use std::{
    fmt::{self, Display},
    iter, mem,
    str::FromStr,
    sync::Arc,
};

use base64::{Engine, prelude::BASE64_STANDARD};
use egui::{
    Color32, ColorImage, CornerRadius, Event, Frame, Id, Mesh, PointerButton, Pos2, Rect, Sense,
    Shape, Stroke, TextureHandle, Theme, Ui, Vec2,
    emath::{self, TSTransform},
    epaint::{Brush, RectShape, TessellationOptions, Tessellator, Vertex},
    load::SizedTexture,
};
use eyre::{Context, bail};
use eyre::{OptionExt, eyre};
use half::f16;
use zerocopy::{FromBytes, Immutable, IntoBytes, KnownLayout};

use crate::{
    custom_code_block::try_from_custom_code_block,
    rasterizer::{self, rasterize, rasterize_onto},
};
use crate::{custom_code_block::write_custom_code_block, util::random_id};

const HANDWRITING_MIN_HEIGHT: f32 = 100.0;
const HANDWRITING_BOTTOM_PADDING: f32 = 80.0;
const HANDWRITING_MARGIN: f32 = 0.05;
const HANDWRITING_LINE_SPACING: f32 = 36.0;

pub const CODE_BLOCK_KEY: &str = "handwriting";

type StrokeBlendMode = rasterizer::blend::Normal;

const TESSELATION_OPTIONS: TessellationOptions = TessellationOptions {
    feathering: true,
    feathering_size_in_pixels: 1.0,
    coarse_tessellation_culling: true,
    prerasterized_discs: true,
    round_text_to_pixels: true,
    round_line_segments_to_pixels: true,
    round_rects_to_pixels: true,
    debug_paint_text_rects: false,
    debug_paint_clip_rects: false,
    debug_ignore_clip_rects: false,
    bezier_tolerance: 0.1,
    epsilon: 1.0e-5,
    parallel_tessellation: true,
    validate_meshes: false,
};

pub struct HandwritingStyle {
    pub stroke: Stroke,
    pub bg_line_stroke: Stroke,
    pub bg_color: Color32,
    pub animate: bool,
}

#[derive(serde::Deserialize, serde::Serialize)]
#[serde(default)]
pub struct Handwriting {
    #[serde(skip, default = "random_id")]
    id: Id,

    strokes: Vec<Vec<Pos2>>,

    /// The stroke that is currently being drawed.
    #[serde(skip)]
    current_stroke: Vec<Pos2>,

    /// The lines that have not been blitted to `texture` yet.
    #[serde(skip)]
    unblitted_lines: Vec<[Pos2; 2]>,

    height: f32,
    desired_height: f32,

    /// Tesselated mesh of all strokes
    #[serde(skip)]
    mesh: Arc<Mesh>,

    #[serde(skip)]
    texture: Option<TextureHandle>,

    #[serde(skip)]
    image: ColorImage,

    #[serde(skip)]
    refresh_texture: bool,

    /// Context of the last mesh render.
    #[serde(skip)]
    last_mesh_ctx: Option<MeshContext>,
}

pub struct HandwritingResponse {
    pub changed: bool,
}

/// Context of a mesh render.
#[derive(Clone, Copy, PartialEq)]
struct MeshContext {
    /// Need to update the mesh when the stroke color changes.
    pub ui_theme: Theme,

    pub pixels_per_point: f32,

    /// Canvas size in points
    pub size: Vec2,

    pub stroke: Stroke,
}

/// Get [Painting::texture], initializing it if necessary.
macro_rules! texture {
    ($self_:expr, $ui:expr, $mesh_context:expr) => {{
        let ui: &Ui = $ui;
        let mesh_context: &MeshContext = $mesh_context;
        let image_size = mesh_context.pixel_size();

        let new_image = || {
            let image = ColorImage::new(image_size, Color32::TRANSPARENT);
            ui.ctx()
                .load_texture("handwriting", image, Default::default())
        };

        let texture = $self_.texture.get_or_insert_with(new_image);

        if texture.size() != image_size {
            $self_.refresh_texture = true;
            // TODO: don't redraw the entire mesh, just blit the old texture onto the new one
            *texture = new_image()
        };

        texture
    }};
}

impl MeshContext {
    /// Calculate canvas size in pixels
    pub fn pixel_size(&self) -> [usize; 2] {
        let Vec2 { x, y } = self.size * self.pixels_per_point;
        [x, y].map(|f| f.ceil() as usize)
    }
}

