refactor+feat: use bitmask approach and cyclic/acyclic dfs for resolution

internal/2025/DayEleven/code.go

@@ -2,6 +2,7 @@ package dayeleven
 
 import (
 	"os"
+	"slices"
 	"strings"
 
 	"advent-of-code/internal/registry"
@@ -16,7 +17,7 @@ func ParseInput(filepath string) []string {
 	return strings.Split(string(content), "\n")
 }
 
-func PartOne(data []string) int {
+func buildGraph(data []string) map[string][]string {
 	graph := make(map[string][]string)
 
 	for _, line := range data {
@@ -24,22 +25,129 @@ func PartOne(data []string) int {
 		graph[device] = strings.Fields(outputsStr)
 	}
 
-	var countPaths func(string) int
-	countPaths = func(node string) int {
-		if node == "out" {
+	return graph
+}
+
+func hasCycle(graph map[string][]string) bool {
+	const (
+		unvisited = iota
+		visiting
+		visited
+	)
+
+	state := make(map[string]uint8)
+
+	var dfs func(node string) bool
+	dfs = func(node string) bool {
+		switch state[node] {
+		case visiting:
+			return true
+		case visited:
+			return false
+		}
+
+		state[node] = visiting
+		if slices.ContainsFunc(graph[node], dfs) {
+			return true
+		}
+		state[node] = visited
+		return false
+	}
+
+	for node := range graph {
+		if state[node] == unvisited {
+			if dfs(node) {
+				return true
+			}
+		}
+	}
+
+	return false
+}
+
+func countPaths(graph map[string][]string, start, end string, requiredNodes map[string]bool) int {
+	requiredIndex := make(map[string]uint)
+	var targetMask uint
+	for node := range requiredNodes {
+		requiredIndex[node] = uint(len(requiredIndex))
+		targetMask |= 1 << requiredIndex[node]
+	}
+
+	if hasCycle(graph) {
+		var dfs func(node string, visited map[string]bool, seenMask uint) int
+		dfs = func(node string, visited map[string]bool, seenMask uint) int {
+			if node == end {
+				if seenMask != targetMask {
+					return 0
+				}
+				return 1
+			}
+
+			if idx, ok := requiredIndex[node]; ok {
+				seenMask |= 1 << idx
+			}
+
+			visited[node] = true
+			defer delete(visited, node)
+
+			count := 0
+			for _, neighbor := range graph[node] {
+				if !visited[neighbor] {
+					count += dfs(neighbor, visited, seenMask)
+				}
+			}
+			return count
+		}
+
+		return dfs(start, make(map[string]bool), 0)
+	}
+
+	type key struct {
+		node string
+		mask uint
+	}
+	memo := make(map[key]int)
+
+	var dfs func(node string, seenMask uint) int
+	dfs = func(node string, seenMask uint) int {
+		if node == end {
+			if seenMask != targetMask {
+				return 0
+			}
 			return 1
 		}
 
+		if idx, ok := requiredIndex[node]; ok {
+			seenMask |= 1 << idx
+		}
+
+		state := key{node: node, mask: seenMask}
+		if cached, ok := memo[state]; ok {
+			return cached
+		}
+
 		count := 0
 		for _, neighbor := range graph[node] {
-			count += countPaths(neighbor)
+			count += dfs(neighbor, seenMask)
 		}
+
+		memo[state] = count
 		return count
 	}
 
-	return countPaths("you")
+	return dfs(start, 0)
+}
+
+func PartOne(data []string) int {
+	graph := buildGraph(data)
+	return countPaths(graph, "you", "out", nil)
 }
 
 func PartTwo(data []string) int {
-	return 0
+	graph := buildGraph(data)
+	requiredNodes := map[string]bool{
+		"dac": true,
+		"fft": true,
+	}
+	return countPaths(graph, "svr", "out", requiredNodes)
 }