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feat: add sub-millisecond timestamp into uuid_generate_v7 #42

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feat: add sub-millisecond timestamp into uuid_generate_v7 #42

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29 changes: 25 additions & 4 deletions pg_uuidv7.c
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Original file line number Diff line number Diff line change
Expand Up @@ -31,10 +31,32 @@ Datum uuid_generate_v7(PG_FUNCTION_ARGS)
errmsg("could not get CLOCK_REALTIME")));

tms = ((uint64_t)ts.tv_sec * 1000) + ((uint64_t)ts.tv_nsec / 1000000);
tms = pg_hton64(tms << 16);
memcpy(&uuid->data[0], &tms, 6);

if (!pg_strong_random(&uuid->data[6], UUID_LEN - 6))
/*
* Generate first 64 bits of UUID
* 48 bits = unix epoch timestamp in milliseconds
* 4 bits = version 7 [0111]
* 12 bits = ~250 nanoseconds precision
*
* Using an OPTIONAL sub-millisecond timestamp fraction (12 bits at maximum) as per Section 6.2 (Method 3).
* https://datatracker.ietf.org/doc/rfc9562/
*/
tms = pg_hton64(
(tms << 16) |
0x7000 |
/*
* ((((uint64_t)ts.tv_nsec << 12) / 1000000) & 0x0fff) is equivalent to
* ((((uint64_t)ts.tv_nsec % 1000000) * 4096) / 1000000)
* but faster, because it replaces a MOD function with an & operation with the same result.
* (<< 12) is used for a faster 4096 multiplication
*/
((((uint64_t)ts.tv_nsec << 12) / 1000000) & 0x0fff));
memcpy(&uuid->data[0], &tms, 8);

/*
* Generate last 62 random bits
*/
if (!pg_strong_random(&uuid->data[8], UUID_LEN - 8))
ereport(ERROR,
(errcode(ERRCODE_INTERNAL_ERROR),
errmsg("could not generate random values")));
Expand All @@ -43,7 +65,6 @@ Datum uuid_generate_v7(PG_FUNCTION_ARGS)
* Set magic numbers for a "version 7" UUID, see
* https://www.ietf.org/archive/id/draft-ietf-uuidrev-rfc4122bis-00.html#name-uuid-version-7
*/
uuid->data[6] = (uuid->data[6] & 0x0f) | 0x70; /* 4 bit version [0111] */
uuid->data[8] = (uuid->data[8] & 0x3f) | 0x80; /* 2 bit variant [10] */

PG_RETURN_UUID_P(uuid);
Expand Down