// Copyright © SixtyFPS GmbH <info@slint.dev>
// SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-1.2 OR LicenseRef-Slint-commercial

#![allow(clippy::identity_op)] // We use x + 0 a lot here for symmetry

//! This is the module for the functions that are drawing the pixels
//! on the line buffer

use super::{PhysicalLength, PhysicalRect};
use crate::graphics::{PixelFormat, Rgb8Pixel};
use crate::lengths::{PointLengths, SizeLengths};
use crate::software_renderer::fixed::Fixed;
use crate::Color;
use derive_more::{Add, Mul, Sub};
use integer_sqrt::IntegerSquareRoot;

/// Draw one line of the texture in the line buffer
///
pub(super) fn draw_texture_line(
    span: &PhysicalRect,
    line: PhysicalLength,
    texture: &super::SceneTexture,
    line_buffer: &mut [impl TargetPixel],
    extra_clip: i16,
) {
    let super::SceneTexture {
        data,
        format,
        pixel_stride,
        extra: super::SceneTextureExtra { colorize, alpha, rotation, dx, dy, off_x, off_y },
    } = *texture;

    let source_size = texture.source_size().cast::<i32>();
    let len = line_buffer.len();
    let y = line - span.origin.y_length();
    let y = if rotation.mirror_width() { span.size.height - y.get() - 1 } else { y.get() } as i32;

    let off_y = Fixed::<i32, 8>::from_fixed(off_y);
    let dx = Fixed::<i32, 8>::from_fixed(dx);
    let dy = Fixed::<i32, 8>::from_fixed(dy);
    let off_x = Fixed::<i32, 8>::from_fixed(off_x) + dx * extra_clip as i32;

    if !rotation.is_transpose() {
        let mut delta = dx;
        // The position where to start in the image array for a this row
        let mut init = Fixed::from_integer((off_y + dy * y).truncate() % source_size.height)
            * pixel_stride as i32;

        // the size of the tile in physical pixels in the target
        let tile_len = (Fixed::from_integer(source_size.width) / delta) as usize;
        // the amount of missing image pixel on one tile
        let mut remainder = Fixed::from_integer(source_size.width) % delta;
        // The position in image pixel where to get the image
        let mut pos;
        // the end index in the target buffer
        let mut end;
        // the accumulated error in image pixels
        let mut acc_err;
        if rotation.mirror_height() {
            let o = (off_x + (delta * (len as i32 - 1))) % Fixed::from_integer(source_size.width);
            pos = init + o;
            init += Fixed::from_integer(source_size.width);
            end = (o / delta) as usize + 1;
            acc_err = -delta + o % delta;
            delta = -delta;
            remainder = -remainder;
        } else {
            let o = off_x % Fixed::from_integer(source_size.width);
            pos = init + o;
            end = ((Fixed::from_integer(source_size.width) - o) / delta) as usize;
            acc_err = (Fixed::from_integer(source_size.width) - o) % delta;
            if acc_err != Fixed::default() {
                acc_err = delta - acc_err;
                end += 1;
            }
        }
        end = end.min(len);
        let mut begin = 0;
        while begin < len {
            fetch_blend_pixel(
                &mut line_buffer[begin..end],
                format,
                data,
                alpha,
                colorize,
                #[inline(always)]
                |bpp| {
                    let p = pos.truncate() as usize * bpp;
                    pos += delta;
                    p
                },
            );
            begin = end;
            end += tile_len;
            pos = init;
            pos += acc_err;
            if remainder != Fixed::from_integer(0) {
                acc_err -= remainder;
                let wrap = if rotation.mirror_height() {
                    acc_err >= Fixed::from_integer(0)
                } else {
                    acc_err < Fixed::from_integer(0)
                };
                if wrap {
                    acc_err += delta;
                    end += 1;
                }
            };
            end = end.min(len);
        }
    } else {
        let bpp = format.bpp();
        let col = off_x + dx * y;
        let col = (col.truncate() % source_size.width) as usize * bpp;
        let stride = pixel_stride as usize * bpp;
        let mut row_delta = dy;
        let tile_len = (Fixed::from_integer(source_size.height) / row_delta) as usize;
        let mut remainder = Fixed::from_integer(source_size.height) % row_delta;
        let mut end;
        let mut row_init = Fixed::default();
        let mut row;
        let mut acc_err;
        if rotation.mirror_height() {
            row_init = Fixed::from_integer(source_size.height);
            row =
                (off_y + (row_delta * (len as i32 - 1))) % Fixed::from_integer(source_size.height);
            end = (row / row_delta) as usize + 1;
            acc_err = -row_delta + row % row_delta;
            row_delta = -row_delta;
            remainder = -remainder;
        } else {
            row = off_y % Fixed::from_integer(source_size.height);
            end = ((Fixed::from_integer(source_size.height) - row) / row_delta) as usize;
            acc_err = (Fixed::from_integer(source_size.height) - row) % row_delta;
            if acc_err != Fixed::default() {
                acc_err = row_delta - acc_err;
                end += 1;
            }
        };
        end = end.min(len);
        let mut begin = 0;
        while begin < len {
            fetch_blend_pixel(
                &mut line_buffer[begin..end],
                format,
                data,
                alpha,
                colorize,
                #[inline(always)]
                |_| {
                    let pos = row.truncate() as usize * stride + col;
                    row += row_delta;
                    pos
                },
            );
            begin = end;
            end += tile_len;
            row = row_init;
            row += acc_err;
            if remainder != Fixed::from_integer(0) {
                acc_err -= remainder;
                let wrap = if rotation.mirror_height() {
                    acc_err >= Fixed::from_integer(0)
                } else {
                    acc_err < Fixed::from_integer(0)
                };
                if wrap {
                    acc_err += row_delta;
                    end += 1;
                }
            };
            end = end.min(len);
        }
    };

