Solution for Day 9

README.md

@@ -93,3 +93,12 @@ The solution for "eight" is:
 There were 476 instances of digits 1, 4, 7, or 8
 The sum of the decoded output values was 1011823
 ```
+
+### Day nine
+
+```sh
+$ ./aoc2021 --nine
+The solution for "nine" is:
+The sum of the risk level of low points is 439
+The product of the three largest basins is 900900
+```

main.go

@@ -8,6 +8,7 @@ import (
 	"unsupervised.ca/aoc2021/eight"
 	"unsupervised.ca/aoc2021/five"
 	"unsupervised.ca/aoc2021/four"
+	"unsupervised.ca/aoc2021/nine"
 	"unsupervised.ca/aoc2021/one"
 	"unsupervised.ca/aoc2021/seven"
 	"unsupervised.ca/aoc2021/six"
@@ -46,6 +47,8 @@ func main() {
 		day = seven.Init("seven/input.txt")
 	case "eight":
 		day = eight.Init("eight/input.txt")
+	case "nine":
+		day = nine.Init("nine/input.txt")
 	default:
 		fmt.Printf("%q does not have a solution.\n", flagParts[1])
 		help()

nine/input.txt

@@ -0,0 +1,100 @@
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nine/main.go

@@ -0,0 +1,24 @@
+package nine
+
+import "fmt"
+
+type Nine struct {
+	vents vents
+}
+
+func Init(filepath string) *Nine {
+	nine := &Nine{
+		vents: vents{},
+	}
+
+	nine.vents.load(filepath)
+	return nine
+}
+
+func (d *Nine) Answer() string {
+	return fmt.Sprintf("The sum of the risk level of low points is %d", d.vents.sumLowPointRisk())
+}
+
+func (d *Nine) FollowUp() string {
+	return fmt.Sprintf("The product of the three largest basins is %d", d.vents.productLargestBasins())
+}

nine/test_input.txt

@@ -0,0 +1,5 @@
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678

nine/vents.go

@@ -0,0 +1,176 @@
+package nine
+
+import (
+	"bufio"
+	"os"
+	"sort"
+	"strconv"
+	"strings"
+)
+
+type vents struct {
+	floor     [][]int
+	counted   [][]bool
+	lowPoints *[]basin
+}
+
+type basin struct {
+	lowPoint int
+	size     int
+}
+
+func (v *vents) load(filename string) error {
+	file, err := os.Open(filename)
+	if err != nil {
+		return err
+	}
+	defer file.Close()
+
+	v.floor = [][]int{}
+	v.counted = [][]bool{}
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		elements := []int{}
+
+		heights := strings.Split(scanner.Text(), "")
+
+		counts := make([]bool, len(heights))
+
+		for _, h := range heights {
+			v, err := strconv.Atoi(h)
+			if err != nil {
+				return nil
+			}
+			elements = append(elements, v)
+		}
+
+		v.floor = append(v.floor, elements)
+		v.counted = append(v.counted, counts)
+	}
+	return nil
+}
+
+func (v *vents) findLowPoints() []basin {
+	if v.lowPoints != nil {
+		return *v.lowPoints
+	}
+
+	lowPoints := []basin{}
+
+	if len(v.floor) == 0 || len(v.floor[0]) == 0 {
+		return lowPoints
+	}
+
+	horizontal := len(v.floor[0])
+	vertical := len(v.floor)
+
+	for y := 0; y < vertical; y++ {
+		for x := 0; x < horizontal; x++ {
+			minimum := true
+			//Check above
+			if y > 0 {
+				if v.floor[y][x] >= v.floor[y-1][x] {
+					minimum = false
+				}
+			}
+
+			//Check below
+			if y < vertical-1 {
+				if v.floor[y][x] >= v.floor[y+1][x] {
+					minimum = false
+				}
+			}
+
+			//Check left
+			if x > 0 {
+				if v.floor[y][x] >= v.floor[y][x-1] {
+					minimum = false
+				}
+			}
+
+			//Check left
+			if x < horizontal-1 {
+				if v.floor[y][x] >= v.floor[y][x+1] {
+					minimum = false
+				}
+			}
+
+			if minimum {
+				b := basin{
+					lowPoint: v.floor[y][x],
+					size:     v.basinSize(x, y),
+				}
+				lowPoints = append(lowPoints, b)
+			}
+		}
+	}
+
+	v.lowPoints = &lowPoints
+	return lowPoints
+}
+
+func (v *vents) basinSize(x int, y int) int {
+	size := 1
+
+	v.counted[y][x] = true
+
+	if v.floor[y][x] == 9 {
+		return 0
+	}
+
+	horizontal := len(v.floor[0])
+	vertical := len(v.floor)
+
+	//Check above
+	if y > 0 && !v.counted[y-1][x] {
+		if v.floor[y][x] < v.floor[y-1][x] {
+			size += v.basinSize(x, y-1)
+		}
+	}
+
+	//Check below
+	if y < vertical-1 && !v.counted[y+1][x] {
+		if v.floor[y][x] < v.floor[y+1][x] {
+			size += v.basinSize(x, y+1)
+		}
+	}
+
+	//Check left
+	if x > 0 && !v.counted[y][x-1] {
+		if v.floor[y][x] < v.floor[y][x-1] {
+			size += v.basinSize(x-1, y)
+		}
+	}
+
+	//Check left
+	if x < horizontal-1 && !v.counted[y][x+1] {
+		if v.floor[y][x] < v.floor[y][x+1] {
+			size += v.basinSize(x+1, y)
+		}
+	}
+
+	return size
+}
+
+func (v *vents) sumLowPointRisk() (sum int) {
+	for _, b := range v.findLowPoints() {
+		sum += (b.lowPoint + 1)
+	}
+
+	return sum
+}
+
+func (v *vents) productLargestBasins() int {
+	sizes := sort.IntSlice{}
+	for _, b := range v.findLowPoints() {
+		sizes = append(sizes, b.size)
+	}
+
+	sort.Sort(sizes)
+
+	if len(sizes) < 3 {
+		return 0
+	}
+
+	return sizes[len(sizes)-1] * sizes[len(sizes)-2] * sizes[len(sizes)-3]
+}

nine/vents_test.go

@@ -0,0 +1,50 @@
+package nine
+
+import "testing"
+
+func Test_read(t *testing.T) {
+	v := vents{}
+	if err := v.load("test_input.txt"); err != nil {
+		t.Log(err)
+		t.FailNow()
+	}
+
+	if len(v.floor) != 5 {
+		t.Logf("Expected 5 rows, found %d", len(v.floor))
+		t.Fail()
+	}
+
+}
+
+func Test_lowPoints(t *testing.T) {
+	v := vents{}
+	v.load("test_input.txt")
+
+	lowPoints := v.findLowPoints()
+
+	if len(lowPoints) != 4 {
+		t.Logf("Expected 4 low points, found %d", len(lowPoints))
+	}
+}
+
+func Test_sumLowPointRisk(t *testing.T) {
+	v := vents{}
+	v.load("test_input.txt")
+
+	sum := v.sumLowPointRisk()
+	if sum != 15 {
+		t.Logf("Expected risk level of 15 rows, found %d", sum)
+		t.Fail()
+	}
+}
+
+func Test_productLargestBasins(t *testing.T) {
+	v := vents{}
+	v.load("test_input.txt")
+
+	product := v.productLargestBasins()
+	if product != 1134 {
+		t.Logf("Expected largest basin product of 1134, found %d", product)
+		t.Fail()
+	}
+}