feat: add Seek() method to Decoder

add new `Seek()` method (satisfying `io.Seeker`) to `Decoder` with associated tests
also add `Mode()` to expose the read mode (fast vs safe) to consumers

Issue: #136

Author: Dylan Bourque

decoder.go

@@ -66,11 +66,44 @@ func NewDecoder(p []byte) *Decoder {
 	}
 }
 
+// Mode returns the current decoding mode, safe vs fastest.
+func (d *Decoder) Mode() DecoderMode {
+	return d.mode
+}
+
 // SetMode configures the decoding behavior, safe vs fastest.
 func (d *Decoder) SetMode(m DecoderMode) {
 	d.mode = m
 }
 
+// Seek sets the position of the next read operation to [offset], interpreted according to [whence]:
+// [io.SeekStart] means relative to the start of the data, [io.SeekCurrent] means relative to the
+// current offset, and [io.SeekEnd] means relative to the end.
+//
+// This low-level operation is provided to support advanced/custom usages of the decoder and it is up
+// to the caller to ensure that the resulting offset will point to a valid location in the data stream.
+func (d *Decoder) Seek(offset int64, whence int) (int64, error) {
+	pos := int(offset)
+	switch whence {
+	case io.SeekStart:
+		// no adjustment needed
+	case io.SeekCurrent:
+		// shift relative to current read offset
+		pos += d.offset
+	case io.SeekEnd:
+		// shift relative to EOF
+		pos += len(d.p)
+	default:
+		return int64(d.offset), fmt.Errorf("invalid value (%d) for whence", whence)
+	}
+	// verify bounds then update the read position
+	if pos < 0 || pos > len(d.p) {
+		return int64(d.offset), fmt.Errorf("seek position (%d) out of bounds", pos)
+	}
+	d.offset = pos
+	return int64(d.offset), nil
+}
+
 // Reset moves the read offset back to the beginning of the encoded data
 func (d *Decoder) Reset() {
 	d.offset = 0

decoder_test.go

@@ -11,6 +11,151 @@ import (
 	"github.com/CrowdStrike/csproto"
 )
 