    fn fetch_blend_pixel(
        line_buffer: &mut [impl TargetPixel],
        format: PixelFormat,
        data: &[u8],
        alpha: u8,
        color: Color,
        mut pos: impl FnMut(usize) -> usize,
    ) {
        match format {
            PixelFormat::Rgb => {
                for pix in line_buffer {
                    let pos = pos(3);
                    let p = &data[pos..pos + 3];
                    if alpha == 0xff {
                        *pix = TargetPixel::from_rgb(p[0], p[1], p[2]);
                    } else {
                        pix.blend(PremultipliedRgbaColor::premultiply(Color::from_argb_u8(
                            alpha, p[0], p[1], p[2],
                        )))
                    }
                }
            }
            PixelFormat::Rgba => {
                if color.alpha() == 0 {
                    for pix in line_buffer {
                        let pos = pos(4);
                        let alpha = ((data[pos + 3] as u16 * alpha as u16) / 255) as u8;
                        let c = PremultipliedRgbaColor::premultiply(Color::from_argb_u8(
                            alpha,
                            data[pos + 0],
                            data[pos + 1],
                            data[pos + 2],
                        ));
                        pix.blend(c);
                    }
                } else {
                    for pix in line_buffer {
                        let pos = pos(4);
                        let alpha = ((data[pos + 3] as u16 * alpha as u16) / 255) as u8;
                        let c = PremultipliedRgbaColor::premultiply(Color::from_argb_u8(
                            alpha,
                            color.red(),
                            color.green(),
                            color.blue(),
                        ));
                        pix.blend(c);
                    }
                }
            }
            PixelFormat::RgbaPremultiplied => {
                if color.alpha() > 0 {
                    for pix in line_buffer {
                        let pos = pos(4);
                        let c = PremultipliedRgbaColor::premultiply(Color::from_argb_u8(
                            ((data[pos + 3] as u16 * alpha as u16) / 255) as u8,
                            color.red(),
                            color.green(),
                            color.blue(),
                        ));
                        pix.blend(c);
                    }
                } else if alpha == 0xff {
                    for pix in line_buffer {
                        let pos = pos(4);
                        let c = PremultipliedRgbaColor {
                            alpha: data[pos + 3],
                            red: data[pos + 0],
                            green: data[pos + 1],
                            blue: data[pos + 2],
                        };
                        pix.blend(c);
                    }
                } else {
                    for pix in line_buffer {
                        let pos = pos(4);
                        let c = PremultipliedRgbaColor {
                            alpha: (data[pos + 3] as u16 * alpha as u16 / 255) as u8,
                            red: (data[pos + 0] as u16 * alpha as u16 / 255) as u8,
                            green: (data[pos + 1] as u16 * alpha as u16 / 255) as u8,
                            blue: (data[pos + 2] as u16 * alpha as u16 / 255) as u8,
                        };
                        pix.blend(c);
                    }
                }
            }
            PixelFormat::AlphaMap => {
                for pix in line_buffer {
                    let pos = pos(1);
                    let c = PremultipliedRgbaColor::premultiply(Color::from_argb_u8(
                        ((data[pos] as u16 * alpha as u16) / 255) as u8,
                        color.red(),
                        color.green(),
                        color.blue(),
                    ));
                    pix.blend(c);
                }
            }
        };
    }
}

