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

vlib/rand/splitmix64/splitmix64.v

@@ -4,14 +4,15 @@
 module splitmix64
 
 import rand.seed
-import rand.constants
+
+pub const seed_len = 2
 
 // SplitMix64RNG ported from http://xoshiro.di.unimi.it/splitmix64.c
 pub struct SplitMix64RNG {
 mut:
-	state     u64 = seed.time_seed_64()
-	has_extra bool
-	extra     u32
+	state      u64 = seed.time_seed_64()
+	bytes_left int
+	buffer     u64
 }
 
 // seed sets the seed of the accepting SplitMix64RNG to the given data
@@ -22,25 +23,57 @@ pub fn (mut rng SplitMix64RNG) seed(seed_data []u32) {
 		exit(1)
 	}
 	rng.state = seed_data[0] | (u64(seed_data[1]) << 32)
-	rng.has_extra = false
+	rng.bytes_left = 0
+	rng.buffer = 0
 }
 
-// u32 updates the PRNG state and returns the next pseudorandom `u32`.
+// byte returns a uniformly distributed pseudorandom 8-bit unsigned positive `byte`.
+[inline]
+pub fn (mut rng SplitMix64RNG) byte() byte {
+	if rng.bytes_left >= 1 {
+		rng.bytes_left -= 1
+		value := byte(rng.buffer)
+		rng.buffer >>= 8
+		return value
+	}
+	rng.buffer = rng.u64()
+	rng.bytes_left = 7
+	value := byte(rng.buffer)
+	rng.buffer >>= 8
+	return value
+}
+
+// u16 returns a pseudorandom 16bit int in range `[0, 2¹⁶)`.
+[inline]
+pub fn (mut rng SplitMix64RNG) u16() u16 {
+	if rng.bytes_left >= 2 {
+		rng.bytes_left -= 2
+		value := u16(rng.buffer)
+		rng.buffer >>= 16
+		return value
+	}
+	ans := rng.u64()
+	rng.buffer = ans >> 16
+	rng.bytes_left = 6
+	return u16(ans)
+}
+
+// u32 returns a pseudorandom 32bit int in range `[0, 2³²)`.
 [inline]
 pub fn (mut rng SplitMix64RNG) u32() u32 {
-	if rng.has_extra {
-		rng.has_extra = false
-		return rng.extra
+	if rng.bytes_left >= 4 {
+		rng.bytes_left -= 4
+		value := u32(rng.buffer)
+		rng.buffer >>= 32
+		return value
 	}
-	full_value := rng.u64()
-	lower := u32(full_value & constants.lower_mask)
-	upper := u32(full_value >> 32)
-	rng.extra = upper
-	rng.has_extra = true
-	return lower
+	ans := rng.u64()
+	rng.buffer = ans >> 32
+	rng.bytes_left = 4
+	return u32(ans)
 }
 
-// u64 updates the PRNG state and returns the next pseudorandom `u64`.
+// u64 returns a pseudorandom 64bit int in range `[0, 2⁶⁴)`.
 [inline]
 pub fn (mut rng SplitMix64RNG) u64() u64 {
 	rng.state += (0x9e3779b97f4a7c15)
@@ -50,6 +83,12 @@ pub fn (mut rng SplitMix64RNG) u64() u64 {
 	return z ^ (z >> (31))
 }
 
+// block_size returns the number of bits that the RNG can produce in a single iteration.
+[inline]
+pub fn (mut rng SplitMix64RNG) block_size() int {
+	return 64
+}
+
 // free should be called when the generator is no longer needed
 [unsafe]
 pub fn (mut rng SplitMix64RNG) free() {