Allow lsDive to recurse with minimal stack usage

Marlin/SdBaseFile.cpp

   if (!isDir() || (0X1F & curPosition_)) return -1;
   
   //If we have a longFilename buffer, mark it as invalid. If we find a long filename it will be filled automaticly.
-  if (longFilename != NULL)
-  {
-  	longFilename[0] = '\0';
-  }
+  if (longFilename != NULL) longFilename[0] = '\0';
 
   while (1) {
+
     n = read(dir, sizeof(dir_t));
     if (n != sizeof(dir_t)) return n == 0 ? 0 : -1;
+
     // last entry if DIR_NAME_FREE
     if (dir->name[0] == DIR_NAME_FREE) return 0;
+
     // skip empty entries and entry for .  and ..
     if (dir->name[0] == DIR_NAME_DELETED || dir->name[0] == '.') continue;
-    //Fill the long filename if we have a long filename entry,
-	// long filename entries are stored before the actual filename.
-	if (DIR_IS_LONG_NAME(dir) && longFilename != NULL)
-    {
+
+    // Fill the long filename if we have a long filename entry.
+    // Long filename entries are stored before the short filename.
+    if (longFilename != NULL && DIR_IS_LONG_NAME(dir)) {
     	vfat_t *VFAT = (vfat_t*)dir;
-		//Sanity check the VFAT entry. The first cluster is always set to zero. And th esequence number should be higher then 0
-    	if (VFAT->firstClusterLow == 0 && (VFAT->sequenceNumber & 0x1F) > 0 && (VFAT->sequenceNumber & 0x1F) <= MAX_VFAT_ENTRIES)
-    	{
-			//TODO: Store the filename checksum to verify if a none-long filename aware system modified the file table.
+  		// Sanity-check the VFAT entry. The first cluster is always set to zero. And the sequence number should be higher than 0
+    	if (VFAT->firstClusterLow == 0 && (VFAT->sequenceNumber & 0x1F) > 0 && (VFAT->sequenceNumber & 0x1F) <= MAX_VFAT_ENTRIES) {
+  			// TODO: Store the filename checksum to verify if a none-long filename aware system modified the file table.
     		n = ((VFAT->sequenceNumber & 0x1F) - 1) * FILENAME_LENGTH;
-			longFilename[n+0] = VFAT->name1[0];
-			longFilename[n+1] = VFAT->name1[1];
-			longFilename[n+2] = VFAT->name1[2];
-			longFilename[n+3] = VFAT->name1[3];
-			longFilename[n+4] = VFAT->name1[4];
-			longFilename[n+5] = VFAT->name2[0];
-			longFilename[n+6] = VFAT->name2[1];
-			longFilename[n+7] = VFAT->name2[2];
-			longFilename[n+8] = VFAT->name2[3];
-			longFilename[n+9] = VFAT->name2[4];
-			longFilename[n+10] = VFAT->name2[5];
-			longFilename[n+11] = VFAT->name3[0];
-			longFilename[n+12] = VFAT->name3[1];
-			//If this VFAT entry is the last one, add a NUL terminator at the end of the string
-			if (VFAT->sequenceNumber & 0x40)
-				longFilename[n+FILENAME_LENGTH] = '\0';
-		}
+        for (uint8_t i=0; i<FILENAME_LENGTH; i++)
+          longFilename[n+i] = (i < 5) ? VFAT->name1[i] : (i < 11) ? VFAT->name2[i-5] : VFAT->name3[i-11];
+        // If this VFAT entry is the last one, add a NUL terminator at the end of the string
+        if (VFAT->sequenceNumber & 0x40) longFilename[n+FILENAME_LENGTH] = '\0';
+  		}
     }
-    // return if normal file or subdirectory
+    // Return if normal file or subdirectory
     if (DIR_IS_FILE_OR_SUBDIR(dir)) return n;
   }
 }
+
 //------------------------------------------------------------------------------
 // Read next directory entry into the cache
 // Assumes file is correctly positioned