/// draw one line of the rounded rectangle in the line buffer
#[allow(clippy::unnecessary_cast)] // Coord
pub(super) fn draw_rounded_rectangle_line(
    span: &PhysicalRect,
    line: PhysicalLength,
    rr: &super::RoundedRectangle,
    line_buffer: &mut [impl TargetPixel],
    extra_left_clip: i16,
    extra_right_clip: i16,
) {
    /// This is an integer shifted by 4 bits.
    /// Note: this is not a "fixed point" because multiplication and sqrt operation operate to
    /// the shifted integer
    #[derive(Clone, Copy, PartialEq, Ord, PartialOrd, Eq, Add, Sub, Mul)]
    struct Shifted(u32);
    impl Shifted {
        const ONE: Self = Shifted(1 << 4);
        pub fn new(value: impl TryInto<u32>) -> Self {
            Self(value.try_into().map_err(|_| ()).unwrap() << 4)
        }
        pub fn floor(self) -> u32 {
            self.0 >> 4
        }
        pub fn ceil(self) -> u32 {
            (self.0 + Self::ONE.0 - 1) >> 4
        }
        pub fn saturating_sub(self, other: Self) -> Self {
            Self(self.0.saturating_sub(other.0))
        }
        pub fn sqrt(self) -> Self {
            Self(self.0.integer_sqrt())
        }
    }
    impl core::ops::Mul for Shifted {
        type Output = Shifted;
        fn mul(self, rhs: Self) -> Self::Output {
            Self(self.0 * rhs.0)
        }
    }
    let width = line_buffer.len();
    let y1 = (line - span.origin.y_length()) + rr.top_clip;
    let y2 = (span.origin.y_length() + span.size.height_length() - line) + rr.bottom_clip
        - PhysicalLength::new(1);
    let y = y1.min(y2);
    debug_assert!(y.get() >= 0,);
    let border = Shifted::new(rr.width.get());
    const ONE: Shifted = Shifted::ONE;
    const ZERO: Shifted = Shifted(0);
    let anti_alias = |x1: Shifted, x2: Shifted, process_pixel: &mut dyn FnMut(usize, u32)| {
        // x1 and x2 are the coordinate on the top and bottom of the intersection of the pixel
        // line and the curve.
        // `process_pixel` be called for the coordinate in the array and a coverage between 0..255
        // This algorithm just go linearly which is not perfect, but good enough.
        for x in x1.floor()..x2.ceil() {
            // the coverage is basically how much of the pixel should be used
            let cov = ((ONE + Shifted::new(x) - x1).0 << 8) / (ONE + x2 - x1).0;
            process_pixel(x as usize, cov);
        }
    };
    let rev = |x: Shifted| {
        (Shifted::new(width) + Shifted::new(rr.right_clip.get() + extra_right_clip))
            .saturating_sub(x)
    };
    let calculate_xxxx = |r: i16, y: i16| {
        let r = Shifted::new(r);
        // `y` is how far away from the center of the circle the current line is.
        let y = r - Shifted::new(y);
        // Circle equation: x = √(r² - y²)
        // Coordinate from the left edge: x' = r - x
        let x2 = r - (r * r).saturating_sub(y * y).sqrt();
        let x1 = r - (r * r).saturating_sub((y - ONE) * (y - ONE)).sqrt();
        let r2 = r.saturating_sub(border);
        let x4 = r - (r2 * r2).saturating_sub(y * y).sqrt();
        let x3 = r - (r2 * r2).saturating_sub((y - ONE) * (y - ONE)).sqrt();
        (x1, x2, x3, x4)
    };

