furmint-runtime 0.1.0

use std::collections::HashMap;
use std::fs;
use std::fs::File;
use std::path::PathBuf;
use std::sync::mpsc::Sender;
use std::time::{Duration, Instant};

use log::{debug, error, info, warn};

use crate::error::{RuntimeError, RuntimeResult};
use crate::events::{AppEvent, AppEventReceivingSystem, Events};
use crate::plugins::{Plugin, PluginBuildContext, PluginUpdateContext};
use crate::scenes::{
    ActiveScene, Scene, SceneCommand, SceneCommands, SceneLibrary, SceneLoader, SceneSystem,
};
use crate::time;
use crate::time::Time;
use furmint_registry::{ComponentFactory, ComponentRegistry};
use furmint_resources::assets::{AssetServer, Handle};
use serde::{Deserialize, Serialize};
use specs::{Component, Dispatcher, DispatcherBuilder, World, WorldExt};

/// Alias for system registration closure
pub type SystemRegistration = Box<dyn FnOnce(&mut DispatcherBuilder<'static, 'static>) + Send>;

/// The application struct which contains [`World`] and `specs`
/// `Dispatcher`
pub struct App {
    /// ECS world
    world: World,
    /// ECS dispatchers, keyed by stage
    dispatchers: HashMap<Stage, Dispatcher<'static, 'static>>,
    /// App plugins
    plugins: Vec<Box<dyn Plugin + 'static + Send + Sync>>,
    /// Is the app running?
    running: bool,
}

/// Builder for [`App`]
#[derive(Default)]
pub struct AppBuilder {
    config: &'static str,
    world: World,
    component_registry: ComponentRegistry,
    dispatcher_builders: HashMap<Stage, DispatcherBuilder<'static, 'static>>,
    dispatcher_config: HashMap<Stage, Vec<SystemRegistration>>,
    plugins: Vec<Box<dyn Plugin + Send + Sync>>,
}

/// Execution stages for systems in a frame
///
/// Systems are grouped by stage and run in this order:
/// `PreUpdate` -> `Update` -> `PostUpdate` -> `Runtime`
#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
pub enum Stage {
    /// Runs before game logic. Use for input handling and preparation
    PreUpdate,
    /// Main game logic. Most user systems should be here
    Update,
    /// Runs after game logic. Use for cleanup or derived updates
    PostUpdate,
    /// Internal engine systems (e.g. scene management)`
    Runtime,
}

impl Stage {
    /// Order of stages
    pub const ORDER: [Stage; 4] = [
        Stage::PreUpdate,
        Stage::Update,
        Stage::PostUpdate,
        Stage::Runtime,
    ];
}

/// Application config defined by user
#[derive(Deserialize, Serialize, Debug)]
struct Config {
    /// Path to assets directory
    assets_path: String,
    /// First scene to display
    initial_scene: String,
    /// ECS system dispatch tick rate
    tick_period: u64,
}

impl App {
    /// Runs the event loop. Must be called only once.
    pub fn run(mut self) -> RuntimeResult<()> {
        self.running = true;
        while self.running {
            self.run_frame()?;
        }
        Ok(())
    }

    fn run_frame(&mut self) -> RuntimeResult<()> {
        let frame_start = Instant::now();

        self.handle_app_events();
        self.dispatch_stage(Stage::PreUpdate);
        self.dispatch_stage(Stage::Update);
        self.dispatch_stage(Stage::PostUpdate);

        self.handle_scene_commands();
        self.dispatch_stage(Stage::Runtime);

        self.world.maintain();

        let dt = self.finish_tick(frame_start.elapsed());

        self.update_plugins(dt);

        Ok(())
    }

    fn handle_app_events(&mut self) {
        let mut receiver = self.world.write_resource::<Events<AppEvent>>();
        receiver.update();
        for ev in receiver.iter() {
            debug!("app event: {:?}", ev);
            match ev {
                AppEvent::ExitRequested => {
                    info!("got ExitRequested, exiting app");
                    self.running = false;
                }
            }
        }
    }

    fn dispatch_stage(&mut self, stage: Stage) {
        self.dispatchers
            .get_mut(&stage)
            .expect("stage dispatcher missing")
            .dispatch(&self.world);
    }

    fn update_plugins(&mut self, dt_seconds: f64) {
        let mut ctx = PluginUpdateContext::new(dt_seconds, &mut self.world);
        for plugin in &mut self.plugins {
            if let Err(e) = plugin.update(&mut ctx) {
                error!("failed to update plugin: {e}");
            }
        }
    }

    fn finish_tick(&mut self, elapsed: Duration) -> f64 {
        let elapsed_nanos = elapsed.as_nanos() as u64;
        let mut time = self.world.write_resource::<Time>();

