rand: extend PRNG interface, add buffering support (#13608)

vlib/rand/pcg32/pcg32.v

@@ -4,15 +4,18 @@
 module pcg32
 
 import rand.seed
-import rand.constants
+
+pub const seed_len = 4
 
 // PCG32RNG ported from http://www.pcg-random.org/download.html,
 // https://github.com/imneme/pcg-c-basic/blob/master/pcg_basic.c, and
 // https://github.com/imneme/pcg-c-basic/blob/master/pcg_basic.h
 pub struct PCG32RNG {
 mut:
-	state u64 = u64(0x853c49e6748fea9b) ^ seed.time_seed_64()
-	inc   u64 = u64(0xda3e39cb94b95bdb) ^ seed.time_seed_64()
+	state      u64 = u64(0x853c49e6748fea9b) ^ seed.time_seed_64()
+	inc        u64 = u64(0xda3e39cb94b95bdb) ^ seed.time_seed_64()
+	bytes_left int
+	buffer     u32
 }
 
 // seed seeds the PCG32RNG with 4 `u32` values.
@@ -30,11 +33,44 @@ pub fn (mut rng PCG32RNG) seed(seed_data []u32) {
 	rng.u32()
 	rng.state += init_state
 	rng.u32()
+	rng.bytes_left = 0
+	rng.buffer = 0
+}
+
+// byte returns a uniformly distributed pseudorandom 8-bit unsigned positive `byte`.
+[inline]
+fn (mut rng PCG32RNG) byte() byte {
+	if rng.bytes_left >= 1 {
+		rng.bytes_left -= 1
+		value := byte(rng.buffer)
+		rng.buffer >>= 8
+		return value
+	}
+	rng.buffer = rng.u32()
+	rng.bytes_left = 3
+	value := byte(rng.buffer)
+	rng.buffer >>= 8
+	return value
+}
+
+// u16 returns a pseudorandom 16-bit unsigned integer (`u16`).
+[inline]
+pub fn (mut rng PCG32RNG) u16() u16 {
+	if rng.bytes_left >= 2 {
+		rng.bytes_left -= 2
+		value := u16(rng.buffer)
+		rng.buffer >>= 16
+		return value
+	}
+	ans := rng.u32()
+	rng.buffer = ans >> 16
+	rng.bytes_left = 2
+	return u16(ans)
 }
 
 // u32 returns a pseudorandom unsigned `u32`.
 [inline]
-pub fn (mut rng PCG32RNG) u32() u32 {
+fn (mut rng PCG32RNG) u32() u32 {
 	oldstate := rng.state
 	rng.state = oldstate * (6364136223846793005) + rng.inc
 	xorshifted := u32(((oldstate >> u64(18)) ^ oldstate) >> u64(27))
@@ -48,6 +84,12 @@ pub fn (mut rng PCG32RNG) u64() u64 {
 	return u64(rng.u32()) | (u64(rng.u32()) << 32)
 }
 
+// block_size returns the number of bits that the RNG can produce in a single iteration.
+[inline]
+pub fn (mut rng PCG32RNG) block_size() int {
+	return 32
+}
+
 // free should be called when the generator is no longer needed
 [unsafe]
 pub fn (mut rng PCG32RNG) free() {