lib/readers: add Seek method to PatternReader

lib/readers/pattern_reader_test.go

@@ -28,3 +28,61 @@ func TestPatternReader(t *testing.T) {
 	require.Equal(t, io.EOF, err)
 	require.Equal(t, 0, n)
 }
+
+func TestPatternReaderSeek(t *testing.T) {
+	r := NewPatternReader(1024)
+	b, err := ioutil.ReadAll(r)
+	require.NoError(t, err)
+
+	for i := range b {
+		assert.Equal(t, byte(i%251), b[i])
+	}
+
+	n, err := r.Seek(1, io.SeekStart)
+	require.NoError(t, err)
+	assert.Equal(t, int64(1), n)
+
+	// pos 1
+
+	b2 := make([]byte, 10)
+	nn, err := r.Read(b2)
+	require.NoError(t, err)
+	assert.Equal(t, 10, nn)
+	assert.Equal(t, b[1:11], b2)
+
+	// pos 11
+
+	n, err = r.Seek(9, io.SeekCurrent)
+	require.NoError(t, err)
+	assert.Equal(t, int64(20), n)
+
+	// pos 20
+
+	nn, err = r.Read(b2)
+	require.NoError(t, err)
+	assert.Equal(t, 10, nn)
+	assert.Equal(t, b[20:30], b2)
+
+	n, err = r.Seek(-24, io.SeekEnd)
+	require.NoError(t, err)
+	assert.Equal(t, int64(1000), n)
+
+	// pos 1000
+
+	nn, err = r.Read(b2)
+	require.NoError(t, err)
+	assert.Equal(t, 10, nn)
+	assert.Equal(t, b[1000:1010], b2)
+
+	// Now test errors
+
+	n, err = r.Seek(1, 400)
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "invalid whence")
+	assert.Equal(t, int64(0), n)
+
+	n, err = r.Seek(-1, io.SeekStart)
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "negative position")
+	assert.Equal(t, int64(0), n)
+}