math/rand: Add xoshiro256**

This is extracted from the previos PR as an improved non-cryptographic
PRNG option.

core/math/rand/rand_xoshiro256.odin

@@ -0,0 +1,123 @@
+package rand
+
+import "base:intrinsics"
+import "base:runtime"
+
+import "core:math/bits"
+
+/*
+The state for a xoshiro256** pseudorandom generator.
+*/
+Xoshiro256_Random_State :: struct {
+	s: [4]u64,
+}
+
+xoshiro256_random_generator_proc :: proc(data: rawptr, mode: runtime.Random_Generator_Mode, p: []byte) {
+	@(require_results)
+	read_u64 :: proc "contextless" (r: ^Xoshiro256_Random_State) -> u64 {
+		// xoshiro256** output function and state transition
+
+		result := bits.rotate_left64(r.s[1] * 5, 7) * 9
+		t := r.s[1] << 17
+
+		r.s[2] = r.s[2] ~ r.s[0]
+		r.s[3] = r.s[3] ~ r.s[1]
+		r.s[1] = r.s[1] ~ r.s[2]
+		r.s[0] = r.s[0] ~ r.s[3]
+		r.s[2] = r.s[2] ~ t
+		r.s[3] = bits.rotate_left64(r.s[3], 45)
+
+		return result
+	}
+
+	@(thread_local)
+	global_rand_seed: Xoshiro256_Random_State
+
+	init :: proc "contextless" (r: ^Xoshiro256_Random_State, seed: u64) {
+		// splitmix64 to expand a 64-bit seed into 256 bits of state
+		sm64_next :: proc "contextless" (s: ^u64) -> u64 {
+			s^ += 0x9E3779B97F4A7C15
+			z := s^
+			z = (z ~ (z >> 30)) * 0xBF58476D1CE4E5B9
+			z = (z ~ (z >> 27)) * 0x94D049BB133111EB
+			return z ~ (z >> 31)
+		}
+
+		local_seed := seed
+		r.s[0] = sm64_next(&local_seed)
+		r.s[1] = sm64_next(&local_seed)
+		r.s[2] = sm64_next(&local_seed)
+		r.s[3] = sm64_next(&local_seed)
+		// Extremely unlikely all zero; ensure non-zero state
+		if (r.s[0] | r.s[1] | r.s[2] | r.s[3]) == 0 {
+			// force a minimal non-zero tweak
+			r.s[0] = 1
+		}
+	}
+
+	r: ^Xoshiro256_Random_State = ---
+	if data == nil {
+		r = &global_rand_seed
+	} else {
+		r = cast(^Xoshiro256_Random_State)data
+	}
+
+	switch mode {
+	case .Read:
+		if (r.s[0] | r.s[1] | r.s[2] | r.s[3]) == 0 {
+			init(r, u64(intrinsics.read_cycle_counter()))
+		}
+
+		switch len(p) {
+		case size_of(u64):
+			// Fast path for a 64-bit destination.
+			intrinsics.unaligned_store((^u64)(raw_data(p)), read_u64(r))
+		case:
+			// All other cases.
+			pos := i8(0)
+			val := u64(0)
+			for &v in p {
+				if pos == 0 {
+					val = read_u64(r)
+					pos = 8
+				}
+				v = byte(val)
+				val >>= 8
+				pos -= 1
+			}
+		}
+
+	case .Reset:
+		seed: u64 = 0
+		runtime.mem_copy_non_overlapping(&seed, raw_data(p), min(size_of(seed), len(p)))
+		init(r, seed)
+
+	case .Query_Info:
+		if len(p) != size_of(Generator_Query_Info) {
+			return
+		}
+		info := (^Generator_Query_Info)(raw_data(p))
+		info^ += {.Uniform, .Resettable}
+	}
+}
+
+/*
+Returns an instance of the xoshiro256** pseudorandom generator.  If no
+initial state is provided, the PRNG will be lazily initialized with the
+system timestamp counter on first-use.
+
+WARNING: This random number generator is NOT cryptographically secure.
+
+Inputs:
+- state: Optional initial PRNG state.
+
+Returns:
+- A `Generator` instance.
+*/
+@(require_results)
+xoshiro256_random_generator :: proc "contextless" (state: ^Xoshiro256_Random_State = nil) -> Generator {
+	return {
+		procedure = xoshiro256_random_generator_proc,
+		data = state,
+	}
+}