    let (x1, x2, x3, x4, x5, x6, x7, x8) = if let Some(r) = rr.radius.as_uniform() {
        let (x1, x2, x3, x4) =
            if y.get() < r { calculate_xxxx(r, y.get()) } else { (ZERO, ZERO, border, border) };
        (x1, x2, x3, x4, rev(x4), rev(x3), rev(x2), rev(x1))
    } else {
        let (x1, x2, x3, x4) = if y1 < PhysicalLength::new(rr.radius.top_left) {
            calculate_xxxx(rr.radius.top_left, y.get())
        } else if y2 < PhysicalLength::new(rr.radius.bottom_left) {
            calculate_xxxx(rr.radius.bottom_left, y.get())
        } else {
            (ZERO, ZERO, border, border)
        };
        let (x5, x6, x7, x8) = if y1 < PhysicalLength::new(rr.radius.top_right) {
            let x = calculate_xxxx(rr.radius.top_right, y.get());
            (x.3, x.2, x.1, x.0)
        } else if y2 < PhysicalLength::new(rr.radius.bottom_right) {
            let x = calculate_xxxx(rr.radius.bottom_right, y.get());
            (x.3, x.2, x.1, x.0)
        } else {
            (border, border, ZERO, ZERO)
        };
        (x1, x2, x3, x4, rev(x5), rev(x6), rev(x7), rev(x8))
    };
    anti_alias(
        x1.saturating_sub(Shifted::new(rr.left_clip.get() + extra_left_clip)),
        x2.saturating_sub(Shifted::new(rr.left_clip.get() + extra_left_clip)),
        &mut |x, cov| {
            if x >= width {
                return;
            }
            let c = if border == ZERO { rr.inner_color } else { rr.border_color };
            let col = PremultipliedRgbaColor {
                alpha: (((c.alpha as u32) * cov as u32) / 255) as u8,
                red: (((c.red as u32) * cov as u32) / 255) as u8,
                green: (((c.green as u32) * cov as u32) / 255) as u8,
                blue: (((c.blue as u32) * cov as u32) / 255) as u8,
            };
            line_buffer[x].blend(col);
        },
    );
    if y < rr.width {
        // up or down border (x2 .. x7)
        let l = x2
            .ceil()
            .saturating_sub((rr.left_clip.get() + extra_left_clip) as u32)
            .min(width as u32) as usize;
        let r = x7.floor().min(width as u32) as usize;
        if l < r {
            TargetPixel::blend_slice(&mut line_buffer[l..r], rr.border_color)
        }
    } else {
        if border > ZERO {
            // 3. draw the border (between x2 and x3)
            if ONE + x2 <= x3 {
                TargetPixel::blend_slice(
                    &mut line_buffer[x2
                        .ceil()
                        .saturating_sub((rr.left_clip.get() + extra_left_clip) as u32)
                        .min(width as u32) as usize
                        ..x3.floor()
                            .saturating_sub((rr.left_clip.get() + extra_left_clip) as u32)
                            .min(width as u32) as usize],
                    rr.border_color,
                )
            }
            // 4. anti-aliasing for the contents (x3 .. x4)
            anti_alias(
                x3.saturating_sub(Shifted::new(rr.left_clip.get() + extra_left_clip)),
                x4.saturating_sub(Shifted::new(rr.left_clip.get() + extra_left_clip)),
                &mut |x, cov| {
                    if x >= width {
                        return;
                    }
                    let col = interpolate_color(cov, rr.border_color, rr.inner_color);
                    line_buffer[x].blend(col);
                },
            );
        }
        if rr.inner_color.alpha > 0 {
            // 5. inside (x4 .. x5)
            let begin = x4
                .ceil()
                .saturating_sub((rr.left_clip.get() + extra_left_clip) as u32)
                .min(width as u32);
            let end = x5.floor().min(width as u32);
            if begin < end {
                TargetPixel::blend_slice(
                    &mut line_buffer[begin as usize..end as usize],
                    rr.inner_color,
                )
            }
        }
        if border > ZERO {
            // 6. border anti-aliasing: x5..x6
            anti_alias(x5, x6, &mut |x, cov| {
                if x >= width {
                    return;
                }
                let col = interpolate_color(cov, rr.inner_color, rr.border_color);
                line_buffer[x].blend(col)
            });
            // 7. border x6 .. x7
            if ONE + x6 <= x7 {
                TargetPixel::blend_slice(
                    &mut line_buffer[x6.ceil().min(width as u32) as usize
                        ..x7.floor().min(width as u32) as usize],
                    rr.border_color,
                )
            }
        }
    }
    anti_alias(x7, x8, &mut |x, cov| {
        if x >= width {
            return;
        }
        let c = if border == ZERO { rr.inner_color } else { rr.border_color };
        let col = PremultipliedRgbaColor {
            alpha: (((c.alpha as u32) * (255 - cov) as u32) / 255) as u8,
            red: (((c.red as u32) * (255 - cov) as u32) / 255) as u8,
            green: (((c.green as u32) * (255 - cov) as u32) / 255) as u8,
            blue: (((c.blue as u32) * (255 - cov) as u32) / 255) as u8,
        };
        line_buffer[x].blend(col);
    });
}

