total-order/lex

/**
 * Generates a binary colexicographical comparator
 * from a binary comparator.
 *
 * https://en.wikipedia.org/wiki/Lexicographical_order#Colexicographic_order
 *
 * compare( a, b ) should always return
 *   - a negative value if a < b
 *   - a positive value if a > b
 *   - zero if a === b
 *
 * compare should express an increasing ordering
 */

export default function colexicographical(compare) {
    /**
     * Compares 2 arrays a and b colexicographically.
     */

    return function (a, b) {
        const m = a.length;
        const n = b.length;

        const length = Math.min(m, n);

        for (let i = 1; i <= length; ++i) {
            const d = compare(a[m - i], b[n - i]);

            if (d !== 0) return d;
        }

        return m - n;
    };
}