Add EntryBytesReader: impl Read + Seek

src/decoder/ifd.rs

@@ -393,7 +393,7 @@ impl Entry {
         self.count
     }
 
-    pub(crate) fn offset(&self) -> &[u8] {
+    pub(crate) fn offset_raw(&self) -> &[u8; 8] {
         &self.offset
     }
 

src/decoder/mod.rs

@@ -20,8 +20,10 @@ mod image;
 mod logluv;
 mod stream;
 mod tag_reader;
+mod value;
 
 pub use self::buffer::{DecodingSampleBuffer, DecodingSampleSlice, DecodingSampleType};
+pub use self::value::EntryBytesReader;
 
 #[deprecated = "Renamed to `DecodingSampleBuffer`"]
 pub type DecodingResult = DecodingSampleBuffer;
@@ -943,6 +945,10 @@ impl<R: Read + Seek> Decoder<R> {
         Ok(val)
     }
 
+    pub fn read_entry(&mut self, entry: ifd::Entry) -> TiffResult<EntryBytesReader<'_, R>> {
+        EntryBytesReader::from_entry(&mut self.value_reader, entry)
+    }
+
     /// Reads a TIFF IFA offset/value field
     #[inline]
     pub fn read_offset_u64(&mut self) -> Result<[u8; 8], io::Error> {