// a is between 0 and 255. When 0, we get color1, when 255 we get color2
fn interpolate_color(
    a: u32,
    color1: PremultipliedRgbaColor,
    color2: PremultipliedRgbaColor,
) -> PremultipliedRgbaColor {
    let b = 255 - a;

    let al1 = color1.alpha as u32;
    let al2 = color2.alpha as u32;

    let a_ = a * al2;
    let b_ = b * al1;
    let m = a_ + b_;

    if m == 0 {
        return PremultipliedRgbaColor::default();
    }

    PremultipliedRgbaColor {
        alpha: (m / 255) as u8,
        red: ((b * color1.red as u32 + a * color2.red as u32) / 255) as u8,
        green: ((b * color1.green as u32 + a * color2.green as u32) / 255) as u8,
        blue: ((b * color1.blue as u32 + a * color2.blue as u32) / 255) as u8,
    }
}

pub(super) fn draw_gradient_line(
    rect: &PhysicalRect,
    line: PhysicalLength,
    g: &super::GradientCommand,
    mut buffer: &mut [impl TargetPixel],
    extra_left_clip: i16,
) {
    let fill_col1 = g.flags & 0b010 != 0;
    let fill_col2 = g.flags & 0b100 != 0;
    let invert_slope = g.flags & 0b1 != 0;

    let y = (line.get() - rect.min_y() + g.top_clip.get()) as i32;
    let size_y = (rect.height() + g.top_clip.get() + g.bottom_clip.get()) as i32;
    let start = g.start as i32;

    let (mut color1, mut color2) = (g.color1, g.color2);

    if g.start == 0 {
        let p = if invert_slope {
            (255 - start) * y / size_y
        } else {
            start + (255 - start) * y / size_y
        };
        if (fill_col1 || p >= 0) && (fill_col2 || p < 255) {
            let col = interpolate_color(p.clamp(0, 255) as u32, color1, color2);
            TargetPixel::blend_slice(buffer, col);
        }
        return;
    }

    let size_x = (rect.width() + g.left_clip.get() + g.right_clip.get()) as i32;

    let mut x = if invert_slope {
        (y * size_x * (255 - start)) / (size_y * start)
    } else {
        (size_y - y) * size_x * (255 - start) / (size_y * start)
    } + g.left_clip.get() as i32
        + extra_left_clip as i32;

    let len = ((255 * size_x) / start) as usize;

    if x < 0 {
        let l = (-x as usize).min(buffer.len());
        if invert_slope {
            if fill_col1 {
                TargetPixel::blend_slice(&mut buffer[..l], g.color1);
            }
        } else if fill_col2 {
            TargetPixel::blend_slice(&mut buffer[..l], g.color2);
        }
        buffer = &mut buffer[l..];
        x = 0;
    }

    if buffer.len() + x as usize > len {
        let l = len.saturating_sub(x as usize);
        if invert_slope {
            if fill_col2 {
                TargetPixel::blend_slice(&mut buffer[l..], g.color2);
            }
        } else if fill_col1 {
            TargetPixel::blend_slice(&mut buffer[l..], g.color1);
        }
        buffer = &mut buffer[..l];
    }

