From b17c80d1dda721f1b70af1e243403228651eb06b Mon Sep 17 00:00:00 2001
From: Gregory <neogen570@gmail.com>
Date: Mon, 21 Feb 2022 22:01:00 +0200
Subject: [PATCH] d9

---
 d9/d9.go      | 105 ++++++++++++++++++++++++++++++++++++++++++++++++++
 d9/d9_test.go |  27 +++++++++++++
 main.go       |   3 ++
 3 files changed, 135 insertions(+)
 create mode 100644 d9/d9.go
 create mode 100644 d9/d9_test.go

diff --git a/d9/d9.go b/d9/d9.go
new file mode 100644
index 0000000..0ce64fd
--- /dev/null
+++ b/d9/d9.go
@@ -0,0 +1,105 @@
+package d9
+
+import (
+	"fmt"
+	"sort"
+	"strconv"
+	"strings"
+)
+
+func bigger(a byte, b byte) bool {
+	aNum, _ := strconv.Atoi(string(a))
+	bNum, _ := strconv.Atoi(string(b))
+
+	return aNum > bNum
+}
+
+func toNum(str byte) int {
+	num, _ := strconv.Atoi(string(str))
+	return num
+}
+
+func addWalls(lines []string) []string {
+	for i, line := range lines {
+		lines[i] = fmt.Sprintf("9%v9", line)
+	}
+
+	nines := strings.Repeat("9", len(lines[0]))
+	return append([]string{nines}, append(lines, nines)...)
+}
+
+func D9(in string) int {
+	lines := addWalls(strings.Split(strings.TrimSpace(in), "\n"))
+	risk := 0
+
+	for i := 1; i < len(lines)-1; i++ {
+		for j := 1; j < len(lines[0])-1; j++ {
+			current := lines[i][j]
+			top := bigger(lines[i-1][j], current)
+			right := bigger(lines[i][j+1], current)
+			bottom := bigger(lines[i+1][j], current)
+			left := bigger(lines[i][j-1], current)
+			if top && right && bottom && left {
+				risk += toNum(current) + 1
+			}
+		}
+	}
+
+	return risk
+}
+
+type exists struct{}
+
+type pos struct {
+	x int
+	y int
+}
+
+type basin map[pos]exists
+
+const wall = '9'
+
+func flooded(x, y int, basins []basin) bool {
+	for _, basin := range basins {
+		if _, ok := basin[pos{x, y}]; ok {
+			return true
+		}
+	}
+
+	return false
+}
+
+func flood(x, y int, heights []string, basin basin) basin {
+	if heights[y][x] == wall {
+		return basin
+	}
+
+	if _, ok := basin[pos{x, y}]; ok {
+		return basin
+	}
+
+	basin[pos{x, y}] = exists{}
+
+	flood(x, y-1, heights, basin)
+	flood(x+1, y, heights, basin)
+	flood(x, y+1, heights, basin)
+	flood(x-1, y, heights, basin)
+	return basin
+}
+
+func D9P2(in string) int {
+	heights := addWalls(strings.Split(strings.TrimSpace(in), "\n"))
+	basins := []basin{}
+
+	for y := 1; y < len(heights)-1; y++ {
+		for x := 1; x < len(heights[0])-1; x++ {
+			if !flooded(x, y, basins) && heights[y][x] != wall {
+				basins = append(basins, flood(x, y, heights, basin{}))
+			}
+		}
+	}
+
+	sort.Slice(basins, func(i, j int) bool { return len(basins[i]) > len(basins[j]) })
+
+	return len(basins[0]) * len(basins[1]) * len(basins[2])
+}
diff --git a/d9/d9_test.go b/d9/d9_test.go
new file mode 100644
index 0000000..5d1e391
--- /dev/null
+++ b/d9/d9_test.go
@@ -0,0 +1,27 @@
+package d9
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestD8(t *testing.T) {
+	input := `2199943210
+3987894921
+9856789892
+8767896789
+9899965678`
+
+	assert.EqualValues(t, 15, D9(input))
+}
+
+func TestD8P2(t *testing.T) {
+	input := `2199943210
+3987894921
+9856789892
+8767896789
+9899965678`
+
+	assert.EqualValues(t, 1134, D9P2(input))
+}
diff --git a/main.go b/main.go
index 34c79b1..c678ae0 100644
--- a/main.go
+++ b/main.go
@@ -15,6 +15,7 @@ import (
 	"github.com/fotonmoton/aoc2021/d6"
 	"github.com/fotonmoton/aoc2021/d7"
 	"github.com/fotonmoton/aoc2021/d8"
+	"github.com/fotonmoton/aoc2021/d9"
 )
 
 func main() {
@@ -43,4 +44,6 @@ func main() {
 	fmt.Printf("day 7 part 2: %v\n", d7.D7P2(client.Day(7)))
 	fmt.Printf("day 8: %v\n", d8.D8(client.Day(8)))
 	fmt.Printf("day 8 part 2: %v\n", d8.D8P2(client.Day(8)))
+	fmt.Printf("day 9: %v\n", d9.D9(client.Day(9)))
+	fmt.Printf("day 9 part 2: %v\n", d9.D9P2(client.Day(9)))
 }