longest_run.js

// Find the length of the longest monotonically increasing or monitonically 
// decreasing sequence of values in an array.

function longest_run(a) {
  const less = (a, b)     => a < b;
  const more = (a, b)     => a > b;
  const cum  = (a, f)     => a.map((v, i) => f(v, a[i-1]));  // compare adjacent elements
  const sum  = (a, t = 0) => a.map(v => t = v ? t + v : 0);  // running sum, reset on zero

  return !a.length ? 0 :
    1 + Math.max(...sum(cum(a, less)), ...sum(cum(a, more)));
}

console.log(longest_run([1,2,3,4,3,2,1, 0, 2,1, 3,4]));
console.log(longest_run([]));
console.log(longest_run([1]));

This code is so beautiful!