From c35022d1955d5f33a909133af31820b12d11c177 Mon Sep 17 00:00:00 2001
From: Andrew Coleman <penguincoder@gmail.com>
Date: Mon, 25 Dec 2017 16:19:04 -0500
Subject: [PATCH] day25 solution

---
 day25/Makefile  |   8 ++++
 day25/day25.erl | 103 ++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 111 insertions(+)
 create mode 100644 day25/Makefile
 create mode 100644 day25/day25.erl

diff --git a/day25/Makefile b/day25/Makefile
new file mode 100644
index 0000000..6548514
--- /dev/null
+++ b/day25/Makefile
@@ -0,0 +1,8 @@
+all: day25.beam
+	erl -noshell -s day25 start -s init stop
+
+day25.beam:
+	erlc day25.erl
+
+clean:
+	rm -f day25.beam
diff --git a/day25/day25.erl b/day25/day25.erl
new file mode 100644
index 0000000..87dca1f
--- /dev/null
+++ b/day25/day25.erl
@@ -0,0 +1,103 @@
+-module(day25).
+-export([start/0]).
+
+part_1()
+  -> state_a(maps:new(), 0, 0, 6).
+
+state_a(Tape, _Index, _CurrentBit, Steps)
+  when Steps == 0 -> Tape;
+state_a(Tape, Index, CurrentBit, Steps)
+  when CurrentBit == 0 -> NewTape = maps:put(Index, 1, Tape),
+                          NewIndex = Index + 1,
+                          state_b(NewTape, NewIndex, maps:get(NewIndex, NewTape, 0), Steps - 1);
+state_a(Tape, Index, CurrentBit, Steps)
+  when CurrentBit == 1 -> NewTape = maps:put(Index, 0, Tape),
+                          NewIndex = Index - 1,
+                          state_b(NewTape, NewIndex, maps:get(NewIndex, NewTape, 0), Steps - 1).
+
+state_b(Tape, _Index, _CurrentBit, Steps)
+  when Steps == 0 -> Tape;
+state_b(Tape, Index, CurrentBit, Steps)
+  when CurrentBit == 0 -> NewTape = maps:put(Index, 1, Tape),
+                          NewIndex = Index - 1,
+                          state_a(NewTape, NewIndex, maps:get(NewIndex, NewTape, 0), Steps - 1);
+state_b(Tape, Index, CurrentBit, Steps)
+  when CurrentBit == 1 -> NewTape = maps:put(Index, 1, Tape),
+                          NewIndex = Index + 1,
+                          state_a(NewTape, NewIndex, maps:get(NewIndex, NewTape, 0), Steps - 1).
+
+diagnostic_checksum(Tape)
+  -> maps:fold(fun(_K, V, Sum) -> Sum + V end, 0, Tape).
+
+part_2()
+  -> state_a2(maps:new(), 0, 0, 12861455).
+
+state_a2(Tape, _Index, _CurrentBit, Steps)
+  when Steps == 0 -> Tape;
+state_a2(Tape, Index, CurrentBit, Steps)
+  when CurrentBit == 0 -> NewTape = maps:put(Index, 1, Tape),
+                          NewIndex = Index + 1,
+                          state_b2(NewTape, NewIndex, maps:get(NewIndex, NewTape, 0), Steps - 1);
+state_a2(Tape, Index, CurrentBit, Steps)
+  when CurrentBit == 1 -> NewTape = maps:put(Index, 0, Tape),
+                          NewIndex = Index - 1,
+                          state_b2(NewTape, NewIndex, maps:get(NewIndex, NewTape, 0), Steps - 1).
+
+state_b2(Tape, _Index, _CurrentBit, Steps)
+  when Steps == 0 -> Tape;
+state_b2(Tape, Index, CurrentBit, Steps)
+  when CurrentBit == 0 -> NewTape = maps:put(Index, 1, Tape),
+                          NewIndex = Index - 1,
+                          state_c2(NewTape, NewIndex, maps:get(NewIndex, NewTape, 0), Steps - 1);
+state_b2(Tape, Index, CurrentBit, Steps)
+  when CurrentBit == 1 -> NewTape = maps:put(Index, 0, Tape),
+                          NewIndex = Index + 1,
+                          state_e2(NewTape, NewIndex, maps:get(NewIndex, NewTape, 0), Steps - 1).
+
+state_c2(Tape, _Index, _CurrentBit, Steps)
+  when Steps == 0 -> Tape;
+state_c2(Tape, Index, CurrentBit, Steps)
+  when CurrentBit == 0 -> NewTape = maps:put(Index, 1, Tape),
+                          NewIndex = Index + 1,
+                          state_e2(NewTape, NewIndex, maps:get(NewIndex, NewTape, 0), Steps - 1);
+state_c2(Tape, Index, CurrentBit, Steps)
+  when CurrentBit == 1 -> NewTape = maps:put(Index, 0, Tape),
+                          NewIndex = Index - 1,
+                          state_d2(NewTape, NewIndex, maps:get(NewIndex, NewTape, 0), Steps - 1).
+
+state_d2(Tape, _Index, _CurrentBit, Steps)
+  when Steps == 0 -> Tape;
+state_d2(Tape, Index, CurrentBit, Steps)
+  when CurrentBit == 0 -> NewTape = maps:put(Index, 1, Tape),
+                          NewIndex = Index - 1,
+                          state_a2(NewTape, NewIndex, maps:get(NewIndex, NewTape, 0), Steps - 1);
+state_d2(Tape, Index, CurrentBit, Steps)
+  when CurrentBit == 1 -> NewTape = maps:put(Index, 1, Tape),
+                          NewIndex = Index - 1,
+                          state_a2(NewTape, NewIndex, maps:get(NewIndex, NewTape, 0), Steps - 1).
+
+state_e2(Tape, _Index, _CurrentBit, Steps)
+  when Steps == 0 -> Tape;
+state_e2(Tape, Index, CurrentBit, Steps)
+  when CurrentBit == 0 -> NewTape = maps:put(Index, 0, Tape),
+                          NewIndex = Index + 1,
+                          state_a2(NewTape, NewIndex, maps:get(NewIndex, NewTape, 0), Steps - 1);
+state_e2(Tape, Index, CurrentBit, Steps)
+  when CurrentBit == 1 -> NewTape = maps:put(Index, 0, Tape),
+                          NewIndex = Index + 1,
+                          state_f2(NewTape, NewIndex, maps:get(NewIndex, NewTape, 0), Steps - 1).
+
+state_f2(Tape, _Index, _CurrentBit, Steps)
+  when Steps == 0 -> Tape;
+state_f2(Tape, Index, CurrentBit, Steps)
+  when CurrentBit == 0 -> NewTape = maps:put(Index, 1, Tape),
+                          NewIndex = Index + 1,
+                          state_e2(NewTape, NewIndex, maps:get(NewIndex, NewTape, 0), Steps - 1);
+state_f2(Tape, Index, CurrentBit, Steps)
+  when CurrentBit == 1 -> NewTape = maps:put(Index, 1, Tape),
+                          NewIndex = Index + 1,
+                          state_a2(NewTape, NewIndex, maps:get(NewIndex, NewTape, 0), Steps - 1).
+
+start()
+  -> io:fwrite("sample diagnostic checksum ~p~n", [diagnostic_checksum(part_1())]),
+     io:fwrite("problem diagnostic checksum ~p~n", [diagnostic_checksum(part_2())]).