Move Directory from image into decoder proper

This prepares allowing to iterate over a TIFF file with invalid images
while `Image` maintains invariants. For instance we precalculate the
amount of buffer space and verify that the count of strips makes sense
in the constructor—so we do not have to redo that for every other entry
point.

src/decoder/mod.rs

@@ -516,8 +516,9 @@ where
     /// There are grouped for borrow checker reasons. This allows us to implement methods that
     /// borrow the stream access and the other fields mutably at the same time.
     value_reader: ValueReader<R>,
-    current_ifd: Option<IfdPointer>,
-    next_ifd: Option<IfdPointer>,
+    current_ifd_pointer: Option<IfdPointer>,
+    next_ifd_pointer: Option<IfdPointer>,
+    directory: Directory,
     /// The IFDs we visited already in this chain of IFDs.
     ifd_offsets: Vec<IfdPointer>,
     /// Map from the ifd into the `ifd_offsets` ordered list.
@@ -821,12 +822,12 @@ impl<R: Read + Seek> Decoder<R> {
                 bigtiff,
                 limits: Default::default(),
             },
-            next_ifd,
+            next_ifd_pointer: next_ifd,
             ifd_offsets,
-            current_ifd: None,
+            current_ifd_pointer: None,
+            directory: Directory::empty(),
             seen_ifds: cycles::IfdCycles::new(),
             image: Image {
-                ifd: None,
                 width: 0,
                 height: 0,
                 bits_per_sample: 1,
@@ -867,7 +868,7 @@ impl<R: Read + Seek> Decoder<R> {
 
     /// The offset of the directory representing the current image.
     pub fn ifd_pointer(&mut self) -> Option<IfdPointer> {
-        self.current_ifd
+        self.current_ifd_pointer
     }
 
     fn image(&self) -> &Image {
@@ -879,8 +880,8 @@ impl<R: Read + Seek> Decoder<R> {
         // Check whether we have seen this IFD before, if so then the index will be less than the length of the list of ifd offsets
         if ifd_index >= self.ifd_offsets.len() {
             // We possibly need to load in the next IFD
-            if self.next_ifd.is_none() {
-                self.current_ifd = None;
+            if self.next_ifd_pointer.is_none() {
+                self.current_ifd_pointer = None;
 
                 return Err(TiffError::FormatError(
                     TiffFormatError::ImageFileDirectoryNotFound,
@@ -903,10 +904,10 @@ impl<R: Read + Seek> Decoder<R> {
 
         // If the index is within the list of ifds then we can load the selected image/IFD
         if let Some(ifd_offset) = self.ifd_offsets.get(ifd_index) {
-            let ifd = self.value_reader.read_directory(*ifd_offset)?;
-            self.next_ifd = ifd.next();
-            self.current_ifd = Some(*ifd_offset);
-            self.image = Image::from_reader(&mut self.value_reader, ifd)?;
+            self.directory = self.value_reader.read_directory(*ifd_offset)?;
+            self.next_ifd_pointer = self.directory.next();
+            self.current_ifd_pointer = Some(*ifd_offset);
+            self.image = Image::from_reader(&mut self.value_reader, &self.directory)?;
 
             Ok(())
         } else {
@@ -917,7 +918,7 @@ impl<R: Read + Seek> Decoder<R> {
     }
 
     fn next_ifd(&mut self) -> TiffResult<Directory> {
-        let Some(next_ifd) = self.next_ifd.take() else {
+        let Some(next_ifd) = self.next_ifd_pointer.take() else {
             return Err(TiffError::FormatError(
                 TiffFormatError::ImageFileDirectoryNotFound,
             ));
@@ -929,12 +930,12 @@ impl<R: Read + Seek> Decoder<R> {
         self.seen_ifds.insert_next(next_ifd, ifd.next())?;
 
         // Extend the list of known IFD offsets in this chain, if needed.
-        if self.ifd_offsets.last().copied() == self.current_ifd {
+        if self.ifd_offsets.last().copied() == self.current_ifd_pointer {
             self.ifd_offsets.push(next_ifd);
         }
 
-        self.current_ifd = Some(next_ifd);
-        self.next_ifd = ifd.next();
+        self.current_ifd_pointer = Some(next_ifd);
+        self.next_ifd_pointer = ifd.next();
 
         Ok(ifd)
     }
@@ -944,13 +945,14 @@ impl<R: Read + Seek> Decoder<R> {
     /// To determine whether there are more images call `TIFFDecoder::more_images` instead.
     pub fn next_image(&mut self) -> TiffResult<()> {
         let ifd = self.next_ifd()?;
-        self.image = Image::from_reader(&mut self.value_reader, ifd)?;
+        self.directory = ifd;
+        self.image = Image::from_reader(&mut self.value_reader, &self.directory)?;
         Ok(())
     }
 
     /// Returns `true` if there is at least one more image available.
     pub fn more_images(&self) -> bool {
-        self.next_ifd.is_some()
+        self.next_ifd_pointer.is_some()
     }
 
     /// Returns the byte_order of the file.
@@ -1527,7 +1529,7 @@ impl<R: Read + Seek> Decoder<R> {
         IfdDecoder {
             inner: tag_reader::TagReader {
                 decoder: &mut self.value_reader,
-                ifd: self.image.ifd.as_ref().unwrap(),
+                ifd: &self.directory,
             },
         }
     }