1 files changed, 85 insertions, 0 deletions
diff --git a/2021/09/second.go b/2021/09/second.go
new file mode 100644
index 0000000..6cba3dd
--- /dev/null
+++ b/2021/09/second.go
@@ -0,0 +1,85 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"sort"
+)
+
+var (
+	grid          [][]byte = make([][]byte, 0)
+	width, height int
+)
+
+func isMin(x, y int) bool {
+	var left, right, top, bottom byte
+	if x == 0 {
+		left = '9'
+	} else {
+		left = grid[x-1][y]
+	}
+	if x == height-1 {
+		right = '9'
+	} else {
+		right = grid[x+1][y]
+	}
+	if y == 0 {
+		top = '9'
+	} else {
+		top = grid[x][y-1]
+	}
+	if y == width-1 {
+		bottom = '9'
+	} else {
+		bottom = grid[x][y+1]
+	}
+	return grid[x][y] < top && grid[x][y] < bottom && grid[x][y] < left && grid[x][y] < right
+}
+
+type Min struct {
+	x, y int
+}
+
+var minimums []Min
+
+func main() {
+	s := bufio.NewScanner(os.Stdin)
+	s.Scan()
+	width = len(s.Text())
+	for {
+		line := make([]byte, width)
+		for i, ch := range s.Text() {
+			line[i] = byte(ch)
+		}
+		grid = append(grid, line)
+		if !s.Scan() {
+			break
+		}
+	}
+	height = len(grid)
+
+	for y := 0; y < width; y++ {
+		for x := 0; x < height; x++ {
+			if isMin(x, y) {
+				minimums = append(minimums, Min{x, y})
+			}
+		}
+	}
+
+	basins := make([]int, 0)
+	for _, m := range minimums {
+		basins = append(basins, computeBasinSize(m.x, m.y))
+	}
+	sort.Ints(basins)
+	basins = basins[len(basins)-3:]
+	fmt.Println(basins[0] * basins[1] * basins[2])
+}
+
+func computeBasinSize(x, y int) int {
+	if x < 0 || x >= height || y < 0 || y >= width || grid[x][y] == '9' {
+		return 0
+	}
+	grid[x][y] = '9'
+	return 1 + computeBasinSize(x-1, y) + computeBasinSize(x+1, y) + computeBasinSize(x, y-1) + computeBasinSize(x, y+1)
+}