alphabet-board-path.ts

const ZERO = 'a'.charCodeAt(0);
const WIDTH = 5;

const f = Math.floor;

function vPath(n: number): string {
  if (n === 0) return '';
  if (n > 0) return 'D'.repeat(n);
  if (n < 0) return 'U'.repeat(-n);
  return '' as never;
}

function hPath(n: number): string {
  if (n === 0) return '';
  if (n > 0) return 'R'.repeat(n);
  if (n < 0) return 'L'.repeat(-n);
  return '' as never;
}

function findPath([a, b]: [string, string]): string {
  if (b === 'z') return findPath([a, 'u']) + 'D';
  const x = a.charCodeAt(0) - ZERO;
  const y = b.charCodeAt(0) - ZERO;
  if (x === y) return '';
  return vPath(f(y / WIDTH) - f(x / WIDTH)) + hPath(y % WIDTH - x % WIDTH);
}

function main(input: string) {
  const cs: string[] = Array.prototype.slice.call(input);
  const [xs] = cs.reduce(
    ([acc, last], curr) => [acc.concat([[last, curr]]), curr],
    [[] as [string, string][], 'a'],
  );
  return xs.reduce((acc, curr) => acc + findPath(curr) + '!', '');
}

console.log(main('leet'), main('code'));