diff --git a/src/rng.lua b/src/rng.lua
new file mode 100644
index 0000000..abf3a9e
--- /dev/null
+++ b/src/rng.lua
@@ -0,0 +1,47 @@
+-- Deterministic 64-bit PRNG (splitmix64). Same seed always yields the same
+-- stream, independent of platform and of the global math.random state.
+
+local Rng = {}
+Rng.__index = Rng
+
+local GOLDEN = 0x9E3779B97F4A7C15
+local MIX1 = 0xBF58476D1CE4E5B9
+local MIX2 = 0x94D049BB133111EB
+
+function Rng.new(seed)
+  return setmetatable({ state = math.tointeger(seed) or 0 }, Rng)
+end
+
+function Rng:nextUint()
+  self.state = self.state + GOLDEN
+  local z = self.state
+  z = (z ~ (z >> 30)) * MIX1
+  z = (z ~ (z >> 27)) * MIX2
+  z = z ~ (z >> 31)
+  return z
+end
+
+-- Inclusive integer in [lo, hi]. Modulo of a signed value by a positive span
+-- lands in [0, span) under Lua's floor-division remainder.
+function Rng:range(lo, hi)
+  local span = hi - lo + 1
+  return lo + self:nextUint() % span
+end
+
+function Rng:chance(numerator, denominator)
+  return self:range(1, denominator) <= numerator
+end
+
+function Rng:pick(list)
+  return list[self:range(1, #list)]
+end
+
+function Rng:serialize()
+  return { state = self.state }
+end
+
+function Rng.deserialize(snapshot)
+  return setmetatable({ state = snapshot.state }, Rng)
+end
+
+return Rng