src/decoder/value.rs

@@ -0,0 +1,196 @@
+use std::io::{self, Cursor, Read, Seek};
+
+use crate::{
+    decoder::{ifd::Entry, stream::EndianReader, ValueReader},
+    tags::{ByteOrder, IfdPointer, Type},
+    TiffResult,
+};
+
+pub struct EntryBytesReader<'lt, R> {
+    inner: &'lt mut EndianReader<R>,
+    variant: Variant,
+}
+
+const OFFSET_BUFFER_LEN: usize = 8;
+
+enum Variant {
+    Immediate {
+        /// An in-value reader if the offset field contains the data. Note that we rotate it to the
+        /// back of this buffer, not the front, so that `Cursor` already contains the right end.
+        /// We do not let you seek back beyond our start.
+        data: Cursor<[u8; OFFSET_BUFFER_LEN]>,
+        value_bytes: u8,
+    },
+    InFile {
+        // the length relative to the base so we can seek relative without querying the position.
+        value_bytes: u64,
+        remainder: u64,
+        base: IfdPointer,
+    },
+}
+
+impl<'lt, R> EntryBytesReader<'lt, R> {
+    pub(super) fn from_entry(inner: &'lt mut ValueReader<R>, entry: Entry) -> TiffResult<Self>
+    where
+        R: Read + Seek,
+    {
+        let offset: [_; OFFSET_BUFFER_LEN] = *entry.offset_raw();
+        let limit = if inner.bigtiff { 8 } else { 4 };
+        assert!(limit as usize <= OFFSET_BUFFER_LEN);
+
+        let value_bytes = entry.field_type().value_bytes(entry.count())?;
+
+        if value_bytes <= limit {
+            // We move the data to the back of the array whereas it is in the front right now.
+            let mut value = offset;
+            value.rotate_left(value_bytes as usize);
+            let mut cursor = Cursor::new(value);
+            cursor.set_position(OFFSET_BUFFER_LEN as u64 - value_bytes);
+
+            Ok(EntryBytesReader {
+                inner: &mut inner.reader,
+                variant: Variant::Immediate {
+                    data: cursor,
+                    value_bytes: value_bytes as u8,
+                },
+            })
+        } else {
+            // remainder > limit >= 0 so this is an unreachable panic.
+            let pointer = if inner.bigtiff {
+                let mut offset = offset;
+
+                inner
+                    .reader
+                    .byte_order
+                    .convert(Type::IFD8, &mut offset[..], ByteOrder::native());
+
+                IfdPointer(u64::from_ne_bytes(offset))
+            } else {
+                let mut offset = *offset[..4].as_array::<4>().unwrap();
+
+                inner
+                    .reader
+                    .byte_order
+                    .convert(Type::IFD, &mut offset[..], ByteOrder::native());
+
+                let offset = u32::from_ne_bytes(offset);
+                IfdPointer(u64::from(offset))
+            };
+
+            inner.reader.goto_offset(pointer.0)?;
+
+            Ok(EntryBytesReader {
+                inner: &mut inner.reader,
+                variant: Variant::InFile {
+                    value_bytes,
+                    remainder: value_bytes,
+                    base: pointer,
+                },
+            })
+        }
+    }
+}
+
+impl<R: Read> Read for EntryBytesReader<'_, R> {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        match &mut self.variant {
+            Variant::Immediate { data, .. } => {
+                return data.read(buf);
+            }
+            Variant::InFile { remainder, .. } => {
+                let bound = usize::try_from(*remainder).unwrap_or(usize::MAX);
+                let avail = buf.len().min(bound);
+                let read = self.inner.inner().read(&mut buf[..avail])?;
+
+                // On a well-behaved `Read`: read <= avail <= bound <=(mathematical) remainder
+                let consume = read as u64;
+                *remainder -= consume;
+                Ok(read)
+            }
+        }
+    }
+
+    fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
+        match &mut self.variant {
+            Variant::Immediate { data, .. } => {
+                return data.read_exact(buf);
+            }
+            Variant::InFile { remainder, .. } => {
+                let bound = usize::try_from(*remainder).unwrap_or(usize::MAX);
+
+                // Fail early if we would be sure to over-read.
+                if buf.len() > bound {
+                    return Err(io::Error::new(
+                        io::ErrorKind::UnexpectedEof,
+                        "Tiff value shorter than the requested buffer size",
+                    ));
+                }
+
+                // since buf.len() <= bound <=(mathematical) remainder
+                let consume = buf.len() as u64;
+                self.inner.inner().read_exact(buf)?;
+                *remainder -= consume;
+
+                Ok(())
+            }
+        }
+    }
+}
+
+impl<R: Read + Seek> Seek for EntryBytesReader<'_, R> {
+    fn seek(&mut self, offset: io::SeekFrom) -> io::Result<u64> {
+        let (value_bytes, remainder) = match &self.variant {
+            Variant::Immediate { data, value_bytes } => {
+                let remainder = data.get_ref().len() as u64 - data.position();
+                (*value_bytes as u64, remainder)
+            }
+            &Variant::InFile {
+                value_bytes,
+                remainder,
+                base: _,
+            } => (value_bytes, remainder),
+        };
+
+        // Normalize to an offset from the start
+        let offset_from_start = match offset {
+            io::SeekFrom::Start(n) => Some(n),
+            io::SeekFrom::Current(n) => {
+                let front = value_bytes - remainder;
+                front.checked_add_signed(n)
+            }
+            io::SeekFrom::End(n) => value_bytes.checked_add_signed(n),
+        };
+
+        let Some((offset, remainder)) = offset_from_start.and_then(|n| {
+            let remainder = value_bytes.checked_sub(n)?;
+            Some((n, remainder))
+        }) else {
+            return Err(io::Error::new(
+                io::ErrorKind::InvalidInput,
+                "seek out of bounds of entry value",
+            ));
+        };
+
+        match &mut self.variant {
+            Variant::Immediate { data, .. } => {
+                let start = OFFSET_BUFFER_LEN as u64 - value_bytes;
+                data.set_position(start + offset);
+                Ok(offset)
+            }
+            &mut Variant::InFile { base, .. } => {
+                // We checked this against overflow on construction. Ignore the offset we get.
+                self.inner
+                    .inner()
+                    .seek(io::SeekFrom::Start(base.0 + offset))?;
+
+                self.variant = Variant::InFile {
+                    value_bytes,
+                    remainder,
+                    base,
+                };
+
+                Ok(offset)
+            }
+        }
+    }
+}

src/encoder/mod.rs

@@ -488,7 +488,8 @@ impl<'a, W: 'a + Write + Seek, K: TiffKind> DirectoryEncoder<'a, W, K> {
             self.writer.write_u16(entry.field_type().to_u16())?;
             let count = K::convert_offset(entry.count())?;
             count.write(self.writer)?;
-            self.writer.write_bytes(&entry.offset()[..offset_bytes])?;
+            self.writer
+                .write_bytes(&entry.offset_raw()[..offset_bytes])?;
         }
 
         Ok(offset)