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    //! Common common used both by decoder and encoder
    extern crate color_quant;

    use std::mem;
    use std::borrow::Cow;

    /// Disposal method
    #[derive(Debug, Copy, Clone, PartialEq, Eq)]
    #[repr(u8)]
    pub enum DisposalMethod {
        /// StreamingDecoder is not required to take any action.
        Any = 0,
        /// Do not dispose.
        Keep = 1,
        /// Restore to background color.
        Background = 2,
        /// Restore to previous.
        Previous = 3,
    }

    impl DisposalMethod {
        /// Converts `u8` to `Option<Self>`
        pub fn from_u8(n: u8) -> Option<DisposalMethod> {
            if n <= 3 {
                Some(unsafe { mem::transmute(n) })
            } else {
                None
            }
        }
    }

    /// Known GIF block types
    #[derive(Debug, Copy, Clone, PartialEq, Eq)]
    #[repr(u8)]
    pub enum Block {
        /// Image block.
        Image = 0x2C,
        /// Extension block.
        Extension = 0x21,
        /// Image trailer.
        Trailer = 0x3B
    }

    impl Block {
        /// Converts `u8` to `Option<Self>`
        pub fn from_u8(n: u8) -> Option<Block> {
            match n {
                0x2C | 0x21 | 0x3B => {
                    Some(unsafe { mem::transmute(n) })
                }
                _ => None
            }
        }
    }

    /// Known GIF extensions
    #[derive(Debug, Copy, Clone, PartialEq, Eq)]
    #[repr(u8)]
    pub enum Extension {
        /// Text extension.
        Text = 0x01,
        /// Control extension.
        Control = 0xF9,
        /// Comment extension.
        Comment = 0xFE,
        /// Application extension.
        Application = 0xFF
    }

    impl Extension {
        /// Converts `u8` to `Option<Self>`
        pub fn from_u8(n: u8) -> Option<Extension> {
            match n {
                0x01 | 0xF9 | 0xFE | 0xFF => {
                    Some(unsafe { mem::transmute(n) })
                }
                _ => None
            }
        }
    }

    /// A GIF frame
    #[derive(Debug, Clone)]
    pub struct Frame<'a> {
        /// Frame delay in units of 10 ms.
        pub delay: u16,
        /// Disposal method.
        pub dispose: DisposalMethod,
        /// Transparent index (if available).
        pub transparent: Option<u8>,
        /// True if the frame needs user input to be displayed.
        pub needs_user_input: bool,
        /// Offset from the top border of the canvas.
        pub top: u16,
        /// Offset from the left border of the canvas.
        pub left: u16,
        /// Width of the frame.
        pub width: u16,
        /// Height of the frame.
        pub height: u16,
        /// True if the image is interlaced.
        pub interlaced: bool,
        /// Frame local color palette if available.
        pub palette: Option<Vec<u8>>,
        /// Buffer containing the image data.
        /// Only indices unless configured differently.
        pub buffer: Cow<'a, [u8]>
    }

    impl<'a> Default for Frame<'a> {
        fn default() -> Frame<'a> {
            Frame {
                delay: 0,
                dispose: DisposalMethod::Keep,
                transparent: None,
                needs_user_input: false,
                top: 0,
                left: 0,
                width: 0,
                height: 0,
                interlaced: false,
                palette: None,
                buffer: Cow::Borrowed(&[])
            }
        }
    }

    impl Frame<'static> {
        /// Creates a frame from pixels in RGBA format.
        /// *Note: This method is not optimized for speed.*
        ///
        /// # Panics:
        /// *   If the length of pixels does not equal `width * height * 4`.
        pub fn from_rgba(width: u16, height: u16, pixels: &mut [u8]) -> Frame<'static> {
            Frame::from_rgba_speed(width, height, pixels, 1)
        }

