From 1445104f24b5ca3ab15bae211a29212154ceef23 Mon Sep 17 00:00:00 2001
From: Andrew Coleman <penguincoder@gmail.com>
Date: Fri, 15 Dec 2017 00:49:58 -0500
Subject: [PATCH] day15 solution

---
 day15/Makefile  |  8 ++++++++
 day15/day15.erl | 43 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+)
 create mode 100644 day15/Makefile
 create mode 100644 day15/day15.erl

diff --git a/day15/Makefile b/day15/Makefile
new file mode 100644
index 0000000..a4ad01b
--- /dev/null
+++ b/day15/Makefile
@@ -0,0 +1,8 @@
+all: day15.beam
+	erl -noshell -s day15 start -s init stop
+
+day15.beam:
+	erlc day15.erl
+
+clean:
+	rm -f day15.beam
diff --git a/day15/day15.erl b/day15/day15.erl
new file mode 100644
index 0000000..c0d394c
--- /dev/null
+++ b/day15/day15.erl
@@ -0,0 +1,43 @@
+-module(day15).
+-export([start/0]).
+
+gen_a_value(Val)
+  -> (Val * 16807) rem 2147483647.
+
+gen_b_value(Val)
+  -> (Val * 48271) rem 2147483647.
+
+gen_a_value2(Val)
+  -> Result = (Val * 16807) rem 2147483647,
+     if Result rem 4 == 0 -> Result;
+        true -> gen_a_value2(Result)
+     end.
+
+gen_b_value2(Val)
+  -> Result = (Val * 48271) rem 2147483647,
+     if Result rem 8 == 0 -> Result;
+        true -> gen_b_value2(Result)
+     end.
+
+lower_16_matches(ValA, ValB)
+  -> PaddedValA = <<ValA:32>>,
+     PaddedValB = <<ValB:32>>,
+     <<_A:16,B:16>> = PaddedValA,
+     <<_C:16,D:16>> = PaddedValB,
+     B == D.
+
+judge_matches(Start, _GenA, _GenB, MatchCount, _GenAFun, _GenBFun)
+  when Start == 0 -> MatchCount;
+judge_matches(Start, GenA, GenB, MatchCount, GenAFun, GenBFun)
+  -> GenAResult = GenAFun(GenA),
+     GenBResult = GenBFun(GenB),
+     Match = lower_16_matches(GenAResult, GenBResult),
+     if Match -> judge_matches(Start - 1, GenAResult, GenBResult, MatchCount + 1, GenAFun, GenBFun);
+        true -> judge_matches(Start - 1, GenAResult, GenBResult, MatchCount, GenAFun, GenBFun)
+     end.
+
+start()
+  -> io:fwrite("~p~n", [judge_matches(40000000, 65, 8921, 0, fun gen_a_value/1, fun gen_b_value/1)]),
+     io:fwrite("~p~n", [judge_matches(40000000, 703, 516, 0, fun gen_a_value/1, fun gen_b_value/1)]),
+     io:fwrite("~p~n", [judge_matches(5000000, 65, 8921, 0, fun gen_a_value2/1, fun gen_b_value2/1)]),
+     io:fwrite("~p~n", [judge_matches(5000000, 703, 516, 0, fun gen_a_value2/1, fun gen_b_value2/1)]).