From d493eb16c8135ffe5e128b9303d88762644b481f Mon Sep 17 00:00:00 2001
From: Jeremy Kaplan <jdkaplan@metagram.net>
Date: Wed, 8 Dec 2021 21:45:23 -0800
Subject: [PATCH] 2021 Day 9

---
 2021/day9/main.go | 110 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 110 insertions(+)
 create mode 100644 2021/day9/main.go

diff --git a/2021/day9/main.go b/2021/day9/main.go
new file mode 100644
index 0000000..ad4abcc
--- /dev/null
+++ b/2021/day9/main.go
@@ -0,0 +1,110 @@
+package main
+
+import (
+	"fmt"
+	"sort"
+
+	"github.com/jdkaplan/advent-of-code/aoc"
+)
+
+func main() {
+	lines := aoc.Input().ReadLines("day9.txt")
+	fmt.Println(part1(lines))
+	fmt.Println(part2(lines))
+}
+
+func part1(lines []string) (risk int) {
+	g := NewGrid(lines)
+	for r := 0; r < g.Rows; r++ {
+		for c := 0; c < g.Cols; c++ {
+			risk += g.Risk(r, c)
+		}
+	}
+	return risk
+}
+
+func part2(lines []string) int {
+	g := NewGrid(lines)
+	var starts []RC
+	for r := 0; r < g.Rows; r++ {
+		for c := 0; c < g.Cols; c++ {
+			if g.Risk(r, c) != 0 {
+				starts = append(starts, RC{r, c})
+			}
+		}
+	}
+
+	var sizes []int
+	for _, rc := range starts {
+		sizes = append(sizes, g.Flood(rc))
+	}
+	sort.Sort(sort.Reverse(sort.IntSlice(sizes)))
+	return sizes[0] * sizes[1] * sizes[2]
+}
+
+type RC struct{ r, c int }
+
+type Grid struct {
+	Rows int
+	Cols int
+
+	m map[RC]int
+}
+
+func NewGrid(lines []string) Grid {
+	g := Grid{
+		m:    make(map[RC]int),
+		Rows: len(lines),
+		Cols: len(lines[0]),
+	}
+	for r, row := range lines {
+		for c, col := range row {
+			g.m[RC{r, c}] = int(col - '0')
+		}
+	}
+	return g
+}
+
+func (g Grid) Risk(r, c int) int {
+	rc := RC{r, c}
+	v, ok := g.m[rc]
+	if !ok {
+		return 0
+	}
+	for _, n := range g.Neighbors(rc) {
+		u, ok := g.m[n]
+		if ok && u <= v {
+			return 0
+		}
+	}
+	return v + 1
+}
+
+func (g Grid) Neighbors(rc RC) []RC {
+	r, c := rc.r, rc.c
+	return []RC{
+		{r - 1, c},
+		{r, c + 1},
+		{r + 1, c},
+		{r, c - 1},
+	}
+}
+
+func (g Grid) Flood(rc RC) (size int) {
+	queue := []RC{rc}
+	seen := make(map[RC]bool)
+	for len(queue) > 0 {
+		rc, queue = queue[0], queue[1:]
+		if seen[rc] {
+			continue
+		}
+		for _, n := range g.Neighbors(rc) {
+			if v, ok := g.m[n]; ok && v < 9 {
+				queue = append(queue, n)
+			}
+		}
+		seen[rc] = true
+		size++
+	}
+	return size
+}