decimal128/examples/rounding-error.ts

Partial implementation of IEEE 754 Decimal128 decimal floating-point numbers

import { Decimal128 } from "../src/Decimal128.mjs";

function getRandomInt(max: number): number {
    return Math.floor(Math.random() * max);
}

function getRandomDigits(n: number): number[] {
    let digits = [];
    for (let i = 0; i < n; i++) {
        digits.push(getRandomInt(10));
    }
    return digits;
}

function getRandomDigitsAsString(n: number): string {
    let digits = getRandomDigits(n + 4);
    return digits.slice(0, n).join("") + "." + digits.slice(n).join("");
}

function compareNumbers(
    digits1: string,
    digits2: string,
    digits3: string
): [string, string] {
    let d1 = new Decimal128(digits1);
    let d2 = new Decimal128(digits2);
    let d3 = new Decimal128(digits3);

    let decimalResult = d1.multiply(d2).add(d3);
    let numberResult = Number(digits1) * Number(digits2) + Number(digits3);
    let numberResultString = numberResult.toFixed(4);
    let decimalResultString = decimalResult.toFixed({ digits: 4 });

    return [numberResultString, decimalResultString];
}

function keepTrying(): boolean {
    let digits1 = getRandomDigitsAsString(5);
    let digits2 = getRandomDigitsAsString(3);
    let digits3 = getRandomDigitsAsString(4);
    let [result1, result2] = compareNumbers(digits1, digits2, digits3);

    if (result1 !== result2) {
        let diff = new Decimal128(result1)
            .subtract(new Decimal128(result2))
            .abs();
        console.log(
            `(${digits1} * ${digits2}) + ${digits3} = ${result1} but it should be ${result2} (difference of ${diff.toString()})`
        );
    }

    return result1 !== result2;
}

let numAttempts = 1000;
let numFailures = 0;
for (let i = 0; i < numAttempts; i++) {
    let failed = keepTrying();
    if (failed) {
        numFailures++;
    }
}

console.log(`Failed ${numFailures} out of ${numAttempts} attempts`);