Marlin/cardreader.cpp

   return buffer;
 }
 
+/**
+ * Dive into a folder and recurse depth-first to perform a pre-set operation lsAction:
+ *   LS_Count       - Add +1 to nrFiles for every file within the parent
+ *   LS_GetFilename - Get the filename of the file indexed by nrFiles
+ *   LS_SerialPrint - Print the full path of each file to serial output
+ */
 void CardReader::lsDive(const char *prepend, SdFile parent, const char * const match/*=NULL*/) {
   dir_t p;
   uint8_t cnt = 0;
 
+  // Read the next entry from a directory
   while (parent.readDir(p, longFilename) > 0) {
-    if (DIR_IS_SUBDIR(&p) && lsAction != LS_Count && lsAction != LS_GetFilename) { // hence LS_SerialPrint
-      char path[FILENAME_LENGTH*2];
+
+    // If the entry is a directory and the action is LS_SerialPrint
+    if (DIR_IS_SUBDIR(&p) && lsAction != LS_Count && lsAction != LS_GetFilename) {
+
+      // Allocate enough stack space for the full path to a folder
+      int len = strlen(prepend) + FILENAME_LENGTH + 1;
+      char path[len];
+
+      // Get the short name for the item, which we know is a folder
       char lfilename[FILENAME_LENGTH];
       createFilename(lfilename, p);
 
-      path[0] = 0;
-      if (prepend[0] == 0) strcat(path, "/"); //avoid leading / if already in prepend
+      // Append the FOLDERNAME12/ to the passed string.
+      // It contains the full path to the "parent" argument.
+      // We now have the full path to the item in this folder.
+      path[0] = '\0';
+      if (prepend[0] == '\0') strcat(path, "/"); // a root slash if prepend is empty
       strcat(path, prepend);
       strcat(path, lfilename);
       strcat(path, "/");
 
-      //Serial.print(path);
+      // Serial.print(path);
 
+      // Get a new directory object using the full path
+      // and dive recursively into it.
       SdFile dir;
       if (!dir.open(parent, lfilename, O_READ)) {
         if (lsAction == LS_SerialPrint) {
         }
       }
       lsDive(path, dir);
-      //close done automatically by destructor of SdFile
+      // close() is done automatically by destructor of SdFile
     }
     else {
       char pn0 = p.name[0];
       if (pn0 == DIR_NAME_FREE) break;
       if (pn0 == DIR_NAME_DELETED || pn0 == '.') continue;
-      char lf0 = longFilename[0];
-      if (lf0 == '.') continue;
+      if (longFilename[0] == '.') continue;
 
       if (!DIR_IS_FILE_OR_SUBDIR(&p)) continue;
 
 
       if (!filenameIsDir && (p.name[8] != 'G' || p.name[9] == '~')) continue;
 
-      //if (cnt++ != nr) continue;
-      createFilename(filename, p);
-      if (lsAction == LS_SerialPrint) {
-        SERIAL_PROTOCOL(prepend);
-        SERIAL_PROTOCOLLN(filename);
-      }
-      else if (lsAction == LS_Count) {
-        nrFiles++;
-      }
-      else if (lsAction == LS_GetFilename) {
-        if (match != NULL) {
-          if (strcasecmp(match, filename) == 0) return;
-        }
-        else if (cnt == nrFiles) return;
-        cnt++;
+      switch (lsAction) {
+        case LS_Count:
+          nrFiles++;
+          break;
+        case LS_SerialPrint:
+          createFilename(filename, p);
+          SERIAL_PROTOCOL(prepend);
+          SERIAL_PROTOCOLLN(filename);
+          break;
+        case LS_GetFilename:
+          createFilename(filename, p);
+          if (match != NULL) {
+            if (strcasecmp(match, filename) == 0) return;
+          }
+          else if (cnt == nrFiles) return;
+          cnt++;
+          break;
       }
+
     }
-  }
+  } // while readDir
 }
 
 void CardReader::ls()  {