//! A fly-camera over one level's terrain.
//!
//! The first thing in the project that puts the disc's own geometry on screen.
//! It is deliberately crude: no GS emulation, no sky, no tie/moby, no fog, no
//! lighting. Its job is to answer the one question parser tests cannot —
//! whether everything decoded so far actually looks like a place.
//!
//! Run it against unpacked level data:
//!
//! ```text
//! cargo run -p gc-engine -- <unpacked-dir> [level]
//! ```
//!
//! Controls: `A`/`D` orbit, `W`/`S` dolly, `Q`/`E` change height, `R` reloads
//! the level so LOD selection can be seen changing.

use std::collections::BTreeMap;
use std::path::PathBuf;

use anyhow::{Context, Result};
use bevy::asset::RenderAssetUsages;
use bevy::image::{ImageFilterMode, ImageSampler, ImageSamplerDescriptor};
use bevy::prelude::*;
use bevy::render::mesh::{Indices, PrimitiveTopology};
use bevy::render::render_resource::{Extent3d, TextureDimension, TextureFormat};
use bevy::render::view::window::screenshot::{CapturedScreenshots, Screenshot};
use bevy::window::WindowResolution;

use gc_engine::level::{build_level, level_bounds, LevelGeometry, LevelSource, TriangleBatch};

fn main() -> Result<()> {
    let args: Vec<String> = std::env::args().skip(1).collect();
    let (dir, level) = match args.len() {
        0 => {
            println!("usage: gc-engine <unpacked-dir> [level]");
            return Ok(());
        }
        1 => (PathBuf::from(&args[0]), 0usize),
        _ => (
            PathBuf::from(&args[0]),
            args[1].parse().context("level number")?,
        ),
    };

    // Read the level once here rather than in a startup system: a resource
    // inserted through `Commands` is deferred to the end of the schedule, so
    // `setup` could not see a focus inserted that way.
    let (files, focus) = load_source(&dir, level)?;
    let geometry = build_level(&files.as_source(), [focus[0], focus[1] + 150.0, focus[2]])?;
    report(&geometry);

    let mut app = App::new();
    app.add_plugins(DefaultPlugins.set(WindowPlugin {
        primary_window: Some(Window {
            title: format!("Going Commando — level{level}"),
            resolution: WindowResolution::new(1280, 720),
            ..default()
        }),
        ..default()
    }))
    .add_systems(Startup, (first_load, setup).chain())
    .add_systems(Update, (orbit, reload, request_shot, save_shot))
    .insert_resource(Level(level))
    .insert_resource(LoadedDir(dir))
    .insert_resource(Focus(focus))
    .insert_resource(LoadedFiles(files))
    .init_resource::<Orbit>();

    if let Ok(path) = std::env::var("GC_ENGINE_SHOT") {
        app.insert_resource(Shot {
            path: PathBuf::from(path),
            frames: 20,
        });
    }
    app.run();
    Ok(())
}

#[derive(Resource)]
struct LoadedDir(PathBuf);

/// The level's bytes, loaded in `main` so every system sees the same data.
#[derive(Resource)]
struct LoadedFiles(LevelFiles);

/// The middle of the level's terrain, which is what the camera orbits.
#[derive(Resource)]
struct Focus([f32; 3]);

/// Set `GC_ENGINE_SHOT` to a path to capture one frame and exit.
///
/// Everything here can be verified by parsing, and the parse can be wrong in
/// ways no assertion catches -- a mirrored axis, a UV in the wrong half of the
/// texture, a hundredfold scale error all produce a well-formed frame. One
/// look is worth more than any of them.
#[derive(Resource)]
struct Shot {
    path: std::path::PathBuf,
    frames: u32,
}

fn request_shot(
    mut commands: Commands,
    shot: Res<Shot>,
    mut frame: Local<u32>,
    mut done: Local<bool>,
) {
    *frame += 1;
    if *done || *frame < shot.frames {
        return;
    }
    *done = true;
    commands.spawn(Screenshot::primary_window());
}

