1 files changed, 102 insertions, 0 deletions
diff --git a/day10/main.go b/day10/main.go
new file mode 100644
index 0000000..e122082
--- /dev/null
+++ b/day10/main.go
@@ -0,0 +1,102 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strings"
+)
+
+func main() {
+	fmt.Println(find(readData("example.txt")))
+	fmt.Println(36, 81)
+	fmt.Println(find(readData("data.txt")))
+}
+
+type Candidate struct {
+	X, Y int
+	Alt  byte
+	Head *TrailHead
+}
+
+func (c Candidate) Identity() int {
+	return (c.X << 16) | c.Y
+}
+
+type TrailHead struct {
+	Reaches map[int]bool
+}
+
+func findNextCandidates(x, y int, targetAlt byte, head *TrailHead, topo [][]byte) (result []Candidate) {
+	if x > 0 && topo[y][x-1] == targetAlt {
+		result = append(result, Candidate{x - 1, y, targetAlt, head})
+	}
+	if x < len(topo[y])-1 && topo[y][x+1] == targetAlt {
+		result = append(result, Candidate{x + 1, y, targetAlt, head})
+	}
+	if y > 0 && topo[y-1][x] == targetAlt {
+		result = append(result, Candidate{x, y - 1, targetAlt, head})
+	}
+	if y < len(topo)-1 && topo[y+1][x] == targetAlt {
+		result = append(result, Candidate{x, y + 1, targetAlt, head})
+	}
+	return
+}
+
+func find(topo [][]byte) (resultPart1, resultPart2 int) {
+	candidates := make([]Candidate, 0)
+	for y, line := range topo {
+		for x, alt := range line {
+			if alt == 0 {
+				trailhead := &TrailHead{make(map[int]bool)}
+				candidate := Candidate{x, y, alt, trailhead}
+				candidates = append(candidates, candidate)
+			}
+		}
+	}
+
+	for len(candidates) > 0 {
+		current := candidates[len(candidates)-1]
+		candidates = candidates[:len(candidates)-1]
+
+		nextCandidates := findNextCandidates(current.X, current.Y, current.Alt+1, current.Head, topo)
+		if current.Alt == 8 {
+			for _, summit := range nextCandidates {
+				id := summit.Identity()
+				_, alreadyIn := summit.Head.Reaches[id]
+				if !alreadyIn {
+					summit.Head.Reaches[id] = true
+					resultPart1++
+				}
+			}
+			resultPart2 += len(nextCandidates)
+		} else {
+			candidates = append(candidates, nextCandidates...)
+		}
+	}
+
+	return
+}
+
+func readData(fileName string) (topo [][]byte) {
+	fp, err := os.Open(fileName)
+	if err != nil {
+		panic(err)
+	}
+
+	scanner := bufio.NewScanner(fp)
+	for scanner.Scan() {
+		line := []byte(strings.TrimSpace(scanner.Text()))
+		if len(line) == 0 {
+			continue
+		}
+
+		for i := range line {
+			line[i] = line[i] - '0'
+		}
+
+		topo = append(topo, line)
+	}
+
+	return
+}