Fix SeekEnd direction and ReadInt24 sign extension

binary.go

@@ -329,7 +329,7 @@ func (r *BinaryReader) Seek(off int64, whence int) (int64, error) {
 		if off < -r.f.Len() || 0 < off {
 			return 0, fmt.Errorf("invalid offset")
 		}
-		r.pos = r.f.Len() - off
+		r.pos = r.f.Len() + off
 	} else {
 		return 0, fmt.Errorf("invalid whence")
 	}
@@ -439,7 +439,8 @@ func (r *BinaryReader) ReadInt16() int16 {
 
 // ReadInt24 reads a int24 into an int32.
 func (r *BinaryReader) ReadInt24() int32 {
-	return int32(r.ReadUint24())
+	// no int24 to convert through, so sign-extend from bit 23
+	return int32(r.ReadUint24()<<8) >> 8
 }
 
 // ReadInt32 reads a int32.

binary_test.go

@@ -1,6 +1,7 @@
 package parse
 
 import (
+	"io"
 	"testing"
 
 	"github.com/tdewolff/test"
@@ -23,3 +24,30 @@ func TestBinaryReaderFullRead(t *testing.T) {
 	test.T(t, NewBinaryReaderBytes([]byte{1, 2, 3, 4}).ReadUint32(), uint32(0x01020304))
 	test.T(t, NewBinaryReaderBytes([]byte{1, 2, 3, 4, 5, 6, 7, 8}).ReadUint64(), uint64(0x0102030405060708))
 }
+
+func TestBinaryReaderSeekEnd(t *testing.T) {
+	// io.SeekEnd counts back from the end, so a negative offset moves earlier.
+	buf := []byte{1, 2, 3, 4, 5, 6, 7, 8}
+	for _, tt := range []struct {
+		off  int64
+		want int64
+	}{{0, 8}, {-1, 7}, {-4, 4}, {-8, 0}} {
+		r := NewBinaryReaderBytes(buf)
+		pos, err := r.Seek(tt.off, io.SeekEnd)
+		test.T(t, err, nil)
+		test.T(t, pos, tt.want)
+	}
+
+	r := NewBinaryReaderBytes(buf)
+	_, err := r.Seek(-4, io.SeekEnd)
+	test.T(t, err, nil)
+	test.T(t, r.ReadUint32(), uint32(0x05060708))
+}
+
+func TestBinaryReaderInt24(t *testing.T) {
+	for _, v := range []int32{-1, -2, -8388608, 0, 1, 8388607} {
+		w := NewBinaryWriter(nil)
+		w.WriteInt24(v)
+		test.T(t, NewBinaryReaderBytes(w.Bytes()).ReadInt24(), v)
+	}
+}