fn save_shot(
    captured: Res<CapturedScreenshots>,
    shot: Res<Shot>,
    mut exit: MessageWriter<AppExit>,
) {
    let Ok((_entity, image)) = captured.0.lock().unwrap().try_recv() else {
        return;
    };
    let (w, h) = (image.width(), image.height());
    // Bevy hands back RGBA; PPM has no alpha, so composite over black rather
    // than pretending the level has an alpha channel.
    let Some(data) = image.data.as_ref() else {
        eprintln!("screenshot returned no pixels");
        let _ = exit.write(bevy::app::AppExit::error());
        return;
    };
    let mut out = format!("P6\n{w} {h}\n255\n").into_bytes();
    for px in data.chunks_exact(4) {
        out.extend_from_slice(&px[..3]);
    }
    if let Err(e) = std::fs::write(&shot.path, &out) {
        eprintln!("screenshot: {e}");
    } else {
        eprintln!("screenshot written to {}", shot.path.display());
    }
    let _ = exit.write(bevy::app::AppExit::Success);
}

#[derive(Resource)]
struct Level(usize);

#[derive(Resource)]
struct Orbit {
    azimuth: f32,
    pitch: f32,
    distance: f32,
    height: f32,
}

impl Default for Orbit {
    fn default() -> Self {
        // Looking down at roughly 30 degrees from a few hundred units out.
        Orbit {
            azimuth: 0.6,
            pitch: 0.5,
            distance: 260.0,
            height: 40.0,
        }
    }
}

fn setup(mut commands: Commands, focus: Res<Focus>, orbit: Res<Orbit>) {
    // Start where `orbit` would put it. `orbit` only runs when a key changes
    // something, so a camera spawned anywhere else sits at its initial pose
    // until the first keypress -- and pointing at the world origin, which is
    // nowhere near the terrain, that is an empty screen.
    let target = Vec3::from_array(focus.0);
    let (sin_p, cos_p) = orbit.pitch.sin_cos();
    let (sin_a, cos_a) = orbit.azimuth.sin_cos();
    let offset = Vec3::new(
        cos_p * cos_a * orbit.distance,
        sin_p * orbit.distance,
        cos_p * sin_a * orbit.distance,
    );
    commands.spawn((
        Camera3d::default(),
        Transform::from_translation(target + offset).looking_at(target, Vec3::Y),
        // A long far plane: levels are hundreds of units across and the whole
        // level is drawn at once, so culling is the patch bounding spheres'
        // job, not the projection's.
        Projection::Perspective(PerspectiveProjection {
            far: 8000.0,
            ..default()
        }),
    ));
    // Ambient only. The game's lighting is a per-vertex light array that is
    // parsed but not yet reproduced, and a directional light would make the
    // terrain look lit when it is not.
    commands.spawn(AmbientLight {
        color: Color::WHITE,
        brightness: 1600.0,
        ..default()
    });
}

/// Load the level once at startup. It is the same path `R` takes afterwards,
/// so the first frame and a reload cannot disagree.
/// Spawn the level `main` already loaded.
fn first_load(
    mut commands: Commands,
    images: ResMut<Assets<Image>>,
    meshes: ResMut<Assets<Mesh>>,
    materials: ResMut<Assets<StandardMaterial>>,
    files: Res<LoadedFiles>,
    focus: Res<Focus>,
) {
    let camera = [focus.0[0], focus.0[1] + 150.0, focus.0[2]];
    match build_level(&files.0.as_source(), camera) {
        Ok(g) => spawn_level(&mut commands, images, meshes, materials, &g),
        Err(e) => eprintln!("could not build level: {e:#}"),
    }
}

fn reload(
    keys: Res<ButtonInput<KeyCode>>,
    mut commands: Commands,
    images: ResMut<Assets<Image>>,
    meshes: ResMut<Assets<Mesh>>,
    materials: ResMut<Assets<StandardMaterial>>,
    level: Res<Level>,
    dir: Res<LoadedDir>,
) {
    if !keys.just_pressed(KeyCode::KeyR) {
        return;
    }
    let src = match load(&dir.0, level.0) {
        Ok(s) => s,
        Err(e) => {
            eprintln!("reload failed: {e:#}");
            return;
        }
    };
    spawn_level(&mut commands, images, meshes, materials, &src);
}

