Ensure palette is trimmed after depth reduction

Fixes #159

src/png/mod.rs

@@ -190,12 +190,13 @@ impl PngData {
         // Palette
         if let Some(ref palette) = self.raw.palette {
             let mut palette_data = Vec::with_capacity(palette.len() * 3);
-            for px in palette {
+            let max_palette_size = 1 << (self.raw.ihdr.bit_depth.as_u8() as usize);
+            for px in palette.iter().take(max_palette_size) {
                 palette_data.extend_from_slice(px.rgb().as_slice());
             }
             write_png_block(b"PLTE", &palette_data, &mut output);
             let num_transparent =
-                palette.iter().enumerate().fold(
+                palette.iter().take(max_palette_size).enumerate().fold(
                     0,
                     |prev, (index, px)| {
                         if px.a != 255 {

src/reduction/bit_depth.rs

@@ -68,7 +68,7 @@ pub fn reduce_bit_depth(png: &PngImage, minimum_bits: usize) -> Option<PngImage>
             bit_depth: BitDepth::Eight,
             ..png.ihdr
         },
-        palette: png.palette.clone(),
+        palette: None,
         transparency_pixel: png.transparency_pixel.clone(),
         aux_headers: png.aux_headers.clone(),
     })

tests/filters.rs

@@ -55,6 +55,11 @@ fn test_it_converts(
 
     assert_eq!(png.raw.ihdr.color_type, color_type_out);
     assert_eq!(png.raw.ihdr.bit_depth, bit_depth_out);
+    if let Some(palette) = png.raw.palette.as_ref() {
+        assert!(palette.len() <= 1 << (png.raw.ihdr.bit_depth.as_u8() as usize));
+    } else {
+        assert!(png.raw.ihdr.color_type != ColorType::Indexed);
+    }
 
     remove_file(output).ok();
 }

tests/regression.rs

@@ -33,8 +33,8 @@ fn test_it_converts(
     let (output, opts) = custom.unwrap_or_else(|| get_opts(&input));
     let png = PngData::new(&input, opts.fix_errors).unwrap();
 
-    assert_eq!(png.raw.ihdr.color_type, color_type_in);
-    assert_eq!(png.raw.ihdr.bit_depth, bit_depth_in);
+    assert_eq!(png.raw.ihdr.color_type, color_type_in, "test file is broken");
+    assert_eq!(png.raw.ihdr.bit_depth, bit_depth_in, "test file is broken");
 
     match oxipng::optimize(&InFile::Path(input), &output, &opts) {
         Ok(_) => (),
@@ -51,8 +51,13 @@ fn test_it_converts(
         }
     };
 
-    assert_eq!(png.raw.ihdr.color_type, color_type_out);
-    assert_eq!(png.raw.ihdr.bit_depth, bit_depth_out);
+    assert_eq!(png.raw.ihdr.color_type, color_type_out, "optimized to wrong color type");
+    assert_eq!(png.raw.ihdr.bit_depth, bit_depth_out, "optimized to wrong bit depth");
+    if let Some(palette) = png.raw.palette.as_ref() {
+        assert!(palette.len() <= 1 << (png.raw.ihdr.bit_depth.as_u8() as usize));
+    } else {
+        assert!(png.raw.ihdr.color_type != ColorType::Indexed);
+    }
 
     remove_file(output).ok();
 }
@@ -572,3 +577,15 @@ fn issue_153() {
         BitDepth::Eight,
     );
 }
+
+#[test]
+fn issue_159() {
+    test_it_converts(
+        "tests/files/issue-159.png",
+        None,
+        ColorType::Indexed,
+        BitDepth::One,
+        ColorType::Indexed,
+        BitDepth::One,
+    );
+}