fix missing sign extension

3 files changed, 27 insertions, 2 deletions

diff --git a/bitreader.cc b/bitreader.cc
index dc92223..8b78bb7 100644
--- a/bitreader.cc
+++ b/bitreader.cc
@@ -18,6 +18,17 @@
 
 #include "bitreader.h"
 
+namespace
+{
+inline qint64 signExtend(quint64 x, int n)
+{
+	if (n < 64 && (x & (1 << (n-1)))) {
+		x |= -(1LL << n);
+	}
+	return x;
+}
+}
+
 BitReader::BitReader(QIODevice *device)
 	: child(device), buf(0), avail(0)
 {
@@ -38,6 +49,12 @@ quint64 BitReader::readBits(int n)
 	return x;
 }
 
+qint64 BitReader::readSignedBits(int n)
+{
+	quint64 x = readBits(n);
+	return signExtend(x, n);
+}
+
 quint64 BitReader::peekBits(int n)
 {
 	while (n > avail) {
@@ -51,6 +68,12 @@ quint64 BitReader::peekBits(int n)
 	return x;
 }
 
+qint64 BitReader::peekSignedBits(int n)
+{
+	quint64 x = peekBits(n);
+	return signExtend(x, n);
+}
+
 void BitReader::skipUntilNextByte()
 {
 	int skip = avail % 8;
diff --git a/bitreader.h b/bitreader.h
index c5ebf3f..327bc04 100644
--- a/bitreader.h
+++ b/bitreader.h
@@ -28,7 +28,9 @@ public:
 	~BitReader();
 
 	quint64 readBits(int n);
+	qint64 readSignedBits(int n);
 	quint64 peekBits(int n);
+	qint64 peekSignedBits(int n);
 
 	void skipUntilNextByte();
 
diff --git a/stfreader.cc b/stfreader.cc
index 7d78635..09967cc 100644
--- a/stfreader.cc
+++ b/stfreader.cc
@@ -130,8 +130,8 @@ bool StfReader::parseV1(BitReader& br)
 					pa = p1 - p0;
 				} else {
 					do_delta = true;
-					delta.setX(br.readBits(8));
-					delta.setY(br.readBits(8));
+					delta.setX(br.readSignedBits(8));
+					delta.setY(br.readSignedBits(8));
 				}
 			} else {
 				do_delta = true;