From 1c8af2a8aadfb3b9a6802109393aafee8581f77a Mon Sep 17 00:00:00 2001
From: Stefan Harmuth <stefan.harmuth@docomodigital.com>
Date: Sun, 24 Oct 2021 16:59:54 +0200
Subject: [PATCH] day16

---
 day16/day.go   | 58 ++++++++++++++++++++++++++++++++++++++++++++++++++
 inputs/16      |  1 +
 inputs/16_test |  1 +
 main.go        |  3 ++-
 4 files changed, 62 insertions(+), 1 deletion(-)
 create mode 100644 day16/day.go
 create mode 100644 inputs/16
 create mode 100644 inputs/16_test

diff --git a/day16/day.go b/day16/day.go
new file mode 100644
index 0000000..234bbd5
--- /dev/null
+++ b/day16/day.go
@@ -0,0 +1,58 @@
+package day16
+
+import (
+	"tools"
+)
+
+func expandState(state string) string {
+	b := []rune(state)
+	for i, j := 0, len(b)-1; i <= j; i, j = i+1, j-1 {
+		x := b[j]
+		if b[i] == 48 {
+			b[j] = 49
+		} else {
+			b[j] = 48
+		}
+		if x == 48 {
+			b[i] = 49
+		} else {
+			b[i] = 48
+		}
+	}
+
+	return state + "0" + string(b)
+}
+
+func getCheckSum(state string) string {
+	checksum := []byte(state)
+	for len(checksum)%2 == 0 {
+		var newsum []byte
+		bytes := checksum
+		for i := 0; i < len(bytes); i += 2 {
+			newsum = append(newsum, 49-(bytes[i]+bytes[i+1])%2)
+		}
+		checksum = newsum
+	}
+
+	return string(checksum)
+}
+
+func Part1(puzzle tools.AoCPuzzle) interface{} {
+	state := puzzle.GetInputArray()[0]
+
+	for len(state) < 272 {
+		state = expandState(state)
+	}
+
+	return getCheckSum(state[:272])
+}
+
+func Part2(puzzle tools.AoCPuzzle) interface{} {
+	state := puzzle.GetInputArray()[0]
+
+	for len(state) < 35651584 {
+		state = expandState(state)
+	}
+
+	return getCheckSum(state[:35651584])
+}
diff --git a/inputs/16 b/inputs/16
new file mode 100644
index 0000000..9f912b4
--- /dev/null
+++ b/inputs/16
@@ -0,0 +1 @@
+01111001100111011
\ No newline at end of file
diff --git a/inputs/16_test b/inputs/16_test
new file mode 100644
index 0000000..1746da6
--- /dev/null
+++ b/inputs/16_test
@@ -0,0 +1 @@
+10000
\ No newline at end of file
diff --git a/main.go b/main.go
index 0bc58cb..1158ff3 100644
--- a/main.go
+++ b/main.go
@@ -15,6 +15,7 @@ import (
 	"aoc2016/day13"
 	"aoc2016/day14"
 	"aoc2016/day15"
+	"aoc2016/day16"
 	"flag"
 	"fmt"
 	"os"
@@ -47,7 +48,7 @@ func initDayFunctions() {
 		13: {1: day13.Part1, 2: day13.Part2},
 		14: {1: day14.Part1, 2: day14.Part2},
 		15: {1: day15.Part1, 2: day15.Part2},
-		//		16: {1: day16.Part1, 2: day16.Part2},
+		16: {1: day16.Part1, 2: day16.Part2},
 		//		17: {1: day17.Part1, 2: day17.Part2},
 		//		18: {1: day18.Part1, 2: day18.Part2},
 		//		19: {1: day19.Part1, 2: day19.Part2},