init

rng/rng.go

@@ -0,0 +1,74 @@
+// Package rng provides a small deterministic PRNG (xoshiro256**) seeded via
+// splitmix64. Streams are derived from a seed plus labels so that independent
+// parts of the simulation never share (and thus never perturb) each other's
+// random sequence.
+package rng
+
+import (
+	"math/bits"
+
+	"wh/fixed"
+)
+
+type R struct{ s [4]uint64 }
+
+func splitmix(x *uint64) uint64 {
+	*x += 0x9e3779b97f4a7c15
+	z := *x
+	z = (z ^ (z >> 30)) * 0xbf58476d1ce4e5b9
+	z = (z ^ (z >> 27)) * 0x94d049bb133111eb
+	return z ^ (z >> 31)
+}
+
+// New returns a generator for the given seed.
+func New(seed uint64) *R {
+	r := &R{}
+	for i := range r.s {
+		r.s[i] = splitmix(&seed)
+	}
+	return r
+}
+
+// Derive returns a generator for an independent stream identified by seed and
+// labels, e.g. Derive(worldSeed, day, shipID).
+func Derive(seed uint64, labels ...uint64) *R {
+	h := seed
+	for _, l := range labels {
+		h ^= l + 0x9e3779b97f4a7c15 + (h << 6) + (h >> 2)
+		splitmix(&h)
+	}
+	return New(h)
+}
+
+func (r *R) Uint64() uint64 {
+	s := &r.s
+	res := bits.RotateLeft64(s[1]*5, 7) * 9
+	t := s[1] << 17
+	s[2] ^= s[0]
+	s[3] ^= s[1]
+	s[1] ^= s[2]
+	s[0] ^= s[3]
+	s[2] ^= t
+	s[3] = bits.RotateLeft64(s[3], 45)
+	return res
+}
+
+// Intn returns a uniform value in [0, n). n must be > 0.
+func (r *R) Intn(n uint64) uint64 {
+	hi, lo := bits.Mul64(r.Uint64(), n)
+	if lo < n {
+		thresh := -n % n
+		for lo < thresh {
+			hi, lo = bits.Mul64(r.Uint64(), n)
+		}
+	}
+	return hi
+}
+
+// Fixed returns a uniform value in [0, 1).
+func (r *R) Fixed() fixed.F { return fixed.F(r.Uint64() >> 32) }
+
+// FixedRange returns a uniform value in [lo, hi).
+func (r *R) FixedRange(lo, hi fixed.F) fixed.F {
+	return lo + r.Fixed().Mul(hi-lo)
+}