impl Default for Handwriting {
    fn default() -> Self {
        Self {
            id: random_id(),
            strokes: Default::default(),
            current_stroke: Default::default(),
            height: HANDWRITING_MIN_HEIGHT,
            desired_height: HANDWRITING_MIN_HEIGHT,
            mesh: Default::default(),
            texture: None,
            image: ColorImage::new([0, 0], Color32::WHITE),
            refresh_texture: true,
            last_mesh_ctx: None,
            unblitted_lines: Default::default(),
        }
    }
}

impl Handwriting {
    pub fn ui_control(
        &mut self,
        style: Option<&mut HandwritingStyle>,
        ui: &mut egui::Ui,
        response: &mut HandwritingResponse,
    ) -> egui::Response {
        ui.horizontal(|ui| {
            if let Some(style) = style {
                ui.label("Stroke:");
                ui.add(&mut style.stroke);
                ui.separator();
            }

            if ui.button("Clear Painting").clicked() {
                self.strokes.clear();
                self.refresh_texture = true;
                response.changed = true;
            }

            ui.add_enabled_ui(!self.strokes.is_empty(), |ui| {
                if ui.button("Undo").clicked() {
                    self.strokes.pop();
                    self.refresh_texture = true;
                    response.changed = true;
                }
            });

            let vertex_count: usize = self.mesh.indices.len() / 3;
            ui.label(format!("vertices: {vertex_count}"));
        })
        .response
    }

    fn commit_current_line(&mut self, response: &mut HandwritingResponse) {
        debug_assert!(!self.current_stroke.is_empty());
        self.strokes.push(mem::take(&mut self.current_stroke));
        response.changed = true;
    }

    pub fn ui_content(
        &mut self,
        style: &HandwritingStyle,
        ui: &mut Ui,
        hw_response: &mut HandwritingResponse,
    ) -> egui::Response {
        if style.animate {
            self.height = ui.ctx().animate_value_with_time(
                self.id.with("height animation"),
                self.desired_height,
                0.4,
            );
        } else {
            self.height = self.desired_height;
        }

        let desired_size = Vec2::new(ui.available_width(), self.height);
        let (response, painter) = ui.allocate_painter(desired_size, Sense::drag());

        let mut response = response
            //.on_hover_cursor(CursorIcon::Crosshair)
            //.on_hover_and_drag_cursor(CursorIcon::None)
            ;

        let size = response.rect.size();

        // Calculate matrices that convert between screen-space and image-space.
        // - image-space: 0,0 is the top-left of the texture.
        // - screen-space: 0,0 is the top-left of the window.
        // Both spaces use the same logical points, not pixels.
        let to_screen =
            emath::RectTransform::from_to(Rect::from_min_size(Pos2::ZERO, size), response.rect);
        let from_screen = to_screen.inverse();

        // Was the user in the process of drawing a stroke last frame?
        let was_drawing = !self.current_stroke.is_empty();

        // Is the user in the process of drawing a stroke now?
        let is_drawing = response.interact_pointer_pos().is_some();

        if !is_drawing {
            if was_drawing {
                // commit current line
                self.commit_current_line(hw_response);
                response.mark_changed();
            }

            // recalculate how tall the widget should be
            let lines_max_y = self
                .strokes
                .iter()
                .flatten()
                .map(|p| p.y + HANDWRITING_BOTTOM_PADDING)
                .fold(HANDWRITING_MIN_HEIGHT, |max, y| max.max(y));

            // Change the height of the handwriting item.
            // We don't do this mid-stroke, only when the user e.g. lifts the pen.
            if self.desired_height != lines_max_y {
                self.desired_height = lines_max_y;
                response.mark_changed();
            }
        } else {
            let events = ui.ctx().input(|input| {
                // If we are getting both MouseMoved and PointerMoved events, ignore the first.
                let mut events = input.raw.events.iter().peekable();
                iter::from_fn(move || {
                    let next = events.next()?;
                    let Some(peek) = events.peek() else {
                        return Some(next);
                    };

                    match next {
                        Event::PointerMoved(..) if matches!(peek, Event::MouseMoved(..)) => {
                            let _ = events.next(); // drop the MouseMoved event
                            Some(next)
                        }
                        Event::MouseMoved(..) if matches!(peek, Event::PointerMoved(..)) => {
                            // return the peeked PointerMoved instead
                            Some(events.next().expect("next is some"))
                        }
                        _ => Some(next),
                    }
                })
                .cloned()
                .filter(|event| {
                    // FIXME: pinenote: PointerMoved are duplicated after the MouseMoved events
                    cfg!(not(feature = "pinenote")) || !matches!(event, Event::PointerMoved(..))
                })
                .collect::<Vec<_>>()
            });

