math/rand: Add a copy of the existing PCG PRNG

The switch to the new RNG will be a breaking change as the outputted
sequence will change for a given seed.  This is intended for backward
compatibility purposes only.

core/math/rand/rand_pcg.odin

@@ -0,0 +1,107 @@
+package rand
+
+import "base:intrinsics"
+import "base:runtime"
+
+/*
+The state for a PCG64 RXS-M-XS pseudorandom generator.
+*/
+PCG_Random_State :: struct {
+	state: u64,
+	inc:   u64,
+}
+
+pcg_random_generator_proc :: proc(data: rawptr, mode: runtime.Random_Generator_Mode, p: []byte) {
+	@(require_results)
+	read_u64 :: proc "contextless" (r: ^PCG_Random_State) -> u64 {
+		old_state := r.state
+		r.state = old_state * 6364136223846793005 + (r.inc|1)
+		xor_shifted := (((old_state >> 59) + 5) ~ old_state) * 12605985483714917081
+		rot := (old_state >> 59)
+		return (xor_shifted >> rot) | (xor_shifted << ((-rot) & 63))
+	}
+
+	@(thread_local)
+	global_rand_seed: PCG_Random_State
+
+	init :: proc "contextless" (r: ^PCG_Random_State, seed: u64) {
+		seed := seed
+		if seed == 0 {
+			seed = u64(intrinsics.read_cycle_counter())
+		}
+		r.state = 0
+		r.inc = (seed << 1) | 1
+		_ = read_u64(r)
+		r.state += seed
+		_ = read_u64(r)
+	}
+
+	r: ^PCG_Random_State = ---
+	if data == nil {
+		r = &global_rand_seed
+	} else {
+		r = cast(^PCG_Random_State)data
+	}
+
+	switch mode {
+	case .Read:
+		if r.state == 0 && r.inc == 0 {
+			init(r, 0)
+		}
+
+		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
+		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 PGC64 RXS-M-XS 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,
+and is additionally known to be flawed.  It is only included for
+backward compatibility with historical releases of Odin.
+See: https://github.com/odin-lang/Odin/issues/5881
+
+Inputs:
+- state: Optional initial PRNG state.
+
+Returns:
+- A `Generator` instance.
+*/
+@(require_results)
+pcg_random_generator :: proc "contextless" (state: ^PCG_Random_State = nil) -> Generator {
+	return {
+		procedure = pcg_random_generator_proc,
+		data = state,
+	}
+}