Add solution day 16 part 2

Author: terminalnode

README.md

@@ -41,13 +41,14 @@ automatically rebuilt and redeployed every time the `common` module or their own
 
 ## Progress
 * ⭐ means solved
-* 🥸 means solved, but takes over a minute to run, or requires some manual tinkering
+* 🥸 means solved, but takes 10 seconds or more to run
+* 💩 means solved, but takes over a minute to run
 
 | Day | Solution | Day | Solution |
 |-----|----------|-----|----------|
 | 01  | ⭐ ⭐      | 14  | ⭐ ⭐      |
 | 02  | ⭐ ⭐      | 15  | ⭐ ⭐      |
-| 03  | ⭐ ⭐      | 16  | ⭐        |
+| 03  | ⭐ ⭐      | 16  | ⭐ 🥸     |
 | 04  | ⭐ ⭐      | 17  |          |
 | 05  | ⭐ ⭐      | 18  |          |
 | 06  | ⭐ ⭐      | 19  |          |

solutions/day16/main.go

@@ -7,7 +7,7 @@ import (
 )
 
 func main() {
-	common.Setup(16, part1, nil)
+	common.Setup(16, part1, part2)
 }
 
 func part1(
@@ -20,50 +20,84 @@ func part1(
 
 	// Initialize visited set with end point
 	set := initializeVisitedSet(i)
-	loop(initialReindeer(i.s), i, set)
+	loop(initialReindeer(i.s), i, set, initializeWinningSet(i), -1)
 
 	return fmt.Sprintf("Cheapest path: %v", lowestSoFar(i, set))
 }
 
+func part2(
+	input string,
+) string {
+	i, err := parse(input)
+	if err != nil {
+		return fmt.Sprintf("Failed to parse input: %v", err)
+	}
+
+	// Initialize visited set with end point
+	visited := initializeVisitedSet(i)
+	loop(initialReindeer(i.s), i, visited, initializeWinningSet(i), -1)
+
+	// Run again with the correct answer, which should weed out
+	// all paths that were the best before better paths were found.
+	winners := initializeWinningSet(i)
+	answer := lowestSoFar(i, visited)
+	loop(initialReindeer(i.s), i, initializeVisitedSet(i), winners, answer)
+	winners[i.e.Y][i.e.X] = true
+
+	count := 0
+	for _, ys := range winners {
+		count += len(ys)
+	}
+
+	return fmt.Sprintf("Best seats: %d", count)
+}
+
 func loop(
 	r reindeer,
 	i parsedInput,
 	set visitedSet,
-) {
-	if r.visitAndCheckIfDead(set) {
-		return
+	winners winningSet,
+	answer int,
+) bool {
+	isWinner := false
+	if r.visitAndCheckIfDead(set, answer != -1) {
+		return false
 	}
 
 	lowest := lowestSoFar(i, set)
-	if lowest != 0 && r.score > lowest {
-		return
+	if answer == -1 && lowest != 0 && r.score > lowest {
+		winners[r.p.Y][r.p.X] = true
+		return false
 	}
 
 	if r.p.Equals(i.e) {
-		return
+		if answer == -1 {
+			return true
+		} else {
+			return r.score == answer
+		}
 	}
 
 	newR, err := r.forward(i.m)
-	if err == nil {
-		loop(newR, i, set)
+	if err == nil && loop(newR, i, set, winners, answer) {
+		isWinner = true
 	}
 
 	newR, err = r.turnClockwise().forward(i.m)
-	if err == nil {
-		loop(newR, i, set)
+	if err == nil && loop(newR, i, set, winners, answer) {
+		isWinner = true
 	}
 
-	newR, err = r.turnClockwise().turnClockwise().forward(i.m)
-	if err == nil {
-		loop(newR, i, set)
+	newR, err = r.turnCounterClockwise().forward(i.m)
+	if err == nil && loop(newR, i, set, winners, answer) {
+		isWinner = true
 	}
 
-	newR, err = r.turnCounterClockwise().forward(i.m)
-	if err == nil {
-		loop(newR, i, set)
+	if isWinner {
+		winners[r.p.Y][r.p.X] = true
 	}
 
-	return
+	return isWinner
 }
 
 func lowestSoFar(
@@ -85,3 +119,13 @@ func initializeVisitedSet(
 	}
 	return set
 }
+
+func initializeWinningSet(
+	i parsedInput,
+) winningSet {
+	set := make(winningSet)
+	for y, _ := range i.m {
+		set[y] = make(map[intX]bool)
+	}
+	return set
+}

solutions/day16/reindeer.go

@@ -10,6 +10,7 @@ type intY = int
 type intDirection = int
 type intScore = int
 type visitedSet = map[intY]map[intX]intScore
+type winningSet = map[intY]map[intX]bool
 
 const (
 	North = iota
@@ -78,12 +79,19 @@ func (r reindeer) turnCounterClockwise() reindeer {
 
 func (r reindeer) visitAndCheckIfDead(
 	set visitedSet,
+	leeway bool,
 ) bool {
 	v := set[r.p.Y][r.p.X]
 	if v == 0 || r.score < v {
 		set[r.p.Y][r.p.X] = r.score
 		return false
 	}
+
+	// This leeway allows to find "temporarily suboptimal paths" when doing part 2
+	if leeway && r.score < (v+2000) {
+		return false
+	}
+
 	return true
 }