fb8c8-040f-7f4d-913c-8ce5c5482990
017fb8c8-040f-7f4d-913c-8ce5c5482991
017fb8c8-040f-7f4d-913c-8ce5c5482992
017fb8c8-040f-7f4d-913c-8ce5c5482993
017fb8c8-040f-7f4d-913c-8ce5c5482994
017fb8c8-040f-7f4d-913c-8ce5c5482995
              ^^^^ ^^^^ ^^^^^^^^^^^^ <- plus 1

Stateful UUID v7 using Method 2 and Type B:

017fb8c8-040f-7f4d-913c-8ce5c54829e9
017fb8c8-040f-7f4d-913c-8ce5c5482a03
017fb8c8-040f-7f4d-913c-8ce5c5482a71
017fb8c8-040f-7f4d-913c-8ce5c5482ac2
017fb8c8-040f-7f4d-913c-8ce5c5482b7b
017fb8c8-040f-7f4d-913c-8ce5c5482bbf
017fb8c8-040f-7f4d-913c-8ce5c5482bd6
017fb8c8-040f-7f4d-913c-8ce5c5482c0d
017fb8c8-040f-7f4d-913c-8ce5c5482c32
017fb8c8-040f-7f4d-913c-8ce5c5482cdd
              ^^^^ ^^^^ ^^^^^^^^^^^^ <- plus n, where 1 <= n <= 255

*/