collinbrewer/expression

// https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSExpression_Class/#//apple_ref/occ/clm/NSExpression/expressionForFunction:arguments:
// https://support.airtable.com/hc/en-us/articles/203255215-Formula-Field-Reference#numeric_operators

function num (n) { return (isNaN(n) ? 0 : n); }

function avg (a) { return sum(a) / a.length; }

function sum (a) {
    return a.reduce(function (p, c) {
        return num(c) + p;
    }, 0);
}

function count (a) { return a.length; }

function min (a) {
    return a.reduce(function (p, c) {
        p = num(p);
        c = num(c);
        return (p < c ? p : c);
    }, undefined);
}

function max (a) {
    return a.reduce(function (p, c) {
        return (p > c ? p : c);
    }, undefined);
}

function median (arr) {
    if (arr.length === 0) return undefined;
    arr.sort(function (a, b) { return num(a) - num(b); });
    var l = arr.length;
    var h = ~~(l / 2);
    return (l % 2 === 1 ? arr[h] : (arr[h - 1] + arr[h]) / 2);
}

function mode (a) {
    return a.reduce(function (p, c) {
        var frequency = ((c in p.mapping) ? (p.mapping[c] + 1) : (p.mapping[c] = 1));

        if (frequency > p.greatestFrequency || p.mode === undefined || p.mode === null) {
            p.greatestFrequency = frequency;
            p.mode = c;
        }

        return p;
    }, {mode: undefined, greatestFrequency: -Infinity, mapping: {}}).mode;
}

function variance (a) {
    var average = avg(a);
    var i = a.length;
    var v = 0;
    var n;

    if (i > 2) {
        while (i--) {
            n = Number(a[i]);

            if (isNaN(n)) {
                n = 0;
            }

            v += Math.pow((n - average), 2);
        }

        v /= a.length;
    }

    return v;
}

function stddev (a) {
    return Math.sqrt(variance(a));
}

function union (a) {
    return a;
}

function distinctUnion (a) {
    return a.reduce(function (p, c) {
        p.distinct.indexOf(c) === -1 && p.distinct.push(c);
        return p;
    }, {distinct: []}).distinct;
}

module.exports = {
    avg: avg,
    average: avg,
    sum: sum,
    count: count,
    min: min,
    max: max,
    median: median,
    mode: mode,
    variance: variance,
    stddev: stddev,
    union: union,
    distinctUnion: distinctUnion
};