        let frame_nanos = if elapsed_nanos < time.tick_period {
            let remaining = time.tick_period - elapsed_nanos;
            std::thread::sleep(Duration::from_nanos(remaining));
            time.tick_period
        } else {
            warn!(
                "cannot keep up, tick took {} > {} nanoseconds",
                elapsed_nanos, time.tick_period
            );
            elapsed_nanos
        };

        let dt = frame_nanos as f64 / time::NANOS_PER_SECOND;
        time.ticks += 1;
        time.delta_seconds = dt;
        time.total_elapsed_time += dt;

        dt
    }

    fn handle_scene_commands(&mut self) {
        let commands = {
            let mut scene_commands = self.world.write_resource::<SceneCommands>();
            scene_commands.drain().collect::<Vec<_>>()
        };

        for command in commands {
            match command {
                SceneCommand::SwitchTo(new_scene) => {
                    let old_entities = {
                        let mut active_scene = self.world.write_resource::<ActiveScene>();
                        std::mem::take(&mut active_scene.entities)
                    };
                    for entity in old_entities {
                        let _ = self.world.delete_entity(entity);
                    }
                    let mut active_scene = self.world.write_resource::<ActiveScene>();
                    active_scene.set(new_scene);
                }
                _ => todo!(),
            }
        }
    }
}

impl AppBuilder {
    /// Create a new [`AppBuilder`]
    pub fn new() -> AppBuilder {
        let mut dispatcher_builders = HashMap::new();
        let mut dispatcher_config = HashMap::new();

        for stage in Stage::ORDER {
            dispatcher_builders.insert(stage, DispatcherBuilder::new());
            dispatcher_config.insert(stage, Vec::new());
        }

        Self {
            config: "",
            world: World::new(),
            component_registry: ComponentRegistry::new(),
            dispatcher_builders,
            dispatcher_config,
            plugins: Vec::new(),
        }
    }

    /// Set the application config
    pub fn with_config(mut self, path: &'static str) -> AppBuilder {
        self.config = path;
        self
    }

    /// Register a system in the default gameplay stage ([`Stage::Update`])
    pub fn with_system<S>(self, sys: S) -> Self
    where
        for<'a> S: specs::System<'a> + Send + 'static,
    {
        self.with_system_in_stage(Stage::Update, sys)
    }

    /// Register a system in a specific stage
    pub fn with_system_in_stage<S>(mut self, stage: Stage, sys: S) -> Self
    where
        for<'a> S: specs::System<'a> + Send + 'static,
    {
        push_system_registration(
            self.dispatcher_config
                .get_mut(&stage)
                .expect("stage config missing"),
            sys,
            &[],
        );

        self
    }

    /// Register a system with dependencies in the default gameplay stage
    pub fn with_system_deps<S>(self, sys: S, deps: &'static [&'static str]) -> Self
    where
        for<'a> S: specs::System<'a> + Send + 'static,
    {
        self.with_system_deps_in_stage(Stage::Update, sys, deps)
    }

    /// Register a system with dependencies in a specific stage
    pub fn with_system_deps_in_stage<S>(
        mut self,
        stage: Stage,
        sys: S,
        deps: &'static [&'static str],
    ) -> Self
    where
        for<'a> S: specs::System<'a> + Send + 'static,
    {
        push_system_registration(
            self.dispatcher_config
                .get_mut(&stage)
                .expect("stage config missing"),
            sys,
            deps,
        );

        self
    }

    /// Register a component in the ECS
    pub fn with_component<C: Component, F: ComponentFactory + Default + 'static>(
        mut self,
    ) -> AppBuilder
    where
        <C as Component>::Storage: Default,
    {
        self.world.register::<C>();
        self.component_registry
            .register(F::name(), Box::new(F::default()));
        self
    }

    /// Register a plugin
    pub fn with_plugin<C: Plugin + 'static + Send + Sync>(mut self, plugin: C) -> AppBuilder {
        self.plugins.push(Box::new(plugin));
        self
    }

