@gandlaf21/bc-ur/src/fountainUtils.ts

A JS implementation of the Uniform Resources (UR) specification from Blockchain Commons

import { intToBytes } from "./utils";
import Xoshiro from "./xoshiro";
// @ts-ignore
import randomSampler from '@apocentre/alias-sampling'
import { Buffer } from "buffer";


export const chooseDegree = (seqLenth: number, rng: Xoshiro): number => {
  const degreeProbabilities = [...new Array(seqLenth)].map((_, index) => 1 / (index + 1));
  const degreeChooser = randomSampler(degreeProbabilities, null, rng.nextDouble);
  return degreeChooser.next() + 1;
}


export const shuffle = (items: any[], rng: Xoshiro): any[] => {
  let remaining = [...items];
  let result = [];

  while (remaining.length > 0) {
    let index = rng.nextInt(0, remaining.length - 1);
    let item = remaining[index];
    // remaining.erase(remaining.begin() + index);
    remaining.splice(index, 1);
    result.push(item);
  }

  return result;
}


export const chooseFragments = (seqNum: number, seqLength: number, checksum: number): number[] => {
  // The first `seqLenth` parts are the "pure" fragments, not mixed with any
  // others. This means that if you only generate the first `seqLenth` parts,
  // then you have all the parts you need to decode the message.
  if (seqNum <= seqLength) {
    return [seqNum - 1];
  } else {
    const seed = Buffer.concat([intToBytes(seqNum), intToBytes(checksum)]);
    const rng = new Xoshiro(seed);
    const degree = chooseDegree(seqLength, rng);
    const indexes = [...new Array(seqLength)].map((_, index) => index);
    const shuffledIndexes = shuffle(indexes, rng);

    return shuffledIndexes.slice(0, degree);
  }
}