//! Display list, software rasterizer, Metal GPU compositor.
//!
//! Walks a layout tree, generates paint commands, and rasterizes them
//! into a BGRA pixel buffer suitable for display via CoreGraphics.

use we_css::values::Color;
use we_layout::{LayoutBox, LayoutTree, TextLine};
use we_style::computed::{BorderStyle, TextDecoration};
use we_text::font::Font;

/// A paint command in the display list.
#[derive(Debug)]
pub enum PaintCommand {
    /// Fill a rectangle with a solid color.
    FillRect {
        x: f32,
        y: f32,
        width: f32,
        height: f32,
        color: Color,
    },
    /// Draw a text fragment at a position with styling.
    DrawGlyphs {
        line: TextLine,
        font_size: f32,
        color: Color,
    },
}

/// A flat list of paint commands in painter's order.
pub type DisplayList = Vec<PaintCommand>;

/// Build a display list from a layout tree.
///
/// Walks the tree in depth-first pre-order (painter's order):
/// backgrounds first, then borders, then text on top.
pub fn build_display_list(tree: &LayoutTree) -> DisplayList {
    let mut list = DisplayList::new();
    paint_box(&tree.root, &mut list);
    list
}

fn paint_box(layout_box: &LayoutBox, list: &mut DisplayList) {
    paint_background(layout_box, list);
    paint_borders(layout_box, list);
    paint_text(layout_box, list);

    // Recurse into children.
    for child in &layout_box.children {
        paint_box(child, list);
    }
}

fn paint_background(layout_box: &LayoutBox, list: &mut DisplayList) {
    let bg = layout_box.background_color;
    // Only paint if the background is not fully transparent and the box has area.
    if bg.a == 0 {
        return;
    }
    if layout_box.rect.width > 0.0 && layout_box.rect.height > 0.0 {
        // Background covers the padding box (content + padding), not including border.
        list.push(PaintCommand::FillRect {
            x: layout_box.rect.x,
            y: layout_box.rect.y,
            width: layout_box.rect.width,
            height: layout_box.rect.height,
            color: bg,
        });
    }
}

fn paint_borders(layout_box: &LayoutBox, list: &mut DisplayList) {
    let b = &layout_box.border;
    let r = &layout_box.rect;
    let styles = &layout_box.border_styles;
    let colors = &layout_box.border_colors;

    // Border box starts at content origin minus padding and border.
    let bx = r.x - layout_box.padding.left - b.left;
    let by = r.y - layout_box.padding.top - b.top;
    let bw = b.left + layout_box.padding.left + r.width + layout_box.padding.right + b.right;
    let bh = b.top + layout_box.padding.top + r.height + layout_box.padding.bottom + b.bottom;

    // Top border
    if b.top > 0.0 && styles[0] != BorderStyle::None && styles[0] != BorderStyle::Hidden {
        list.push(PaintCommand::FillRect {
            x: bx,
            y: by,
            width: bw,
            height: b.top,
            color: colors[0],
        });
    }
    // Right border
    if b.right > 0.0 && styles[1] != BorderStyle::None && styles[1] != BorderStyle::Hidden {
        list.push(PaintCommand::FillRect {
            x: bx + bw - b.right,
            y: by,
            width: b.right,
            height: bh,
            color: colors[1],
        });
    }
    // Bottom border
    if b.bottom > 0.0 && styles[2] != BorderStyle::None && styles[2] != BorderStyle::Hidden {
        list.push(PaintCommand::FillRect {
            x: bx,
            y: by + bh - b.bottom,
            width: bw,
            height: b.bottom,
            color: colors[2],
        });
    }
    // Left border
    if b.left > 0.0 && styles[3] != BorderStyle::None && styles[3] != BorderStyle::Hidden {
        list.push(PaintCommand::FillRect {
            x: bx,
            y: by,
            width: b.left,
            height: bh,
            color: colors[3],
        });
    }
}

fn paint_text(layout_box: &LayoutBox, list: &mut DisplayList) {
    for line in &layout_box.lines {
        let color = line.color;
        let font_size = line.font_size;

        // Record index before pushing glyphs so we can insert the background
        // before the text in painter's order.
        let glyph_idx = list.len();

        list.push(PaintCommand::DrawGlyphs {
            line: line.clone(),
            font_size,
            color,
        });

