fix(pdfcpu): correct sorting for the output paths of the split method

pkg/gotenberg/sort.go

@@ -7,7 +7,8 @@ import (
 )
 
 // AlphanumericSort implements sort.Interface and helps to sort strings
-// alphanumerically.
+// alphanumerically by either a numeric prefix or, if missing, a numeric
+// suffix.
 //
 // See: https://github.com/gotenberg/gotenberg/issues/805.
 type AlphanumericSort []string
@@ -21,20 +22,20 @@ func (s AlphanumericSort) Swap(i, j int) {
 }
 
 func (s AlphanumericSort) Less(i, j int) bool {
-	numI, restI := extractPrefix(s[i])
-	numJ, restJ := extractPrefix(s[j])
+	numI, restI := extractNumber(s[i])
+	numJ, restJ := extractNumber(s[j])
 
-	// Compares numerical prefixes if they exist.
+	// If both strings contain a number, compare them numerically.
 	if numI != -1 && numJ != -1 {
 		if numI != numJ {
 			return numI < numJ
 		}
-		// If numbers are equal, falls back to string comparison of the rest.
+		// If the numbers are equal, compare the "rest" strings.
 		return restI < restJ
 	}
 
-	// If one has a numerical prefix and the other doesn't, the one with the
-	// number comes first.
+	// If one contains a number and the other doesn't, the one with the number
+	// comes first.
 	if numI != -1 {
 		return true
 	}
@@ -42,28 +43,52 @@ func (s AlphanumericSort) Less(i, j int) bool {
 		return false
 	}
 
-	// If neither has a numerical prefix, compare as strings
+	// Neither has a number; fall back to lexicographical order.
 	return s[i] < s[j]
 }
 
-// extractPrefix attempts to extract a numerical prefix and the rest of the filename
-func extractPrefix(filename string) (int, string) {
-	matches := numPrefixRegexp.FindStringSubmatch(filename)
-	if len(matches) > 2 {
-		prefix, err := strconv.Atoi(matches[1])
-		if err == nil {
-			return prefix, matches[2]
+// extractNumber attempts to extract a numeric portion from the filename.
+// It first checks for a numeric prefix (digits at the beginning).
+// If none is found, it next attempts to match a number immediately before the
+// extension (for filenames such as "sample1_1.pdf").
+// If that fails, it then attempts a trailing numeric pattern.
+// If no number is found, it returns -1 and the original string.
+func extractNumber(str string) (int, string) {
+	// Check for a numeric prefix.
+	if matches := prefixRegexp.FindStringSubmatch(str); len(matches) > 2 {
+		if num, err := strconv.Atoi(matches[1]); err == nil {
+			return num, matches[2]
 		}
 	}
 
-	// Returns -1 if no numerical prefix is found, indicating to just compare
-	// as strings.
-	return -1, filename
+	// Check for a number immediately before an extension.
+	if matches := extensionSuffixRegexp.FindStringSubmatch(str); len(matches) > 3 {
+		if num, err := strconv.Atoi(matches[2]); err == nil {
+			// Remove the numeric block but keep the extension.
+			return num, matches[1] + matches[3]
+		}
+	}
+
+	// Check for a trailing number (with no extension following).
+	if matches := suffixRegexp.FindStringSubmatch(str); len(matches) > 2 {
+		if num, err := strconv.Atoi(matches[2]); err == nil {
+			return num, matches[1]
+		}
+	}
+
+	// No numeric portion found.
+	return -1, str
 }
 
-var numPrefixRegexp = regexp.MustCompile(`^(\d+)(.*)$`)
+// Regular expressions used by extractNumber.
+var (
+	// Matches a numeric prefix: one or more digits at the start.
+	prefixRegexp = regexp.MustCompile(`^(\d+)(.*)$`)
+	// Matches a numeric block immediately before a file extension.
+	extensionSuffixRegexp = regexp.MustCompile(`^(.*?)(\d+)(\.[^.]+)$`)
+	// Matches a trailing numeric sequence when there is no extension.
+	suffixRegexp = regexp.MustCompile(`^(.*?)(\d+)$`)
+)
 
 // Interface guard.
-var (
-	_ sort.Interface = (*AlphanumericSort)(nil)
-)
+var _ sort.Interface = (*AlphanumericSort)(nil)