/* @flow */

import variance from "./variance";

/**
 * The [standard deviation](http://en.wikipedia.org/wiki/Standard_deviation)
 * is the square root of the variance. This is also known as the population
 * standard deviation. It's useful for measuring the amount
 * of variation or dispersion in a set of values.
 *
 * Standard deviation is only appropriate for full-population knowledge: for
 * samples of a population, {@link sampleStandardDeviation} is
 * more appropriate.
 *
 * @param {Array<number>} x input
 * @returns {number} standard deviation
 * @example
 * variance([2, 4, 4, 4, 5, 5, 7, 9]); // => 4
 * standardDeviation([2, 4, 4, 4, 5, 5, 7, 9]); // => 2
 */
function standardDeviation(x /*: Array<number> */) /*:number*/ {
    if (x.length === 1) {
        return 0;
    }
    const v = variance(x);
    return Math.sqrt(v);
}

export default standardDeviation;