edit-distance/myers-1986

import assert from 'assert';

/**
 * Computes a lower bound for the signed distance.
 *
 * @see {@link uBound} for more details on usage and invariants.
 *
 * @param {number} D >= 1
 * @param {number} N >= 1
 * @return {number} A number -N <= k < D.
 */
export default function lBound(D, N) {
    assert(N >= 1);
    assert(D >= 1);
    const k = D <= N ? -D | 0 : (D - (N << 1)) | 0;
    assert(k >= -N);
    assert(k < D);
    assert((D & 1) === (k & 1));
    return k;
}