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    //!  Decoding of GIF Images
    //!
    //!  GIF (Graphics Interchange Format) is an image format that supports lossless compression.
    //!
    //!  # Related Links
    //!  * <http://www.w3.org/Graphics/GIF/spec-gif89a.txt> - The GIF Specification
    //!
    //! # Examples
    //! ```rust,no_run
    //! use image::gif::{Decoder, Encoder};
    //! use image::{ImageDecoder, AnimationDecoder};
    //! use std::fs::File;
    //! # fn main() -> std::io::Result<()> {
    //! // Decode a gif into frames
    //! let file_in = File::open("foo.gif")?;
    //! let mut decoder = Decoder::new(file_in).unwrap();
    //! let frames = decoder.into_frames();
    //! let frames = frames.collect_frames().expect("error decoding gif");
    //!
    //! // Encode frames into a gif and save to a file
    //! let mut file_out = File::open("out.gif")?;
    //! let mut encoder = Encoder::new(file_out);
    //! encoder.encode_frames(frames.into_iter());
    //! # Ok(())
    //! # }
    //! ```
    #![cfg_attr(feature = "cargo-clippy", allow(while_let_loop))]

    extern crate gif;
    extern crate num_rational;

    use std::clone::Clone;
    use std::io::{Cursor, Read, Write};

    use self::gif::{ColorOutput, SetParameter};
    pub use self::gif::{DisposalMethod, Frame};

    use animation;
    use buffer::{ImageBuffer, Pixel};
    use color;
    use color::Rgba;
    use image::{AnimationDecoder, ImageDecoder, ImageError, ImageResult};
    use num_rational::Ratio;

    /// GIF decoder
    pub struct Decoder<R: Read> {
        reader: gif::Reader<R>,
    }

    impl<R: Read> Decoder<R> {
        /// Creates a new decoder that decodes the input steam ```r```
        pub fn new(r: R) -> ImageResult<Decoder<R>> {
            let mut decoder = gif::Decoder::new(r);
            decoder.set(ColorOutput::RGBA);

            Ok(Decoder {
                reader: decoder.read_info()?,
            })
        }
    }

    impl<R: Read> ImageDecoder for Decoder<R> {
        type Reader = Cursor<Vec<u8>>;

        fn dimensions(&self) -> (u64, u64) {
            (self.reader.width() as u64, self.reader.height() as u64)
        }

        fn colortype(&self) -> color::ColorType {
            color::ColorType::RGBA(8)
        }

        fn into_reader(self) -> ImageResult<Self::Reader> {
            Ok(Cursor::new(self.read_image()?))
        }

        fn read_image(mut self) -> ImageResult<Vec<u8>> {
            if self.reader.next_frame_info()?.is_some() {
                let mut buf = vec![0; self.reader.buffer_size()];
                self.reader.read_into_buffer(&mut buf)?;
                Ok(buf)
            } else {
                Err(ImageError::ImageEnd)
            }
        }
    }

    struct GifFrameIterator<R: Read> {
        reader: gif::Reader<R>,

        width: u32,
        height: u32,

        background_img: ImageBuffer<Rgba<u8>, Vec<u8>>,
        non_disposed_frame: ImageBuffer<Rgba<u8>, Vec<u8>>,

        left: u32,
        top: u32,
        delay: Ratio<u16>,
        dispose: DisposalMethod,
    }

    impl<R: Read> GifFrameIterator<R> {
        fn new(decoder: Decoder<R>) -> GifFrameIterator<R> {
            let (width, height) = decoder.dimensions();

            // TODO: Avoid this cast
            let (width, height) = (width as u32, height as u32);

            // set the background color to be either the bg_color defined in the gif
            // or a transparent pixel.
            let background_color_option = decoder.reader.bg_color();
            let mut background_color = vec![0; 4];
            let background_pixel = {
                let global_palette = decoder.reader.global_palette();
                match background_color_option {
                    Some(index) => {
                        // find the color by looking in the global palette
                        match global_palette {
                            // take the color from the palette that is at the index defined
                            // by background_color_option
                            Some(slice) => {
                                background_color.clone_from_slice(&slice[index..(index + 4)]);
                                Rgba::from_slice(&background_color)
                            }
                            // if there is no global palette, assign the background color to be
                            // transparent
                            None => Rgba::from_slice(&[0, 0, 0, 0]),
                        }
                    }
                    // return a transparent background color
                    None => Rgba::from_slice(&[0, 0, 0, 0]),
                }
            };

            // create the background image to use later
            let background_img = ImageBuffer::from_pixel(width, height, *background_pixel);

            // the background image is the first non disposed frame
            let non_disposed_frame = background_img.clone();

            GifFrameIterator {
                reader: decoder.reader,
                width,
                height,
                background_img,
                non_disposed_frame,
                left: 0,
                top: 0,
                delay: Ratio::new(0, 1),
                dispose: DisposalMethod::Any
            }
        }
    }

    impl<R: Read> Iterator for GifFrameIterator<R> {
        type Item = ImageResult<animation::Frame>;

        fn next(&mut self) -> Option<ImageResult<animation::Frame>> {
            // begin looping over each frame
            match self.reader.next_frame_info() {
                Ok(frame_info) => {
                    if let Some(frame) = frame_info {
                        self.left = u32::from(frame.left);
                        self.top = u32::from(frame.top);

