AUDIO: Forbid adding timestamps with differing framerate

We used to allow this, but the result is a timestamp with a framerate
potentially different from that of both original timestamps, which can
lead to completely unexpected behavior. For example, consider this code
snippet:
  a = a + b;
  a = a.addFrames(1);  // frame rate changed!

svn-id: r55840

2 files changed, 11 insertions, 35 deletions

diff --git a/sound/timestamp.cpp b/sound/timestamp.cpp
index 1eb5483fc2..1e49e1b476 100644
--- a/sound/timestamp.cpp
+++ b/sound/timestamp.cpp
@@ -148,21 +148,9 @@ Timestamp Timestamp::addMsecs(int ms) const {
 }
 
 void Timestamp::addIntern(const Timestamp &ts) {
+	assert(_framerate == ts._framerate);
 	_secs += ts._secs;
-
-	if (_framerate == ts._framerate) {
-		_numFrames += ts._numFrames;
-	} else {
-		// We need to multiply by the quotient of the two framerates.
-		// We cancel the GCD in this fraction to reduce the risk of
-		// overflows.
-		const uint g = Common::gcd(_framerate, ts._framerate);
-		const uint p = _framerate / g;
-		const uint q = ts._framerate / g;
-
-		_framerate *= q;
-		_numFrames = _numFrames * q + ts._numFrames * p;
-	}
+	_numFrames += ts._numFrames;
 
 	normalize();
 }
@@ -187,24 +175,6 @@ Timestamp Timestamp::operator-(const Timestamp &ts) const {
 	return result;
 }
 
-/*
-Timestamp &Timestamp::operator+=(const Timestamp &ts) {
-	addIntern(ts);
-	return *this;
-}
-
-Timestamp &Timestamp::operator-=(const Timestamp &ts) {
-	addIntern(-ts);
-	return *this;
-}
-*/
-
-/*
-int Timestamp::frameDiff(const Timestamp &ts) const {
-	return (*this - ts).totalNumberOfFrames();
-}
-*/
-
 int Timestamp::frameDiff(const Timestamp &ts) const {
 
 	int delta = 0;
diff --git a/sound/timestamp.h b/sound/timestamp.h
index 410445eccd..fef4107535 100644
--- a/sound/timestamp.h
+++ b/sound/timestamp.h
@@ -126,11 +126,17 @@ public:
 	// unary minus
 	Timestamp operator-() const;
 
+	/**
+	 * Compute the sum of two timestamps. This is only
+	 * allowed if they use the same framerate.
+	 */
 	Timestamp operator+(const Timestamp &ts) const;
-	Timestamp operator-(const Timestamp &ts) const;
 
-//	Timestamp &operator+=(const Timestamp &ts);
-//	Timestamp &operator-=(const Timestamp &ts);
+	/**
+	 * Compute the difference between two timestamps. This is only
+	 * allowed if they use the same framerate.
+	 */
+	Timestamp operator-(const Timestamp &ts) const;
 
 	/**
 	 * Computes the number of frames between this timestamp and ts.