rand: add uuid_v7(), session function, simplify uuid_v4() (#24313)

vlib/rand/random_identifiers_test.v

@@ -21,6 +21,58 @@ fn test_rand_uuid_v4() {
 	}
 }
 
+// uuid_v7:
+fn test_rand_uuid_v7() {
+	uuid1 := rand.uuid_v7()
+	uuid2 := rand.uuid_v7()
+	uuid3 := rand.uuid_v7()
+	assert uuid1 != uuid2
+	assert uuid1 != uuid3
+	assert uuid2 != uuid3
+	assert uuid1.len == 36
+	assert uuid2.len == 36
+	assert uuid3.len == 36
+	for i in 0 .. 1000 {
+		x := rand.uuid_v7()
+		// check the version field is always 7:
+		assert x[14] == `7`
+		// and variant field is always 0b10:
+		assert x[19] in [`8`, `9`, `a`, `b`]
+	}
+}
+
+// uuid_v7_session:
+fn test_rand_uuid_v7_session() {
+	mut u := rand.new_uuid_v7_session()
+	uuid1 := u.next()
+	uuid2 := u.next()
+	uuid3 := u.next()
+	assert uuid1 != uuid2
+	assert uuid1 != uuid3
+	assert uuid2 != uuid3
+	assert uuid1.len == 36
+	assert uuid2.len == 36
+	assert uuid3.len == 36
+	mut prev_counter := `3`
+	for i in 0 .. 1000 {
+		x := u.next()
+		// check the version field is always 7:
+		assert x[14] == `7`
+		// and variant field is always 0b10:
+		assert x[19] in [`8`, `9`, `a`, `b`]
+
+		// verify counter increase
+		assert x[17] == prev_counter
+		if prev_counter == `9` {
+			prev_counter = `a`
+		} else if prev_counter == `f` {
+			prev_counter = `0`
+		} else {
+			prev_counter++
+		}
+	}
+}
+
 // ulids:
 fn test_ulids_are_unique() {
 	ulid1 := rand.ulid()