Add solution day 21 part 2 - AoC 2024 is DONE!

Author: terminalnode

README.md

@@ -53,9 +53,9 @@ automatically rebuilt and redeployed every time the `common` module or their own
 | 05  | ⭐ ⭐      | 18  | ⭐ ⭐      |
 | 06  | ⭐ ⭐      | 19  | ⭐ ⭐      |
 | 07  | ⭐ ⭐      | 20  | ⭐ ⭐      |
-| 08  | ⭐ ⭐      | 21  | ⭐        |
+| 08  | ⭐ ⭐      | 21  | ⭐ ⭐      |
 | 09  | ⭐ ⭐      | 22  | ⭐ ⭐      |
 | 10  | ⭐ ⭐      | 23  | ⭐ ⭐      |
 | 11  | ⭐ ⭐      | 24  | ⭐ ⭐      |
-| 12  | ⭐ ⭐      | 25  | ⭐        |
+| 12  | ⭐ ⭐      | 25  | ⭐ ⭐      |
 | 13  | ⭐ ⭐      |     |          |

solutions/day21/keypad.go

@@ -7,9 +7,15 @@ import (
 var numStart = util.Coordinate{X: 2, Y: 3}
 var dirStart = util.Coordinate{X: 2, Y: 0}
 
+type cacheMap = map[cacheKey]int
+type cacheKey struct {
+	path  string
+	depth int
+}
+
 // Char to Pos on numeric keypad
 func charToPosNum(
-	c int,
+	c byte,
 ) util.Coordinate {
 	switch c {
 	case '0':
@@ -25,7 +31,7 @@ func charToPosNum(
 
 // Char to Pos on directional keypad
 func charToPosDir(
-	c int,
+	c byte,
 ) util.Coordinate {
 	switch c {
 	case '^':
@@ -44,8 +50,8 @@ func charToPosDir(
 func shortestPath(
 	start, end util.Coordinate,
 	useNumPad bool,
-) []int {
-	var path []int
+) []byte {
+	var path []byte
 	dx := end.X - start.X
 	dy := end.Y - start.Y
 	pVert, pHor := buildPath(dx, dy)
@@ -71,14 +77,40 @@ func shortestPath(
 	return path
 }
 
+func dfs(
+	cache cacheMap,
+	key cacheKey,
+) int {
+	out := 0
+	if v, ok := cache[key]; ok {
+		return v
+	} else if key.depth == 0 {
+		return len(key.path)
+	}
+
+	subPaths := make([][]byte, len(key.path))
+	prev := dirStart
+	for i, c := range key.path {
+		curr := charToPosDir(byte(c))
+		subPaths[i] = shortestPath(prev, curr, false)
+		prev = curr
+	}
+
+	for _, subPath := range subPaths {
+		out += dfs(cache, cacheKey{path: string(subPath), depth: key.depth - 1})
+	}
+	cache[key] = out
+	return out
+}
+
 func buildPath(
 	dx int,
 	dy int,
-) ([]int, []int) {
+) ([]byte, []byte) {
 	adx := util.AbsInt(dx)
 	ady := util.AbsInt(dy)
-	pVert := make([]int, ady)
-	pHor := make([]int, adx)
+	pVert := make([]byte, ady)
+	pHor := make([]byte, adx)
 
 	for i := range adx {
 		if dx < 0 {

solutions/day21/main.go

@@ -7,47 +7,41 @@ import (
 	"strings"
 )
 
-func main() {
-	common.Setup(21, part1, nil)
-}
+func main() { common.Setup(21, part1, part2) }
+
+func part1(input string) string { return solve(input, 2) }
+func part2(input string) string { return solve(input, 25) }
 
-func part1(
+func solve(
 	input string,
+	robots int,
 ) string {
-	var out string
+	cache := make(cacheMap)
 
 	sequences := strings.Split(input, "\n")
 	sum := 0
 	for _, code := range sequences {
+		codeNumeric, err := strconv.Atoi(code[:len(code)-1])
+		if err != nil {
+			return fmt.Sprintf("Failed to parse code %s as int: %v", code, err)
+		}
+
 		// Five is the longest path between two keys on numeric pad
-		path := make([]int, 0, 5*len(code))
+		path := make([][]byte, 0, 5*len(code))
 		prev := numStart
 		for _, c := range code {
-			curr := charToPosNum(int(c))
-			path = append(path, shortestPath(prev, curr, true)...)
+			curr := charToPosNum(byte(c))
+			path = append(path, shortestPath(prev, curr, true))
 			prev = curr
 		}
 
-		for range 2 {
-			// Three is the longest path between two keys on directional pad
-			newPath := make([]int, 0, 3*len(path))
-			prev = dirStart
-			for _, c := range path {
-				curr := charToPosDir(c)
-				newPath = append(newPath, shortestPath(prev, curr, false)...)
-				prev = curr
-			}
-			path = newPath
-		}
-
-		codeNumeric, err := strconv.Atoi(code[:len(code)-1])
-		if err != nil {
-			return fmt.Sprintf("Failed to parse code %s as int: %v", code, err)
+		pathLen := 0
+		for _, c := range path {
+			pathLen += dfs(cache, cacheKey{path: string(c), depth: robots})
 		}
 
-		sum += len(path) * codeNumeric
+		sum += pathLen * codeNumeric
 	}
 
-	out += fmt.Sprintf("Sum: %d", sum)
-	return out
+	return fmt.Sprintf("Sum: %d", sum)
 }