use std::borrow::Cow;
use std::collections::HashMap;
use std::collections::HashSet;

/// 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> {
        match n {
            0 => Some(DisposalMethod::Any),
            1 => Some(DisposalMethod::Keep),
            2 => Some(DisposalMethod::Background),
            3 => Some(DisposalMethod::Previous),
            _ => None
        }
    }
}

/// Known GIF block labels.
///
/// Note that the block uniquely specifies the layout of bytes that follow and how they are
/// framed. For example, the header always has a fixed length but is followed by a variable amount
/// of additional data. An image descriptor may be followed by a local color table depending on
/// information read in it. Therefore, it doesn't make sense to continue parsing after encountering
/// an unknown block as the semantics of following bytes are unclear.
///
/// The extension block provides a common framing for an arbitrary amount of application specific
/// data which may be ignored.
#[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 => Some(Block::Image),
            0x21 => Some(Block::Extension),
            0x3B => Some(Block::Trailer),
            _ => None
        }
    }
}

/// A newtype wrapper around an arbitrary extension ID.
///
/// An extension is some amount of byte data organized in sub-blocks so that one can skip over it
/// without knowing the semantics. Though technically you likely want to use a `Application`
/// extension, the library tries to stay flexible here.
///
/// This allows us to customize the set of impls compared to a raw `u8`. It also clarifies the
/// intent and gives some inherent methods for interoperability with known extension types.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub struct AnyExtension(pub u8);

/// Known GIF extension labels.
///
/// These are extensions which may be interpreted by the library and to which a specification with
/// the internal data layout is known.
#[derive(Debug, Copy, Clone, PartialEq, Eq)]
#[repr(u8)]
pub enum Extension {
    /// Plain Text extension.
    ///
    /// This instructs the decoder to render a text as characters in a grid of cells, in a
    /// mono-spaced font of its choosing. This is seldom actually implemented and ignored by
    /// ImageMagick. The color is always taken from the global table which further complicates any
    /// use. No real information on the frame sequencing of this block is available in the
    /// standard.
    Text = 0x01,
    /// Control extension.
    Control = 0xF9,
    /// Comment extension.
    Comment = 0xFE,
    /// Application extension.
    ///
    /// See [ImageMagick] for an idea of commonly recognized extensions.
    ///
    /// [ImageMagick]: https://github.com/ImageMagick/ImageMagick/blob/b0b58c6303195928060f55f9c3ca8233ab7f7733/coders/gif.c#L1128
    Application = 0xFF,
}

impl AnyExtension {
    /// Decode the label as a known extension.
    pub fn into_known(self) -> Option<Extension> {
        Extension::from_u8(self.0)
    }
}

impl From<Extension> for AnyExtension {
    fn from(ext: Extension) -> Self {
        AnyExtension(ext as u8)
    }
}

impl Extension {
    /// Converts `u8` to a `Extension` if it is known.
    pub fn from_u8(n: u8) -> Option<Extension> {
        match n {
            0x01 => Some(Extension::Text),
            0xF9 => Some(Extension::Control),
            0xFE => Some(Extension::Comment),
            0xFF => Some(Extension::Application),
            _ => 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.
    ///
    /// This is a lossy method. The `gif` format does not support arbitrary alpha but only a 1-bit
    /// transparency mask per pixel. Any non-zero alpha value will be interpreted as a fully opaque
    /// pixel. Additionally, only 256 colors can appear in a single frame. The palette will be
    /// reduced by the NeuQuant algorithm if necessary. Different frames have independent palettes.
    ///
    /// *Note: This method is not optimized for speed.*
    ///
    /// # Panics:
    /// *   If the length of pixels does not equal `width * height * 4`.
    #[cfg(feature = "color_quant")]
    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.
    ///
    /// This is a lossy method. The `gif` format does not support arbitrary alpha but only a 1-bit
    /// transparency mask per pixel. Any non-zero alpha value will be interpreted as a fully opaque
    /// pixel. Additionally, only 256 colors can appear in a single frame. The palette will be
    /// reduced by the NeuQuant algorithm if necessary. Different frames have independent palettes.
    ///
    /// # Panics:
    /// *   If the length of pixels does not equal `width * height * 4`.
    /// *   If `speed < 1` or `speed > 30`
    #[cfg(feature = "color_quant")]
    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 transparent = None;
        for pix in pixels.chunks_exact_mut(4) {
            if pix[3] != 0 {
                pix[3] = 0xFF;
            } else {
                transparent = Some([pix[0], pix[1], pix[2], pix[3]])
            }
        }

