Fix failure to optimize on certain grayscale images

Closes #89

CHANGELOG.md

@@ -1,5 +1,6 @@
 ### Version 0.18.1 (unreleased)
  - Bump `rayon` to 0.9
+ - Fix failure to optimize on certain grayscale images ([#89](https://github.com/shssoichiro/oxipng/issues/89))
 
 ### Version 0.18.0
  - Bump `itertools` to 0.7

src/reduction.rs

@@ -10,7 +10,11 @@ pub fn reduce_bit_depth_8_or_less(png: &mut PngData) -> bool {
     for line in png.scan_lines() {
         let bit_vec = BitVec::from_bytes(&line.data);
         for (i, bit) in bit_vec.iter().enumerate() {
-            let bit_index = bit_depth - (i % bit_depth);
+            let bit_index = if png.ihdr_data.color_type == ColorType::Indexed {
+                bit_depth - (i % bit_depth)
+            } else {
+                i % bit_depth
+            };
             if bit && bit_index > allowed_bits {
                 allowed_bits = bit_index.next_power_of_two();
                 if allowed_bits == bit_depth {

tests/regression.rs

@@ -308,3 +308,20 @@ fn issue_82() {
         BitDepth::Four,
     );
 }
+
+#[test]
+fn issue_89() {
+    let input = PathBuf::from("tests/files/issue-89.png");
+    let opts = get_opts(&input);
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::RGBA,
+        BitDepth::Eight,
+        ColorType::Grayscale,
+        BitDepth::Eight,
+    );
+}