Day 23 in Go, just 'cause

Also resaved 23input.txt in code page 1252 to remove the UTF-8 BOM

23input.txt

@@ -1 +1 @@
-538914762
+538914762

days/day16.go

@@ -37,21 +37,21 @@ func atol(val string) int64 {
 	return iVal
 }
 
-func msDuration(startTime time.Time) float64 {
-	return float64(time.Since(startTime)) / 1000000
+func msDuration(startTime time.Time) time.Duration {
+	return time.Since(startTime)
 }
 
 // Day16 runs day 16
 func Day16() {
 	start := time.Now()
 	makeList()
-	log.Printf("Q16MakeList took %fms\n", msDuration(start))
+	log.Printf("Q16MakeList took %s\n", msDuration(start))
 	start = time.Now()
 	part1()
-	log.Printf("Q16Part1 took %fms\n", msDuration(start))
+	log.Printf("Q16Part1 took %s\n", msDuration(start))
 	start = time.Now()
 	part2()
-	log.Printf("Q16Part2 took %fms\n", msDuration(start))
+	log.Printf("Q16Part2 took %s\n", msDuration(start))
 }
 
 func makeList() {

days/day23.go

@@ -0,0 +1,133 @@
+package days
+
+import (
+	"fmt"
+	"io/ioutil"
+	"log"
+	"time"
+)
+
+var (
+	d23List []int
+)
+
+// Day23 runs day 23
+func Day23() {
+	start := time.Now()
+	d23MakeList()
+	log.Printf("Q23MakeList took %s\n", msDuration(start))
+	start = time.Now()
+	d23Part1()
+	log.Printf("Q23Part1 took %s\n", msDuration(start))
+	start = time.Now()
+	d23Part2()
+	log.Printf("Q23Part2 took %s\n", msDuration(start))
+}
+
+func d23MakeList() {
+	bytes, err := ioutil.ReadFile("23input.txt")
+	if err != nil {
+		panic(err)
+	}
+	for _, ch := range string(bytes) {
+		d23List = append(d23List, int(ch-'0'))
+	}
+}
+
+func d23Part1() {
+
+	cupList := make([]int, len(d23List)+1)
+	currIdx := 0
+	for i := 0; i < len(d23List); i++ {
+		cupList[currIdx] = d23List[i]
+		currIdx = d23List[i]
+	}
+
+	d23Solve(cupList, 100)
+
+	cup := cupList[1]
+	scoreStr := ""
+	for cup != 1 {
+		scoreStr += fmt.Sprintf("%d", cup)
+		cup = getNextCup(cupList, cup)
+	}
+
+	log.Printf("Q23Part1: labels=%s\n", scoreStr)
+}
+
+func d23Part2() {
+	cupList := make([]int, 1_000_001)
+	currIdx := 0
+	for _, cup := range d23List {
+		cupList[currIdx] = cup
+		currIdx = cup
+	}
+	highest := 0
+	for idx := range d23List {
+		if cupList[idx] == 0 {
+			highest = idx
+			break
+		}
+	}
+	cupList[highest] = len(d23List) + 1
+	for i := cupList[highest]; i < len(cupList); i++ {
+		if i == len(cupList)-1 {
+			cupList[i] = 0
+		} else {
+			cupList[i] = i + 1
+		}
+	}
+
+	d23Solve(cupList, 10_000_000)
+
+	first := cupList[1]
+	second := cupList[first]
+	log.Printf("Q23Part2: first=%d, second=%d, mult=%d", first, second, first*second)
+}
+
+func getNextCup(cupList []int, currCup int) int {
+	next := cupList[currCup]
+	if next == 0 {
+		return getNextCup(cupList, next)
+	}
+
+	return next
+}
+
+func hasPickedUp(pickedUpCups []int, find int) bool {
+	for _, val := range pickedUpCups {
+		if val == find {
+			return true
+		}
+	}
+
+	return false
+}
+
+func d23Solve(cupList []int, iterations int) {
+	currentCup := cupList[0]
+	pickedUpCups := make([]int, 3)
+	for move := 0; move < iterations; move++ {
+		pickedUp := getNextCup(cupList, currentCup)
+		nextCup := pickedUp
+		for i := 0; i < len(pickedUpCups); i++ {
+			pickedUpCups[i] = nextCup
+			nextCup = getNextCup(cupList, nextCup)
+		}
+
+		cupList[currentCup] = nextCup
+
+		destinationCup := currentCup - 1
+		for hasPickedUp(pickedUpCups, destinationCup) || destinationCup == 0 {
+			destinationCup--
+			if destinationCup <= 0 {
+				destinationCup = len(cupList) - 1
+			}
+		}
+
+		oldDestinationNext := getNextCup(cupList, destinationCup)
+		cupList[destinationCup] = pickedUp
+		cupList[pickedUpCups[len(pickedUpCups)-1]] = oldDestinationNext
+		currentCup = getNextCup(cupList, currentCup)
+	}
+}

main.go

@@ -4,4 +4,5 @@ import "aoc2020/days"
 
 func main() {
 	days.Day16()
+	days.Day23()
 }