advent-code-day-3.js

var ar = document.getElementsByTagName('pre')[0].textContent.split('\n').filter(Boolean)

function makeWalk(steps) {
  return function walk(l, pos) { 
    const lastPos = l.length - 1;
    
    if ((pos.x + steps) > lastPos) {
      return makeWalk(pos.x + steps - lastPos - 1)(l, {x: 0, y: pos.y })
    }

    return { 
      y: pos.y, 
      x: pos.x + steps 
    };
  }
}

function makeTraverse(right, down) {
  const walk = makeWalk(right);

  return function traverse(list) {
    let cursor = { x: 0, y: 0 };
    let count = 0;

    while(cursor.y < list.length - 1) {
        cursor = walk(list[cursor.y], cursor);
        cursor.y = cursor.y + down;
        const node = list[cursor.y][cursor.x];

        if(node === '#') {
          count += 1;
        }
    }
    return count;
  }
}

console.log(
  makeTraverse(1, 1)(ar) * 
  makeTraverse(3, 1)(ar) * 
  makeTraverse(5, 1)(ar) * 
  makeTraverse(7, 1)(ar) * 
  makeTraverse(1, 2)(ar)
)