day15 solution

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

day15/day15.erl

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+-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)]).