Advent of Code, 2017 Day 1: A more general solution

dayOne.re

let explode = (input) =>
  Js.String.split("", input)
  |> Array.map(int_of_string);

let rec solver = (digits, len, step, xPos, total) => {
  let x = digits[xPos];
  let yPos = (xPos + step) mod len;
  let y = digits[yPos];

  /* curry the next call to solver */
  let next = solver(digits, len, step);

  let isEndOfList = xPos == len - 1;
  switch (x == y, isEndOfList) {
  | (false, true) => total
  | (true, true) => total + x
  | (false, false) => next(xPos + 1, total)
  | (true, false) => next(xPos + 1, total + x)
  }
};

let neighbor = (input) => {
  let step = 1;
  let digits = explode(input);
  solver(digits, Array.length(digits), step, 0, 0)
};

let halfway = (input) => {
  let len = String.length(input);
  let step = len / 2;
  let digits = explode(input);
  solver(digits, len, step, 0, 0)
};