Also, this is what Gemini provided which looks basically correct - but I think encapsulating it with a Rewind() method would be easier to understand.
While Go doesn't have a built-in PushbackReader like some other languages (e.g., Java), you can implement similar functionality using a custom struct and a buffer. Here's an example implementation: package main import ( "bytes" "io" ) type PushbackReader struct { reader io.Reader buffer *bytes.Buffer } func NewPushbackReader(r io.Reader) *PushbackReader { return &PushbackReader{ reader: r, buffer: new(bytes.Buffer), } } func (p *PushbackReader) Read(b []byte) (n int, err error) { if p.buffer.Len() > 0 { return p.buffer.Read(b) } return p.reader.Read(b) } func (p *PushbackReader) UnreadByte() error { if p.buffer.Len() == 0 { return io.EOF } lastByte := p.buffer.Bytes()[p.buffer.Len()-1] p.buffer.Truncate(p.buffer.Len() - 1) p.buffer.WriteByte(lastByte) return nil } func (p *PushbackReader) Unread(buf []byte) error { if p.buffer.Len() == 0 { return io.EOF } p.buffer.Write(buf) return nil } func main() { // Example usage r := NewPushbackReader(bytes.NewBufferString("Hello, World!")) buf := make([]byte, 5) r.Read(buf) r.UnreadByte() r.Read(buf) } Explanation: PushbackReader struct: This struct holds the underlying io.Reader and a buffer to store the pushed-back bytes. NewPushbackReader: This function creates a new PushbackReader from an existing io.Reader. Read method: This method reads bytes from either the buffer (if it contains data) or the underlying reader. UnreadByte method: This method pushes back a single byte into the buffer. Unread method: This method pushes back a slice of bytes into the buffer. Important Considerations: The buffer size is not managed automatically. You may need to adjust the buffer size based on your use case. This implementation does not handle pushing back beyond the initially read data. If you need to support arbitrary pushback, you'll need a more complex solution. Generative AI is experimental. > On Jan 12, 2025, at 2:53 PM, Robert Engels <reng...@ix.netcom.com> wrote: > > You can see the two pass reader here > https://stackoverflow.com/questions/20666594/how-can-i-push-bytes-into-a-reader-in-go > > But yea, the basic premise is that you buffer the data so you can rewind if > needed > > Are you certain it is reading to the end to return EOF? It may be returning > eof once the parsing fails. > > Otherwise I would expect this is being decoded wrong - eg the mime type or > encoding type should tell you the correct format before you start decoding. > >> On Jan 12, 2025, at 2:46 PM, Rory Campbell-Lange <r...@campbell-lange.net> >> wrote: >> >> Thanks for the suggestion of a ReadSeeker to wrap an io.Reader. >> >> My google fu must be deserting me. I can find PushbackReader implementations >> in Java, but the only similar thing for Go I could find was >> https://gitlab.com/osaki-lab/iowrapper. If you have a specific >> recommendation for a ReadSeeker wrapper to an io.Reader that would be great >> to know. >> >> Since the base64 decoding error I'm looking for is an EOF, I guess the >> wrapper approach will not work when the EOF byte position is > than the >> io.ReadSeeker buffer size. >> >> Rory >> >> On 12/01/25, robert engels (reng...@ix.netcom.com) wrote: >>> create a ReadSeeker that wraps the Reader providing the buffering (mark & >>> reset) - normally the buffer only needs to be large enough to detect the >>> format contained in the Reader. >>> >>> You can search Google for PushbackReader in Go and you’ll get a basic >>> implementation. >>> >>>> On Jan 12, 2025, at 12:52 PM, Rory Campbell-Lange >>>> <r...@campbell-lange.net> wrote: >> ... >>>> I'm attempting to rationalise the process [of avoiding reading email parts >>>> into byte slices] by simply wrapping the provided io.Reader with the >>>> necessary decoders to reduce memory usage and unnecessary processing. >>>> >>>> The wrapping strategy seems to work ok. However there is a particular >>>> issue in detecting base64.StdEncoding versus base64.RawStdEncoding, which >>>> requires draining the io.Reader using base64.StdEncoding and (based on the >>>> current implementation) switching to base64.RawStdEncoding if an >>>> io.ErrUnexpectedEOF is found. >>>> > > > -- > You received this message because you are subscribed to the Google Groups > "golang-nuts" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to golang-nuts+unsubscr...@googlegroups.com > <mailto:golang-nuts+unsubscr...@googlegroups.com>. > To view this discussion visit > https://groups.google.com/d/msgid/golang-nuts/DD0C1480-D237-447A-B978-78FC8951FE05%40ix.netcom.com > > <https://groups.google.com/d/msgid/golang-nuts/DD0C1480-D237-447A-B978-78FC8951FE05%40ix.netcom.com?utm_medium=email&utm_source=footer>. -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. 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