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/*
exif-terminator
Copyright (C) 2022 SuperSeriousBusiness admin@gotosocial.org
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Affero General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU Affero General Public License for more details.
You should have received a copy of the GNU Affero General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
package terminator
import (
"io"
pngstructure "github.com/superseriousbusiness/go-png-image-structure/v2"
)
type pngVisitor struct {
ps *pngstructure.PngSplitter
writer io.Writer
lastWrittenChunk int
}
func (v *pngVisitor) split(data []byte, atEOF bool) (int, []byte, error) {
// execute the ps split function to read in data
advance, token, err := v.ps.Split(data, atEOF)
if err != nil {
return advance, token, err
}
// if we haven't written anything at all yet, then write the png header back into the writer first
if v.lastWrittenChunk == -1 {
if _, err := v.writer.Write(pngstructure.PngSignature[:]); err != nil {
return advance, token, err
}
}
// Check if the splitter now has
// any new chunks in it for us.
chunkSlice, err := v.ps.Chunks()
if err != nil {
return advance, token, err
}
// Write each chunk by passing it
// through our custom write func,
// which strips out exif and fixes
// the CRC of each chunk.
chunks := chunkSlice.Chunks()
for i, chunk := range chunks {
if i <= v.lastWrittenChunk {
// Skip already
// written chunks.
continue
}
// Write this new chunk.
if err := v.writeChunk(chunk); err != nil {
return advance, token, err
}
v.lastWrittenChunk = i
// Zero data; here you
// go garbage collector.
chunk.Data = nil
}
return advance, token, err
}
func (v *pngVisitor) writeChunk(chunk *pngstructure.Chunk) error {
if chunk.Type == pngstructure.EXifChunkType {
// Replace exif data
// with zero bytes.
clear(chunk.Data)
}
// Fix CRC of each chunk.
chunk.UpdateCrc32()
// finally, write chunk to writer.
_, err := chunk.WriteTo(v.writer)
return err
}
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