        // Draw underline as a 1px line below the baseline.
        if line.text_decoration == TextDecoration::Underline && line.width > 0.0 {
            let baseline_y = line.y + font_size;
            let underline_y = baseline_y + 2.0;
            list.push(PaintCommand::FillRect {
                x: line.x,
                y: underline_y,
                width: line.width,
                height: 1.0,
                color,
            });
        }

        // Draw inline background if not transparent.
        if line.background_color.a > 0 && line.width > 0.0 {
            list.insert(
                glyph_idx,
                PaintCommand::FillRect {
                    x: line.x,
                    y: line.y,
                    width: line.width,
                    height: font_size * 1.2,
                    color: line.background_color,
                },
            );
        }
    }
}

/// Software renderer that paints a display list into a BGRA pixel buffer.
pub struct Renderer {
    width: u32,
    height: u32,
    /// BGRA pixel data, row-major, top-to-bottom.
    buffer: Vec<u8>,
}

impl Renderer {
    /// Create a new renderer with the given dimensions.
    /// The buffer is initialized to white.
    pub fn new(width: u32, height: u32) -> Renderer {
        let size = (width as usize) * (height as usize) * 4;
        let mut buffer = vec![0u8; size];
        // Fill with white (BGRA: B=255, G=255, R=255, A=255).
        for pixel in buffer.chunks_exact_mut(4) {
            pixel[0] = 255; // B
            pixel[1] = 255; // G
            pixel[2] = 255; // R
            pixel[3] = 255; // A
        }
        Renderer {
            width,
            height,
            buffer,
        }
    }

    /// Paint a layout tree into the pixel buffer.
    pub fn paint(&mut self, layout_tree: &LayoutTree, font: &Font) {
        let display_list = build_display_list(layout_tree);
        for cmd in &display_list {
            match cmd {
                PaintCommand::FillRect {
                    x,
                    y,
                    width,
                    height,
                    color,
                } => {
                    self.fill_rect(*x, *y, *width, *height, *color);
                }
                PaintCommand::DrawGlyphs {
                    line,
                    font_size,
                    color,
                } => {
                    self.draw_text_line(line, *font_size, *color, font);
                }
            }
        }
    }

    /// Get the BGRA pixel data.
    pub fn pixels(&self) -> &[u8] {
        &self.buffer
    }

    /// Width in pixels.
    pub fn width(&self) -> u32 {
        self.width
    }

    /// Height in pixels.
    pub fn height(&self) -> u32 {
        self.height
    }

    /// Fill a rectangle with a solid color.
    pub fn fill_rect(&mut self, x: f32, y: f32, width: f32, height: f32, color: Color) {
        let x0 = (x as i32).max(0) as u32;
        let y0 = (y as i32).max(0) as u32;
        let x1 = ((x + width) as i32).max(0).min(self.width as i32) as u32;
        let y1 = ((y + height) as i32).max(0).min(self.height as i32) as u32;

        if color.a == 255 {
            // Fully opaque — direct write.
            for py in y0..y1 {
                for px in x0..x1 {
                    self.set_pixel(px, py, color);
                }
            }
        } else if color.a > 0 {
            // Semi-transparent — alpha blend.
            let alpha = color.a as u32;
            let inv_alpha = 255 - alpha;
            for py in y0..y1 {
                for px in x0..x1 {
                    let offset = ((py * self.width + px) * 4) as usize;
                    let dst_b = self.buffer[offset] as u32;
                    let dst_g = self.buffer[offset + 1] as u32;
                    let dst_r = self.buffer[offset + 2] as u32;
                    self.buffer[offset] =
                        ((color.b as u32 * alpha + dst_b * inv_alpha) / 255) as u8;
                    self.buffer[offset + 1] =
                        ((color.g as u32 * alpha + dst_g * inv_alpha) / 255) as u8;
                    self.buffer[offset + 2] =
                        ((color.r as u32 * alpha + dst_r * inv_alpha) / 255) as u8;
                    self.buffer[offset + 3] = 255;
                }
            }
        }
    }

