day20: ignore walls in pathsearch

this was the missing piece, solution is now right

Author: fxnn

day20/main.go

@@ -91,9 +91,9 @@ type cheat struct {
 	end   util.Point
 }
 
-func findCheats(course [][]byte, moves []util.Point, startIdx int, minSaving int, debug bool) []cheat {
+func findCheats(course [][]byte, moves []util.Point, startIdx int, minSaving int, debug bool) map[cheat]int {
 	var s = moves[startIdx]
-	var cheats []cheat
+	var cheats = make(map[cheat]int)
 	var visited = make(map[util.Point]int)
 	var recents = make(map[util.Point]int)
 	recents[s] = 0
@@ -111,9 +111,9 @@ func findCheats(course [][]byte, moves []util.Point, startIdx int, minSaving int
 				if _, ok := visited[p2]; ok {
 					continue
 				}
+				visited[p2] = cost + 1
+				found[p2] = cost + 1
 				if course[p2.Y][p2.X] == WALL {
-					visited[p2] = cost + 1
-					found[p2] = cost + 1
 					continue
 				}
 				var endIdx = findMoveIdx(moves, p2)
@@ -124,7 +124,7 @@ func findCheats(course [][]byte, moves []util.Point, startIdx int, minSaving int
 				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})
+				cheats[cheat{start: s, end: p2}] = saving
 			}
 		}
 		recents = found
@@ -155,15 +155,29 @@ func main() {
 	var time = len(moves) - 1
 	fmt.Printf("course takes %d picoseconds\n", time)
 
-	var cheats = make(map[cheat]util.Void)
+	var cheats = make(map[cheat]int)
 	for startIdx := range moves {
-		for _, c := range findCheats(course, moves, startIdx, *minSaving, *debug) {
-			cheats[c] = util.Void{}
+		for c, s := range findCheats(course, moves, startIdx, *minSaving, *debug) {
+			if so, ok := cheats[c]; !ok || so < s {
+				cheats[c] = s
+			}
 		}
 		if startIdx%100 == 0 {
 			fmt.Printf("searched cheats for %d/%d moves\n", startIdx+1, len(moves))
 		}
 	}
 
+	var savingCheats = make(map[int]int)
+	for _, s := range cheats {
+		if n, ok := savingCheats[s]; ok {
+			savingCheats[s] = n + 1
+		} else {
+			savingCheats[s] = 1
+		}
+	}
+	for s, n := range savingCheats {
+		fmt.Printf("found %d cheats that save %d picoseconds\n", n, s)
+	}
+
 	fmt.Printf("found %d unique cheats saving >= %d picoseconds", len(cheats), *minSaving)
 }