discourse/plugins/poll/assets/javascripts/lib/even-round.js

// works as described on http://stackoverflow.com/a/13483710
function sumsUpTo100(percentages) {
  return percentages.map((p) => Math.floor(p)).reduce((a, b) => a + b) === 100;
}

export default function (percentages) {
  let decimals = percentages.map((a) => a % 1);
  const sumOfDecimals = Math.ceil(decimals.reduce((a, b) => a + b));
  // compensate error by adding 1 to n items with the greatest decimal part
  for (let i = 0, max = decimals.length; i < sumOfDecimals && i < max; i++) {
    // find the greatest item in the decimals array, set it to 0,
    // and increase the corresponding item in the percentages array by 1
    let greatest = 0;
    let index = 0;
    for (let j = 0; j < decimals.length; j++) {
      if (decimals[j] > greatest) {
        index = j;
        greatest = decimals[j];
      }
    }
    ++percentages[index];
    decimals[index] = 0;
    // quit early when there is a rounding issue
    if (sumsUpTo100(percentages)) {
      break;
    }
  }

  return percentages.map((p) => Math.floor(p));
}