fix transparent background bug (#17)

* fix transparent background bug

* handle edge case before loop for slices

Author: Martomate

src/decode.rs

@@ -35,6 +35,11 @@ where
     let mut px = Pixel::<N>::new().with_a(0xff);
     let mut px_rgba: Pixel<4>;
 
+    if matches!(data, [QOI_OP_RUN..=QOI_OP_RUN_END, ..]) {
+        px_rgba = px.as_rgba(0xff);
+        index[px_rgba.hash_index() as usize] = px_rgba;
+    }
+
     while let [px_out, ptail @ ..] = pixels {
         pixels = ptail;
         match data {
@@ -178,6 +183,7 @@ where
                 let (phead, ptail) = pixels.split_at_mut(run); // can't panic
                 phead.fill(px.into());
                 pixels = ptail;
+                index[px.hash_index() as usize] = px;
                 continue;
             }
             QOI_OP_DIFF..=QOI_OP_DIFF_END => {

tests/test_misc.rs

@@ -1,5 +1,5 @@
 use qoi::{
-    consts::{QOI_OP_RGB, QOI_OP_RUN},
+    consts::{QOI_OP_RGB, QOI_OP_RUN, QOI_OP_INDEX},
     decode_to_vec, Channels, ColorSpace, Header, Result,
 };
 
@@ -22,6 +22,18 @@ fn test_start_with_qoi_op_run() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn test_start_with_qoi_op_run_and_use_index() -> Result<()> {
+    let header = Header::try_new(4, 1, Channels::Rgba, ColorSpace::Linear)?;
+    let mut qoi_data: Vec<_> = header.encode().into_iter().collect();
+    qoi_data.extend([QOI_OP_RUN | 1, QOI_OP_RGB, 10, 20, 30, QOI_OP_INDEX | 53]);
+    qoi_data.extend([0; 7]);
+    qoi_data.push(1);
+    let (_, decoded) = decode_to_vec(&qoi_data)?;
+    assert_eq!(decoded, vec![0, 0, 0, 255, 0, 0, 0, 255, 10, 20, 30, 255, 0, 0, 0, 255]);
+    Ok(())
+}
+
 #[cfg(target_endian = "big")]
 #[test]
 fn test_big_endian() {