improvements to bit reader

formats/dcc/dcc.go

@@ -88,52 +88,52 @@ func NewFromReader(reader io.ReadSeeker) (*Sprite, error) {
 func readDirectionHeader(reader io.ReadSeeker) (*directionHeader, error) {
 	r := streaming.NewBitReader(reader)
 
-	codedSize, err := r.ReadBits(32)
+	codedSize, err := r.ReadBitsUnsigned(32)
 	if err != nil {
 		return nil, err
 	}
 
-	hasRawPixelEncoding, err := r.ReadBits(1)
+	hasRawPixelEncoding, err := r.ReadBitsUnsigned(1)
 	if err != nil {
 		return nil, err
 	}
 
-	compressEqualCells, err := r.ReadBits(1)
+	compressEqualCells, err := r.ReadBitsUnsigned(1)
 	if err != nil {
 		return nil, err
 	}
 
-	variable0Bits, err := r.ReadBits(4)
+	variable0Bits, err := r.ReadBitsUnsigned(4)
 	if err != nil {
 		return nil, err
 	}
 
-	widthBits, err := r.ReadBits(4)
+	widthBits, err := r.ReadBitsUnsigned(4)
 	if err != nil {
 		return nil, err
 	}
 
-	heightBits, err := r.ReadBits(4)
+	heightBits, err := r.ReadBitsUnsigned(4)
 	if err != nil {
 		return nil, err
 	}
 
-	offsetXBits, err := r.ReadBits(4)
+	offsetXBits, err := r.ReadBitsUnsigned(4)
 	if err != nil {
 		return nil, err
 	}
 
-	offsetYBits, err := r.ReadBits(4)
+	offsetYBits, err := r.ReadBitsUnsigned(4)
 	if err != nil {
 		return nil, err
 	}
 
-	optionalBytesBits, err := r.ReadBits(4)
+	optionalBytesBits, err := r.ReadBitsUnsigned(4)
 	if err != nil {
 		return nil, err
 	}
 
-	codedBytesBits, err := r.ReadBits(4)
+	codedBytesBits, err := r.ReadBitsUnsigned(4)
 	if err != nil {
 		return nil, err
 	}

streaming/bitreader.go

@@ -15,7 +15,36 @@ func NewBitReader(reader io.Reader) *BitReader {
 	return &BitReader{reader: reader}
 }
 
-func (r *BitReader) ReadBits(count int) (uint64, error) {
+func (r *BitReader) ReadBitsSigned(count int) (int64, error) {
+	value, err := r.readBits(count)
+	if err != nil {
+		return 0, err
+	}
+
+	if count > 0 {
+		valueMasked := value &^ (1 << uint(count-1))
+		if valueMasked != value {
+			return -int64(valueMasked), nil
+		}
+	}
+
+	return int64(value), nil
+}
+
+func (r *BitReader) ReadBitsUnsigned(count int) (uint64, error) {
+	return r.readBits(count)
+}
+
+func (r *BitReader) ReadBitFlag() (bool, error) {
+	value, err := r.readBits(1)
+	if err != nil {
+		return false, err
+	}
+
+	return value == 1, nil
+}
+
+func (r *BitReader) readBits(count int) (uint64, error) {
 	if count > 64 {
 		return 0, errors.New("cannot read more than 64 bits at a time")
 	}

streaming/streaming_test.go

@@ -18,13 +18,13 @@ func TestBitReader(t *testing.T) {
 	r := NewBitReader(bytes.NewReader(data))
 
 	readPass := func(c int, v uint64) {
-		if value, err := r.ReadBits(c); value != v || err != nil {
+		if value, err := r.ReadBitsUnsigned(c); value != v || err != nil {
 			t.Fail()
 		}
 	}
 
 	readFail := func(c int) {
-		if value, err := r.ReadBits(c); value != 0 || err == nil {
+		if value, err := r.ReadBitsUnsigned(c); value != 0 || err == nil {
 			t.Fail()
 		}
 	}