rand: use time microseconds too in default rand initialization; generate proper uuid_v4

vlib/rand/pcg32/pcg32_test.v

@@ -25,12 +25,15 @@ fn gen_randoms(seed_data []u32, bound int) []u32 {
 }
 
 fn test_pcg32_reproducibility() {
-	randoms1 := gen_randoms(util.time_seed_array(4), 1000)
+	seed_data := util.time_seed_array(4)
-	randoms2 := gen_randoms(util.time_seed_array(4), 1000)
+	randoms1 := gen_randoms(seed_data, 1000)
+	randoms2 := gen_randoms(seed_data, 1000)
 	assert randoms1.len == randoms2.len
 	len := randoms1.len
 	for i in 0 .. len {
-		assert randoms1[i] == randoms2[i]
+		r1 := randoms1[i]
+		r2 := randoms2[i]
+		assert r1 == r2
 	}
 }
 

vlib/rand/rand.v

@@ -149,13 +149,18 @@ pub fn string(len int) string {
 pub fn uuid_v4() string {
 	mut buf := malloc(37)
 	for i in 0..36 {
+		mut v := intn(16)
+		if i == 19 {
+			v = (v & 0x3) | 0x8
+		}
 		unsafe {
-			buf[i] = hex_chars[intn(16)]
+			buf[i] = hex_chars[v]
 		}
 	}
 	unsafe {
 		buf[8] = `-`
 		buf[13] = `-`
+		buf[14] = `4`
 		buf[18] = `-`
 		buf[23] = `-`
 		buf[36] = 0

vlib/rand/random_numbers_test.v

@@ -186,4 +186,7 @@ fn test_rand_uuid_v4() {
 	assert uuid1.len == 36    
 	assert uuid2.len == 36    
 	assert uuid3.len == 36
+	assert uuid1[14] == `4`
+	assert uuid2[14] == `4`
+	assert uuid3[14] == `4`
 }

vlib/rand/util/util.v

@@ -25,7 +25,8 @@ fn nr_next(prev u32) u32 {
 // time_seed_array is a utility function that returns the required number of u32s generated from system time
 [inline]
 pub fn time_seed_array(count int) []u32 {
-	mut seed := u32(time.now().unix_time())
+	ctime := time.now()
+	mut seed := u32(ctime.unix_time() ^ ctime.microsecond)
 	mut seed_data := []u32{cap: count}
 	for _ in 0 .. count {
 		seed = nr_next(seed)