UTF longname followup (#20115)

Marlin/src/sd/SdBaseFile.cpp

@@ -1122,45 +1122,44 @@ int8_t SdBaseFile::readDir(dir_t* dir, char* longFilename) {
       }
     }
 
-    // Return if normal file or subdirectory
+    // Post-process normal file or subdirectory longname, if any
     if (DIR_IS_FILE_OR_SUBDIR(dir)) {
       #if ENABLED(UTF_FILENAME_SUPPORT)
-        // Convert filename from utf-16 to utf-8 as Marlin expects
         #if LONG_FILENAME_CHARSIZE > 2
           // Add warning for developers for currently not supported 3-byte cases (Conversion series of 2-byte
           // codepoints to 3-byte in-place will break the rest of filename)
           #error "Currently filename re-encoding is done in-place. It may break the remaining chars to use 3-byte codepoints."
         #endif
-        uint16_t currentPos = 0;
-        LOOP_L_N(i, (LONG_FILENAME_LENGTH / 2)) {
-          uint16_t idx = i * 2; // This is fixed as FAT LFN always contain UTF-16LE encoding
 
+        // Is there a long filename to decode?
+        if (longFilename) {
+          // Reset n to the start of the long name
+          n = 0;
+          for (uint16_t idx = 0; idx < (LONG_FILENAME_LENGTH) / 2; idx += 2) {    // idx is fixed since FAT LFN always contains UTF-16LE encoding
             uint16_t utf16_ch = longFilename[idx] | (longFilename[idx + 1] << 8);
             if (0xD800 == (utf16_ch & 0xF800))                                    // Surrogate pair - encode as '_'
-            longFilename[currentPos++] = '_';
-          else if (0 == (utf16_ch & 0xFF80))                                    // Encode as 1-byte utf-8 char
-            longFilename[currentPos++] = utf16_ch & 0x007F;
-          else if (0 == (utf16_ch & 0xF800)) {                                  // Encode as 2-byte utf-8 char
-            longFilename[currentPos++] = 0xC0 | ((utf16_ch >> 6) & 0x1F);
-            longFilename[currentPos++] = 0x80 | (utf16_ch & 0x3F);
+              longFilename[n++] = '_';
+            else if (0 == (utf16_ch & 0xFF80))                                    // Encode as 1-byte UTF-8 char
+              longFilename[n++] = utf16_ch & 0x007F;
+            else if (0 == (utf16_ch & 0xF800)) {                                  // Encode as 2-byte UTF-8 char
+              longFilename[n++] = 0xC0 | ((utf16_ch >> 6) & 0x1F);
+              longFilename[n++] = 0x80 | ( utf16_ch       & 0x3F);
             }
             else {
-            #if LONG_FILENAME_CHARSIZE > 2                                      // Encode as 3-byte utf-8 char
-              longFilename[currentPos++] = 0xE0 | ((utf16_ch >> 12) & 0x0F);
-              longFilename[currentPos++] = 0xC0 | ((utf16_ch >> 6) & 0x3F);
-              longFilename[currentPos++] = 0xC0 | (utf16_ch & 0x3F);
+              #if LONG_FILENAME_CHARSIZE > 2                                      // Encode as 3-byte UTF-8 char
+                longFilename[n++] = 0xE0 | ((utf16_ch >> 12) & 0x0F);
+                longFilename[n++] = 0xC0 | ((utf16_ch >>  6) & 0x3F);
+                longFilename[n++] = 0xC0 | ( utf16_ch        & 0x3F);
               #else                                                               // Encode as '_'
-              longFilename[currentPos++] = '_';
+                longFilename[n++] = '_';
               #endif
             }
-
             if (0 == utf16_ch) break; // End of filename
-        }
-        return currentPos;
-      #else
-        return n;
+          } // idx
+        } // longFilename
       #endif
-    }
+      return n;
+    } // DIR_IS_FILE_OR_SUBDIR
   }
 }