    /// Draw a line of text using the font to render glyphs.
    fn draw_text_line(&mut self, line: &TextLine, font_size: f32, color: Color, font: &Font) {
        let positioned = font.render_text(&line.text, font_size);

        for glyph in &positioned {
            if let Some(ref bitmap) = glyph.bitmap {
                // Glyph origin: line position + glyph horizontal offset.
                let gx = line.x + glyph.x + bitmap.bearing_x as f32;
                // Baseline is at line.y + font_size (approximate: baseline at ~80% of font_size).
                // bearing_y is distance from baseline to top of glyph (positive = above baseline).
                let baseline_y = line.y + font_size;
                let gy = baseline_y - bitmap.bearing_y as f32;

                self.composite_glyph(gx, gy, bitmap, color);
            }
        }
    }

    /// Composite a grayscale glyph bitmap onto the buffer with anti-aliasing.
    fn composite_glyph(
        &mut self,
        x: f32,
        y: f32,
        bitmap: &we_text::font::rasterizer::GlyphBitmap,
        color: Color,
    ) {
        let x0 = x as i32;
        let y0 = y as i32;

        for by in 0..bitmap.height {
            for bx in 0..bitmap.width {
                let px = x0 + bx as i32;
                let py = y0 + by as i32;

                if px < 0 || py < 0 || px >= self.width as i32 || py >= self.height as i32 {
                    continue;
                }

                let coverage = bitmap.data[(by * bitmap.width + bx) as usize];
                if coverage == 0 {
                    continue;
                }

                let px = px as u32;
                let py = py as u32;

                // Alpha-blend the glyph coverage with the text color onto the background.
                let alpha = (coverage as u32 * color.a as u32) / 255;
                if alpha == 0 {
                    continue;
                }

                let offset = ((py * self.width + px) * 4) as usize;
                let dst_b = self.buffer[offset] as u32;
                let dst_g = self.buffer[offset + 1] as u32;
                let dst_r = self.buffer[offset + 2] as u32;

                // Source-over compositing.
                let inv_alpha = 255 - alpha;
                self.buffer[offset] = ((color.b as u32 * alpha + dst_b * inv_alpha) / 255) as u8;
                self.buffer[offset + 1] =
                    ((color.g as u32 * alpha + dst_g * inv_alpha) / 255) as u8;
                self.buffer[offset + 2] =
                    ((color.r as u32 * alpha + dst_r * inv_alpha) / 255) as u8;
                self.buffer[offset + 3] = 255; // Fully opaque.
            }
        }
    }

    /// Set a single pixel to the given color (no blending).
    fn set_pixel(&mut self, x: u32, y: u32, color: Color) {
        if x >= self.width || y >= self.height {
            return;
        }
        let offset = ((y * self.width + x) * 4) as usize;
        self.buffer[offset] = color.b; // B
        self.buffer[offset + 1] = color.g; // G
        self.buffer[offset + 2] = color.r; // R
        self.buffer[offset + 3] = color.a; // A
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use we_dom::Document;
    use we_style::computed::{extract_stylesheets, resolve_styles};
    use we_text::font::Font;

    fn test_font() -> Font {
        let paths = [
            "/System/Library/Fonts/Geneva.ttf",
            "/System/Library/Fonts/Monaco.ttf",
        ];
        for path in &paths {
            let p = std::path::Path::new(path);
            if p.exists() {
                return Font::from_file(p).expect("failed to parse font");
            }
        }
        panic!("no test font found");
    }

    fn layout_doc(doc: &Document) -> we_layout::LayoutTree {
        let font = test_font();
        let sheets = extract_stylesheets(doc);
        let styled = resolve_styles(doc, &sheets).unwrap();
        we_layout::layout(&styled, doc, 800.0, 600.0, &font)
    }

    #[test]
    fn renderer_new_white_background() {
        let r = Renderer::new(10, 10);
        assert_eq!(r.width(), 10);
        assert_eq!(r.height(), 10);
        assert_eq!(r.pixels().len(), 10 * 10 * 4);
        // All pixels should be white (BGRA: 255, 255, 255, 255).
        for pixel in r.pixels().chunks_exact(4) {
            assert_eq!(pixel, [255, 255, 255, 255]);
        }
    }

    #[test]
    fn fill_rect_basic() {
        let mut r = Renderer::new(20, 20);
        let red = Color::new(255, 0, 0, 255);
        r.fill_rect(5.0, 5.0, 10.0, 10.0, red);

        // Pixel at (7, 7) should be red (BGRA: 0, 0, 255, 255).
        let offset = ((7 * 20 + 7) * 4) as usize;
        assert_eq!(r.pixels()[offset], 0); // B
        assert_eq!(r.pixels()[offset + 1], 0); // G
        assert_eq!(r.pixels()[offset + 2], 255); // R
        assert_eq!(r.pixels()[offset + 3], 255); // A

