Day 10 solution.

2020/day10.go

@@ -19,8 +19,75 @@ func getMax(values map[int]bool) int {
 	return max
 }
 
+type Node struct {
+	Value    int
+	Children []*Node
+	Paths    int // the total number of paths from here to some target
+}
+
+// Build a tree of possible paths, then count them.
+func findArrangements(values map[int]bool, target int) int {
+	root := buildTree(values, target)
+	return countPaths(root, target)
+}
+
+func buildTree(values map[int]bool, target int) *Node {
+	values[0] = true
+	nodeMap := make(map[int]*Node)
+
+	for i := 0; i < target; i++ {
+		if values[i] {
+			node := createOrFetchNode(nodeMap, i)
+
+			for j := 1; j < 4; j++ {
+				if i+j > target {
+					break
+				}
+
+				if values[i+j] {
+					child := createOrFetchNode(nodeMap, i+j)
+					node.Children = append(node.Children, child)
+				}
+			}
+		}
+	}
+
+	return nodeMap[0]
+}
+
+func createOrFetchNode(nodeMap map[int]*Node, value int) *Node {
+	if node, ok := nodeMap[value]; ok {
+		return node
+	}
+
+	node := &Node{
+		Value:    value,
+		Children: make([]*Node, 0),
+	}
+	nodeMap[value] = node
+
+	return node
+}
+
+func countPaths(node *Node, target int) int {
+	if node.Value == target {
+		return 1
+	}
+
+	paths := 0
+	for _, child := range node.Children {
+		if child.Paths != 0 {
+			paths += child.Paths
+		} else {
+			paths += countPaths(child, target)
+		}
+	}
+	node.Paths = paths
+	return paths
+}
+
 func main() {
-	// step := os.Args[1]
+	step := os.Args[1]
 	values := fileutils.InputParserIntMap(os.Args[2])
 
 	diffMap := make(map[int]int)
@@ -28,6 +95,8 @@ func main() {
 	device := getMax(values) + 3
 	values[device] = true
 
+	switch step {
+	case "1":
 		diff := 0
 		for i := 0; i < device; i++ {
 			// increment diff, make sure we haven't broken the chain
@@ -45,4 +114,8 @@ func main() {
 			}
 		}
 		fmt.Println(diffMap[1] * diffMap[3])
+
+	case "2":
+		fmt.Println(findArrangements(values, device))
+	}
 }