CODEWITHSUNDEEP
mathdart
gmosx
gmosx

Reputation: 351

How can I create a random 256 bit integer in Dart?

I would like to create a random (uniformly distributed) 256bit integer in Dart. Dart's Random only support 32 bit integers. Any ideas?

Upvotes: 6

Views: 1667

Answers (2)

MarioP
MarioP

Reputation: 3832

import "dart:math";

// 16bit, because random.nextInt() only supports (2^32)-1 possible values.
const PARTS = 16;  // 256bit / 16bit

void main() {
  Random rand = new Random();
  int combinedVal = 0;
  // random parts
  for(var i=0;i<PARTS;i++) {
    int part = rand.nextInt(1<<16); // 2^16
    print("Part $i: $part");
    // shift the 16bit blocks to the left and append the new block
    combinedVal <<= 16;
    combinedVal += part;
    print("Combined: $combinedVal");
  }
  print("Final Result: $combinedVal");
}

Output (console application):

Part 0: 4273569419
Combined: 4273569419
Part 1: 2298770505
Combined: 18354840894089491529
Part 2: 1076269765
Combined: 78833441363397765815400305349
Part 3: 500743884
Combined: 338587052486927055616611084622869610188
Part 4: 1660193956
Combined: 1454220317280387171410917722806313469431388605604
Part 5: 1335995533
Combined: 6245828703898006563427837796799909693532109776937558322317
Part 6: 2409230726
Combined: 26825630019660005909515912993248305589473794217828668028446551175558
Part 7: 3743170719
...

EDIT

As Darshan Computing pointed out in the comments, to make this work with dart2js, some modifications are needed, and this would result in lost precision. To use this with browser, an external library and js interop would be needed. As an example, i used Leemon Baird's public domain BigInt library

HTML file:

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Web Playground</title>
    <link rel="stylesheet" href="web_playground.css">
    <script src="BigInt.js"></script> <!-- this is the important part -->
  </head>
  <body>
    <h1>Web Playground</h1>
    <script type="application/dart" src="web_playground.dart"></script>
    <script src="packages/browser/dart.js"></script>
  </body>
</html>

Dart file:

import "dart:html";
import "package:js/js.dart" as js;

void main() {
  var rand = js.context.randBigInt(256,0);
  window.alert(js.context.bigInt2str(rand,10));
}

Upvotes: 7

MvG
MvG

Reputation: 60858

Thanks to the comments below, I found that depending on the execution environment, you int class might be arbitrary size or restricted to 53 bits.

In the former case, you can simply construct a random value from multiple snippets using shift operators. Since nextInt does not support an inclusive maximum of 232 − 1 (its max argument allows that value, but it is exclusive, so you get one less posisble value), you might want to do this in chunks of 16 bits. Start with zero, and in every step shift the current value left by 16 bits before adding another 16-bit integer (i.e. one with 216 as the max argument). After 16 iterations you'll have accumulated 256 bits.

When compiled to JavaScript, int has only 53 significant bits. In that case, you'll have to have an arbitrary size integer implementation, either using some third party library or written yourself, perhaps based on ideas from http://en.wikipedia.org/wiki/Arbitrary-precision_arithmetic. Once you have a big integer class, generating a random element should be easy, as most likely the internal representation of the big integers will consist of 16 or 32 bit units in any case.

Upvotes: 3

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