alvarocastro/pick-random-weighted

class Picker {
    /**
     * @param {Function} [randomFn] Random number generator.
     */
    constructor (randomFn) {
        if (typeof randomFn === 'function') {
            this.random = randomFn;
        }
    }

    /**
     * Generates a random number.
     * @return {Number}
     */
    random () {
        return Math.random();
    }

    /**
     * Pick a random value based on its weight.
     * @param {Array} data Array of values.
     * @return {Mixed}
     */
    pick (data) {
        if (!Array.isArray(data)) {
            throw new TypeError('Expected an Array as the first argument');
        }

        // Split input into two separate arrays of values and weights.
        const values = data.map(d => d[0]);
        const weights = data.map(d => d[1]);

        // Will contain the sum of all weights
        let sum = 0;
        // Will contain an array of each weight accumilating the sum of previous weights
        const accumulatedWeights = [];
        let weight;

        for (weight of weights) {
            sum += weight;
            accumulatedWeights.push(sum);
        }

        const rand = this.random() * sum;
        const value = values[accumulatedWeights.filter(element => element <= rand).length];

        return value;
    }
}

/**
 * All-in-one function that picks a random value based on its weight.
 * @param {Array} data Array of values.
 * @return {Mixed}
 */
const pick = function (data) {
    const picker = new Picker();

    return picker.pick(data);
};

export default pick;

export { pick, Picker };