    /// Builds an [`App`]
    pub fn build(mut self) -> RuntimeResult<App> {
        if self.config.is_empty() {
            return Err(RuntimeError::EmptyConfigPath);
        }

        let config_reader =
            fs::read(self.config).map_err(|source| RuntimeError::ConfigIo { source })?;

        let config: Config = toml::from_slice(&config_reader)?;

        let mut asset_server = AssetServer::with_root(config.assets_path.clone());
        asset_server.register_loader::<Scene>(Box::new(SceneLoader));

        self.world.register::<Handle<Scene>>();

        self.world.insert(Time {
            ticks: 0,
            tick_period: config.tick_period,
            total_elapsed_time: 0.0,
            delta_seconds: 0.0,
        });

        self.world.insert(SceneCommands::default());

        self.load_scenes(&config, &mut asset_server)?;

        self.world.insert(ActiveScene {
            name: Some(config.initial_scene),
            loaded: false,
            entities: Vec::new(),
        });

        self.world.insert(asset_server);
        self.world.insert(Events::<AppEvent>::default());

        self.register_internal_systems();
        self.build_plugins()?;
        self.build_dispatcher_configs();

        let dispatchers = self.build_dispatchers();
        let sender = self.world.write_resource::<Sender<AppEvent>>().clone();
        ctrlc::set_handler(move || {
            sender
                .send(AppEvent::ExitRequested)
                .expect("failed to send exit request message");
        })
        .expect("error setting Ctrl-C handler");

        self.world.insert(self.component_registry);
        // systems shouldn't have access to this, and it already got passed to plugins by this point
        self.world.remove::<Sender<AppEvent>>();

        Ok(App {
            world: self.world,
            dispatchers,
            plugins: self.plugins,
            running: false,
        })
    }

    fn load_scenes(
        &mut self,
        config: &Config,
        asset_server: &mut AssetServer,
    ) -> RuntimeResult<()> {
        let scenes_path = PathBuf::from(&config.assets_path).join("scenes");

        debug!("config.assets_path={}", config.assets_path);
        debug!("scenes_path={}", scenes_path.display());

        let mut library = SceneLibrary::default();

        for entry in fs::read_dir(scenes_path)? {
            let entry = entry?;
            let mut reader = File::open(entry.path())?;

            let scene = asset_server.load_reader::<Scene>(
                entry
                    .file_name()
                    .to_str()
                    .expect("failed to convert scene path to str"),
                &mut reader,
            )?;
            let name = scene.read().name.clone();
            library.scenes.insert(name.clone(), scene);
        }

        self.world.insert(library);

        Ok(())
    }

    fn build_plugins(&mut self) -> RuntimeResult<()> {
        for plugin in self.plugins.iter_mut() {
            let event_sender = self.world.write_resource::<Sender<AppEvent>>().clone();
            let mut ctx = PluginBuildContext {
                world: &mut self.world,
                dispatcher_config: &mut self.dispatcher_config,
                component_registry: &mut self.component_registry,
                event_sender,
            };

            plugin.build(&mut ctx).map_err(|source| {
                error!("failed to build plugin `{}`: {}", plugin.name(), source);

                RuntimeError::PluginSetup {
                    plugin: plugin.name().to_string(),
                    source,
                }
            })?;

            debug!("registered plugin {}", plugin.name());
        }

        Ok(())
    }

    fn register_internal_systems(&mut self) {
        let (sender, receiver) = std::sync::mpsc::channel::<AppEvent>();
        self.world.insert(sender);

        let runtime = self
            .dispatcher_builders
            .get_mut(&Stage::Runtime)
            .expect("runtime dispatcher builder missing");

        runtime.add(SceneSystem, "furmint.scene_system", &[]);

        let pre_update = self
            .dispatcher_builders
            .get_mut(&Stage::PreUpdate)
            .expect("pre-update dispatcher builder missing");
        pre_update.add(
            AppEventReceivingSystem::new(receiver),
            "furmint.app_event_receiving",
            &[],
        );
    }

    fn build_dispatcher_configs(&mut self) {
        for stage in Stage::ORDER {
            let builder = self
                .dispatcher_builders
                .get_mut(&stage)
                .expect("dispatcher builder missing");

            let regs = self
                .dispatcher_config
                .get_mut(&stage)
                .expect("dispatcher config missing");

            for reg in regs.drain(..) {
                reg(builder);
            }

            #[cfg(debug_assertions)]
            {
                debug!("system graph for stage {:?}:", stage);
                builder.print_par_seq();
            }
        }
    }

    fn build_dispatchers(&mut self) -> HashMap<Stage, Dispatcher<'static, 'static>> {
        let mut dispatchers = HashMap::new();

        for stage in Stage::ORDER {
            let builder = self
                .dispatcher_builders
                .remove(&stage)
                .expect("dispatcher builder missing");

            dispatchers.insert(stage, builder.build());
        }

        dispatchers
    }
}

pub(crate) fn push_system_registration<S>(
    config: &mut Vec<SystemRegistration>,
    sys: S,
    deps: &'static [&'static str],
) where
    for<'a> S: specs::System<'a> + Send + 'static,
{
    config.push(Box::new(move |builder| {
        let name = std::any::type_name::<S>();
        builder.add(sys, name, deps);
    }));
}