rtmigo/xrandom_dart

Dart library with random number generators focused on the consistency, performance and reproducibility

xrandom_dart's Issues

Int is not a true 64 bit - other type needed

Hello

The four seed parameters defined for Xoshiro256ss are int each.
However, in dart the int type is only 2^63-1 max value, because 1 bit is used for defining the sign (https://stackoverflow.com/a/53591126/346488).
Which means int is not a proper type to use as an input, as it is not possible to pass each part as a true 64-bit integer.

E.g. I have an issue passing one as:
int.parse('f78207dec08eba34', radix:16)
It returns an error:
[ERROR:flutter/lib/ui/ui_dart_state.cc(209)] Unhandled Exception: FormatException: Positive input exceeds the limit of integer

It looks like the right approach would be to use BigInt as input parameters type and then check inside that each parameter is within 64 bits.

Could you please check on this?
Thank you!

Add Xoshiro256StarStar

Hello

Could you please add xoshiro256ss pseudorandom generator?
https://xoshiro.di.unimi.it/xoshiro256starstar.c

I understand it is mostly reusing xoshiro256pp but with a slight change.

Thank you in advance!

Limit for max in nextInt should be 2^31 inclusive, not exclusive

If nextInt(1 << 32) is called on Xrandom() a RangeError is thrown. The same call on the dart Random() works fine because the limit for max is defined as 1 << 32 inclusively. The generated values will of course be in the range from 0 (inclusive) to 1 << 32 exclusive, so exactly 32 random bits.

The Problem is in the lines:

xrandom_dart/lib/src/21_base32.dart

Lines 83 to 84 in 8b4b47b

	if (max < 1 || max > 0xFFFFFFFF) {
	throw RangeError.range(max, 1, 0xFFFFFFFF);

nextInt is not uniform

nextInt(n) does not generate uniform random numbers for larger n.

The issue is with the following line:

xrandom_dart/lib/src/21_base32.dart

Line 88 in 8b4b47b

for (int u = r; u - (r = u % max) + m < 0; u = nextRaw32()) {}

If I'm not mistaken the line should be for (int u = r; u - (r = u % max) + max > (1 << 32); u = nextRaw32()) {}. Not sure what the point of m = max-1 is, but i don't think its needed.

Since u % max is always <= u, u-(u%max) is never negative. And since m is also never negative, u-(r=u%max)+m will never be less than 0. So essentially only nextRaw32() % max will every be returned. This is approximately uniform for small m but for larger m, it is not. This can easily experimentally be verified.

The following program prints lower: 6665204, upper: 3334796 even though there should be approximately an equal amout of numers below the midpoint as above.

import 'package:xrandom/xrandom.dart';

void main() {
  final random = Xrandom(42);

  const mid = (1 << 32) ~/ 3;
  const max = mid * 2;
  var lower = 0;
  var upper = 0;
  for (var i = 0; i < 10000000; i++) {
    if (random.nextInt(max) < mid) {
      lower++;
    } else {
      upper++;
    }
  }
  print('lower: $lower, upper: $upper');
}