rand: add pcg32 and splitmix64 implementations

vlib/rand/splitmix64.v

@@ -0,0 +1,42 @@
+module rand
+
+// Ported from http://xoshiro.di.unimi.it/splitmix64.c
+
+struct Splitmix64 {
+mut:
+	state u64
+}
+/**
+ * new_splitmix64 - a Splitmix64 PRNG generator
+ * @param seed the initial seed of the PRNG. 
+ * @return a new Splitmix64 PRNG instance
+*/
+pub fn new_splitmix64(seed u64) Splitmix64 {
+	return Splitmix64{ seed }
+}
+/**
+ * Splitmix64.next - update the PRNG state and get back the next random number
+ * @return the generated pseudo random number
+*/
+[inline] pub fn (rng mut Splitmix64) next() u64 {
+	rng.state += u64(0x9e3779b97f4a7c15)
+	mut z := rng.state
+	z = (z ^ u64((z >> u64(30)))) * u64(0xbf58476d1ce4e5b9)
+	z = (z ^ u64((z >> u64(27)))) * u64(0x94d049bb133111eb)
+	return z ^ u64(z >> u64(31))
+}
+/**
+ * Splitmix64.bounded_next - Get the next random number < bound
+ * @param bound - the returned random number will be < bound
+ * @return the generated pseudo random number
+*/
+[inline] pub fn (rng mut Splitmix64) bounded_next(bound u64) u64 {
+	threshold := u64( -bound % bound )
+	for {
+		r := rng.next()
+		if r >= threshold {
+			return u64( r % bound )
+		}			
+	}
+	return u64(0)
+}