-module(day12).

-export([solve/1]).

solve(Input) ->
    Board = parse_board(Input),
    Result1 = find_path(find_start(Board), find_goal(Board), Board),
    %Part
    Part2Goal = fun(Node) -> value(Board, Node) == $a end,
    Part2Neighbors = fun(Node) -> neighbors2(Board, Node) end,
    {ok, Result2} = bfs(find_goal(Board), Part2Neighbors, Part2Goal),
    {Result1, Result2}.

find_start({W, H, _Data} = Board) ->
    [Pos] = lists:filter(fun(P) -> raw_value(Board, P) == $S end, [ {X, Y} || X <- lists:seq(0, W-1), Y <- lists:seq(0, H-1) ]),
    Pos.
find_goal({W, H, _Data} = Board) ->
    [Pos] = lists:filter(fun(P) -> raw_value(Board, P) == $E end, [ {X, Y} || X <- lists:seq(0, W-1), Y <- lists:seq(0, H-1) ]),
    Pos.

parse_board(Input) ->
    Parsed = binary:split(Input, <<"\n">>, [global, trim_all]),
    {byte_size(hd(Parsed)), length(Parsed), Input}.

raw_value({W, _H, Data}, {X, Y}) ->
    Pos = Y * (W + 1) + X,
    <<_Before:Pos/binary, RawValue, _After/binary>> = Data,
    RawValue.

value(Board, Pos) ->
    case raw_value(Board, Pos) of
        $S -> $a;
        $E -> $z;
        Letter -> Letter
    end.

valid_position({_W, _H, _Data}, {X, Y}) when X < 0 orelse Y < 0 -> false;
valid_position({W, H, _Data}, {X, Y}) when X >= W orelse Y >= H -> false;
valid_position({_W, _H, _Data}, {_X, _Y}) -> true.

valid_move(State, {StartX, StartY}=Start, {MoveX, MoveY} = Move) ->
    StartValue = value(State, Start),
    MoveValue = value(State, Move),
    if
        abs(StartX - MoveX) + abs(StartY - MoveY) > 1 -> false;
        MoveValue - StartValue > 1 -> false;
        true -> true
    end.

valid_move2(State, Start, Move) ->
    value(State, Move) - value(State, Start) >= -1.

neighbors(State, Pos) ->
    PossibleNeighbors0 = [ vec_add(Pos, Dir) || Dir <- [{0,-1}, {0, 1}, {1, 0}, {-1, 0}]],
    PossibleNeighbors1 = lists:filter(fun(Move) -> valid_position(State, Move) end, PossibleNeighbors0),
    lists:filter(fun(Move) -> valid_move(State, Pos, Move) end, PossibleNeighbors1).

neighbors2(State, Pos) ->
    PossibleNeighbors0 = [ vec_add(Pos, Dir) || Dir <- [{0,-1}, {0, 1}, {1, 0}, {-1, 0}]],
    PossibleNeighbors1 = lists:filter(fun(Move) -> valid_position(State, Move) end, PossibleNeighbors0),
    lists:filter(fun(Move) -> valid_move2(State, Pos, Move) end, PossibleNeighbors1).

vec_add({X1, Y1}, {X2, Y2}) -> {X1+X2, Y1+Y2}.
vec_sub({X1, Y1}, {X2, Y2}) -> {X1-X2, Y1-Y2}.
vec_norm({X1, Y1}) -> abs(X1)+abs(Y1).

cartesian(P1, P2) -> vec_norm(vec_sub(P1, P2)).

enqueue({_Priority, _Distance, _Parent, Pos}, {Map, Priq}) when is_map_key(Pos, Map) -> {Map, Priq};
enqueue({Priority, Distance, Parent, Pos}, {Map, Priq}) ->
    {Map#{Pos => {Distance, Parent}}, priq:insert({Priority, Pos}, Priq)}.

dequeue({_Map, empty}) -> error;
dequeue({Map, Priq}) ->
    {ok, {Priority, Pos}} = priq:peek_min(Priq),
    {ok, Rest} = priq:delete_min(Priq),
    #{Pos := {Distance, Parent}} = Map,
    {ok, {Priority, Distance, Parent, Pos}, {Map, Rest}}.

find_path(Start, Goal, Board) ->
    a_star({cartesian(Start, Goal), 0, start, Start}, Goal, {#{}, empty}, #{}, Board).

% Closed: A map of Node -> Node (parent). Used to check for visitation. Forms the path.
a_star({_Heuristic, Distance, _Parent, Goal}, Goal, _Open, _Closed, _State) ->
    Distance;
a_star({_Heuristic, _Distance, _Parent, Curr}, Goal, Open, Closed, State) when is_map_key(Curr, Closed) ->
    case dequeue(Open) of
        error -> {error, no_path};
        {ok, NextCurrNode, NextOpen} -> a_star(NextCurrNode, Goal, NextOpen, Closed, State)
    end;
a_star({_Heuristic, Distance, Parent, Curr}, Goal, Open, Closed, State) ->
    OpenNeighbors = neighbors(State, Curr),
    OpenNeighborNodes = lists:map(fun(Neighbor) -> {cartesian(Neighbor, Goal)+Distance+1, Distance+1, Curr, Neighbor} end, OpenNeighbors),
    AddedOpen = lists:foldl(fun enqueue/2, Open, OpenNeighborNodes),
    case dequeue(AddedOpen) of
        error -> {error, no_path};
        {ok, NextCurrNode, NextOpen} -> a_star(NextCurrNode, Goal, NextOpen, Closed#{Curr=>{Distance, Parent}}, State)
    end.

bfs(Position, NeighborFn, GoalFn) -> 
    do_bfs0(Position, NeighborFn, GoalFn, 0, priq:new(), #{}).
do_bfs0(Position, NeighborFn, GoalFn, Distance, Queue, Visited) when is_map_key(Position, Visited) ->
    do_bfs2(Position, NeighborFn, GoalFn, Distance, Queue, Visited);
do_bfs0(Position, NeighborFn, GoalFn, Distance, Queue, Visited) ->
    case GoalFn(Position) of
        true -> {ok, Distance};
        false -> do_bfs1(Position, NeighborFn, GoalFn, Distance, Queue, Visited)
    end.

do_bfs1(Position, NeighborFn, GoalFn, Distance, Queue, Visited) ->
    WeightedNeighbors = lists:map(fun (Node) -> {Distance+1, Node} end, NeighborFn(Position)),
    UpdatedQueue = lists:foldl(fun priq:insert/2, Queue, WeightedNeighbors),
    do_bfs2(Position, NeighborFn, GoalFn, Distance, UpdatedQueue, Visited).

do_bfs2(_Position, _NeighborFn, _GoalFn, _Distance, empty, _Visited) ->
    {error, no_path};
do_bfs2(Position, NeighborFn, GoalFn, _Distance, Queue, Visited) ->
    {ok, {NextDistance, NextPosition}} = priq:peek_min(Queue),
    {ok, NextQueue} = priq:delete_min(Queue),
    do_bfs0(NextPosition, NeighborFn, GoalFn, NextDistance, NextQueue, Visited#{Position => []}).

%calculate_path(Goal, Closed) -> {Goal, Closed}.