util/buffer: add new seeker implementation

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

util/buffer/seeker_buffer.go

@@ -0,0 +1,78 @@
+package buffer
+
+import "io"
+
+var _ interface {
+	io.ReadCloser
+	io.ReadSeeker
+} = (*SeekerBuffer)(nil)
+
+// Buffer is a ReadWriteCloser that supports seeking. It's intended to
+// replicate the functionality of bytes.Buffer that I use in my projects.
+//
+// Note that the seeking is limited to the read marker; all writes are
+// append-only.
+type SeekerBuffer struct {
+	data []byte
+	pos  int64
+}
+
+func NewSeekerBuffer(data []byte) *SeekerBuffer {
+	return &SeekerBuffer{
+		data: data,
+	}
+}
+
+func (b *SeekerBuffer) Read(p []byte) (int, error) {
+	if b.pos >= int64(len(b.data)) {
+		return 0, io.EOF
+	}
+
+	n := copy(p, b.data[b.pos:])
+	b.pos += int64(n)
+	return n, nil
+}
+
+func (b *SeekerBuffer) Write(p []byte) (int, error) {
+	b.data = append(b.data, p...)
+	return len(p), nil
+}
+
+// Seek sets the read pointer to pos.
+func (b *SeekerBuffer) Seek(offset int64, whence int) (int64, error) {
+	switch whence {
+	case io.SeekStart:
+		b.pos = offset
+	case io.SeekEnd:
+		b.pos = int64(len(b.data)) + offset
+	case io.SeekCurrent:
+		b.pos += offset
+	}
+
+	return b.pos, nil
+}
+
+// Rewind resets the read pointer to 0.
+func (b *SeekerBuffer) Rewind() error {
+	if _, err := b.Seek(0, io.SeekStart); err != nil {
+		return err
+	}
+	return nil
+}
+
+// Close clears all the data out of the buffer and sets the read position to 0.
+func (b *SeekerBuffer) Close() error {
+	b.data = nil
+	b.pos = 0
+	return nil
+}
+
+// Len returns the length of data remaining to be read.
+func (b *SeekerBuffer) Len() int {
+	return len(b.data[b.pos:])
+}
+
+// Bytes returns the underlying bytes from the current position.
+func (b *SeekerBuffer) Bytes() []byte {
+	return b.data[b.pos:]
+}