Reuse encoder::write_chunk from test_utils::write_chunk.

src/test_utils.rs

@@ -1,6 +1,13 @@
 //! A set of test utilities.
+//!
+//! There is some overlap between the `test_utils::write_...` functions and the code in the
+//! `encoder.rs` module.  The main difference is that the `test_utils` function perform no
+//! validation - this allows testcases to construct arbitrary, potentially invalid PNG inputs.
+//! OTOH, we should try to reuse code whenever possible - e.g.  `test_utils::write_chunk` is just a
+//! thin wrapper around `encoder::write_chunk`.
 
 use byteorder::WriteBytesExt;
+use std::convert::TryInto;
 use std::io::Write;
 
 /// Generates a store-only, non-compressed image:
@@ -37,20 +44,12 @@ pub fn write_png_sig(w: &mut impl Write) {
 
 /// Writes an arbitrary PNG chunk.
 pub fn write_chunk(w: &mut impl Write, chunk_type: &[u8], data: &[u8]) {
-    assert_eq!(chunk_type.len(), 4);
-    let crc = {
-        let input = chunk_type
-            .iter()
-            .copied()
-            .chain(data.iter().copied())
-            .collect::<Vec<_>>();
-        crc32fast::hash(input.as_slice())
-    };
-    w.write_u32::<byteorder::BigEndian>(data.len() as u32)
-        .unwrap();
-    w.write_all(chunk_type).unwrap();
-    w.write_all(data).unwrap();
-    w.write_u32::<byteorder::BigEndian>(crc).unwrap();
+    let chunk_type = crate::chunk::ChunkType(
+        chunk_type
+            .try_into()
+            .expect("`chunk_type` needs to be 4 bytes long"),
+    );
+    crate::encoder::write_chunk(w, chunk_type, data).unwrap();
 }
 
 /// Writes an IHDR chunk that indicates a non-interlaced RGBA8 that uses the same height and