Round Half-Even

roundHalfEven.js

//
// Round Half-Even
// Refer: https://en.wikipedia.org/wiki/Rounding#Round_half_to_even
// Demo: https://codepen.io/schowdhuri/pen/wPXdJe
//
// Author: Subir Chowdhuri
// Updated: 2018-02-12
// License: MIT
//
// Copyright (c) 2018 Subir Chowdhuri
//
// Permission is hereby granted, free of charge, to any person
// obtaining a copy of this software and associated documentation
// files (the "Software"), to deal in the Software without
// restriction, including without limitation the rights to use,
// copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the
// Software is furnished to do so, subject to the following
// conditions:
//
// The above copyright notice and this permission notice shall be
// included in all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
// OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
// HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
// WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
// OTHER DEALINGS IN THE SOFTWARE.
//


const isEven = d => d%2===0;

const round = (num, numDecimals=2) => {
    if(typeof(num)!=="number")
        throw new Error("Unrecognised format");
    if(num < 0)
        return -round(-num);
    // convert to string; remove trailing 0s
    const strNum = `${num}`.replace(/0+$/, "");
    const decimalIndex = strNum.indexOf(".");
    if(decimalIndex < 0) // no fractional part
        return num;
    let intPart = strNum.slice(0, decimalIndex);
    if(intPart.length==0)
        intPart = 0;
    const fractPart = strNum.slice(decimalIndex + 1) // extract fractional part
    if(fractPart.length < numDecimals)
        return num;
    const followingDig = parseInt(fractPart[numDecimals], 10);
    if(followingDig < 5) {
        // rounding not required
        const newFractPart = fractPart.slice(0, numDecimals);
        return parseFloat(`${intPart}.${newFractPart}`);
    }
    if(followingDig === 5) {
        const newFractPart = fractPart.slice(0, numDecimals + 1);
        if(parseInt(fractPart.slice(numDecimals + 1), 10) > 0) {
            fractPart = `${newFractPart}9`;
        } else {
            fractPart = newFractPart;
        }
    }
    let nextDig = parseInt(fractPart[fractPart.length-1], 10);
    let carriedOver = 0;
    for(let ptr=fractPart.length-1; ptr>=numDecimals; ptr--) {
        let dig = parseInt(fractPart[ptr-1], 10) + carriedOver;
        if(nextDig > 5 ||(nextDig==5 && !isEven(dig))) {
            ++dig;
        }
        if(dig > 9) {
            dig -= 10;
            carriedOver = 1;
        } else {
            carriedOver = 0;
        }
        nextDig = dig;
    }
    let newFractPart = "";
    for(let ptr=numDecimals-2; ptr >=0; ptr--) {
        let d = parseInt(fractPart[ptr], 10) + carriedOver;
        if(d > 9) {
            d -= 10;
            carriedOver = 1;
        } else {
            carriedOver = 0;
        }
        newFractPart = `${d}${newFractPart}`
    }
    intPart = parseInt(intPart, 10) + carriedOver;
    return parseFloat(`${intPart}.${newFractPart}${nextDig}`);
};

export default round;