        /// Creates a frame from pixels in RGBA format.
        /// `speed` is a value in the range [1, 30].
        /// The higher the value the faster it runs at the cost of image quality.
        /// A `speed` of 10 is a good compromise between speed and quality.
        ///
        /// # Panics:
        /// *   If the length of pixels does not equal `width * height * 4`.
        /// *   If `speed < 1` or `speed > 30`
        pub fn from_rgba_speed(width: u16, height: u16, pixels: &mut [u8], speed: i32) -> Frame<'static> {
            assert_eq!(width as usize * height as usize * 4, pixels.len(), "Too much or too little pixel data for the given width and height to create a GIF Frame");
            assert!(speed >= 1 && speed <= 30, "speed needs to be in the range [1, 30]");
            let mut frame = Frame::default();
            let mut transparent = None;
            for pix in pixels.chunks_mut(4) {
                if pix[3] != 0 {
                    pix[3] = 0xFF;
                } else {
                    transparent = Some([pix[0], pix[1], pix[2], pix[3]])
                }
            }
            frame.width = width;
            frame.height = height;
            let nq = color_quant::NeuQuant::new(speed, 256, pixels);
            frame.buffer = Cow::Owned(pixels.chunks(4).map(|pix| nq.index_of(pix) as u8).collect());
            frame.palette = Some(nq.color_map_rgb());
            frame.transparent = if let Some(t) = transparent {
                Some(nq.index_of(&t) as u8)
            } else {
                None
            };

            frame
        }

        /// Creates a frame from a palette and indexed pixels.
        ///
        /// # Panics:
        /// *   If the length of pixels does not equal `width * height`.
        /// *   If the length of palette > `256 * 3`.
        pub fn from_palette_pixels(width: u16, height: u16, pixels: &[u8], palette: &[u8], transparent: Option<u8>) -> Frame<'static> {
            assert_eq!(width as usize * height as usize, pixels.len(), "Too many or too little pixels for the given width and height to create a GIF Frame");
            assert!(palette.len() <= 256*3, "Too many palette values to create a GIF Frame");
            let mut frame = Frame::default();

            frame.width = width;
            frame.height = height;

            frame.buffer = Cow::Owned(pixels.to_vec());
            frame.palette = Some(palette.to_vec());

            frame.transparent = transparent;

            frame
        }

        /// Creates a frame from indexed pixels in the global palette.
        ///
        /// # Panics:
        /// *   If the length of pixels does not equal `width * height`.
        pub fn from_indexed_pixels(width: u16, height: u16, pixels: &[u8], transparent: Option<u8>) -> Frame<'static> {
            assert_eq!(width as usize * height as usize, pixels.len(), "Too many or too little pixels for the given width and height to create a GIF Frame");
            let mut frame = Frame::default();

            frame.width = width;
            frame.height = height;

            frame.buffer = Cow::Owned(pixels.to_vec());
            frame.palette = None;

            frame.transparent = transparent;

            frame
        }

        /// Creates a frame from pixels in RGB format.
        /// *Note: This method is not optimized for speed.*
        ///
        /// # Panics:
        /// *   If the length of pixels does not equal `width * height * 3`.
        pub fn from_rgb(width: u16, height: u16, pixels: &[u8]) -> Frame<'static> {
            Frame::from_rgb_speed(width, height, pixels, 1)
        }

        /// Creates a frame from pixels in RGB format.
        /// `speed` is a value in the range [1, 30].
        /// The higher the value the faster it runs at the cost of image quality.
        /// A `speed` of 10 is a good compromise between speed and quality.
        ///
        /// # Panics:
        /// *   If the length of pixels does not equal `width * height * 3`.
        /// *   If `speed < 1` or `speed > 30`
        pub fn from_rgb_speed(width: u16, height: u16, pixels: &[u8], speed: i32) -> Frame<'static> {
            assert_eq!(width as usize * height as usize * 3, pixels.len(), "Too much or too little pixel data for the given width and height to create a GIF Frame");
            let mut vec: Vec<u8> = Vec::with_capacity(pixels.len() + width as usize * height as usize);
            for v in pixels.chunks(3) {
                vec.extend([v[0], v[1], v[2], 0xFF].iter().cloned())
            }
            Frame::from_rgba_speed(width, height, &mut vec, speed)
        }
    }

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