Codibre/preciser-calc

import {
    defaultCorrectionFactor,
    maxSafeFactor,
    maxSafeInteger,
} from './constants';

function convertBack(acc: bigint, correction: bigint, factor: bigint): number {
    const q = acc / correction;
    const r = acc % correction;

    return (
        Number(q) +
        (r > maxSafeInteger
            ? convertBack(r, correction / factor, factor)
            : Number(r) / Number(correction))
    );
}

function bigIntFallback(
    init: number,
    values: (string | number)[],
    accBase: number,
    correctionBase: number,
    correctionFactorBase: number,
) {
    let acc = BigInt(Math.round(accBase));
    let correction = BigInt(correctionBase);
    const correctionFactor = BigInt(correctionFactorBase);

    for (let i = init; i < values.length; i++) {
        acc *= BigInt((values[i] as number) * correctionFactorBase);
        if (acc % correctionFactor !== 0n) {
            correction *= correctionFactor;
        } else {
            acc /= correctionFactor;
        }
    }

    return convertBack(acc, correction, correctionFactor);
}

/**
 * Sums the informed value applying the correction factor for better precision
 * @param values the list of values to sum
 * @param correctionFactor a power of 10. The number of zeroes is the number of decimal places you want as precision
 * @returns the sum
 */
export function multiply(
    values: (number | string)[],
    correctionFactor: number = defaultCorrectionFactor,
) {
    const { length } = values;
    let acc = 1;
    let correction = 1;
    for (let i = 0; i < length; i++) {
        const newValue = (values[i] as number) * correctionFactor;
        const result = acc * newValue;
        if (result > Number.MAX_SAFE_INTEGER) {
            return bigIntFallback(i, values, acc, correction, correctionFactor);
        }
        acc = result;
        if (acc % correctionFactor === 0) {
            acc /= correctionFactor;
        } else {
            const newCorrection = correction * correctionFactor;
            if (newCorrection > maxSafeFactor) {
                return bigIntFallback(i, values, acc, correction, correctionFactor);
            }
            correction = newCorrection;
        }
    }
    return acc / correction;
}