added txspace() method to FastSerial

this returns the number of bytes available in the transmit buffer

git-svn-id: https://arducopter.googlecode.com/svn/trunk@3245 f9c3cf11-9bcb-44bc-f272-b75c42450872

libraries/FastSerial/FastSerial.cpp

@@ -149,6 +149,13 @@ int FastSerial::space(void)
 	return (((_rxBuffer->tail - _rxBuffer->head) & _rxBuffer->mask) - 1);
 }
 
+int FastSerial::txspace(void)
+{
+	if (!_open)
+		return (-1);
+	return ((_txBuffer->mask+1) - ((_txBuffer->head - _txBuffer->tail) & _txBuffer->mask));
+}
+
 int FastSerial::read(void)
 {
 	uint8_t c;

libraries/FastSerial/FastSerial.h

@@ -111,6 +111,7 @@ public:
 	virtual void end(void);
 	virtual int available(void);
 	virtual int space(void);
+	virtual int txspace(void);
 	virtual int read(void);
 	virtual int peek(void);
 	virtual void flush(void);