rand: further bridge gap b/w rand module and PRNG interface (#13719)

vlib/rand/random_bytes_test.v

@@ -21,6 +21,28 @@ fn test_rand_bytes() ? {
 	dump(differences)
 }
 
+fn test_prng_rand_bytes() ? {
+	mut randoms := []string{}
+	mut rng := rand.get_current_rng()
+	for i in 0 .. 100 {
+		x := rng.bytes(i) ?.hex()
+		if x.len > 0 {
+			randoms << x
+		}
+		assert x.len == i * 2
+	}
+	mut differences := 0
+	for idx in 1 .. randoms.len {
+		start := randoms[idx]#[0..8]
+		prev_start := randoms[idx - 1]#[0..8]
+		if start != prev_start {
+			differences++
+		}
+	}
+	assert differences > 95 // normally around 98
+	dump(differences)
+}
+
 fn test_rand_read() ? {
 	max := 50
 	mut a := []byte{len: max}
@@ -49,3 +71,33 @@ fn test_rand_read() ? {
 	dump(differences)
 	assert differences > 11700 // normally around 11758
 }
+
+fn test_prng_rand_read() ? {
+	max := 50
+	mut a := []byte{len: max}
+	mut differences := 0
+	mut rng := rand.get_current_rng()
+	for j in 1 .. max {
+		start := '00'.repeat(j)
+		for k in j + 1 .. max {
+			end := '00'.repeat(max - k)
+			middle := '00'.repeat(k - j)
+			// eprintln('> j: $j | k: $k | start: $start | middle: $middle | end: $end')
+			for i in 0 .. max {
+				a[i] = 0
+			}
+			assert a[j..k].hex() == middle
+			for i in 0 .. 10 {
+				rng.read(mut a[j..k])
+				// dump(a.hex())
+				assert a[0..j].hex() == start
+				assert a[k..].hex() == end
+				if a[j..k].hex() != middle {
+					differences++
+				}
+			}
+		}
+	}
+	dump(differences)
+	assert differences > 11700 // normally around 11758
+}