        // Pixel at (0, 0) should still be white.
        assert_eq!(r.pixels()[0], 255); // B
        assert_eq!(r.pixels()[1], 255); // G
        assert_eq!(r.pixels()[2], 255); // R
        assert_eq!(r.pixels()[3], 255); // A
    }

    #[test]
    fn fill_rect_clipping() {
        let mut r = Renderer::new(10, 10);
        let blue = Color::new(0, 0, 255, 255);
        // Rect extends beyond the buffer — should not panic.
        r.fill_rect(-5.0, -5.0, 20.0, 20.0, blue);

        // All pixels should be blue (BGRA: 255, 0, 0, 255).
        for pixel in r.pixels().chunks_exact(4) {
            assert_eq!(pixel, [255, 0, 0, 255]);
        }
    }

    #[test]
    fn display_list_from_empty_layout() {
        let doc = Document::new();
        let font = test_font();
        let sheets = extract_stylesheets(&doc);
        let styled = resolve_styles(&doc, &sheets);
        if let Some(styled) = styled {
            let tree = we_layout::layout(&styled, &doc, 800.0, 600.0, &font);
            let list = build_display_list(&tree);
            assert!(list.len() <= 1);
        }
    }

    #[test]
    fn display_list_has_background_and_text() {
        let mut doc = Document::new();
        let root = doc.root();
        let html = doc.create_element("html");
        let body = doc.create_element("body");
        let p = doc.create_element("p");
        let text = doc.create_text("Hello world");
        doc.append_child(root, html);
        doc.append_child(html, body);
        doc.append_child(body, p);
        doc.append_child(p, text);

        let tree = layout_doc(&doc);
        let list = build_display_list(&tree);

        let has_text = list
            .iter()
            .any(|c| matches!(c, PaintCommand::DrawGlyphs { .. }));

        assert!(has_text, "should have at least one DrawGlyphs");
    }

    #[test]
    fn paint_simple_page() {
        let mut doc = Document::new();
        let root = doc.root();
        let html = doc.create_element("html");
        let body = doc.create_element("body");
        let p = doc.create_element("p");
        let text = doc.create_text("Hello");
        doc.append_child(root, html);
        doc.append_child(html, body);
        doc.append_child(body, p);
        doc.append_child(p, text);

        let font = test_font();
        let tree = layout_doc(&doc);
        let mut renderer = Renderer::new(800, 600);
        renderer.paint(&tree, &font);

        let pixels = renderer.pixels();

        // The buffer should have some non-white pixels (from text rendering).
        let has_non_white = pixels.chunks_exact(4).any(|p| p != [255, 255, 255, 255]);
        assert!(
            has_non_white,
            "rendered page should have non-white pixels from text"
        );
    }

    #[test]
    fn bgra_format_correct() {
        let mut r = Renderer::new(1, 1);
        let color = Color::new(100, 150, 200, 255);
        r.set_pixel(0, 0, color);
        let pixels = r.pixels();
        // BGRA format.
        assert_eq!(pixels[0], 200); // B
        assert_eq!(pixels[1], 150); // G
        assert_eq!(pixels[2], 100); // R
        assert_eq!(pixels[3], 255); // A
    }

    #[test]
    fn paint_heading_produces_larger_glyphs() {
        let mut doc = Document::new();
        let root = doc.root();
        let html = doc.create_element("html");
        let body = doc.create_element("body");
        let h1 = doc.create_element("h1");
        let h1_text = doc.create_text("Big");
        let p = doc.create_element("p");
        let p_text = doc.create_text("Small");
        doc.append_child(root, html);
        doc.append_child(html, body);
        doc.append_child(body, h1);
        doc.append_child(h1, h1_text);
        doc.append_child(body, p);
        doc.append_child(p, p_text);

        let tree = layout_doc(&doc);
        let list = build_display_list(&tree);

        // There should be DrawGlyphs commands with different font sizes.
        let font_sizes: Vec<f32> = list
            .iter()
            .filter_map(|c| match c {
                PaintCommand::DrawGlyphs { font_size, .. } => Some(*font_size),
                _ => None,
            })
            .collect();

        assert!(font_sizes.len() >= 2, "should have at least 2 text lines");
        // h1 has font_size 32, p has font_size 16.
        assert!(
            font_sizes.iter().any(|&s| s > 20.0),
            "should have a heading with large font size"
        );
        assert!(
            font_sizes.iter().any(|&s| s < 20.0),
            "should have a paragraph with normal font size"
        );
    }

