clockProblem.js

/**
 * Steps To Run
 * 1. Install NodeJS
 * 2. Dowload Gist
 * 3. Run from terminal node clockProblem.js
 */
const CLOCK_MAX = 12;
const CLOCK_HALF = 6;

const add = (x, y) => x + y;

const addHours = (currentHour, index) =>
  (currentHour + index) > CLOCK_MAX ? (currentHour + index) % CLOCK_MAX : (currentHour + index);

const halfRange = (startHour) => {
  if (startHour > CLOCK_MAX || startHour < 1) {
    throw new Error('invalid hour');
  }
  let range = [];
  for (x = 0; x < CLOCK_HALF; x++) {
    range = [].concat(range, addHours(startHour, x));
  }
  return {
    range,
    start: range[0],
    end: range[range.length - 1],
  };
}

let clockSum = 0;
for (hour = 1; hour <= CLOCK_MAX; hour++) {
  clockSum += hour;
}
const expectedSum = clockSum / 2;

for (hour = 1; hour <= CLOCK_HALF; hour++) {
  const halfOne = halfRange(hour);
  if (halfOne.range.reduce(add, 0) === expectedSum) {
    const halfTwo = halfRange(addHours(hour, CLOCK_HALF));
    console.log(`Draw line between ${halfOne.start}-${halfTwo.end} and ${halfTwo.start}-${halfOne.end}`);
    console.log(`Sum is ${expectedSum}`);
  }
}