Trio/trio-oref/lib/percentile.js

// From https://gist.github.com/IceCreamYou/6ffa1b18c4c8f6aeaad2
// Returns the value at a given percentile in a sorted numeric array.
// "Linear interpolation between closest ranks" method
module.exports = function percentile(arr, p) {
    if (arr.length === 0) return 0;
    if (typeof p !== 'number') throw new TypeError('p must be a number');
    if (p <= 0) return arr[0];
    if (p >= 1) return arr[arr.length - 1];

    var index = arr.length * p,
        lower = Math.floor(index),
        upper = lower + 1,
        weight = index % 1;

    if (upper >= arr.length) return arr[lower];
    return arr[lower] * (1 - weight) + arr[upper] * weight;
}