From 3b0f5ab20ce312b2aa467425873cb178f670f99b Mon Sep 17 00:00:00 2001
From: Dreaded_X <tim.huizinga@gmail.com>
Date: Thu, 9 Dec 2021 18:09:22 +0100
Subject: [PATCH] 2021 - Day 9

---
 2021/9/go.mod  |   9 ++++
 2021/9/go.sum  |   2 +
 2021/9/main.go | 127 +++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 138 insertions(+)
 create mode 100644 2021/9/go.mod
 create mode 100644 2021/9/go.sum
 create mode 100644 2021/9/main.go

diff --git a/2021/9/go.mod b/2021/9/go.mod
new file mode 100644
index 0000000..226512f
--- /dev/null
+++ b/2021/9/go.mod
@@ -0,0 +1,9 @@
+module AoC/2021/9
+
+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/2021/9/go.sum b/2021/9/go.sum
new file mode 100644
index 0000000..8c9f290
--- /dev/null
+++ b/2021/9/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/2021/9/main.go b/2021/9/main.go
new file mode 100644
index 0000000..3a62e5d
--- /dev/null
+++ b/2021/9/main.go
@@ -0,0 +1,127 @@
+package main
+
+import (
+	aoc "AoC/2021/common"
+	"bufio"
+	"sort"
+	"strconv"
+	"strings"
+)
+
+func processInput(input *bufio.Scanner) ([][]int, int, int) {
+	var heightmap [][]int
+	for input.Scan() {
+		heights := strings.Split(input.Text(), "")
+
+		var row []int
+		for _, height := range heights {
+			// Ignore any errors
+			h, _ := strconv.Atoi(height)
+			row = append(row, h)
+		}
+
+		heightmap = append(heightmap, row)
+	}
+
+	ymax := len(heightmap) - 1
+	// This assumes that at least one row exists and that all rows have the same length
+	xmax := len(heightmap[0]) - 1
+
+	return heightmap, xmax, ymax
+}
+
+func floodFill(heightmap [][]int, marked []bool, size int, x int, y int, xmax int, ymax int) ([]bool, int) {
+	if heightmap[y][x] != 9 && !marked[y*(xmax+1) + x] {
+		marked[y*(xmax+1) + x] = true
+		size++
+
+		if (x != 0) {
+			marked, size = floodFill(heightmap, marked, size, x-1, y, xmax, ymax)
+		}
+
+		if (x != xmax) {
+			marked, size = floodFill(heightmap, marked, size, x+1, y, xmax, ymax)
+		}
+
+		if (y != 0) {
+			marked, size = floodFill(heightmap, marked, size, x, y-1, xmax, ymax)
+		}
+
+		if (y != ymax) {
+			marked, size = floodFill(heightmap, marked, size, x, y+1, xmax, ymax)
+		}
+	}
+
+	return marked, size
+}
+
+func main() {
+	challenge := aoc.New(2021, 9)
+
+	challenge.Test(`2199943210
+3987894921
+9856789892
+8767896789
+9899965678`, []int{15, 1134})
+
+	challenge.Solution(1, func (input *bufio.Scanner) int {
+		heightmap, xmax, ymax := processInput(input)
+
+		sum := 0
+		for y := 0; y <= ymax; y++ {
+			for x := 0; x <= xmax; x++ {
+				height := heightmap[y][x]
+				if (x != 0 && heightmap[y][x-1] <= height) || (x != xmax && heightmap[y][x+1] <= height) {
+					// Neighbour on the x-axis is lower
+					continue
+				}
+
+				if (y != 0 && heightmap[y-1][x] <= height) || (y != ymax && heightmap[y+1][x] <= height) {
+					// Neighbour on the y-axis is lower
+					continue
+				}
+
+				// Found a low point
+				sum += 1 + height
+			}
+		}
+
+		return sum
+	})
+
+	challenge.Solution(2, func (input *bufio.Scanner) int {
+		heightmap, xmax, ymax := processInput(input)
+
+		// Find the size of all basins
+		var sizes []int
+		for y := 0; y <= ymax; y++ {
+			for x := 0; x <= xmax; x++ {
+				height := heightmap[y][x]
+				if (x != 0 && heightmap[y][x-1] <= height) || (x != xmax && heightmap[y][x+1] <= height) {
+					// Neighbour on the x-axis is lower
+					continue
+				}
+
+				if (y != 0 && heightmap[y-1][x] <= height) || (y != ymax && heightmap[y+1][x] <= height) {
+					// Neighbour on the y-axis is lower
+					continue
+				}
+
+				// Found a low point, find the size of the basin using flood fill
+				_, size := floodFill(heightmap, make([]bool, (ymax+1)*(xmax+1)), 0, x, y, xmax, ymax)
+				sizes = append(sizes, size)
+			}
+		}
+
+		// Sort the basin sizes
+		sort.Ints(sizes)
+
+		// Multiply the largest three
+		answer := 1
+		for _, size := range sizes[len(sizes)-3:] {
+			answer *= size
+		}
+
+		return answer
+	})
+}