RingStream RAM saver

RingStream.cpp

@@ -18,9 +18,17 @@
  *  along with CommandStation.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+
+// NOTE: The use of a marker byte without an escape algorithm means
+// RingStream is unsuitable for binary data. Should binary data need to be 
+// streamed it will be necessary to implementr an escape strategy to handle the 
+// marker char when embedded in data. 
+
 #include "RingStream.h"
 #include "DIAG.h"
 
+const byte FLASH_INSERT_MARKER=0xff;
+
 RingStream::RingStream( const uint16_t len)
 {
   _len=len;
@@ -31,6 +39,7 @@ RingStream::RingStream( const uint16_t len)
   _overflow=false;
   _mark=0;
   _count=0; 
+  _flashInsert=0;
 }
 
 size_t RingStream::write(uint8_t b) {
@@ -46,8 +55,74 @@ size_t RingStream::write(uint8_t b) {
   return 1;
 }
 
+// Ideally, I would prefer to override the Print:print(_FlashStringHelper) function
+// but the library authors omitted to make this virtual.
+// Therefore we obveride the only other simple function that has no side effects
+// in order that StringFormatter can recognise a RingStream and call its 
+// printFlash() directly.  
+int RingStream::availableForWrite()  { 
+  return  THIS_IS_A_RINGSTREAM; 
+  }
+
+size_t RingStream::printFlash(const FSH * flashBuffer) {
+// We are about to add a PROGMEM string to the buffer. 
+// To save RAM we can insert a marker and the
+// progmem address into the buffer instead.
+// The buffer reading code must recognise this marker and 
+// silently extract the progmem bytes.
+// In addition, we must make the count correct as if the 
+// string had been embedded so that things like the wifi code
+// can read the expected count before reading the buffer. 
+
+// Establish the actual length of the progmem string.
+char * flash=(char *)flashBuffer;
+int16_t plength=strlen_P(flash);
+if (plength==0) return 0; // just ignore empty string
+
+// Retain the buffer count as it will be modified by the marker+address insert
+int prevCount=_count;
+write(FLASH_INSERT_MARKER); // write the marker
+uintptr_t iFlash=reinterpret_cast<uintptr_t>(flash); // expect size match with pointer
+
+// write address bytes LSB first (size depends on CPU) 
+for (byte f=0;f<sizeof(iFlash); f++) {
+    write((byte) (iFlash & 0xFF));
+    iFlash>>=8;
+}
+
+// correct the buffer count to reflect the flash length, not the marker/addr.
+_count=prevCount+plength;  
+return plength;
+}
+
 int RingStream::read() {
+  if (_flashInsert) {
+    // we are reading out of a flash string 
+    byte fb=GETFLASH(_flashInsert);
+    _flashInsert++;
+    if (fb) return fb; // we have a byte from the flash
+    // flash insert complete, clear and drop through to next buffer byte
+    _flashInsert=NULL; 
+  }
   if ((_pos_read==_pos_write) && !_overflow) return -1;  // empty  
+  byte b=readRawByte();
+  if (b!=FLASH_INSERT_MARKER) return b; 
+  
+  // Detected a flash insert 
+  // read address bytes LSB first (size depends on CPU) 
+  uintptr_t iFlash=0; 
+  for (byte f=0; f<sizeof(iFlash); f++) {
+    uintptr_t bf=readRawByte();
+    bf&=0x00ff;
+    bf<<= (8*f); // shift byte to correct position in iFlash
+    iFlash |= bf;  
+  }
+  _flashInsert=reinterpret_cast<char * >( iFlash);
+  // and try again... so will read the first byte of the insert. 
+  return read();     
+}
+
+byte RingStream::readRawByte() {
   byte b=_buffer[_pos_read];
   _pos_read++;
   if (_pos_read==_len) _pos_read=0;
@@ -55,9 +130,8 @@ int RingStream::read() {
   return b;
 }
 
-
 int RingStream::count() {
-  return (read()<<8) | read(); 
+  return (readRawByte()<<8) | readRawByte(); 
   }
 
 int RingStream::freeSpace() {
@@ -83,6 +157,7 @@ uint8_t RingStream::peekTargetMark() {
 }
 
 bool RingStream::commit() {
+  _flashInsert=NULL; // prepared for first read
   if (_overflow) {
         DIAG(F("RingStream(%d) commit(%d) OVERFLOW"),_len, _count);
         // just throw it away 
@@ -108,8 +183,7 @@ void RingStream::flush() {
   _pos_write=0;
   _pos_read=0;
   _buffer[0]=0;
+  _flashInsert=NULL; // prepared for first read
+  
 }
-void RingStream::printBuffer(Print * stream) {
-  _buffer[_pos_write]='\0';
-  stream->print((char *)_buffer);
-}
+  

RingStream.h

@@ -21,22 +21,32 @@
  */
 
 #include <Arduino.h>
+#include "FSH.h"
   
 class RingStream : public Print {
 
   public:
     RingStream( const uint16_t len);
-  
+    static const int THIS_IS_A_RINGSTREAM=77;
     virtual size_t write(uint8_t b);
+
+    // This availableForWrite function is subverted from its original intention so that a caller 
+    // can destinguish between a normal stream and a RingStream. 
+    // The Arduino compiler does not support runtime dynamic cast to perform
+    // an instranceOf check. 
+    // This is necessary since the Print functions are mostly not virtual so 
+    // we cant override the print(__FlashStringHelper *) function.
+   virtual int availableForWrite() override;
     using Print::write;
+    size_t printFlash(const FSH * flashBuffer);
     int read();
     int count();
     int freeSpace();
     void mark(uint8_t b);
     bool commit();
     uint8_t peekTargetMark();
-    void printBuffer(Print * streamer);
     void flush();
+    byte readRawByte();
  private:
    int _len;
    int _pos_write;
@@ -45,6 +55,7 @@ class RingStream : public Print {
    int _mark;
    int _count;
    byte * _buffer;
+   char * _flashInsert;
 };
 
 #endif

StringFormatter.cpp

@@ -97,7 +97,21 @@ void StringFormatter::send2(Print * stream,const FSH* format, va_list args) {
       case 's': stream->print(va_arg(args, char*)); break;
       case 'e': printEscapes(stream,va_arg(args, char*)); break;
       case 'E': printEscapes(stream,(const FSH*)va_arg(args, char*)); break;
-      case 'S': stream->print((const FSH*)va_arg(args, char*)); break;
+      case 'S':
+      { 
+        const FSH*  flash= (const FSH*)va_arg(args, char*);
+
+#if WIFI_ON | ETHERNET_ON
+        // RingStream has special logic to handle flash strings
+        // but is not implemented unless wifi or ethernet are enabled.
+        // The define prevents RingStream code being added unnecessariliy.        
+        if (stream->availableForWrite()==RingStream::THIS_IS_A_RINGSTREAM)
+              ((RingStream *)stream)->printFlash(flash);
+              else 
+#endif
+        stream->print(flash);
+        break;
+             }
       case 'd': printPadded(stream,va_arg(args, int), formatWidth, formatLeft); break;
       case 'u': printPadded(stream,va_arg(args, unsigned int), formatWidth, formatLeft); break;
       case 'l': printPadded(stream,va_arg(args, long), formatWidth, formatLeft); break;

StringFormatter.h

@@ -20,6 +20,8 @@
 #define StringFormatter_h
 #include <Arduino.h>
 #include "FSH.h"
+#include "RingStream.h"
+ 
 #if defined(ARDUINO_ARCH_SAMD)
    // Some processors use a gcc compiler that renames va_list!!!
   #include <cstdarg>