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package webp
import (
"bytes"
"errors"
"io"
"golang.org/x/image/riff"
)
var (
errInvalidHeader = errors.New("could not read an 8 byte header, sub-chunk is not valid")
)
// SubChunkReader helps in reading riff data from an existing chunk which are comprised of sub-chunks.
// A good example would be ANMF chunks of animated webp files. These chunks can contain headers, ALPH chunks
// and VP8 or VP8L chunks within the main riff data chunk.
type SubChunkReader struct {
r io.Reader
}
// Next will return the FourCC, data, and data length of a subchunk.
// The io.Reader returned for data will not be the same as the provided reader
// and is safe to discord without fully reading the contents.
// Will return an error if the format is invalid or an underlying read operation fails.
func (c SubChunkReader) Next() (riff.FourCC, io.Reader, uint32, error) {
header := make([]byte, 8)
n, err := io.ReadFull(c.r, header)
if err != nil {
if err == io.ErrUnexpectedEOF {
return riff.FourCC{}, nil, 0, errInvalidHeader
}
return riff.FourCC{}, nil, 0, err
}
if n != 8 {
return riff.FourCC{}, nil, 0, errInvalidHeader
}
fourCC := riff.FourCC{header[0], header[1], header[2], header[3]}
chunkLen := u32(header[4:8])
buf := make([]byte, chunkLen)
n, err = io.ReadFull(c.r, buf)
if err != nil {
if err == io.ErrUnexpectedEOF {
return riff.FourCC{}, nil, 0, errInvalidFormat
}
return riff.FourCC{}, nil, 0, err
}
if n != int(chunkLen) {
return riff.FourCC{}, nil, 0, errInvalidFormat
}
// if chunkLen was odd, we need to maintain a 2-byte boundary per RIFF spec.
// in this case read off a single byte of padding to re-align with the next
// fourCC header
if chunkLen%2 == 1 {
n, err := c.r.Read([]byte{0})
if err != nil {
return riff.FourCC{}, nil, 0, err
}
if n != 1 {
return riff.FourCC{}, nil, 0, errInvalidFormat
}
}
return fourCC, bytes.NewReader(buf), chunkLen, nil
}
func NewSubChunkReader(r io.Reader) *SubChunkReader {
return &SubChunkReader{
r: r,
}
}
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