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

vlib/rand/musl/musl_rng.v

@@ -5,10 +5,14 @@ module musl
 
 import rand.seed
 
+pub const seed_len = 1
+
 // MuslRNG ported from https://git.musl-libc.org/cgit/musl/tree/src/prng/rand_r.c
 pub struct MuslRNG {
 mut:
-	state u32 = seed.time_seed_32()
+	state      u32 = seed.time_seed_32()
+	bytes_left int
+	buffer     u32
 }
 
 // seed sets the current random state based on `seed_data`.
@@ -19,6 +23,39 @@ pub fn (mut rng MuslRNG) seed(seed_data []u32) {
 		exit(1)
 	}
 	rng.state = seed_data[0]
+	rng.bytes_left = 0
+	rng.buffer = 0
+}
+
+// byte returns a uniformly distributed pseudorandom 8-bit unsigned positive `byte`.
+[inline]
+fn (mut rng MuslRNG) 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 MuslRNG) 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)
 }
 
 // temper returns a tempered value based on `prev` value.
@@ -33,8 +70,7 @@ fn temper(prev u32) u32 {
 }
 
 // u32 returns a pseudorandom 32-bit unsigned integer (`u32`).
-[inline]
-pub fn (mut rng MuslRNG) u32() u32 {
+fn (mut rng MuslRNG) u32() u32 {
 	rng.state = rng.state * 1103515245 + 12345
 	// We are not dividing by 2 (or shifting right by 1)
 	// because we want all 32-bits of random data
@@ -47,6 +83,12 @@ pub fn (mut rng MuslRNG) 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 MuslRNG) block_size() int {
+	return 32
+}
+
 // free should be called when the generator is no longer needed
 [unsafe]
 pub fn (mut rng MuslRNG) free() {