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// Website: romu-random.org
// Reference paper: http://arxiv.org/abs/2002.11331
// Beware: this PRNG is trivially predictable. While fast, it should *never* be used for cryptographic purposes.
const std = @import("std");
const math = std.math;
const RomuTrio = @This();
x_state: u64,
y_state: u64,
z_state: u64, // set to nonzero seed
pub fn init(init_s: u64) RomuTrio {
var x = RomuTrio{ .x_state = undefined, .y_state = undefined, .z_state = undefined };
x.seed(init_s);
return x;
}
pub fn random(self: *RomuTrio) std.Random {
return std.Random.init(self, fill);
}
fn next(self: *RomuTrio) u64 {
const xp = self.x_state;
const yp = self.y_state;
const zp = self.z_state;
self.x_state = 15241094284759029579 *% zp;
self.y_state = yp -% xp;
self.y_state = std.math.rotl(u64, self.y_state, 12);
self.z_state = zp -% yp;
self.z_state = std.math.rotl(u64, self.z_state, 44);
return xp;
}
pub fn seedWithBuf(self: *RomuTrio, buf: [24]u8) void {
const seed_buf = @as([3]u64, @bitCast(buf));
self.x_state = seed_buf[0];
self.y_state = seed_buf[1];
self.z_state = seed_buf[2];
}
pub fn seed(self: *RomuTrio, init_s: u64) void {
// RomuTrio requires 192-bits of seed.
var gen = std.Random.SplitMix64.init(init_s);
self.x_state = gen.next();
self.y_state = gen.next();
self.z_state = gen.next();
}
pub fn fill(self: *RomuTrio, buf: []u8) void {
var i: usize = 0;
const aligned_len = buf.len - (buf.len & 7);
// Complete 8 byte segments.
while (i < aligned_len) : (i += 8) {
var n = self.next();
comptime var j: usize = 0;
inline while (j < 8) : (j += 1) {
buf[i + j] = @as(u8, @truncate(n));
n >>= 8;
}
}
// Remaining. (cuts the stream)
if (i != buf.len) {
var n = self.next();
while (i < buf.len) : (i += 1) {
buf[i] = @as(u8, @truncate(n));
n >>= 8;
}
}
}
test "sequence" {
// Unfortunately there does not seem to be an official test sequence.
var r = RomuTrio.init(0);
const seq = [_]u64{
16294208416658607535,
13964609475759908645,
4703697494102998476,
3425221541186733346,
2285772463536419399,
9454187757529463048,
13695907680080547496,
8328236714879408626,
12323357569716880909,
12375466223337721820,
};
for (seq) |s| {
try std.testing.expectEqual(s, r.next());
}
}
test fill {
// Unfortunately there does not seem to be an official test sequence.
var r = RomuTrio.init(0);
const seq = [_]u64{
16294208416658607535,
13964609475759908645,
4703697494102998476,
3425221541186733346,
2285772463536419399,
9454187757529463048,
13695907680080547496,
8328236714879408626,
12323357569716880909,
12375466223337721820,
};
for (seq) |s| {
var buf0: [8]u8 = undefined;
var buf1: [7]u8 = undefined;
std.mem.writeInt(u64, &buf0, s, .little);
r.fill(&buf1);
try std.testing.expect(std.mem.eql(u8, buf0[0..7], buf1[0..]));
}
}
test "buf seeding test" {
const buf0 = @as([24]u8, @bitCast([3]u64{ 16294208416658607535, 13964609475759908645, 4703697494102998476 }));
const resulting_state = .{ .x = 16294208416658607535, .y = 13964609475759908645, .z = 4703697494102998476 };
var r = RomuTrio.init(0);
r.seedWithBuf(buf0);
try std.testing.expect(r.x_state == resulting_state.x);
try std.testing.expect(r.y_state == resulting_state.y);
try std.testing.expect(r.z_state == resulting_state.z);
}
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