@extra-array/is-prefix/index.js

Checks if array starts with a prefix.

'use strict';

/**
 * Gives same value.
 * @param v a value
 * @returns v
 */
function id(v) {
    return v;
}

/**
 * Compares two values.
 * @param a a value
 * @param b another value
 * @returns a<b: -1, a=b: 0, a>b: 1
 */
function cmp(a, b) {
    return a < b ? -1 : (a > b ? 1 : 0);
}

/**
 * Compares two arrays.
 * @param x an array
 * @param y another array
 * @param fc compare function (a, b)
 * @param fm map function (v, i, x)
 * @returns x<y: -1, x=y: 0, x>y: 1
 */
function compare(x, y, fc = null, fm = null) {
    var fc = fc || cmp, fm = fm || id;
    var X = x.length, Y = y.length;
    for (var i = 0, I = Math.min(X, Y); i < I; i++) {
        var u1 = fm(x[i], i, x);
        var v1 = fm(y[i], i, y);
        var c = fc(u1, v1);
        if (c !== 0)
            return c;
    }
    return Math.sign(X - Y);
}

/**
 * Checks if array starts with a prefix.
 * @param x an array
 * @param y prefix?
 * @param fc compare function (a, b)
 * @param fm map function (v, i, x)
 */
function isPrefix(x, y, fc = null, fm = null) {
    var Y = y.length;
    return Y === 0 || compare(x.slice(0, Y), y, fc, fm) === 0;
}

module.exports = isPrefix;