Day 12-2

day12.erl

@@ -4,8 +4,12 @@
 
 solve(Input) ->
     Board = parse_board(Input),
-    Result = find_path(find_start(Board), find_goal(Board), Board),
-    {Result, none}.
+    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) ]),
@@ -43,11 +47,19 @@ valid_move(State, {StartX, StartY}=Start, {MoveX, MoveY} = Move) ->
         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).
@@ -85,4 +97,26 @@ a_star({_Heuristic, Distance, Parent, Curr}, Goal, Open, Closed, State) ->
         {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}.