                        // frame.delay is in units of 10ms so frame.delay*10 is in ms
                        self.delay = Ratio::new(frame.delay * 10, 1);
                        self.dispose = frame.dispose;
                    } else {
                        // no more frames
                        return None;
                    }
                },
                Err(err) => return Some(Err(err.into())),
            }

            let mut vec = vec![0; self.reader.buffer_size()];
            if let Err(err) = self.reader.fill_buffer(&mut vec) {
                return Some(Err(err.into()));
            }

            // create the image buffer from the raw frame
            let mut image_buffer = match ImageBuffer::from_raw(self.width, self.height, vec) {
                Some(buffer) => buffer,
                None => return Some(Err(ImageError::UnsupportedError(
                    "Unknown error occured while reading gif frame".into()
                ))),
            };

            // loop over all pixels, checking if any pixels from the non disposed
            // frame need to be used
            for (x, y, pixel) in image_buffer.enumerate_pixels_mut() {
                let previous_img_buffer = &self.non_disposed_frame;
                let mut adjusted_pixel: &mut Rgba<u8> = pixel;
                let previous_pixel: &Rgba<u8> = previous_img_buffer.get_pixel(x, y);

                let pixel_alpha = adjusted_pixel.channels()[3];

                // If a pixel is not visible then we show the non disposed frame pixel instead
                if pixel_alpha == 0 {
                    adjusted_pixel.blend(previous_pixel);
                }
            }

            let frame = animation::Frame::from_parts(
                image_buffer.clone(), self.left, self.top, self.delay
            );

            match self.dispose {
                DisposalMethod::Any => {
                    // do nothing
                    // (completely replace this frame with the next)
                }
                DisposalMethod::Keep => {
                    // do not dispose
                    // (keep pixels from this frame)
                    self.non_disposed_frame = image_buffer;
                }
                DisposalMethod::Background => {
                    // restore to background color
                    // (background shows through transparent pixels in the next frame)
                    self.non_disposed_frame = self.background_img.clone();
                }
                DisposalMethod::Previous => {
                    // restore to previous
                    // (dispose frames leaving the last none disposal frame)
                }
            };

            Some(Ok(frame))
        }
    }

    impl<'a, R: Read + 'a> AnimationDecoder<'a> for Decoder<R> {
        fn into_frames(self) -> animation::Frames<'a> {
            animation::Frames::new(Box::new(GifFrameIterator::new(self)))
        }
    }

    /// GIF encoder.
    pub struct Encoder<W: Write> {
        w: Option<W>,
        gif_encoder: Option<gif::Encoder<W>>,
    }

    impl<W: Write> Encoder<W> {
        /// Creates a new GIF encoder.
        pub fn new(w: W) -> Encoder<W> {
            Encoder {
                w: Some(w),
                gif_encoder: None,
            }
        }
        /// Encodes a frame.
        pub fn encode(&mut self, frame: &Frame) -> ImageResult<()> {
            let result;
            if let Some(ref mut encoder) = self.gif_encoder {
                result = encoder.write_frame(frame).map_err(|err| err.into());
            } else {
                let writer = self.w.take().unwrap();
                let mut encoder = try!(gif::Encoder::new(writer, frame.width, frame.height, &[]));
                result = encoder.write_frame(&frame).map_err(|err| err.into());
                self.gif_encoder = Some(encoder);
            }
            result
        }

        /// Encodes Frames.
        /// Consider using `try_encode_frames` instead to encode an `animation::Frames` like iterator.
        pub fn encode_frames<F>(&mut self, frames: F) -> ImageResult<()>
        where
            F: IntoIterator<Item = animation::Frame>
        {
            for img_frame in frames {
                self.encode_single_frame(img_frame)?;
            }
            Ok(())
        }

        /// Try to encode a collection of `ImageResult<animation::Frame>` objects.
        /// Use this function to encode an `animation::Frames` like iterator.
        /// Whenever an `Err` item is encountered, that value is returned without further actions.
        pub fn try_encode_frames<F>(&mut self, frames: F) -> ImageResult<()>
        where
            F: IntoIterator<Item = ImageResult<animation::Frame>>
        {
            for img_frame in frames {
                self.encode_single_frame(img_frame?)?;
            }
            Ok(())
        }

        fn encode_single_frame(&mut self, img_frame: animation::Frame) -> ImageResult<()> {
            // get the delay before converting img_frame
            let frame_delay = img_frame.delay().to_integer();
            // convert img_frame into RgbaImage
            let rbga_frame = img_frame.into_buffer();

            // Create the gif::Frame from the animation::Frame
            let mut frame = Frame::from_rgba(rbga_frame.width() as u16, rbga_frame.height() as u16, &mut rbga_frame.into_raw());
            frame.delay = frame_delay;

            // encode the gif::Frame
            self.encode(&frame)
        }
    }

    impl From<gif::DecodingError> for ImageError {
        fn from(err: gif::DecodingError) -> ImageError {
            use self::gif::DecodingError::*;
            match err {
                Format(desc) | Internal(desc) => ImageError::FormatError(desc.into()),
                Io(io_err) => ImageError::IoError(io_err),
            }
        }
    }

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