            // Process input events and turn them into strokes
            for event in events {
                let last_canvas_pos = self.current_stroke.last();

                match event {
                    Event::PointerMoved(new_position) => {
                        let new_canvas_pos = from_screen * new_position;
                        if let Some(&last_canvas_pos) = last_canvas_pos {
                            if last_canvas_pos != new_canvas_pos {
                                self.push_to_stroke(new_canvas_pos);
                                response.mark_changed();
                            }
                        }
                    }

                    Event::MouseMoved(mut delta) => {
                        if delta.length() == 0.0 {
                            continue;
                        }

                        // FIXME: pinenote: MouseMovement delta does *not* take into account screen
                        // scaling and rotation, so unless you've scaling=1 and no rotation, the
                        // MouseMoved values will be all wrong.
                        if cfg!(feature = "pinenote") {
                            delta /= 1.8;
                            delta = -delta.rot90();
                        }

                        if let Some(&last_canvas_pos) = last_canvas_pos {
                            self.push_to_stroke(last_canvas_pos + delta);
                            response.mark_changed();
                        } else {
                            println!("Got `MouseMoved`, but have no previous pos");
                        }
                    }

                    Event::PointerButton {
                        pos,
                        button,
                        pressed,
                        modifiers: _,
                    } => match (button, pressed) {
                        (PointerButton::Primary, true) => {
                            if last_canvas_pos.is_none() {
                                self.current_stroke.push(from_screen * pos);
                            }
                        }
                        (PointerButton::Primary, false) => {
                            if last_canvas_pos.is_some() {
                                self.push_to_stroke(from_screen * pos);
                                self.commit_current_line(hw_response);
                                response.mark_changed();
                            }

                            // Stop reading events.
                            // TODO: In theory, we can get multiple press->draw->release series
                            // in the same frame. Should handle this.
                            break;
                        }
                        (_, _) => continue,
                    },

                    // Stop drawing after pointer disappears or the window is unfocused
                    // TODO: In theory, we can get multiple press->draw->release series
                    // in the same frame. Should handle this.
                    Event::PointerGone | Event::WindowFocused(false) => {
                        if !self.current_stroke.is_empty() {
                            self.commit_current_line(hw_response);
                            break;
                        }
                    }

                    Event::WindowFocused(true)
                    | Event::Copy
                    | Event::Cut
                    | Event::Paste(..)
                    | Event::Text(..)
                    | Event::Key { .. }
                    | Event::Zoom(..)
                    | Event::Ime(..)
                    | Event::Touch { .. }
                    | Event::MouseWheel { .. }
                    | Event::Screenshot { .. } => continue,
                }
            }
        }

        // Draw the horizontal ruled lines
        (1..)
            .map(|n| n as f32 * HANDWRITING_LINE_SPACING)
            .take_while(|&y| y < size.y)
            .map(|y| {
                let l = to_screen * Pos2::new(HANDWRITING_MARGIN * size.x, y);
                let r = to_screen * Pos2::new((1.0 - HANDWRITING_MARGIN) * size.x, y);
                Shape::hline(l.x..=r.x, l.y, style.bg_line_stroke)
            })
            .for_each(|shape| {
                painter.add(shape);
            });

        // Get the dimensions of the image
        let mesh_rect = response
            .rect
            .with_max_y(response.rect.min.y + self.desired_height);

        // These are the values that, if changed, would require the mesh to be re-rendered.
        let new_context = MeshContext {
            ui_theme: ui.ctx().theme(),
            pixels_per_point: ui.pixels_per_point(),
            size: mesh_rect.size(),
            stroke: style.stroke,
        };

        // Figure out if we need to re-rasterize the mesh.
        if Some(&new_context) != self.last_mesh_ctx.as_ref() {
            self.refresh_texture = true;
        }

        if self.refresh_texture {
            // ...if we do, rasterize the entire texture from scratch
            self.refresh_texture(style, new_context, ui);
            self.unblitted_lines.clear();
        } else if !self.unblitted_lines.is_empty() {
            // ...if we don't, we can get away with  only rasterizing the *new* lines onto the
            // existing texture.
            for [from, to] in std::mem::take(&mut self.unblitted_lines) {
                self.draw_line_to_texture(from, to, &new_context, ui);
            }
            self.unblitted_lines.clear();
        }

