day20: probably solved part two, but too slow

Author: fxnn

day20/main.go

@@ -91,60 +91,72 @@ type cheat struct {
 	end   util.Point
 }
 
-func findCheats(course [][]byte, moves []util.Point, startIdx int, minSaving int, cheatedWalls map[util.Point]util.Void) []cheat {
+func findCheats(course [][]byte, moves []util.Point, startIdx int, minSaving int, debug bool) []cheat {
 	var s = moves[startIdx]
 	var cheats []cheat
-	for _, d := range directions {
-		var w = s.Add(d)
-		if !w.IsInBounds(len(course), len(course[w.Y])) {
-			continue
-		}
-		if course[w.Y][w.X] != WALL {
-			continue
-		}
-		var e = w.Add(d)
-		var ei = findMoveIdx(moves, e)
-		// HINT: save 100 picoseconds, but this includes two additional steps
-		if ei-startIdx < minSaving+2 {
-			// TODO: might there be multiple cheats forward?
-			continue
+	var visited = make(map[util.Point]int)
+	var recents = make(map[util.Point]int)
+	recents[s] = 0
+	for len(recents) > 0 {
+		var found = make(map[util.Point]int)
+		for p1, cost := range recents {
+			for _, d := range directions {
+				var p2 = p1.Add(d)
+				if !p2.IsInBounds(len(course), len(course[p1.Y])) {
+					continue
+				}
+				if _, ok := visited[p2]; ok {
+					continue
+				}
+				if course[p2.Y][p2.X] == WALL {
+					visited[p2] = cost + 1
+					found[p2] = cost + 1
+					continue
+				}
+				var endIdx = findMoveIdx(moves, p2)
+				var saving = endIdx - startIdx - cost - 1
+				if saving < minSaving {
+					continue
+				}
+				if debug {
+					fmt.Printf("%d picoseconds saved by cheat: %d,%d -> %d,%d\n", saving, s.Y, s.X, p2.Y, p2.X)
+				}
+				cheats = append(cheats, cheat{start: s, end: p2})
+			}
 		}
-		cheatedWalls[w] = util.Void{}
-		fmt.Printf("%d picoseconds saved by cheat: %d,%d -> %d,%d\n", ei-startIdx-2, s.Y, s.X, e.Y, e.X)
-		cheats = append(cheats, cheat{start: s, end: e})
+		recents = found
 	}
-	return cheats
-}
-
-func printCourseWithCheats(course [][]byte, cheats map[util.Point]util.Void) {
-	for y := range course {
-		for x := range course[y] {
-			if _, ok := cheats[util.Point{Y: y, X: x}]; ok {
-				fmt.Print("C")
-			} else {
-				fmt.Print(string(course[y][x]))
+	if debug {
+		for y := range course {
+			for x := range course[y] {
+				if cost, ok := visited[util.Point{Y: y, X: x}]; ok {
+					fmt.Print(cost % 10)
+				} else {
+					fmt.Print(string(course[y][x]))
+				}
 			}
+			fmt.Println()
 		}
 		fmt.Println()
 	}
-	fmt.Println()
+	return cheats
 }
 
 func main() {
 	var minSaving = flag.Int("minSaving", 100, "consider cheats that save at least minSaving picoseconds")
+	var debug = flag.Bool("debug", false, "verbose output")
 	flag.Parse()
 
 	var course = readCourse()
 	var moves = findMoves(course)
 	var time = len(moves) - 1
 	fmt.Printf("course takes %d picoseconds\n", time)
 	var cheats = make(map[cheat]util.Void)
-	var cheatedWalls = make(map[util.Point]util.Void)
 	for i := range moves {
-		for _, c := range findCheats(course, moves, i, *minSaving, cheatedWalls) {
+		for _, c := range findCheats(course, moves, i, *minSaving, *debug) {
 			cheats[c] = util.Void{}
 		}
+		fmt.Printf("searched cheats for %d/%d moves\n", i+1, len(moves))
 	}
-	printCourseWithCheats(course, cheatedWalls)
 	fmt.Printf("found %d unique cheats saving >= %d picoseconds", len(cheats), *minSaving)
 }