    if buffer.is_empty() {
        return;
    }

    if !invert_slope {
        core::mem::swap(&mut color1, &mut color2);
    }

    let dr = (((color2.red as i32 - color1.red as i32) * start) << 15) / (255 * size_x);
    let dg = (((color2.green as i32 - color1.green as i32) * start) << 15) / (255 * size_x);
    let db = (((color2.blue as i32 - color1.blue as i32) * start) << 15) / (255 * size_x);
    let da = (((color2.alpha as i32 - color1.alpha as i32) * start) << 15) / (255 * size_x);

    let mut r = ((color1.red as u32) << 15).wrapping_add((x * dr) as _);
    let mut g = ((color1.green as u32) << 15).wrapping_add((x * dg) as _);
    let mut b = ((color1.blue as u32) << 15).wrapping_add((x * db) as _);
    let mut a = ((color1.alpha as u32) << 15).wrapping_add((x * da) as _);

    if color1.alpha == 255 && color2.alpha == 255 {
        buffer.fill_with(|| {
            let pix = TargetPixel::from_rgb((r >> 15) as u8, (g >> 15) as u8, (b >> 15) as u8);
            r = r.wrapping_add(dr as _);
            g = g.wrapping_add(dg as _);
            b = b.wrapping_add(db as _);
            pix
        })
    } else {
        for pix in buffer {
            pix.blend(PremultipliedRgbaColor {
                red: (r >> 15) as u8,
                green: (g >> 15) as u8,
                blue: (b >> 15) as u8,
                alpha: (a >> 15) as u8,
            });
            r = r.wrapping_add(dr as _);
            g = g.wrapping_add(dg as _);
            b = b.wrapping_add(db as _);
            a = a.wrapping_add(da as _);
        }
    }
}

/// A color whose component have been pre-multiplied by alpha
///
/// The renderer operates faster on pre-multiplied color since it
/// caches the multiplication of its component
///
/// PremultipliedRgbaColor can be constructed from a [`Color`] with
/// the [`From`] trait. This conversion will pre-multiply the color
/// components
#[allow(missing_docs)]
#[derive(Clone, Copy, Debug, Default, bytemuck::Pod, bytemuck::Zeroable)]
#[repr(C)]
pub struct PremultipliedRgbaColor {
    pub red: u8,
    pub green: u8,
    pub blue: u8,
    pub alpha: u8,
}

/// Convert a non-premultiplied color to a premultiplied one
impl From<Color> for PremultipliedRgbaColor {
    fn from(col: Color) -> Self {
        Self::premultiply(col)
    }
}

impl PremultipliedRgbaColor {
    /// Convert a non premultiplied color to a premultiplied one
    fn premultiply(col: Color) -> Self {
        let a = col.alpha() as u16;
        Self {
            alpha: col.alpha(),
            red: (col.red() as u16 * a / 255) as u8,
            green: (col.green() as u16 * a / 255) as u8,
            blue: (col.blue() as u16 * a / 255) as u8,
        }
    }
}

/// Trait for the pixels in the buffer
pub trait TargetPixel: Sized + Copy {
    /// Blend a single pixel with a color
    fn blend(&mut self, color: PremultipliedRgbaColor);
    /// Blend a color to all the pixel in the slice.
    fn blend_slice(slice: &mut [Self], color: PremultipliedRgbaColor) {
        if color.alpha == u8::MAX {
            slice.fill(Self::from_rgb(color.red, color.green, color.blue))
        } else {
            for x in slice {
                Self::blend(x, color);
            }
        }
    }
    /// Create a pixel from the red, gree, blue component in the range 0..=255
    fn from_rgb(red: u8, green: u8, blue: u8) -> Self;

