2016-day19: circular LL for removing elves from a table

2016/day19/main.go

@@ -0,0 +1,77 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+
+	"github.com/alexchao26/advent-of-code-go/cast"
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+func main() {
+	var part int
+	flag.IntVar(&part, "part", 1, "part 1 or 2")
+	flag.Parse()
+	fmt.Println("Running part", part)
+
+	ans := elephant(util.ReadFile("./input.txt"), part)
+	fmt.Println("Output:", ans)
+}
+
+// LLNode represents an elf
+type LLNode struct {
+	elfNum   int
+	presents int
+	next     *LLNode
+}
+
+func elephant(input string, part int) int {
+	startingElves := cast.ToInt(input)
+	root := &LLNode{
+		elfNum:   1,
+		presents: 1,
+	}
+	iter := root
+	for i := 2; i <= startingElves; i++ {
+		iter.next = &LLNode{
+			elfNum:   i,
+			presents: 1,
+		}
+		iter = iter.next
+	}
+	iter.next = root
+
+	if part == 1 {
+		for root.next != root {
+			root.presents += root.next.presents
+			root.next = root.next.next
+			root = root.next
+		}
+		return root.elfNum
+	}
+
+	// initialize a pointer to the node before the node across from the start
+	// need the node before b/c removing a node is like reassigning beforeNode.next
+	// if there are an odd number of starting elves, this points to the first one
+	// which is also the one of "the left."
+	isOddLength := startingElves%2 == 1
+	beforeAcross := root
+	for i := 0; i < startingElves/2-1; i++ {
+		beforeAcross = beforeAcross.next
+	}
+
+	for root.next != root {
+		root.presents += beforeAcross.next.presents
+		// remove beforeAcross node
+		beforeAcross.next = beforeAcross.next.next
+		// if odd number of total nodes, beforeAcross node skips the node
+		// that was previously the "right" side of the across pair
+		if isOddLength {
+			beforeAcross = beforeAcross.next
+		}
+		isOddLength = !isOddLength
+		root = root.next
+	}
+
+	return root.elfNum
+}