Relax Decoder, new interface documentation

The bounds `Read + Seek` are not representational so we may remove them
from the struct itself. This also allows us to use more precise bounds
for direct access to the inner stream, especially were that does not
seek.

Groups operations according to their use case.

tests/decode_bigtiff_images.rs

@@ -26,21 +26,20 @@ fn test_big_tiff() {
             decoder.colortype().expect("Cannot get colortype"),
             ColorType::RGB(8)
         );
+
+        let mut ifd = decoder.current_ifd();
         assert_eq!(
-            decoder
-                .get_tag_u64(Tag::StripOffsets)
+            ifd.get_tag_u64(Tag::StripOffsets)
                 .expect("Cannot get StripOffsets"),
             16
         );
         assert_eq!(
-            decoder
-                .get_tag_u64(Tag::RowsPerStrip)
+            ifd.get_tag_u64(Tag::RowsPerStrip)
                 .expect("Cannot get RowsPerStrip"),
             64
         );
         assert_eq!(
-            decoder
-                .get_tag_u64(Tag::StripByteCounts)
+            ifd.get_tag_u64(Tag::StripByteCounts)
                 .expect("Cannot get StripByteCounts"),
             12288
         )

tests/decode_geotiff_images.rs

@@ -30,27 +30,25 @@ fn test_geo_tiff() {
                 num_samples: 5
             }
         );
+
+        let mut ifd = decoder.current_ifd();
         assert_eq!(
-            decoder
-                .get_tag_u64(Tag::StripOffsets)
+            ifd.get_tag_u64(Tag::StripOffsets)
                 .expect("Cannot get StripOffsets"),
             418
         );
         assert_eq!(
-            decoder
-                .get_tag_u64(Tag::RowsPerStrip)
+            ifd.get_tag_u64(Tag::RowsPerStrip)
                 .expect("Cannot get RowsPerStrip"),
             10
         );
         assert_eq!(
-            decoder
-                .get_tag_u64(Tag::StripByteCounts)
+            ifd.get_tag_u64(Tag::StripByteCounts)
                 .expect("Cannot get StripByteCounts"),
             1000
         );
         assert_eq!(
-            decoder
-                .get_tag(Tag::ModelPixelScaleTag)
+            ifd.get_tag(Tag::ModelPixelScaleTag)
                 .expect("Cannot get pixel scale")
                 .into_f64_vec()
                 .expect("Cannot get pixel scale"),

tests/decode_images.rs

@@ -404,7 +404,10 @@ fn test_string_tags() {
         let img_file = File::open(path).expect("Cannot find test image!");
         let mut decoder = Decoder::open(img_file).expect("Cannot create decoder");
         decoder.next_image().expect("Cannot read image IFD");
-        let software = decoder.get_tag(tiff::tags::Tag::Software).unwrap();
+        let software = decoder
+            .current_ifd()
+            .get_tag(tiff::tags::Tag::Software)
+            .unwrap();
         match software {
             ifd::Value::Ascii(s) => assert_eq!(
                 &s,
@@ -478,7 +481,8 @@ fn test_inner_access() {
     decoder.next_image().expect("Cannot read image IFD");
     assert_eq!(decoder.colortype().unwrap(), ColorType::RGB(8));
 
-    let c = decoder
+    let mut ifd = decoder.current_ifd();
+    let c = ifd
         .get_tag(tiff::tags::Tag::Compression)
         .unwrap()
         .into_u16()
@@ -487,13 +491,13 @@ fn test_inner_access() {
 
     // Because the image is uncompressed, reading the first tile directly with the inner reader
     // should yield the same result as reading it with the decoder's read_chunk method.
-    let first_offset = decoder
+    let first_offset = ifd
         .get_tag_u32_vec(tiff::tags::Tag::TileOffsets)
         .unwrap()
         .into_iter()
         .next()
         .unwrap();
-    let first_byte_count = decoder
+    let first_byte_count = ifd
         .get_tag_u32_vec(tiff::tags::Tag::TileByteCounts)
         .unwrap()
         .into_iter()

tests/encode_images.rs

@@ -40,6 +40,7 @@ fn encode_decode() {
         assert_eq!(decoder.dimensions().unwrap(), (100, 100));
 
         let mut all_tags = decoder
+            .current_ifd()
             .tag_iter()
             .filter_map(Result::ok)
             .collect::<Vec<_>>();
@@ -80,7 +81,7 @@ fn encode_decode() {
         );
 
         assert_eq!(
-            decoder.get_tag(Tag::Artist).unwrap(),
+            decoder.current_ifd().get_tag(Tag::Artist).unwrap(),
             ifd::Value::Ascii("Image-tiff".into())
         );
 