    /// Pixel which will be filled as the background in case the slint view has transparency
    fn background() -> Self {
        Self::from_rgb(0, 0, 0)
    }
}

impl TargetPixel for crate::graphics::image::Rgb8Pixel {
    fn blend(&mut self, color: PremultipliedRgbaColor) {
        let a = (u8::MAX - color.alpha) as u16;
        self.r = (self.r as u16 * a / 255) as u8 + color.red;
        self.g = (self.g as u16 * a / 255) as u8 + color.green;
        self.b = (self.b as u16 * a / 255) as u8 + color.blue;
    }

    fn from_rgb(r: u8, g: u8, b: u8) -> Self {
        Self::new(r, g, b)
    }
}

impl TargetPixel for PremultipliedRgbaColor {
    fn blend(&mut self, color: PremultipliedRgbaColor) {
        let a = (u8::MAX - color.alpha) as u16;
        self.red = (self.red as u16 * a / 255) as u8 + color.red;
        self.green = (self.green as u16 * a / 255) as u8 + color.green;
        self.blue = (self.blue as u16 * a / 255) as u8 + color.blue;
        self.alpha = (self.alpha as u16 + color.alpha as u16
            - (self.alpha as u16 * color.alpha as u16) / 255) as u8;
    }

    fn from_rgb(r: u8, g: u8, b: u8) -> Self {
        Self { red: r, green: g, blue: b, alpha: 255 }
    }

    fn background() -> Self {
        Self { red: 0, green: 0, blue: 0, alpha: 0 }
    }
}

/// A 16bit pixel that has 5 red bits, 6 green bits and  5 blue bits
#[repr(transparent)]
#[derive(Copy, Clone, Debug, PartialEq, Eq, Default, bytemuck::Pod, bytemuck::Zeroable)]
pub struct Rgb565Pixel(pub u16);

impl Rgb565Pixel {
    const R_MASK: u16 = 0b1111_1000_0000_0000;
    const G_MASK: u16 = 0b0000_0111_1110_0000;
    const B_MASK: u16 = 0b0000_0000_0001_1111;

    /// Return the red component as a u8.
    ///
    /// The bits are shifted so that the result is between 0 and 255
    fn red(self) -> u8 {
        ((self.0 & Self::R_MASK) >> 8) as u8
    }
    /// Return the green component as a u8.
    ///
    /// The bits are shifted so that the result is between 0 and 255
    fn green(self) -> u8 {
        ((self.0 & Self::G_MASK) >> 3) as u8
    }
    /// Return the blue component as a u8.
    ///
    /// The bits are shifted so that the result is between 0 and 255
    fn blue(self) -> u8 {
        ((self.0 & Self::B_MASK) << 3) as u8
    }
}

impl TargetPixel for Rgb565Pixel {
    fn blend(&mut self, color: PremultipliedRgbaColor) {
        let a = (u8::MAX - color.alpha) as u32;
        // convert to 5 bits
        let a = (a + 4) >> 3;

        // 00000ggg_ggg00000_rrrrr000_000bbbbb
        let expanded = (self.0 & (Self::R_MASK | Self::B_MASK)) as u32
            | (((self.0 & Self::G_MASK) as u32) << 16);

        // gggggggg_000rrrrr_rrr000bb_bbbbbb00
        let c =
            ((color.red as u32) << 13) | ((color.green as u32) << 24) | ((color.blue as u32) << 2);
        // gggggg00_000rrrrr_000000bb_bbb00000
        let c = c & 0b11111100_00011111_00000011_11100000;

        let res = expanded * a + c;

        self.0 = ((res >> 21) as u16 & Self::G_MASK)
            | ((res >> 5) as u16 & (Self::R_MASK | Self::B_MASK));
    }

    fn from_rgb(r: u8, g: u8, b: u8) -> Self {
        Self(((r as u16 & 0b11111000) << 8) | ((g as u16 & 0b11111100) << 3) | (b as u16 >> 3))
    }
}

impl From<Rgb8Pixel> for Rgb565Pixel {
    fn from(p: Rgb8Pixel) -> Self {
        Self::from_rgb(p.r, p.g, p.b)
    }
}

impl From<Rgb565Pixel> for Rgb8Pixel {
    fn from(p: Rgb565Pixel) -> Self {
        Rgb8Pixel { r: p.red(), g: p.green(), b: p.blue() }
    }
}

#[test]
fn rgb565() {
    let pix565 = Rgb565Pixel::from_rgb(0xff, 0x25, 0);
    let pix888: Rgb8Pixel = pix565.into();
    assert_eq!(pix565, pix888.into());

    let pix565 = Rgb565Pixel::from_rgb(0x56, 0x42, 0xe3);
    let pix888: Rgb8Pixel = pix565.into();
    assert_eq!(pix565, pix888.into());
}