+func TestDecoderSeek(t *testing.T) {
+	testData := []byte{0x08, 0x01, 0x10, 0x00, 0x1A, 0xE, 0x74, 0x68, 0x69, 0x73, 0x20, 0x69, 0x73, 0x20, 0x61, 0x20, 0x74, 0x65, 0x73, 0x74}
+	dec := csproto.NewDecoder(testData)
+
+	t.Run("seek start", func(t *testing.T) {
+		t.Run("negative seek", func(t *testing.T) {
+			dec.Reset()
+			_, _, _ = dec.DecodeTag()
+			_, _ = dec.DecodeBool()
+
+			startPos := int64(dec.Offset())
+			pos, err := dec.Seek(-1, io.SeekStart)
+			assert.Error(t, err, "cannot seek to before BOF")
+			assert.Equal(t, startPos, pos, "read position should not change")
+		})
+		t.Run("zero seek", func(t *testing.T) {
+			dec.Reset()
+			_, _, _ = dec.DecodeTag()
+			_, _ = dec.DecodeBool()
+
+			pos, err := dec.Seek(0, io.SeekStart)
+			assert.NoError(t, err)
+			assert.Equal(t, int64(0), pos, "new read position should be BOF")
+		})
+		t.Run("invalid positive seek", func(t *testing.T) {
+			dec.Reset()
+			_, _, _ = dec.DecodeTag()
+			_, _ = dec.DecodeBool()
+
+			startPos := int64(dec.Offset())
+			pos, err := dec.Seek(int64(len(testData)+1), io.SeekStart)
+			assert.Error(t, err, "cannot seek to after EOF")
+			assert.Equal(t, startPos, pos, "read position should not change")
+		})
+		t.Run("valid positive seek", func(t *testing.T) {
+			dec.Reset()
+
+			pos, err := dec.Seek(2, io.SeekStart)
+			assert.NoError(t, err)
+			assert.Equal(t, int(pos), dec.Offset())
+			assert.True(t, dec.More())
+		})
+	})
+	t.Run("seek current", func(t *testing.T) {
+		t.Run("invalid negative seek", func(t *testing.T) {
+			dec.Reset()
+			_, _, _ = dec.DecodeTag()
+			_, _ = dec.DecodeBool()
+
+			startPos := int64(dec.Offset())
+			pos, err := dec.Seek(-1*(startPos+1), io.SeekCurrent)
+			assert.Error(t, err, "cannot seek to before BOF")
+			assert.Equal(t, startPos, pos, "read position should not change")
+		})
+		t.Run("valid negative seek", func(t *testing.T) {
+			dec.Reset()
+			_, _, _ = dec.DecodeTag()
+			_, _ = dec.DecodeBool()
+
+			startPos := int64(dec.Offset())
+			pos, err := dec.Seek(-1*startPos, io.SeekCurrent)
+			assert.NoError(t, err)
+			assert.Equal(t, int64(0), pos)
+		})
+		t.Run("zero seek", func(t *testing.T) {
+			dec.Reset()
+			_, _, _ = dec.DecodeTag()
+			_, _ = dec.DecodeBool()
+
+			startPos := int64(dec.Offset())
+			pos, err := dec.Seek(0, io.SeekCurrent)
+			assert.NoError(t, err)
+			assert.Equal(t, int64(startPos), pos)
+		})
+		t.Run("invalid positive seek", func(t *testing.T) {
+			dec.Reset()
+			_, _, _ = dec.DecodeTag()
+			_, _ = dec.DecodeBool()
+
+			startPos := int64(dec.Offset())
+			pos, err := dec.Seek(int64(len(testData)), io.SeekCurrent)
+			assert.Error(t, err, "cannot seek to after EOF")
+			assert.Equal(t, startPos, pos, "read position should not change")
+		})
+		t.Run("valid positive seek", func(t *testing.T) {
+			dec.Reset()
+			_, _, _ = dec.DecodeTag()
+			_, _ = dec.DecodeBool()
+
+			pos, err := dec.Seek(2, io.SeekCurrent)
+			assert.NoError(t, err)
+			assert.Equal(t, int(pos), dec.Offset())
+			assert.True(t, dec.More())
+		})
+	})
+	t.Run("seek end", func(t *testing.T) {
+		t.Run("positive seek", func(t *testing.T) {
+			dec.Reset()
+			_, _, _ = dec.DecodeTag()
+			_, _ = dec.DecodeBool()
+			startPos := dec.Offset()
+
+			pos, err := dec.Seek(1, io.SeekEnd)
+			assert.Error(t, err, "cannot seek to after EOF")
+			assert.Equal(t, int64(startPos), pos, "read position should not change")
+		})
+		t.Run("zero seek", func(t *testing.T) {
+			dec.Reset()
+			_, _, _ = dec.DecodeTag()
+			_, _ = dec.DecodeBool()
+
+			pos, err := dec.Seek(0, io.SeekEnd)
+			assert.NoError(t, err)
+			assert.Equal(t, int64(len(testData)), pos, "read position should be at EOF")
+			assert.False(t, dec.More())
+		})
+		t.Run("invalid negative seek", func(t *testing.T) {
+			dec.Reset()
+			_, _, _ = dec.DecodeTag()
+			_, _ = dec.DecodeBool()
+			startPos := dec.Offset()
+
+			pos, err := dec.Seek(int64(-1*(len(testData)+1)), io.SeekEnd)
+			assert.Error(t, err, "cannot seek to before BOF")
+			assert.Equal(t, int64(startPos), pos, "read position should be at EOF")
+		})
+		t.Run("valid negative seek", func(t *testing.T) {
+			dec.Reset()
+			_, _, _ = dec.DecodeTag()
+			_, _ = dec.DecodeBool()
+
+			pos, err := dec.Seek(-16, io.SeekEnd)
+			assert.NoError(t, err)
+			assert.Equal(t, int64(4), pos, "read position should be at EOF")
+		})
+	})
+	t.Run("invalid whence", func(t *testing.T) {
+		dec.Reset()
+		startPos := dec.Offset()
+		pos, err := dec.Seek(0, 1138)
+		assert.Error(t, err, "cannot seek with invalid 'whence'")
+		assert.Equal(t, int64(startPos), pos, "read position should not change")
+	})
+}
+
 func TestDecodeBool(t *testing.T) {
 	cases := []struct {
 		name     string