/// Read a level's three files and find the middle of its terrain.
fn load_source(dir: &std::path::Path, level: usize) -> Result<(LevelFiles, [f32; 3])> {
    let level_dir = dir.join(format!("level{level}"));
    let core_data = std::fs::read(level_dir.join("core_data.bin")).context("core_data.bin")?;
    let core_index = std::fs::read(level_dir.join("core_index.bin")).context("core_index.bin")?;
    let gs_ram = std::fs::read(level_dir.join("gs_ram.bin")).context("gs_ram.bin")?;
    let overlay = std::fs::read(level_dir.join("overlay.bin")).ok();
    let gameplay = std::fs::read(level_dir.join("gameplay.bin")).ok();
    let src = LevelSource {
        core_data: &core_data,
        core_index: &core_index,
        gs_ram: &gs_ram,
        overlay: overlay.as_deref(),
        gameplay: gameplay.as_deref(),
    };
    let (centre, _radius) = level_bounds(&src)?;
    Ok((
        LevelFiles {
            core_data,
            core_index,
            gs_ram,
            overlay,
            gameplay,
        },
        centre,
    ))
}

/// The level's bytes, owned so a system can outlive the load call.
struct LevelFiles {
    core_data: Vec<u8>,
    core_index: Vec<u8>,
    gs_ram: Vec<u8>,
    overlay: Option<Vec<u8>>,
    gameplay: Option<Vec<u8>>,
}

impl LevelFiles {
    fn as_source(&self) -> LevelSource<'_> {
        LevelSource {
            core_data: &self.core_data,
            core_index: &self.core_index,
            gs_ram: &self.gs_ram,
            overlay: self.overlay.as_deref(),
            gameplay: self.gameplay.as_deref(),
        }
    }
}

/// Load a level as seen from above the middle of its terrain.
fn load(dir: &std::path::Path, level: usize) -> Result<LevelGeometry> {
    let (files, focus) = load_source(dir, level)?;
    build_level(&files.as_source(), [focus[0], focus[1] + 150.0, focus[2]])
}

fn spawn_level(
    commands: &mut Commands,
    mut images: ResMut<Assets<Image>>,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
    g: &LevelGeometry,
) {
    report(g);
    for (image, mesh) in to_gpu(g) {
        let texture = images.add(image);
        let material = materials.add(StandardMaterial {
            base_color_texture: Some(texture),
            // Double-sided: the strips as stored do not carry a consistent
            // winding, and the game relies on GS defaults rather than culling.
            cull_mode: None,
            ..default()
        });
        commands.spawn((
            Mesh3d(meshes.add(mesh)),
            MeshMaterial3d(material),
            Transform::IDENTITY,
        ));
    }
}

fn report(g: &LevelGeometry) {
    let s = &g.stats;
    eprintln!(
        "level: {} patches, {} drawn, {} unparsed | {tri} triangles, {v} vertices, {t} textures\n\
         LOD distances {d:?} (raw {r:?})",
        s.patches,
        s.drawn,
        s.skipped_unparsed,
        tri = s.triangles,
        v = s.vertices,
        t = g.textures.len(),
        d = g.lod_distances,
        r = g.lod_thresholds_raw,
    );
    let mut modes: BTreeMap<&str, usize> = BTreeMap::new();
    for p in &g.patches {
        *modes.entry(mode_name(p)).or_default() += 1;
    }
    for (m, n) in modes {
        eprintln!("  {m}: {n} patches");
    }
    // Say so when the level is not lit. Falling back silently to the tfrag's
    // stored colours produces a plausible but wrong picture.
    if s.lit_vertices < s.vertices {
        eprintln!(
            "  lighting: {} of {} vertices shaded by LightTfrags; the rest fall back to the \
             tfrag's stored colours (no usable light bank)",
            s.lit_vertices, s.vertices
        );
    }
}

fn mode_name(p: &gc_engine::level::PatchBounds) -> &'static str {
    use gc_formats::tfrag::TfragDrawMode::*;
    match p.mode {
        Lod2 => "LOD2",
        Lod1Collapse => "LOD1 collapse",
        Lod1Morph => "LOD1 morph",
        Lod0Collapse => "LOD0 collapse",
        Lod0Morph => "LOD0 morph",
        Lod0 => "LOD0",
    }
}