    #[test]
    fn renderer_zero_size() {
        let r = Renderer::new(0, 0);
        assert_eq!(r.pixels().len(), 0);
    }

    #[test]
    fn glyph_compositing_anti_aliased() {
        // Render text and verify we get anti-aliased (partially transparent) pixels.
        let mut doc = Document::new();
        let root = doc.root();
        let html = doc.create_element("html");
        let body = doc.create_element("body");
        let p = doc.create_element("p");
        let text = doc.create_text("MMMMM");
        doc.append_child(root, html);
        doc.append_child(html, body);
        doc.append_child(body, p);
        doc.append_child(p, text);

        let font = test_font();
        let tree = layout_doc(&doc);
        let mut renderer = Renderer::new(800, 600);
        renderer.paint(&tree, &font);

        let pixels = renderer.pixels();
        // Find pixels that are not pure white and not pure black.
        // These represent anti-aliased edges of glyphs.
        let mut has_intermediate = false;
        for pixel in pixels.chunks_exact(4) {
            let b = pixel[0];
            let g = pixel[1];
            let r = pixel[2];
            // Anti-aliased pixel: gray between white and black.
            if r > 0 && r < 255 && r == g && g == b {
                has_intermediate = true;
                break;
            }
        }
        assert!(
            has_intermediate,
            "should have anti-aliased (gray) pixels from glyph compositing"
        );
    }

    #[test]
    fn css_color_renders_correctly() {
        let html_str = r#"<!DOCTYPE html>
<html>
<head><style>p { color: red; }</style></head>
<body><p>Red text</p></body>
</html>"#;
        let doc = we_html::parse_html(html_str);
        let font = test_font();
        let sheets = extract_stylesheets(&doc);
        let styled = resolve_styles(&doc, &sheets).unwrap();
        let tree = we_layout::layout(&styled, &doc, 800.0, 600.0, &font);

        let list = build_display_list(&tree);
        let text_colors: Vec<&Color> = list
            .iter()
            .filter_map(|c| match c {
                PaintCommand::DrawGlyphs { color, .. } => Some(color),
                _ => None,
            })
            .collect();

        assert!(!text_colors.is_empty());
        // Text should be red.
        assert_eq!(*text_colors[0], Color::rgb(255, 0, 0));
    }

    #[test]
    fn css_background_color_renders() {
        let html_str = r#"<!DOCTYPE html>
<html>
<head><style>div { background-color: yellow; }</style></head>
<body><div>Content</div></body>
</html>"#;
        let doc = we_html::parse_html(html_str);
        let font = test_font();
        let sheets = extract_stylesheets(&doc);
        let styled = resolve_styles(&doc, &sheets).unwrap();
        let tree = we_layout::layout(&styled, &doc, 800.0, 600.0, &font);

        let list = build_display_list(&tree);
        let fill_colors: Vec<&Color> = list
            .iter()
            .filter_map(|c| match c {
                PaintCommand::FillRect { color, .. } => Some(color),
                _ => None,
            })
            .collect();

        // Should have a yellow fill rect for the div background.
        assert!(fill_colors.iter().any(|c| **c == Color::rgb(255, 255, 0)));
    }

    #[test]
    fn border_rendering() {
        let html_str = r#"<!DOCTYPE html>
<html>
<head><style>div { border: 2px solid red; }</style></head>
<body><div>Bordered</div></body>
</html>"#;
        let doc = we_html::parse_html(html_str);
        let font = test_font();
        let sheets = extract_stylesheets(&doc);
        let styled = resolve_styles(&doc, &sheets).unwrap();
        let tree = we_layout::layout(&styled, &doc, 800.0, 600.0, &font);

        let list = build_display_list(&tree);
        let red_fills: Vec<_> = list
            .iter()
            .filter(|c| matches!(c, PaintCommand::FillRect { color, .. } if *color == Color::rgb(255, 0, 0)))
            .collect();

        // Should have 4 border fills (top, right, bottom, left).
        assert_eq!(red_fills.len(), 4, "should have 4 border edges");
    }
}