Utils for working with tags

CHANGES.md

@@ -1,3 +1,16 @@
+# Version 0.11.1 (Unreleased)
+
+Fixes:
+- The enumeration types in `tags` are now all marked with a representation of
+  their underlying TIFF type (e.g. `repr(u16)`) and variants are explicitly
+  assigned their corresponding values. That is you may _read_ the raw
+  discriminant and interpret that as the value—except for `Unknown` variants.
+
+Additions:
+- Types in `tags` now generally implement `TiffValue` and can be handed to the
+  `DirectoryEncoder::write_tag` method. Unlike primitive types they do _not_
+  always implement the trait for slices of themselves.
+
 # Version 0.11.0
 
 - `Directory` now implements `FromIterator<(Tag, Value)>`.

src/encoder/mod.rs

@@ -582,7 +582,7 @@ impl<'a, W: 'a + Write + Seek, T: ColorType, K: TiffKind> ImageEncoder<'a, W, T,
         )?;
         encoder.write_tag(Tag::XResolution, Rational { n: 1, d: 1 })?;
         encoder.write_tag(Tag::YResolution, Rational { n: 1, d: 1 })?;
-        encoder.write_tag(Tag::ResolutionUnit, ResolutionUnit::None.to_u16())?;
+        encoder.write_tag(Tag::ResolutionUnit, ResolutionUnit::None)?;
 
         Ok(ImageEncoder {
             encoder,

src/encoder/tiff_value.rs

@@ -1,6 +1,6 @@
 use std::{borrow::Cow, io::Write, slice::from_ref};
 
-use crate::{bytecast, tags::Type, TiffError, TiffFormatError, TiffResult};
+use crate::{bytecast, tags, tags::Type, TiffError, TiffFormatError, TiffResult};
 
 use super::writer::TiffWriter;
 
@@ -458,6 +458,128 @@ impl TiffValue for str {
     }
 }
 
+// FIXME: held up on reading the discriminant for all but the unknown variant.
+//
+// It needs to have `repr(u16)` to have a defined layout and then we may read the tag from its
+// value representation by casting a u16 pointer to a pointer to the enum type.
+//     unsafe { *<*const _>::from(self).cast::<u16>() }
+// But that is quite unsafe and needs careful review to maintain the safety requirements. Maybe we
+// could get `bytemuck` to add a trait for this (it has `TransparentWrapper`) with a derive for
+// enum types that have an explicit repr.
+//
+// This would allow returning borrowed data in more cases. For all types without an unknown, in all
+// cases including as slices, and for all types with and unknown variant when we have a single
+// value (either returning a borrowed discriminant or a borrowed value within `Unknown`).
+impl TiffValue for tags::CompressionMethod {
+    const BYTE_LEN: u8 = <u16 as TiffValue>::BYTE_LEN;
+    const FIELD_TYPE: Type = Type::SHORT;
+
+    fn count(&self) -> usize {
+        1
+    }
+
+    fn data(&self) -> Cow<'_, [u8]> {
+        let bytes = self.to_u16().to_ne_bytes();
+        Cow::Owned(bytes.to_vec())
+    }
+}
+
+impl TiffValue for tags::PhotometricInterpretation {
+    const BYTE_LEN: u8 = <u16 as TiffValue>::BYTE_LEN;
+    const FIELD_TYPE: Type = Type::SHORT;
+
+    fn count(&self) -> usize {
+        1
+    }
+
+    fn data(&self) -> Cow<'_, [u8]> {
+        self.to_u16().to_ne_bytes().to_vec().into()
+    }
+}
+
+impl TiffValue for tags::PlanarConfiguration {
+    const BYTE_LEN: u8 = 2;
+    const FIELD_TYPE: Type = Type::SHORT;
+
+    fn count(&self) -> usize {
+        1
+    }
+
+    fn data(&self) -> Cow<'_, [u8]> {
+        self.to_u16().to_ne_bytes().to_vec().into()
+    }
+}
+
+impl TiffValue for tags::Predictor {
+    const BYTE_LEN: u8 = <u16 as TiffValue>::BYTE_LEN;
+    const FIELD_TYPE: Type = Type::SHORT;
+
+    fn count(&self) -> usize {
+        1
+    }
+
+    fn data(&self) -> Cow<'_, [u8]> {
+        self.to_u16().to_ne_bytes().to_vec().into()
+    }
+}
+
+impl TiffValue for tags::ResolutionUnit {
+    const BYTE_LEN: u8 = <u16 as TiffValue>::BYTE_LEN;
+    const FIELD_TYPE: Type = Type::SHORT;
+
+    fn count(&self) -> usize {
+        1
+    }
+
+    fn data(&self) -> Cow<'_, [u8]> {
+        self.to_u16().to_ne_bytes().to_vec().into()
+    }
+}
+
+/// This is implemented for slices as the `SampleFormat` tag takes a count of `N`.
+///
+/// Use `core::slice::from_ref` if you really need to write a single element.
+///
+/// See: <https://web.archive.org/web/20191120220815/https://www.awaresystems.be/imaging/tiff/tifftags/sampleformat.html>
+impl TiffValue for [tags::SampleFormat] {
+    const BYTE_LEN: u8 = <u16 as TiffValue>::BYTE_LEN;
+    const FIELD_TYPE: Type = Type::SHORT;
+
+    fn count(&self) -> usize {
+        self.len()
+    }
+
+    fn data(&self) -> Cow<'_, [u8]> {
+        let mut buf: Vec<u8> = Vec::with_capacity(self.len() * 2);
+        for x in self {
+            buf.extend_from_slice(&x.to_u16().to_ne_bytes());
+        }
+        Cow::Owned(buf)
+    }
+}
+
+/// This is implemented for slices as the `ExtraSamples` tag takes a count of `N`.
+///
+/// Use `core::slice::from_ref` if you really need to write a single element.
+///
+/// See: <https://web.archive.org/web/20191120220815/https://www.awaresystems.be/imaging/tiff/tifftags/extrasamples.html>
+impl TiffValue for [tags::ExtraSamples] {
+    const BYTE_LEN: u8 = <u16 as TiffValue>::BYTE_LEN;
+    const FIELD_TYPE: Type = Type::SHORT;
+
+    fn count(&self) -> usize {
+        self.len()
+    }
+
+    fn data(&self) -> Cow<'_, [u8]> {
+        let mut buf: Vec<u8> = Vec::with_capacity(self.len() * 2);
+        for x in self {
+            buf.extend_from_slice(&x.to_u16().to_ne_bytes());
+        }
+        Cow::Owned(buf)
+    }
+}
+
 // If you pass `&Vec<_>` then you'd get at first sight the  complaint:
 //
 //  `Vec<T>` does not implement `TiffValue`

src/tags.rs

@@ -10,8 +10,9 @@ macro_rules! tags {
         $( #[$enum_attr] )*
         #[derive(Clone, Copy, PartialEq, Eq, Debug, Hash)]
         #[non_exhaustive]
+        #[repr($ty)]
         pub enum $name {
-            $($(#[$ident_attr])* $tag,)*
+            $($(#[$ident_attr])* $tag = $val,)*
             $(
                 #[$unknown_meta]
                 Unknown($ty),