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// SPDX-License-Identifier: MIT
// Copyright (c) 2015-2021 Zig Contributors
// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
// The MIT license requires this copyright notice to be included in all copies
// and substantial portions of the software.
//! PCG32 - http://www.pcg-random.org/
//!
//! PRNG
const std = @import("std");
const Random = std.rand.Random;
const Pcg = @This();
const default_multiplier = 6364136223846793005;
random: Random,
s: u64,
i: u64,
pub fn init(init_s: u64) Pcg {
var pcg = Pcg{
.random = Random{ .fillFn = fill },
.s = undefined,
.i = undefined,
};
pcg.seed(init_s);
return pcg;
}
fn next(self: *Pcg) u32 {
const l = self.s;
self.s = l *% default_multiplier +% (self.i | 1);
const xor_s = @truncate(u32, ((l >> 18) ^ l) >> 27);
const rot = @intCast(u32, l >> 59);
return (xor_s >> @intCast(u5, rot)) | (xor_s << @intCast(u5, (0 -% rot) & 31));
}
fn seed(self: *Pcg, init_s: u64) void {
// Pcg requires 128-bits of seed.
var gen = std.rand.SplitMix64.init(init_s);
self.seedTwo(gen.next(), gen.next());
}
fn seedTwo(self: *Pcg, init_s: u64, init_i: u64) void {
self.s = 0;
self.i = (init_s << 1) | 1;
self.s = self.s *% default_multiplier +% self.i;
self.s +%= init_i;
self.s = self.s *% default_multiplier +% self.i;
}
fn fill(r: *Random, buf: []u8) void {
const self = @fieldParentPtr(Pcg, "random", r);
var i: usize = 0;
const aligned_len = buf.len - (buf.len & 7);
// Complete 4 byte segments.
while (i < aligned_len) : (i += 4) {
var n = self.next();
comptime var j: usize = 0;
inline while (j < 4) : (j += 1) {
buf[i + j] = @truncate(u8, n);
n >>= 8;
}
}
// Remaining. (cuts the stream)
if (i != buf.len) {
var n = self.next();
while (i < buf.len) : (i += 1) {
buf[i] = @truncate(u8, n);
n >>= 4;
}
}
}
test "pcg sequence" {
var r = Pcg.init(0);
const s0: u64 = 0x9394bf54ce5d79de;
const s1: u64 = 0x84e9c579ef59bbf7;
r.seedTwo(s0, s1);
const seq = [_]u32{
2881561918,
3063928540,
1199791034,
2487695858,
1479648952,
3247963454,
};
for (seq) |s| {
std.testing.expect(s == r.next());
}
}
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