Simplify and avoid allocation in jpeg decompression

src/decoder/image.rs

@@ -1,5 +1,5 @@
 use super::ifd::{Directory, Value};
-use super::stream::{ByteOrder, DeflateReader, JpegReader, LZWReader, PackBitsReader};
+use super::stream::{ByteOrder, DeflateReader, LZWReader, PackBitsReader};
 use super::tag_reader::TagReader;
 use super::{fp_predict_f32, fp_predict_f64, DecodingBuffer, Limits};
 use super::{stream::SmartReader, ChunkType};
@@ -326,7 +326,7 @@ impl Image {
         photometric_interpretation: PhotometricInterpretation,
         compression_method: CompressionMethod,
         compressed_length: u64,
-        jpeg_tables: Option<Arc<Vec<u8>>>,
+        jpeg_tables: Option<&[u8]>,
     ) -> TiffResult<Box<dyn Read + 'r>> {
         Ok(match compression_method {
             CompressionMethod::None => Box::new(reader),
@@ -344,7 +344,29 @@ impl Image {
                     ));
                 }
 
-                let jpeg_reader = JpegReader::new(reader, compressed_length, jpeg_tables)?;
+                // Construct new jpeg_reader wrapping a SmartReader.
+                //
+                // JPEG compression in TIFF allows saving quantization and/or huffman tables in one
+                // central location. These `jpeg_tables` are simply prepended to the remaining jpeg image data.
+                // Because these `jpeg_tables` start with a `SOI` (HEX: `0xFFD8`) or __start of image__ marker
+                // which is also at the beginning of the remaining JPEG image data and would
+                // confuse the JPEG renderer, one of these has to be taken off. In this case the first two
+                // bytes of the remaining JPEG data is removed because it follows `jpeg_tables`.
+                // Similary, `jpeg_tables` ends with a `EOI` (HEX: `0xFFD9`) or __end of image__ marker,
+                // this has to be removed as well (last two bytes of `jpeg_tables`).
+                let jpeg_reader = match jpeg_tables {
+                    Some(jpeg_tables) => {
+                        let mut reader = reader.take(compressed_length);
+                        reader.read_exact(&mut [0; 2])?;
+
+                        Box::new(
+                            Cursor::new(&jpeg_tables[..jpeg_tables.len() - 2])
+                                .chain(reader.take(compressed_length)),
+                        ) as Box<dyn Read>
+                    }
+                    None => Box::new(reader.take(compressed_length)),
+                };
+
                 let mut decoder = jpeg::Decoder::new(jpeg_reader);
 
                 match photometric_interpretation {
@@ -522,13 +544,12 @@ impl Image {
 
         let padding_right = chunk_dims.0 - data_dims.0;
 
-        let jpeg_tables = self.jpeg_tables.clone();
         let mut reader = Self::create_reader(
             reader,
             photometric_interpretation,
             compression_method,
             *compressed_bytes,
-            jpeg_tables,
+            self.jpeg_tables.as_deref().map(|a| &**a),
         )?;
 
         if output_width == data_dims.0 as usize && padding_right == 0 {