        // Attempt to build a palette of all colors. If we go over 256 colors,
        // switch to the NeuQuant algorithm.
        let mut colors: HashSet<(u8, u8, u8, u8)> = HashSet::new();
        for pixel in pixels.chunks_exact(4) {
            if colors.insert((pixel[0], pixel[1], pixel[2], pixel[3])) && colors.len() > 256 {
                // > 256 colours, let's use NeuQuant.
                let nq = color_quant::NeuQuant::new(speed, 256, pixels);

                return Frame {
                    width,
                    height,
                    buffer: Cow::Owned(pixels.chunks_exact(4).map(|pix| nq.index_of(pix) as u8).collect()),
                    palette: Some(nq.color_map_rgb()),
                    transparent: transparent.map(|t| nq.index_of(&t) as u8),
                    ..Frame::default()
                };
            }
        }

        // Palette size <= 256 elements, we can build an exact palette.
        let mut colors_vec: Vec<(u8, u8, u8, u8)> = colors.into_iter().collect();
        colors_vec.sort();
        let palette = colors_vec.iter().flat_map(|&(r, g, b, _a)| [r, g, b]).collect();
        let colors_lookup: HashMap<(u8, u8, u8, u8), u8> =  colors_vec.into_iter().zip(0..=255).collect();

        let index_of = | pixel: &[u8] |
            *colors_lookup.get(&(pixel[0], pixel[1], pixel[2], pixel[3])).unwrap();

        return Frame {
            width,
            height,
            buffer: Cow::Owned(pixels.chunks_exact(4).map(|pix| index_of(pix)).collect()),
            palette: Some(palette),
            transparent: transparent.map(|t| index_of(&t)),
            ..Frame::default()
        }
    }

    /// 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");

        Frame {
            width,
            height,
            buffer: Cow::Owned(pixels.to_vec()),
            palette: Some(palette.to_vec()),
            transparent,
            ..Frame::default()
        }
    }

    /// 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");

        Frame {
            width,
            height,
            buffer: Cow::Owned(pixels.to_vec()),
            palette: None,
            transparent,
            ..Frame::default()
        }
    }

    /// Creates a frame from pixels in RGB format.
    ///
    /// This is a lossy method. In the `gif` format only 256 colors can appear in a single frame.
    /// The palette will be reduced by the NeuQuant algorithm if necessary. Different frames have
    /// independent palettes.
    ///
    /// *Note: This method is not optimized for speed.*
    ///
    /// # Panics:
    /// *   If the length of pixels does not equal `width * height * 3`.
    #[cfg(feature = "color_quant")]
    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].
    ///
    /// This is a lossy method. In the `gif` format only 256 colors can appear in a single frame.
    /// The palette will be reduced by the NeuQuant algorithm if necessary. Different frames have
    /// independent palettes.
    ///
    /// 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`
    #[cfg(feature = "color_quant")]
    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_exact(3) {
            vec.extend_from_slice(&[v[0], v[1], v[2], 0xFF])
        }
        Frame::from_rgba_speed(width, height, &mut vec, speed)
    }

    pub(crate) fn required_bytes(&self) -> usize {
        usize::from(self.width) * usize::from(self.height)
    }
}

#[test]
#[cfg(feature = "color_quant")]
// Creating the `colors_lookup` hashmap in Frame::from_rgba_speed panics due to
// overflow while bypassing NeuQuant and zipping a RangeFrom with 256 colors.
// Changing .zip(0_u8..) to .zip(0_u8..=255) fixes this issue.
fn rgba_speed_avoid_panic_256_colors() {
    let side = 16;
    let pixel_data: Vec<u8> = (0..=255).map(|a| vec![a, a, a]).flatten().collect();
    Frame::from_rgb(side, side, &pixel_data);
}