From bcc5488cbcddc8c3d7125fb59ac7edc054673ebc Mon Sep 17 00:00:00 2001
From: PaulStoffregen <paul@pjrc.com>
Date: Fri, 18 Jul 2014 07:01:26 -0700
Subject: Add availableForWrite() to HardwareSerial

---
 cores/arduino/HardwareSerial.cpp | 15 +++++++++++++++
 cores/arduino/HardwareSerial.h   |  1 +
 2 files changed, 16 insertions(+)

diff --git a/cores/arduino/HardwareSerial.cpp b/cores/arduino/HardwareSerial.cpp
index ed29641..29a3366 100644
--- a/cores/arduino/HardwareSerial.cpp
+++ b/cores/arduino/HardwareSerial.cpp
@@ -176,6 +176,21 @@ int HardwareSerial::read(void)
   }
 }
 
+int HardwareSerial::availableForWrite(void)
+{
+#if (SERIAL_TX_BUFFER_SIZE>256)
+  uint8_t oldSREG = SREG;
+  cli();
+#endif
+  tx_buffer_index_t head = _tx_buffer_head;
+  tx_buffer_index_t tail = _tx_buffer_tail;
+#if (SERIAL_TX_BUFFER_SIZE>256)
+  SREG = oldSREG;
+#endif
+  if (head >= tail) return SERIAL_TX_BUFFER_SIZE - 1 - head + tail;
+  return tail - head - 1;
+}
+
 void HardwareSerial::flush()
 {
   // If we have never written a byte, no need to flush. This special
diff --git a/cores/arduino/HardwareSerial.h b/cores/arduino/HardwareSerial.h
index b96e5d0..935934b 100644
--- a/cores/arduino/HardwareSerial.h
+++ b/cores/arduino/HardwareSerial.h
@@ -112,6 +112,7 @@ class HardwareSerial : public Stream
     virtual int available(void);
     virtual int peek(void);
     virtual int read(void);
+    int availableForWrite(void);
     virtual void flush(void);
     virtual size_t write(uint8_t);
     inline size_t write(unsigned long n) { return write((uint8_t)n); }
-- 
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