From c1c4dcd7cf4cbb3d47a7c59bfc0ada6f2e09e276 Mon Sep 17 00:00:00 2001
From: Dreaded_X <tim.huizinga@gmail.com>
Date: Mon, 20 Dec 2021 19:16:06 +0100
Subject: [PATCH] 2021 - Day 20

---
 2021/20/go.mod  |   9 +++
 2021/20/go.sum  |   2 +
 2021/20/main.go | 160 ++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 171 insertions(+)
 create mode 100644 2021/20/go.mod
 create mode 100644 2021/20/go.sum
 create mode 100644 2021/20/main.go

diff --git a/2021/20/go.mod b/2021/20/go.mod
new file mode 100644
index 0000000..7c70587
--- /dev/null
+++ b/2021/20/go.mod
@@ -0,0 +1,9 @@
+module AoC/2021/20
+
+require AoC/2021/common v0.0.0
+
+require github.com/joho/godotenv v1.4.0 // indirect
+
+replace AoC/2021/common v0.0.0 => ../common
+
+go 1.17
diff --git a/2021/20/go.sum b/2021/20/go.sum
new file mode 100644
index 0000000..8c9f290
--- /dev/null
+++ b/2021/20/go.sum
@@ -0,0 +1,2 @@
+github.com/joho/godotenv v1.4.0 h1:3l4+N6zfMWnkbPEXKng2o2/MR5mSwTrBih4ZEkkz1lg=
+github.com/joho/godotenv v1.4.0/go.mod h1:f4LDr5Voq0i2e/R5DDNOoa2zzDfwtkZa6DnEwAbqwq4=
diff --git a/2021/20/main.go b/2021/20/main.go
new file mode 100644
index 0000000..b825ae1
--- /dev/null
+++ b/2021/20/main.go
@@ -0,0 +1,160 @@
+package main
+
+import (
+	aoc "AoC/2021/common"
+	"bufio"
+	"fmt"
+)
+
+type Vec struct {
+	x int
+	y int
+}
+
+func (a Vec) Add(b Vec) Vec {
+	return Vec{a.x+b.x, a.y+b.y}
+}
+
+type Image struct {
+	data map[Vec]bool
+	min Vec
+	max Vec
+}
+
+func NewImage() Image {
+	var image Image
+	image.data = make(map[Vec]bool)
+
+	return image
+}
+
+func processInput(input *bufio.Scanner) (Image, string) {
+	image := NewImage()
+
+	input.Scan()
+	algorithm := input.Text()
+
+	input.Scan()
+	for y := 0; input.Scan(); y++ {
+		if y > image.max.y {
+			image.max.y = y
+		}
+		line := input.Text()
+		for x, c := range line {
+			if x > image.max.x {
+				image.max.x = x
+			}
+
+			// Only store non-zero values
+			if c == '#' {
+				image.data[Vec{x,y}] = true
+			}
+		}
+	}
+
+	return image, algorithm
+}
+
+func (image *Image) Print() {
+	for y := image.min.y; y <= image.max.y; y++ {
+		for x := image.min.x; x <= image.max.x; x++ {
+			if image.Get(x, y, false) {
+				fmt.Print("#")
+			} else {
+				fmt.Print(".")
+			}
+		}
+		fmt.Print("\n")
+	}
+	fmt.Print("\n")
+}
+
+func (image *Image) Get(x int, y int, outside bool) bool {
+	if x < image.min.x || x > image.max.x || y < image.min.y || y > image.max.y {
+		return outside
+	}
+	return image.data[Vec{x, y}]
+}
+
+func (image *Image) Kernel(x int, y int, algorithm string, outside bool) bool {
+	index := 0
+	for dy := -1; dy <= 1; dy++ {
+		for dx := -1; dx <= 1; dx++ {
+			index <<= 1
+			if image.Get(x+dx, y+dy, outside) {
+				index += 1
+			}
+		}
+	}
+
+	if algorithm[index] == '#' {
+		return true
+	}
+
+	return false
+}
+
+func (image *Image) Enhance(algorithm string, outside bool) Image {
+	newImage := NewImage()
+	newImage.min = image.min.Add(Vec{-1, -1})
+	newImage.max = image.max.Add(Vec{1, 1})
+
+	for y := newImage.min.y; y <= newImage.max.y; y++ {
+		for x := newImage.min.x; x <= newImage.max.x; x++ {
+			if image.Kernel(x, y, algorithm, outside) {
+				newImage.data[Vec{x,y}] = true
+			}
+		}
+	}
+
+	return newImage
+}
+
+func (image *Image) Count() int {
+	return len(image.data)
+}
+
+func main() {
+	challenge := aoc.New(2021, 20)
+
+	challenge.Test(`..#.#..#####.#.#.#.###.##.....###.##.#..###.####..#####..#....#..#..##..###..######.###...####..#..#####..##..#.#####...##.#.#..#.##..#.#......#.###.######.###.####...#.##.##..#..#..#####.....#.#....###..#.##......#.....#..#..#..##..#...##.######.####.####.#.#...#.......#..#.#.#...####.##.#......#..#...##.#.##..#...##.#.##..###.#......#.#.......#.#.#.####.###.##...#.....####.#..#..#.##.#....##..#.####....##...##..#...#......#.#.......#.......##..####..#...#.#.#...##..#.#..###..#####........#..####......#..#
+
+#..#.
+#....
+##..#
+..#..
+..###`, []int{35, 3351})
+
+	challenge.Solution(1, func (input *bufio.Scanner) int {
+		image, algorithm := processInput(input)
+
+		round1 := image.Enhance(algorithm, false)
+
+		alternate := false
+		if algorithm[0] == '#' {
+			alternate = true
+		}
+
+		round2 := round1.Enhance(algorithm, alternate)
+
+		return round2.Count()
+	})
+
+	// @NOTE This solution does not actually properly handle the algorithm[0] == '#' edge case
+	// Here we assume that the last char in the algorithm is '.' and therefore alternate betwee # and . outside
+	// However if the last char is # we keep # outside after the first round
+	// For my input this was not the case, but it is an oversight
+	challenge.Solution(2, func (input *bufio.Scanner) int {
+		image, algorithm := processInput(input)
+
+		alternate := false
+		for round := 0; round < 50; round++ {
+			image = image.Enhance(algorithm, alternate)
+			if algorithm[0] == '#' {
+				alternate = !alternate
+			}
+		}
+
+		return image.Count()
+	})
+}