From 656e975cb1bd65cd9cb7b6cb15d6f86081d30f2f Mon Sep 17 00:00:00 2001
From: Anna Wiggins <annabunches@gmail.com>
Date: Mon, 10 Dec 2018 21:15:33 -0500
Subject: [PATCH] Day 8.2 solution.

---
 2018/day08-1.go             |  5 +----
 2018/day08-2.go             | 36 ++++++++++++++++++++++++++++++++++++
 2018/internal/day08/tree.go |  8 ++++++++
 3 files changed, 45 insertions(+), 4 deletions(-)
 create mode 100644 2018/day08-2.go

diff --git a/2018/day08-1.go b/2018/day08-1.go
index 082a1ce..f4058f8 100644
--- a/2018/day08-1.go
+++ b/2018/day08-1.go
@@ -14,10 +14,7 @@ func main() {
 }
 
 func sumMetadata(node *day08.Node) int {
-	sum := 0
-	for _, v := range node.Metadata {
-		sum += v
-	}
+	sum := node.SumMetadata()
 
 	for _, child := range node.Children {
 		sum += sumMetadata(child)
diff --git a/2018/day08-2.go b/2018/day08-2.go
new file mode 100644
index 0000000..2a469ac
--- /dev/null
+++ b/2018/day08-2.go
@@ -0,0 +1,36 @@
+package main
+
+import (
+	"fmt"
+
+	"internal/day08"
+	"internal/util"
+)
+
+func main() {
+	data := util.ReadInputInts()
+	root := day08.BuildTree(data)
+	fmt.Println(generateChecksum(root))
+}
+
+func generateChecksum(node *day08.Node) int {
+	// If a node has no child nodes, its value is the sum of its metadata entries.
+	if len(node.Children) == 0 {
+		return node.SumMetadata()
+	}
+
+	// if a node does have child nodes, the metadata entries become (1-indexed) indexes
+	// which refer to those child nodes.
+	// The value of the node is the sum of the values of its children.
+	value := 0
+	for _, md := range node.Metadata {
+		// Skip invalid metadata entries
+		if md == 0 || md > len(node.Children) {
+			continue
+		}
+
+		value += generateChecksum(node.Children[md-1])
+	}
+
+	return value
+}
diff --git a/2018/internal/day08/tree.go b/2018/internal/day08/tree.go
index 47da31a..5772a75 100644
--- a/2018/internal/day08/tree.go
+++ b/2018/internal/day08/tree.go
@@ -5,6 +5,14 @@ type Node struct {
 	Metadata []int
 }
 
+func (node Node) SumMetadata() int {
+	sum := 0
+	for _, v := range node.Metadata {
+		sum += v
+	}
+	return sum
+}
+
 func BuildTree(data []int) *Node {
 	root, _ := buildTreeR(data, 0)
 	return root