Advent of Code, 2017 Day 1 (2/2)

halfway.re

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

let solve = (input) => {
  let len = String.length(input);
  let step = len / 2;
  let listInput = explode(input);
  let nth = List.nth(listInput);
  let rec solver = (currentPos, total) => {
    let current = nth(currentPos);
    let halfwayPos = (currentPos + step) mod len;
    let halfway = nth(halfwayPos);
    switch (current == halfway, currentPos == len - 1) {
    | (false, true) => total /* current doesn't equal halfway, end of list */
    | (true, true) => total + current /* current does equal halfway */
    | (false, false) => solver(currentPos + 1, total) /* doesn't equal halfway, and not the end */
    | (true, false) => solver(currentPos + 1, total + current) /* does equal halfway, not the end */
    }
  };
  solver(0, 0)
};

/*
 // Solution re-written in JS; won't work for big numbers D:
 const explode =
  (input) => input.split("").map((s) => parseInt(s, 10));
 
 const solve = (input) {
   const len = input.length;
   const step = len / 2;
   const listInput = explode(input);
   const nth = (n) => listInput[n];
   
   const solver = (currentPos, total) => {
     let current = nth(currentPos);
     let halfwayPos = (currentPos + step) % len;
     let halfway = nth(halfwayPos);
     if (current === halfway && currentPos === len - 1) {
       return total + current;
     } else if(currentPos === len - 1) {
       return total;
     } else if(current === halfway) {
       return solver(current + 1, total + current);
     } else {
       return solver(current + 1, total);
     }
   };
   
   solver(0, 0);
 };
*/