@@ -125,6 +126,7 @@ fn encode_decode_big() {
         assert_eq!(decoder.dimensions().unwrap(), (100, 100));
 
         let mut all_tags = decoder
+            .current_ifd()
             .tag_iter()
             .filter_map(Result::ok)
             .collect::<Vec<_>>();
@@ -165,7 +167,7 @@ fn encode_decode_big() {
         );
 
         assert_eq!(
-            decoder.get_tag(Tag::Artist).unwrap(),
+            decoder.current_ifd().get_tag(Tag::Artist).unwrap(),
             ifd::Value::Ascii("Image-tiff".into())
         );
         let mut buffer = tiff::decoder::DecodingSampleBuffer::U8(vec![]);
@@ -539,37 +541,57 @@ trait AssertDecode {
 
 impl<R: std::io::Read + std::io::Seek> AssertDecode for Decoder<R> {
     fn assert_tag_u32(&mut self, tag: u16) -> u32 {
-        self.get_tag(Tag::Unknown(tag)).unwrap().into_u32().unwrap()
+        self.current_ifd()
+            .get_tag(Tag::Unknown(tag))
+            .unwrap()
+            .into_u32()
+            .unwrap()
     }
     fn assert_tag_u32_vec(&mut self, tag: u16) -> Vec<u32> {
-        self.get_tag(Tag::Unknown(tag))
+        self.current_ifd()
+            .get_tag(Tag::Unknown(tag))
             .unwrap()
             .into_u32_vec()
             .unwrap()
     }
     fn assert_tag_i32(&mut self, tag: u16) -> i32 {
-        self.get_tag(Tag::Unknown(tag)).unwrap().into_i32().unwrap()
+        self.current_ifd()
+            .get_tag(Tag::Unknown(tag))
+            .unwrap()
+            .into_i32()
+            .unwrap()
     }
     fn assert_tag_i32_vec(&mut self, tag: u16) -> Vec<i32> {
-        self.get_tag(Tag::Unknown(tag))
+        self.current_ifd()
+            .get_tag(Tag::Unknown(tag))
             .unwrap()
             .into_i32_vec()
             .unwrap()
     }
     fn assert_tag_u64(&mut self, tag: u16) -> u64 {
-        self.get_tag(Tag::Unknown(tag)).unwrap().into_u64().unwrap()
+        self.current_ifd()
+            .get_tag(Tag::Unknown(tag))
+            .unwrap()
+            .into_u64()
+            .unwrap()
     }
     fn assert_tag_u64_vec(&mut self, tag: u16) -> Vec<u64> {
-        self.get_tag(Tag::Unknown(tag))
+        self.current_ifd()
+            .get_tag(Tag::Unknown(tag))
             .unwrap()
             .into_u64_vec()
             .unwrap()
     }
     fn assert_tag_i64(&mut self, tag: u16) -> i64 {
-        self.get_tag(Tag::Unknown(tag)).unwrap().into_i64().unwrap()
+        self.current_ifd()
+            .get_tag(Tag::Unknown(tag))
+            .unwrap()
+            .into_i64()
+            .unwrap()
     }
     fn assert_tag_i64_vec(&mut self, tag: u16) -> Vec<i64> {
-        self.get_tag(Tag::Unknown(tag))
+        self.current_ifd()
+            .get_tag(Tag::Unknown(tag))
             .unwrap()
             .into_i64_vec()
             .unwrap()
@@ -776,7 +798,13 @@ fn test_rows_per_strip() {
     {
         let mut decoder = Decoder::open(&mut file).unwrap();
         decoder.next_image().unwrap();
-        assert_eq!(decoder.get_tag_u64(Tag::RowsPerStrip).unwrap(), 2);
+        assert_eq!(
+            decoder
+                .current_ifd()
+                .get_tag_u64(Tag::RowsPerStrip)
+                .unwrap(),
+            2
+        );
         assert_eq!(decoder.strip_count().unwrap(), 50);
 
         for i in 0..50 {
@@ -829,6 +857,7 @@ fn test_auxiliary_directory() {
         let _ = decoder.read_image().unwrap();
 
         let exif_location = decoder
+            .current_ifd()
             .get_tag(Tag::ExifDirectory)
             .expect("second directory missing Exif")
             .into_ifd_pointer()