Advent of Code 2021 - Day 5

Questions & Discussions Coding Challenges
4

Yes, or perhaps just well suited to Erlang. And unlike yesterday, it’s not a test of how efficiently you can write a file parser or manipulate text in your favourite editor.

I feel happy enough with my code today to post it (although having seen it, I like @ danilagamma’s from_to/2 better than my dir/2 - though I’d implement it as clauses rather than a case):

p1(InputData) ->
    count_points(fun points_p1/2, InputData).

p2(InputData) ->
    count_points(fun points_p2/2, InputData).

count_points(F, InputData) ->
    AllPoints = lists:foldl(
        fun(E, A) ->
            maps:put(E, maps:get(E, A, 0) + 1, A)
        end,
        #{},
        lists:flatten([F(StartPos, EndPos) || {StartPos, EndPos} <- InputData])),
    maps:size(maps:filter(fun(_K, V) -> V >= 2 end, AllPoints)).

points_p1({X1, Y}, {X2, Y}) ->
    [{X, Y} || X <- lists:seq(X1, X2, dir(X1, X2))];
points_p1({X, Y1}, {X, Y2}) ->
    [{X, Y} || Y <- lists:seq(Y1, Y2, dir(Y1, Y2))];
points_p1(_, _) ->
    [].

points_p2({X1, Y1}, {X2, Y2}) when abs(X2 - X1) =:= abs(Y2 - Y1) ->
    Xs = [X || X <- lists:seq(X1, X2, dir(X1, X2))],
    Ys = [Y || Y <- lists:seq(Y1, Y2, dir(Y1, Y2))],
    lists:zip(Xs, Ys);
points_p2(P1, P2) ->
    points_p1(P1, P2).

dir(A, B) when A < B ->
    1;
dir(_, _) ->
    -1.
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