re-organise repo

2021/day12/day12.erl

@@ -0,0 +1,100 @@
+%% to compile: erlc day3A.erl
+%% to run: erl -noshell -s day5 solve
+%%
+-module(day12).
+
+-export ([solve/0, solve/1, solve/2]).
+-export ([read_input/0]).
+
+solve() ->
+    solve(['1']),
+    solve(['2']),
+    init:stop().
+
+solve(A) ->
+    solve(A, read_input()).
+
+solve(['1'], D) ->
+    io:format("The solution to ~p puzzle1 is: ~p~n", [?MODULE, solve(1, D)]);
+solve(1, D) ->
+    solution1(D);
+solve(['2'], D) ->
+    io:format("The solution to ~p puzzle2 is: ~p~n", [?MODULE, solve(2, D)]);
+solve(2, D) ->
+    solution2(D).
+
+read_input() ->
+    {ok, IO} = file:open("input.txt", 'read'),
+    Data = read_input(IO),
+    file:close(IO),
+    Data.
+
+read_input(IO) ->
+	read_input(IO, []).
+
+read_input(IO, Input) ->
+    case read_line(IO) of
+	'eof' -> Input;
+	Line -> read_input(IO, Input ++ [Line])
+    end.
+
+read_line(IO) ->
+	case file:read_line(IO) of
+		'eof' -> 'eof';
+		{ok, Line} -> parse_line(Line)
+	end.
+
+parse_line(Line) ->
+    [Cave1, Cave2] = string:tokens(Line, "-\n"),
+    {Cave1, Cave2}.
+
+solution1(Caves) ->
+   CaveMap = insert(Caves),
+   count_paths("start", #{}, 'some', CaveMap).
+
+solution2(Caves) ->
+   CaveMap = insert(Caves),
+   count_paths("start", #{}, 'none', CaveMap).
+
+
+insert(Caves) -> insert(Caves, maps:new()).
+
+insert([], CaveMap) -> CaveMap;
+insert([{Cave1, Cave2}|Rest], CaveMap) ->
+	NewMap = 
+	case maps:is_key(Cave1, CaveMap) of
+		'false' -> maps:put(Cave1, [Cave2], CaveMap);
+		'true' -> maps:put(Cave1, [Cave2|maps:get(Cave1, CaveMap)], CaveMap)
+	end,
+	NewMap2 = 
+        case maps:is_key(Cave2, NewMap) of
+                'false' -> maps:put(Cave2, [Cave1], NewMap);
+                'true' -> maps:put(Cave2, [Cave1|maps:get(Cave2, CaveMap)], NewMap)
+        end,
+	insert(Rest, NewMap2).
+
+visit_vertex(Name, Visited, VisitedTwice, Graph) ->
+    NewVisited =
+        case hd(Name) >= $a of
+            true -> Visited#{Name => true};
+            false -> Visited
+        end,
+    #{Name := Neighbours} = Graph,
+    lists:sum([count_paths(N, NewVisited, VisitedTwice, Graph) || N <- Neighbours]).
+
+count_paths("end", _, _, _) ->
+    % We have found an entire path through the graph.
+    1;
+count_paths("start", Visited, _, _) when map_size(Visited) > 0 ->
+    % We cannot visit the start cave more than once.
+    0;
+count_paths(Name, Visited, VisitedTwice, Graph) when hd(Name) >= $a andalso is_map_key(Name, Visited) ->
+    % We can visit only one small cave twice.
+    case VisitedTwice of
+        'none' -> visit_vertex(Name, Visited, Name, Graph);
+         _ -> 0
+    end;
+count_paths(Name, Visited, VisitedTwice, Graph) ->
+    % Small caves can only be visited once, but large caves may be visited any
+    % number of times.
+    visit_vertex(Name, Visited, VisitedTwice, Graph).