From 0e058b5c42ba6b009a949fc412fe5b8d7c12667c Mon Sep 17 00:00:00 2001
From: alexchao26 <alexchaoo@gmail.com>
Date: Thu, 24 Dec 2020 02:06:16 -0500
Subject: [PATCH] 2016-day16: strings.Builder to the rescue

---
 2016/day16/main.go      | 55 +++++++++++++++++++++++++++++++++++++++++
 2016/day16/main_test.go | 26 +++++++++++++++++++
 2 files changed, 81 insertions(+)
 create mode 100644 2016/day16/main.go
 create mode 100644 2016/day16/main_test.go

diff --git a/2016/day16/main.go b/2016/day16/main.go
new file mode 100644
index 0000000..c40caa9
--- /dev/null
+++ b/2016/day16/main.go
@@ -0,0 +1,55 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"strings"
+
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+func main() {
+	var part int
+	flag.IntVar(&part, "part", 1, "part 1 or 2")
+	flag.Parse()
+	fmt.Println("Running part", part)
+
+	ans := dragonChecksum(util.ReadFile("./input.txt"), part)
+	fmt.Println("Output:", ans)
+}
+
+func dragonChecksum(input string, part int) string {
+	disk := input
+	diskLength := 272
+	if part == 2 {
+		diskLength = 35651584
+	}
+	for len(disk) < diskLength {
+		var sb strings.Builder
+		sb.WriteString(disk)
+		sb.WriteByte('0')
+		for i := len(disk) - 1; i >= 0; i-- {
+			if disk[i] == '1' {
+				sb.WriteByte('0')
+			} else {
+				sb.WriteByte('1')
+			}
+		}
+		disk = sb.String()
+	}
+
+	disk = disk[0:diskLength]
+	for len(disk)%2 == 0 {
+		var sb strings.Builder
+		for i := 0; i < len(disk); i += 2 {
+			if disk[i] == disk[i+1] {
+				sb.WriteByte('1')
+			} else {
+				sb.WriteByte('0')
+			}
+		}
+		disk = sb.String()
+	}
+
+	return disk
+}
diff --git a/2016/day16/main_test.go b/2016/day16/main_test.go
new file mode 100644
index 0000000..6d8ffda
--- /dev/null
+++ b/2016/day16/main_test.go
@@ -0,0 +1,26 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+func Test_dragonChecksum(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		part  int
+		want  string
+	}{
+		{"part1", util.ReadFile("input.txt"), 1, "10010110010011110"},
+		{"part2", util.ReadFile("input.txt"), 2, "01101011101100011"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := dragonChecksum(tt.input, tt.part); got != tt.want {
+				t.Errorf("dragonChecksum() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}