day 13 solution

day13/Makefile

@@ -0,0 +1,8 @@
+all: day13.beam
+	erl -noshell -s day13 start -s init stop
+
+day13.beam:
+	erlc day13.erl
+
+clean:
+	rm -f day13.beam

day13/day13.erl

@@ -0,0 +1,94 @@
+-module(day13).
+-export([start/0]).
+
+calc_severity(Input)
+  -> CalcMap = lists:foldl(fun(X, Sum) ->
+                               [Index|Depth] = re:split(X, ": "),
+                               { IndexInt, _ } = string:to_integer(Index),
+                               { DepthInt, _ } = string:to_integer(Depth),
+                               maps:put(IndexInt, DepthInt, Sum)
+                           end, maps:new(), re:split(Input, "\n")),
+     LastStep = lists:max(maps:keys(CalcMap)),
+     InitialSeverity = calc_severity(CalcMap, update_positions(CalcMap, 0), 0, 0, LastStep, 0),
+     SafeDelay = calc_delay(CalcMap, LastStep, 0),
+     { InitialSeverity, SafeDelay }.
+
+calc_delay(CalcMap, LastStep, Delay)
+  -> PositionMap = update_positions(CalcMap, Delay),
+     CurrentSeverity = calc_delay(CalcMap, PositionMap, Delay, 0, LastStep),
+     if CurrentSeverity == 0 -> Delay;
+        true -> calc_delay(CalcMap, LastStep, Delay + 1)
+     end.
+calc_delay(_CalcMap, _PositionMap, _Picosecond, PacketIndex, LastStep)
+  when PacketIndex > LastStep -> 0;
+calc_delay(CalcMap, PositionMap, Picosecond, PacketIndex, LastStep)
+  -> CurrentPosition = maps:get(PacketIndex, PositionMap, -1),
+     if CurrentPosition == 0 -> 1;
+        true -> calc_delay(CalcMap, update_positions(CalcMap, Picosecond + 1), Picosecond + 1, PacketIndex + 1, LastStep)
+     end.
+
+calc_severity(_CalcMap, _PositionMap, _Picosecond, PacketIndex, LastStep, Score)
+  when PacketIndex > LastStep -> Score;
+calc_severity(CalcMap, PositionMap, Picosecond, PacketIndex, LastStep, Score)
+  -> CurrentPosition = maps:get(PacketIndex, PositionMap, -1),
+     UpdatedPositions = update_positions(CalcMap, Picosecond + 1),
+     if CurrentPosition == 0 -> calc_severity(CalcMap, UpdatedPositions, Picosecond + 1, PacketIndex + 1, LastStep, Score + (PacketIndex * maps:get(PacketIndex, CalcMap)));
+        true -> calc_severity(CalcMap, UpdatedPositions, Picosecond + 1, PacketIndex + 1, LastStep, Score)
+     end.
+
+update_positions(CalcMap, Picosecond)
+  -> lists:foldl(fun(X, Sum) ->
+                     CurrentDepth = maps:get(X, CalcMap),
+                     if (Picosecond div (CurrentDepth - 1)) rem 2 == 0 -> maps:put(X, (Picosecond rem (CurrentDepth - 1)), Sum);
+                        true -> maps:put(X, (CurrentDepth - (Picosecond rem CurrentDepth)), Sum)
+                     end
+                 end, maps:new(), maps:keys(CalcMap)).
+
+start()
+  -> io:fwrite("~p~n", [calc_severity("0: 3
+1: 2
+4: 4
+6: 4")]),
+     io:fwrite("~p~n", [calc_severity("0: 3
+1: 2
+2: 4
+4: 6
+6: 4
+8: 6
+10: 5
+12: 6
+14: 9
+16: 6
+18: 8
+20: 8
+22: 8
+24: 8
+26: 8
+28: 8
+30: 12
+32: 14
+34: 10
+36: 12
+38: 12
+40: 10
+42: 12
+44: 12
+46: 12
+48: 12
+50: 12
+52: 14
+54: 14
+56: 12
+62: 12
+64: 14
+66: 14
+68: 14
+70: 17
+72: 14
+74: 14
+76: 14
+82: 14
+86: 18
+88: 14
+96: 14
+98: 44")]).