refactor decoder::refine_non_zeroes

 - Fixes image-rs#132
 - improve readability
 - improve performance

 Criterion report on my computer:

 Benchmarking decode a 512x512 progressive JPEG: Collecting 100 samples in estimated 27.108 s (5050 i                                                                                                    decode a 512x512 progressive JPEG
                         time:   [5.3440 ms 5.3549 ms 5.3669 ms]
                         change: [-24.307% -23.085% -21.920%] (p = 0.00 < 0.05)
                         Performance has improved.
 Found 3 outliers among 100 measurements (3.00%)
   2 (2.00%) high mild
   1 (1.00%) high severe

src/decoder.rs

@@ -739,26 +739,19 @@ fn refine_non_zeroes<R: Read>(reader: &mut R,
                               bit: i16) -> Result<u8> {
     debug_assert_eq!(coefficients.len(), 64);
 
-    let last = range.end - 1;
+    let mut last= range.start;
     let mut zero_run_length = zrl;
 
     for i in range {
+        last = i;
         let index = UNZIGZAG[i as usize] as usize;
+        let coefficient = &mut coefficients[index];
 
-        if coefficients[index] == 0 {
-            if zero_run_length == 0 {
-                return Ok(i);
-            }
-
+        if *coefficient == 0 {
+            if zero_run_length == 0 { break; }
             zero_run_length -= 1;
-        }
-        else if huffman.get_bits(reader, 1)? == 1 && coefficients[index] & bit == 0 {
-            if coefficients[index] > 0 {
-                coefficients[index] += bit;
-            }
-            else {
-                coefficients[index] -= bit;
-            }
+        } else if huffman.get_bits(reader, 1)? == 1 && *coefficient & bit == 0 {
+            *coefficient |= bit;
         }
     }
 

tests/reftest/mod.rs

@@ -91,6 +91,13 @@ fn reftest_decoder<T: std::io::Read>(mut decoder: jpeg::Decoder<T>, path: &Path,
     }).collect();
 
     if pixels.iter().any(|&a| a < 255) {
+        let output_path = path.with_file_name(format!("{}-actual.png", path.file_stem().unwrap().to_str().unwrap()));
+        let output = File::create(&output_path).unwrap();
+        let mut encoder = png::Encoder::new(output, info.width as u32, info.height as u32);
+        encoder.set(png::BitDepth::Eight);
+        encoder.set(png::ColorType::RGB);
+        encoder.write_header().expect("png failed to write header").write_image_data(&data).expect("png failed to write data");
+
         let output_path = path.with_file_name(format!("{}-diff.png", path.file_stem().unwrap().to_str().unwrap()));
         let output = File::create(&output_path).unwrap();
         let mut encoder = png::Encoder::new(output, info.width as u32, info.height as u32);