:- module day3.
:- interface.
:- import_module basics.
:- import_module list.

:- pred run(part::in, list(string)::in, answer::out) is cc_multi.

:- implementation.
:- import_module int.
:- import_module char.
:- import_module string.

:- pred split(string::in, string::out, string::out) is semidet.
split(X, L, H) :-
  Len = length(X), even(Len),
  split(X, Len / 2, L, H).

:- pred member(char::out, string::in) is nondet.
member(C, S) :-
  nondet_int_in_range(0, length(S) - 1, I),
  index(S, I, C).

:- pred common(list(string)::in, char::out) is nondet.
common([S | Ss], C) :- member(C, S), (Ss = [] ; common(Ss, C)).

:- func prio(char) = int.
prio(C) = I :-
  if is_lower(C) then
    I = to_int(C) - to_int('a') + 1
  else
    I = to_int(C) - to_int('A') + 27.


:- type triple(T) ---> t(T, T, T).

:- pred threes(list(T)::in, list(triple(T))::out) is semidet.
threes(In, Out) :- threes(Out, In, []).

:- pred threes(list(triple(T))::out, list(T)::in, list(T)::out) is semidet.
threes([])                  --> =([]).
threes([t(X, Y, Z) | Rest]) --> [X, Y, Z], threes(Rest).


:- pred go1(string::in, int::out) is cc_multi.
go1(Line, Out) :-
  if split(Line, Fst, Snd),
     common([Fst, Snd], C)
  then
    Out = prio(C)
  else
    die("invalid line: ""%s""", [s(Line)]).


:- pred go2(triple(string)::in, int::out) is cc_multi.
go2(t(X, Y, Z), Prio) :-
  if common([X, Y, Z], C) then
    Prio = prio(C)
  else
    die("no item in common in:\n - %s\n - %s\n - %s", [s(X), s(Y), s(Z)]).


run(one, Lines, int(sum(Prios))) :- map(go1, Lines, Prios).
run(two, Lines, int(sum(Prios))) :-
  if threes(Lines, Groups) then
    map(go2, Groups, Prios)
  else
    die("%d lines given (not a multiple of 3)", [i(length(Lines))]).