        // Draw the texture
        if let Some(texture) = &self.texture {
            let texture = SizedTexture::new(texture.id(), texture.size_vec2());
            let shape = RectShape {
                rect: mesh_rect,
                corner_radius: CornerRadius::ZERO,
                fill: Color32::WHITE,
                stroke: Stroke::NONE,
                stroke_kind: egui::StrokeKind::Inside,
                round_to_pixels: None,
                blur_width: 0.0,
                brush: Some(Arc::new(Brush {
                    fill_texture_id: texture.id,
                    uv: Rect {
                        min: Pos2::ZERO,
                        max: Pos2::new(1.0, 1.0),
                    },
                })),
            };
            painter.add(shape);
        }

        response
    }

    fn refresh_texture(
        &mut self,
        style: &HandwritingStyle,
        mesh_context: MeshContext,
        ui: &mut Ui,
    ) {
        self.last_mesh_ctx = Some(mesh_context);

        self.refresh_texture = false;

        #[cfg(not(target_arch = "wasm32"))]
        let start_time = std::time::Instant::now();

        let mut tesselator = Tessellator::new(
            mesh_context.pixels_per_point,
            TESSELATION_OPTIONS,
            Default::default(), // we don't tesselate fonts
            vec![],
        );

        let mesh = Arc::make_mut(&mut self.mesh);
        mesh.clear();

        self.strokes
            .iter()
            .chain([&self.current_stroke])
            .filter(|stroke| stroke.len() >= 2)
            .map(|stroke| {
                //let points: Vec<Pos2> = stroke.iter().map(|&p| to_screen * p).collect();
                egui::Shape::line(stroke.clone(), style.stroke)
            })
            .for_each(|shape| {
                tesselator.tessellate_shape(shape, mesh);
            });

        let texture = texture!(self, ui, &mesh_context);
        let triangles = mesh_triangles(&self.mesh);

        let [px_x, px_y] = mesh_context.pixel_size();
        let point_to_pixel = TSTransform::from_scaling(mesh_context.pixels_per_point);
        self.image = rasterize::<StrokeBlendMode>(px_x, px_y, point_to_pixel, triangles);
        texture.set(self.image.clone(), Default::default());

        #[cfg(not(target_arch = "wasm32"))]
        {
            let elapsed = start_time.elapsed();
            println!("refreshed mesh in {:.3}s", elapsed.as_secs_f32());
        }
    }

    pub fn ui(&mut self, style: &HandwritingStyle, ui: &mut Ui) -> HandwritingResponse {
        let mut response = HandwritingResponse { changed: false };

        ui.vertical_centered_justified(|ui| {
            self.ui_control(None, ui, &mut response);

            //ui.label("Paint with your mouse/touch!");
            Frame::canvas(ui.style())
                .corner_radius(20.0)
                .stroke(Stroke::new(5.0, Color32::from_black_alpha(40)))
                .fill(style.bg_color)
                .show(ui, |ui| {
                    self.ui_content(style, ui, &mut response);
                });
        });

        response
    }

    fn push_to_stroke(&mut self, new_canvas_pos: Pos2) {
        if let Some(&last_canvas_pos) = self.current_stroke.last() {
            if last_canvas_pos == new_canvas_pos {
                return;
            }

            self.unblitted_lines.push([last_canvas_pos, new_canvas_pos]);
        }

        self.current_stroke.push(new_canvas_pos);
    }

    /// Draw a single line onto the existing texture.
    fn draw_line_to_texture(
        &mut self,
        from: Pos2,
        to: Pos2,
        mesh_context: &MeshContext,
        ui: &mut Ui,
    ) {
        let mut tesselator = Tessellator::new(
            mesh_context.pixels_per_point,
            TESSELATION_OPTIONS,
            Default::default(), // we don't tesselate fonts
            vec![],
        );

        let mut mesh = Mesh::default();
        let line = egui::Shape::line_segment([from, to], mesh_context.stroke);
        tesselator.tessellate_shape(line, &mut mesh);

        self.draw_mesh_to_texture(&mesh, mesh_context, ui);
    }

    /// Draw a single mesh onto the existing texture.
    fn draw_mesh_to_texture(&mut self, mesh: &Mesh, mesh_context: &MeshContext, ui: &mut Ui) {
        let triangles = mesh_triangles(mesh);
        let point_to_pixel = TSTransform::from_scaling(mesh_context.pixels_per_point);
        rasterize_onto::<StrokeBlendMode>(&mut self.image, point_to_pixel, triangles);
        texture!(self, ui, mesh_context).set(self.image.clone(), Default::default());
    }

