78 lines
1.5 KiB
Go
78 lines
1.5 KiB
Go
package pngmeta
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import (
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"encoding/binary"
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"errors"
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"hash"
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"hash/crc32"
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"io"
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)
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var (
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ErrCRC32Mismatch = errors.New("crc32 mismatch")
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ErrNotPNG = errors.New("not png")
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ErrBadLength = errors.New("bad length")
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)
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const header = "\x89PNG\r\n\x1a\n"
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type PngChunk struct {
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typ string
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length int32
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r io.Reader
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realR io.Reader
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checksummer hash.Hash32
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}
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func (c *PngChunk) Read(p []byte) (int, error) {
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return io.TeeReader(c.r, c.checksummer).Read(p)
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}
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func (c *PngChunk) Close() error {
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var crc32 uint32
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if err := binary.Read(c.realR, binary.BigEndian, &crc32); err != nil {
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return err
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}
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if crc32 != c.checksummer.Sum32() {
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return ErrCRC32Mismatch
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}
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return nil
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}
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func (c *PngChunk) Type() string {
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return c.typ
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}
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type Reader struct {
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r io.Reader
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}
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func NewPNGStepReader(r io.Reader) (*Reader, error) {
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expectedHeader := make([]byte, len(header))
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if _, err := io.ReadFull(r, expectedHeader); err != nil {
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return nil, err
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}
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if string(expectedHeader) != header {
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return nil, ErrNotPNG
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}
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return &Reader{r}, nil
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}
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func (r *Reader) Next() (*PngChunk, error) {
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var length int32
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if err := binary.Read(r.r, binary.BigEndian, &length); err != nil {
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return nil, err
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}
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if length < 0 {
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return nil, ErrBadLength
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}
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var rawTyp [4]byte
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if _, err := io.ReadFull(r.r, rawTyp[:]); err != nil {
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return nil, err
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}
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typ := string(rawTyp[:])
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checksummer := crc32.NewIEEE()
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checksummer.Write([]byte(typ))
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return &PngChunk{typ, length, io.LimitReader(r.r, int64(length)), r.r, checksummer}, nil
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}
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