Advent of Code 2015, Day 25

advent25.erl

-module(advent25).
-export([main/1]).

-define(Start, {{1,1}, 20151125}).

nextN(Last) -> Last * 252533 rem 33554393.

nextXY({ X, 1 }) -> { 1, X + 1 };
nextXY({ X, Y }) -> { X + 1, Y - 1 }.

nextZip(Fa, Fb) -> fun ({A,B}) -> {Fa(A), Fb(B)} end.

next() -> nextZip(fun nextXY/1, fun nextN/1).

find(Start, Fn, Pred) ->
  case Pred(Start) of
    true -> Start;
    false -> find(Fn(Start), Fn, Pred)
  end.

findXY(X,Y) -> find(?Start, next(), fun ({{Xx, Yy}, _}) -> (X =:= Xx) and (Y =:= Yy) end).

main([]) -> main(["3075", "2981"]);
main([X, Y]) ->
  { _, Part1 } = findXY(list_to_integer(X), list_to_integer(Y)),
  io:format("Part 1: ~B~n", [Part1]).