fix day 2 part 2; when in doubt, use brute force

2024/day02.go

@@ -39,19 +39,10 @@ func (d Day2) Part1(r io.Reader) string {
 	return printi(sum)
 }
 
-// part 2 does not yet work because this code makes a false assumption.
-// that assumption is that in order to fix a broken level, you can only skip the
-// one that made it broken.
-//
-// this is not true because of the following level
-// 5 6 3 2
-// 5 and 6 would be OK, so the algorithm will blame the 3 and attempt to skip it.
-// the real solution however, is to skip the 6 because it would be fine if all descending
 func (d Day2) Part2(r io.Reader) string {
 	s := bufio.NewScanner(r)
 	var sum int
 
-Scan:
 	for s.Scan() {
 
 		sz := strings.Count(s.Text(), " ") + 1
@@ -64,51 +55,15 @@ Scan:
 
 		fmt.Sscan(s.Text(), levelprts...)
 
-		increasing := level[1]-level[0] > 0
+		if layerOkWithDampen(level) {
-		var dampened bool
-		if increasing {
-			for i := 0; i < len(level)-1; i++ {
-				if diff := level[i+1] - level[i]; !inrange(diff) {
-					if dampened {
-						continue Scan
-					}
-
-					if i+2 > len(level)-1 {
-						continue Scan
-					}
-
-					d2 := level[i+2] - level[i]
-					if !inrange(d2) {
-						continue Scan
-					}
-
-					dampened = true
-					i++
-				}
-			}
 			sum++
 		} else {
-			for i := 0; i < len(level)-1; i++ {
+			slices.Reverse(level)
-				if diff := level[i] - level[i+1]; !inrange(diff) {
+			if layerOkWithDampen(level) {
-					if dampened {
+				sum++
-						continue Scan
-					}
-
-					if i+2 > len(level)-1 {
-						continue Scan
-					}
-
-					d2 := level[i] - level[i+2]
-					if !inrange(d2) {
-						continue Scan
-					}
-
-					dampened = true
-					i++
-				}
 			}
-			sum++
 		}
+
 	}
 	return printi(sum)
 }
@@ -125,3 +80,21 @@ func layerOk(layer []int) bool {
 	}
 	return true
 }
+
+func layerOkWithDampen(layer []int) bool {
+	layerc := make([]int, len(layer)-1)
+	for i := 0; i < len(layer)-1; i++ {
+		if !inrange(layer[i] - layer[i+1]) {
+			copy(layerc, layer[i:i+1])
+			for i := range layer {
+				copy(layerc, layer[:i])
+				copy(layerc[i:], layer[i+1:])
+				if layerOk(layerc) {
+					return true
+				}
+			}
+			return false
+		}
+	}
+	return true
+}