wilks-calculator/src/index.js

This library calculates performs calculations with the Wilks coefficient, which is a coefficient used by the International Powerlifting Federation to determine the relative strength of competitors. They also use this coefficient for determining the qual

'use strict';

const maleValues = [
	-216.0475144,
	16.2606339,
	-0.002388645,
	-0.00113732,
	7.01863E-06,
	-1.291E-08
];

const femaleValues = [
	594.31747775582,
	-27.23842536447,
	0.82112226871,
	-0.00930733913,
	4.731582E-05,
	-9.054E-08
];

const imperial = 2.20462262185;

/**
 * Returns a Wilks score based on the body weight of the lifter and the weight they have lifted.
 *
 * @param gender {string} The gender of the lifter the wilks score is calculated for ('m' for male, 'f' for female).
 * @param bodyWeight {number} The body weight of the lifter the wilks score is calculated for.
 * @param liftedWeight {number} The weight the lifter has lifted.
 * @param unitType {string} Optional parameter for lifters using the imperial unit system ('kg' is default, 'imperial' for the imperial system).
 *
 * @returns {number} The Wilks score.
 */
function calculateWilksScore (gender, bodyWeight, liftedWeight, unitType = 'metric') {
    if (!gender || !bodyWeight || !liftedWeight) {
    	throw new Error('Missing parameters, please fill in gender, body weight and weight.');
	}

	if (unitType === 'imperial') {
		liftedWeight /= imperial;
		bodyWeight /= imperial;
	}

	validateInput({gender: gender, bodyWeight: bodyWeight, liftedWeight: liftedWeight, unitType: unitType});

	let coeff = 500 / calculateCoefficient(gender, bodyWeight);

    return liftedWeight * coeff;
}

/**
 * Returns a total amount of weight to lift based on the body weight of the lifter and the preferred Wilks score.
 *
 * @param gender {string} The gender of the lifter the wilks score is calculated for ('m' for male, 'f' for female).
 * @param bodyWeight {number} The body weight of the lifter the wilks score is calculated for.
 * @param wilksScore {number} The preferred Wilks score.
 * @param unitType {string} Optional parameter for lifters using the imperial unit system ('kg' is default, 'imperial' for the imperial system).
 *
 * @returns {number} The total amount of weight to lift.
 */
function calculateWeightToLift (gender, bodyWeight, wilksScore, unitType = 'metric') {
	if (!gender || !bodyWeight || !wilksScore) {
		throw new Error('Missing parameters, please fill in gender, body weight and Wilks score.');
	}

	validateInput({gender: gender, bodyWeight: bodyWeight, wilksScore: wilksScore, unitType: unitType});

	if (unitType === 'imperial') {
		bodyWeight /= imperial;
	}

	let coeff = 500 / calculateCoefficient(gender, bodyWeight);

	return unitType === 'imperial' ? imperial * (wilksScore / coeff) : wilksScore / coeff;
}

/**
 * Returns the needed body weight based on the total amount of weight to lift and the preferred Wilks score.
 *
 * @param gender {string} The gender of the lifter the wilks score is calculated for ('m' for male, 'f' for female).
 * @param liftedWeight {number} liftedWeight {number} The weight the lifter has lifted.
 * @param wilksScore {number} The preferred Wilks score.
 * @param unitType {string} Optional parameter for lifters using the imperial unit system ('kg' is default, 'imperial' for the imperial system).
 *
 * @returns {number} The total amount of weight to lift.
 */
function calculateNeededBodyWeight (gender, liftedWeight, wilksScore, unitType = 'metric') {
	if (!gender || !liftedWeight || !wilksScore) {
		throw new Error('Missing parameters, please fill in gender, lifted weight and Wilks score.');
	}

	validateInput({gender: gender, liftedWeight: liftedWeight, wilksScore: wilksScore, unitType: unitType});

	if (unitType === 'imperial') {
		liftedWeight /= imperial;
	}

	let coeff = 500 / (wilksScore / liftedWeight);
	let bodyWeight = 0.0;
	let result = 0.0;

	do {
		bodyWeight += 0.1;
		result = calculateCoefficient(gender, bodyWeight);
	} while (calculateDifference(coeff, result) > 0.5);

	return unitType === 'imperial' ? imperial * bodyWeight : bodyWeight;
}

/**
 * A helper function to determine the difference between the calculated coefficient and the input.
 *
 * @param a {number}
 * @param b {number}
 *
 * @returns {number} The absolute difference between a and b.
 *
 * @private
 */
function calculateDifference(a, b) {
	return Math.abs(a - b);
}

/**
 * Calculates the coefficient based on the body weight and the gender.
 *
 * @param gender {string}
 * @param bodyWeight {number}
 *
 * @returns {number} The coefficient.
 *
 * @private
 */
function calculateCoefficient(gender, bodyWeight) {
	let coeff = 0;
	let values = gender === 'm' ? maleValues : femaleValues;

	for (let i = 0; i <= 5; i++) {
		coeff += i === 0 ? values[i] : (values[i]  * (bodyWeight ** i));
	}

	return coeff;
}

/**
 * A helper function to validate the input.
 *
 * @param gender {string}
 * @param bodyWeight {number}
 * @param liftedWeight {number}
 * @param wilksScore {number}
 * @param unitType {string}
 *
 * @private
 */
function validateInput ({gender, bodyWeight = 0, liftedWeight = 0, wilksScore = 0, unitType}) {
	if (typeof gender !== 'string' || (gender !== 'm' && gender !== 'f')) {
		throw new Error('Gender is not valid. Please select m for Male or f for Female.')
	}

	if (typeof bodyWeight !== 'number' || bodyWeight < 0) {
		throw new Error('Body weight is not valid.');
	}

	if (typeof liftedWeight !== 'number' || liftedWeight < 0) {
		throw new Error('Weight is not valid.');
	}

	if (typeof wilksScore !== 'number' || wilksScore < 0) {
		throw new Error('Wilks score is not valid.');
	}

	if (typeof unitType !== 'string' || (unitType !== 'metric' && unitType !== 'imperial')) {
		throw new Error('Unit type is not valid. Please select metric or imperial.');
	}
}

module.exports = {
	calculateWilksScore: calculateWilksScore,
	calculateWeightToLift: calculateWeightToLift,
	calculateNeededBodyWeight: calculateNeededBodyWeight
};