fn to_gpu(g: &LevelGeometry) -> Vec<(Image, Mesh)> {
    let mut out = Vec::new();
    for (&tex, batch) in &g.batches {
        let Some(image) = g.textures.get(&tex) else {
            continue;
        };
        let Some(mesh) = mesh_of(batch) else { continue };
        out.push((image_of(image), mesh));
    }
    out
}

fn image_of(t: &gc_formats::texture::Texture) -> Image {
    let mut image = Image::new(
        Extent3d {
            width: t.width,
            height: t.height,
            depth_or_array_layers: 1,
        },
        TextureDimension::D2,
        t.rgba.clone(),
        TextureFormat::Rgba8UnormSrgb,
        RenderAssetUsages::default(),
    );
    // Only mip level 0 is decoded, so minification must clamp rather than
    // invent a level the disc does not have here.
    image.sampler = ImageSampler::Descriptor(ImageSamplerDescriptor {
        label: Some("tfrag texture".into()),
        mipmap_filter: ImageFilterMode::Linear,
        mag_filter: ImageFilterMode::Linear,
        ..default()
    });
    image
}

fn mesh_of(b: &TriangleBatch) -> Option<Mesh> {
    if b.indices.is_empty() {
        return None;
    }
    let mut mesh = Mesh::new(
        PrimitiveTopology::TriangleList,
        RenderAssetUsages::default(),
    );
    mesh.insert_attribute(Mesh::ATTRIBUTE_POSITION, b.positions.clone());
    mesh.insert_attribute(Mesh::ATTRIBUTE_UV_0, b.uvs.clone());
    mesh.insert_attribute(Mesh::ATTRIBUTE_COLOR, b.colors.clone());
    mesh.insert_attribute(Mesh::ATTRIBUTE_NORMAL, b.normals.clone());
    // No normals and no colours: lighting is the game's own per-vertex light
    // array, which is parsed but not yet reproduced. Inventing normals would
    // make unlit terrain look lit and hide that gap.
    mesh.insert_indices(Indices::U32(b.indices.clone()));
    Some(mesh)
}

fn orbit(
    keys: Res<ButtonInput<KeyCode>>,
    time: Res<Time>,
    mut cam: Query<&mut Transform, With<Camera3d>>,
    mut o: ResMut<Orbit>,
    focus: Res<Focus>,
) {
    let dt = time.delta_secs();
    let mut changed = false;
    if keys.pressed(KeyCode::KeyA) {
        o.azimuth += dt;
        changed = true;
    }
    if keys.pressed(KeyCode::KeyD) {
        o.azimuth -= dt;
        changed = true;
    }
    if keys.pressed(KeyCode::KeyW) {
        o.distance = (o.distance - dt * 150.0).max(5.0);
        changed = true;
    }
    if keys.pressed(KeyCode::KeyS) {
        o.distance += dt * 150.0;
        changed = true;
    }
    if keys.pressed(KeyCode::KeyQ) {
        o.height += dt * 40.0;
        changed = true;
    }
    if keys.pressed(KeyCode::KeyE) {
        o.height -= dt * 40.0;
        changed = true;
    }
    if keys.pressed(KeyCode::ArrowUp) {
        o.pitch = (o.pitch + dt * 0.8).clamp(-1.5, 1.5);
        changed = true;
    }
    if keys.pressed(KeyCode::ArrowDown) {
        o.pitch = (o.pitch - dt * 0.8).clamp(-1.5, 1.5);
        changed = true;
    }
    if !changed {
        return;
    }
    let (sin_p, cos_p) = o.pitch.sin_cos();
    let (sin_a, cos_a) = o.azimuth.sin_cos();
    let offset = Vec3::new(
        cos_p * cos_a * o.distance,
        sin_p * o.distance,
        cos_p * sin_a * o.distance,
    );
    // Orbit the middle of the terrain rather than the world origin: level
    // geometry is absolutely positioned and nowhere near the origin.
    let target = Vec3::from_array(focus.0);
    for mut t in &mut cam {
        *t = Transform::from_translation(target + offset + Vec3::new(0.0, o.height, 0.0))
            .looking_at(target, Vec3::Y);
    }
}