From c1087f6f0895304ef1494039e52ffd2e6f0c4278 Mon Sep 17 00:00:00 2001
From: Stephen Judkins <stephen.judkins@gmail.com>
Date: Tue, 9 Apr 2024 15:25:42 -0700
Subject: [PATCH] `Reader` instance for group 4-decoded images, that buffer one
 line at a time

---
 src/decoder/image.rs | 81 +++++++++++++++++++++++++++++++-------------
 1 file changed, 58 insertions(+), 23 deletions(-)

diff --git a/src/decoder/image.rs b/src/decoder/image.rs
index 3dcd03a..12752e4 100644
--- a/src/decoder/image.rs
+++ b/src/decoder/image.rs
@@ -9,6 +9,10 @@ use crate::tags::{
 use crate::{
     ColorType, Directory, TiffError, TiffFormatError, TiffResult, TiffUnsupportedError, UsageError,
 };
+
+#[cfg(feature = "fax")]
+use std::collections::VecDeque;
+
 use std::io::{self, Cursor, Read, Seek};
 use std::num::NonZeroUsize;
 use std::sync::Arc;
@@ -381,12 +385,12 @@ impl Image {
     }
 
     fn create_reader<'r, R: 'r + Read>(
-        width: u32,
-        height: u32,
         reader: R,
         compression_method: CompressionMethod,
         compressed_length: u64,
-        #[allow(unused_variables)] jpeg_tables: Option<&[u8]>,
+        // FIXME: these should be `expect` attributes or we choose another way of passing them.
+        #[cfg_attr(not(feature = "jpeg"), allow(unused_variables))] jpeg_tables: Option<&[u8]>,
+        #[cfg_attr(not(feature = "fax"), allow(unused_variables))] dimensions: (u32, u32),
     ) -> TiffResult<Box<dyn Read + 'r>> {
         Ok(match compression_method {
             CompressionMethod::None => Box::new(reader),
@@ -454,26 +458,58 @@ impl Image {
 
                 Box::new(Cursor::new(data))
             }
+            #[cfg(feature = "fax")]
             CompressionMethod::Fax4 => {
-                let width = u16::try_from(width)?;
-                let height = u16::try_from(height)?;
-                let mut out: Vec<u8> = Vec::with_capacity(usize::from(width) * usize::from(height));
+                let width = u16::try_from(dimensions.0)?;
+                let height = u16::try_from(dimensions.1)?;
 
-                // all extant tiff/fax4 decoders I've found always assume that the photometric interpretation
-                // is `WhiteIsZero`, ignoring the tag. ImageMagick appears to generate fax4-encoded tiffs
-                // with the tag incorrectly set to `BlackIsZero`.
-                fax::decoder::decode_g4(
-                    reader.bytes().map(|b| b.unwrap()),
-                    width,
-                    Some(height),
-                    |transitions| {
-                        out.extend(fax::decoder::pels(transitions, width).map(|c| match c {
-                            fax::Color::Black => 255,
-                            fax::Color::White => 0,
-                        }))
-                    },
-                );
-                Box::new(Cursor::new(out))
+                struct Group4Reader<R2> {
+                    decoder: fax::decoder::Group4Decoder<std::io::Bytes<R2>>,
+                    line_buf: VecDeque<u8>,
+                    y: u16,
+                    height: u16,
+                    width: u16,
+                }
+
+                impl<R2: Read> Read for Group4Reader<R2> {
+                    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+                        if self.line_buf.is_empty() && self.y < self.height {
+                            let next = self
+                                .decoder
+                                .advance()
+                                .map_err(|e| std::io::Error::new(std::io::ErrorKind::Other, e))?;
+
+                            match next {
+                                fax::decoder::DecodeStatus::End => (),
+                                fax::decoder::DecodeStatus::Incomplete => {
+                                    self.y += 1;
+                                    self.line_buf.extend(
+                                        // all extant tiff/fax4 decoders I've found always assume that the photometric interpretation
+                                        // is `WhiteIsZero`, ignoring the tag. ImageMagick appears to generate fax4-encoded tiffs
+                                        // with the tag incorrectly set to `BlackIsZero`.
+                                        fax::decoder::pels(self.decoder.transition(), self.width)
+                                            .map(|c| match c {
+                                                fax::Color::Black => 255,
+                                                fax::Color::White => 0,
+                                            }),
+                                    );
+                                }
+                            }
+                        }
+                        self.line_buf.read(buf)
+                    }
+                }
+
+                Box::new(Group4Reader {
+                    decoder: fax::decoder::Group4Decoder::new(
+                        reader.take(compressed_length).bytes(),
+                        width,
+                    )?,
+                    line_buf: VecDeque::with_capacity(usize::from(width)),
+                    y: 0,
+                    width: width,
+                    height: height,
+                })
             }
             method => {
                 return Err(TiffError::UnsupportedError(
@@ -677,12 +713,11 @@ impl Image {
         assert!(buf.len() >= output_row_stride * (data_dims.1 as usize - 1) + data_row_bytes);
 
         let mut reader = Self::create_reader(
-            self.width,
-            self.height,
             reader,
             compression_method,
             *compressed_bytes,
             self.jpeg_tables.as_deref().map(|a| &**a),
+            chunk_dims,
         )?;
 
         if output_row_stride == chunk_row_bytes {