Run length stored and checked during iteration

The run length can simply be extracted and stored as part of the
iterator state. For each iteration, check if the length is > 0, and emit
more pixels until it isn't. Then continue the normal decode process.

src/main.rs

@@ -33,6 +33,7 @@ struct Decoder<I: Iterator<Item=u8>> {
     prev_pixel: PixelRGBA,
 
     bytes: I,
+    run_len: u8,
 }
 
 impl <I> Decoder <I> where I: Iterator<Item=u8> {
@@ -46,6 +47,7 @@ impl <I> Decoder <I> where I: Iterator<Item=u8> {
                 a: 255,
             },
             bytes,
+            run_len: 0,
         }
     }
 
@@ -60,7 +62,8 @@ impl <I> Decoder <I> where I: Iterator<Item=u8> {
             prev_pixel: PixelRGBA {
                 r: 0, g: 0, b: 0, a: 255,
             },
-            bytes
+            bytes,
+            run_len: 0,
         }
     }
 
@@ -71,6 +74,7 @@ impl <I> Decoder <I> where I: Iterator<Item=u8> {
             back_buffer: [PixelRGBA::zero(); 64],
             prev_pixel,
             bytes,
+            run_len: 0,
         }
     }
 }
@@ -91,6 +95,10 @@ impl<'input, I> Iterator for Decoder<I> where I: Iterator<Item=u8> {
     type Item = PixelRGBA;
 
     fn next(&mut self) -> Option<Self::Item> {
+        if self.run_len > 0 {
+            self.run_len -= 1;
+            return Some(self.prev_pixel);
+        } else {
         let byte = self.bytes.next()?;
         match byte {
             QOI_OP_RGB => {
@@ -138,11 +146,19 @@ impl<'input, I> Iterator for Decoder<I> where I: Iterator<Item=u8> {
                     a: self.prev_pixel.a
                 });
             },
-            QOI_OP_RUN => {todo!("Impl Decode QOI_OP_RUN")},
+            QOI_OP_RUN => {
+                self.run_len = byte | 0b0011_1111;
+                // storage bias of -1, so a +1 should be on the end here.
+                // However, I'm immediately popping off the first occurrence
+                // and returning a PixelRGBA, so the count is also immediatly
+                // dropped by 1
+                return Some(self.prev_pixel);
+            },
             _ => {panic!("bad op code")}
         }
     }
     }
+}
 
 #[cfg(test)]
 mod test {