    pub fn strokes(&self) -> &[Vec<Pos2>] {
        &self.strokes
    }

    #[cfg(test)]
    pub fn example() -> Self {
        Handwriting {
            strokes: vec![
                vec![
                    Pos2::new(-1.0, 1.0),
                    Pos2::new(3.0, 1.0),
                    Pos2::new(3.0, 3.0),
                    Pos2::new(1.5, 2.0),
                    Pos2::new(0.0, 0.0),
                ],
                vec![
                    Pos2::new(3.0, 3.0),
                    Pos2::new(-1.0, 1.0),
                    Pos2::new(0.0, 0.0),
                    Pos2::new(3.0, 1.0),
                ],
            ],
            ..Default::default()
        }
    }
}

impl Display for Handwriting {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let mut raw = vec![];

        for stroke in &self.strokes {
            raw.push((stroke.len() as u16).to_le_bytes());
            for position in stroke {
                let x = half::f16::from_f32(position.x);
                let y = half::f16::from_f32(position.y);
                raw.push(x.to_bits().to_le_bytes());
                raw.push(y.to_bits().to_le_bytes());
            }
        }

        let raw = raw.as_slice().as_bytes();

        write_custom_code_block(f, CODE_BLOCK_KEY, BASE64_STANDARD.encode(raw))
    }
}

impl FromStr for Handwriting {
    type Err = eyre::Report;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let s = try_from_custom_code_block(CODE_BLOCK_KEY, s)
            .ok_or_eyre("Not a valid ```handwriting-block")?;

        let bytes = BASE64_STANDARD
            .decode(s)
            .wrap_err("Failed to decode painting data from base64")?;

        #[allow(non_camel_case_types)]
        type u16_le = [u8; 2];

        #[allow(non_camel_case_types)]
        type f16_le = [u8; 2];

        #[derive(FromBytes, KnownLayout, Immutable)]
        #[repr(C, packed)]
        struct Stroke {
            pub len: u16_le,
            pub positions: [f16_le],
        }

        let mut bytes = &bytes[..];
        let mut strokes = vec![];

        while !bytes.is_empty() {
            let header_len = size_of::<u16_le>();
            if bytes.len() < header_len {
                bail!("Invalid remaining length: {}", bytes.len());
            }

            let stroke = Stroke::ref_from_bytes(&bytes[..header_len]).expect("length is correct");
            let len = usize::from(u16::from_le_bytes(stroke.len));
            let len = len * size_of::<f16_le>() * 2;

            if bytes.len() < len {
                bail!("Invalid remaining length: {}", bytes.len());
            }

            let (stroke, rest) = bytes.split_at(header_len + len);
            bytes = rest;
            let stroke = Stroke::ref_from_bytes(stroke)
                .map_err(|e| eyre!("Failed to decode stroke bytes: {e}"))?;

            let mut positions = stroke
                .positions
                .iter()
                .map(|&position| f16::from_bits(u16::from_le_bytes(position)));

            let mut stroke = vec![];
            while let Some(x) = positions.next() {
                let Some(y) = positions.next() else {
                    unreachable!("len is a multiple of two");
                };
                stroke.push(Pos2::new(x.into(), y.into()));
            }
            strokes.push(stroke);
        }

        Ok(Handwriting {
            strokes,
            ..Default::default()
        })
    }
}

impl HandwritingStyle {
    pub fn from_theme(theme: Theme) -> Self {
        let stroke_color;
        let bg_color;
        let line_color;

        match theme {
            Theme::Dark => {
                stroke_color = Color32::WHITE;
                bg_color = Color32::from_gray(30);
                line_color = Color32::from_rgb(100, 100, 100);
            }
            Theme::Light => {
                stroke_color = Color32::BLACK;
                bg_color = Color32::WHITE;
                line_color = Color32::from_rgb(130, 130, 130); // TODO
            }
        }

        HandwritingStyle {
            stroke: Stroke::new(1.6, stroke_color),
            bg_color,
            bg_line_stroke: Stroke::new(0.5, line_color),
            animate: true,
        }
    }
}

fn mesh_triangles(mesh: &Mesh) -> impl Iterator<Item = [&Vertex; 3]> {
    mesh.triangles()
        .map(|indices| indices.map(|i| &mesh.vertices[i as usize]))
}

#[cfg(test)]
mod test {
    use std::str::FromStr;

    use super::Handwriting;

    #[test]
    fn serialize_handwriting() {
        let handwriting = Handwriting::example();
        insta::assert_debug_snapshot!("handwriting example", handwriting.strokes);

        let serialized = handwriting.to_string();
        insta::assert_snapshot!("serialized handwriting", serialized);

        let deserialized =
            Handwriting::from_str(&serialized).expect("Handwriting must de/serialize correctly");
        insta::assert_debug_snapshot!("deserialized handwriting", deserialized.strokes);
    }
}