From f5ab3b363b33a8f4f6978473514b164210b7bdaa Mon Sep 17 00:00:00 2001
From: "A. Molzer" <5550310+197g@users.noreply.github.com>
Date: Tue, 8 Jul 2025 12:10:52 +0200
Subject: [PATCH] Expose interacting with secondary IFDs

---
 src/decoder/ifd.rs | 32 +++++++++++++++++++++++++++++++-
 src/decoder/mod.rs | 38 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 69 insertions(+), 1 deletion(-)

diff --git a/src/decoder/ifd.rs b/src/decoder/ifd.rs
index 455a3e8..8b9aa8f 100644
--- a/src/decoder/ifd.rs
+++ b/src/decoder/ifd.rs
@@ -6,7 +6,7 @@ use std::mem;
 use std::str;
 
 use super::stream::{ByteOrder, EndianReader};
-use crate::tags::{Tag, Type};
+use crate::tags::{IfdPointer, Tag, Type};
 use crate::{TiffError, TiffFormatError, TiffResult};
 
 use self::Value::{
@@ -130,6 +130,25 @@ impl Value {
         }
     }
 
+    /// Turn this value into an `IfdPointer`.
+    ///
+    /// Notice that this does not take an argument, a 64-bit IFD is always allowed. If the
+    /// difference is crucial and you do not want to be permissive you're expected to filter this
+    /// out before.
+    ///
+    /// For compatibility the smaller sized tags should always be allowed i.e. you might use a
+    /// non-bigtiff's directory and its tag types and move it straight to a bigtiff. For instance
+    /// the SubIFD tag is defined as `LONG or IFD`:
+    ///
+    /// <https://web.archive.org/web/20181105221012/https://www.awaresystems.be/imaging/tiff/tifftags/subifds.html>
+    pub fn into_ifd_pointer(self) -> TiffResult<IfdPointer> {
+        match self {
+            Unsigned(val) | Ifd(val) => Ok(IfdPointer(val.into())),
+            IfdBig(val) => Ok(IfdPointer(val)),
+            _ => Err(TiffError::FormatError(TiffFormatError::InvalidTypeForTag)),
+        }
+    }
+
     pub fn into_string(self) -> TiffResult<String> {
         match self {
             Ascii(val) => Ok(val),
@@ -300,6 +319,17 @@ impl Value {
             _ => Err(TiffError::FormatError(TiffFormatError::InvalidTypeForTag)),
         }
     }
+
+    pub fn into_ifd_vec(self) -> TiffResult<Vec<IfdPointer>> {
+        let vec = match self {
+            Unsigned(val) | Ifd(val) => return Ok(vec![IfdPointer(val.into())]),
+            IfdBig(val) => return Ok(vec![IfdPointer(val)]),
+            List(vec) => vec,
+            _ => return Err(TiffError::FormatError(TiffFormatError::InvalidTypeForTag)),
+        };
+
+        vec.into_iter().map(Self::into_ifd_pointer).collect()
+    }
 }
 
 #[derive(Clone)]
diff --git a/src/decoder/mod.rs b/src/decoder/mod.rs
index af09f1f..3ed7d2d 100644
--- a/src/decoder/mod.rs
+++ b/src/decoder/mod.rs
@@ -765,6 +765,13 @@ impl<R: Read + Seek> Decoder<R> {
         self.value_reader.reader.goto_offset(offset)
     }
 
+    /// Read a directory from a known offset.
+    ///
+    /// This may always modify the position of the reader.
+    pub fn read_directory(&mut self, ptr: IfdPointer) -> TiffResult<Directory> {
+        self.value_reader.read_directory(ptr)
+    }
+
     fn check_chunk_type(&self, expected: ChunkType) -> TiffResult<()> {
         if expected != self.image().chunk_type {
             return Err(TiffError::UsageError(UsageError::InvalidChunkType(
@@ -985,6 +992,37 @@ impl<R: Read + Seek> Decoder<R> {
         }
     }
 
+    /// Prepare reading tag values for some externally given directory.
+    ///
+    /// # Examples
+    ///
+    /// This method may be used to read the values of tags in directories that have been previously
+    /// read with [`Decoder::read_directory`].
+    ///
+    /// ```no_run
+    /// use tiff::decoder::Decoder;
+    /// use tiff::tags::Tag;
+    ///
+    /// # use std::io::Cursor;
+    /// # let mut data = Cursor::new(vec![0]);
+    /// let mut decoder = Decoder::new(&mut data).unwrap();
+    /// let sub_ifds = decoder.get_tag(Tag::SubIfd)?.into_ifd_vec()?;
+    ///
+    /// for ifd in sub_ifds {
+    ///     let subdir = decoder.read_directory(ifd)?;
+    ///     let subfile = decoder.with_ifd(&subdir).find_tag(Tag::SubfileType)?;
+    ///     // todo: handle the subfiles, e.g. thumbnails
+    /// }
+    ///
+    /// # Ok::<_, tiff::TiffError>(())
+    /// ```
+    pub fn with_ifd<'ifd>(&'ifd mut self, ifd: &'ifd Directory) -> IfdDecoder<'ifd> {
+        IfdDecoder {
+            decoder: &mut self.value_reader,
+            ifd,
+        }
+    }
+
     /// Tries to retrieve a tag from the current image directory.
     /// Return `Ok(None)` if the tag is not present.
     pub fn find_tag(&mut self, tag: Tag) -> TiffResult<Option<ifd::Value>> {