use crate::{CoreStage, Events, Plugin, PluginGroup, PluginGroupBuilder, StartupStage};
use bevy_ecs::{
    component::{Component, ComponentDescriptor},
    prelude::{FromWorld, IntoExclusiveSystem, IntoSystem},
    schedule::{
        IntoSystemDescriptor, RunOnce, Schedule, Stage, StageLabel, State, SystemSet, SystemStage,
    },
    world::World,
};
use bevy_utils::tracing::debug;
use std::{fmt::Debug, hash::Hash};

#[cfg(feature = "trace")]
use bevy_utils::tracing::info_span;

#[allow(clippy::needless_doctest_main)]
/// Containers of app logic and data
///
/// App store the ECS World, Resources, Schedule, and Executor. They also store the "run" function
/// of the App, which by default executes the App schedule once. Apps are constructed using the
/// builder pattern.
///
/// ## Example
/// Here is a simple "Hello World" Bevy app:
/// ```
/// # use bevy_app::prelude::*;
/// # use bevy_ecs::prelude::*;
///
/// fn main() {
///    App::new()
///        .add_system(hello_world_system.system())
///        .run();
/// }
///
/// fn hello_world_system() {
///    println!("hello world");
/// }
/// ```
pub struct App {
    pub world: World,
    pub runner: Box<dyn Fn(App)>,
    pub schedule: Schedule,
    sub_apps: Vec<SubApp>,
}

struct SubApp {
    app: App,
    runner: Box<dyn Fn(&mut World, &mut App)>,
}

impl Default for App {
    fn default() -> Self {
        let mut app = App::empty();
        #[cfg(feature = "bevy_reflect")]
        app.init_resource::<bevy_reflect::TypeRegistryArc>();

        app.add_default_stages()
            .add_event::<AppExit>()
            .add_system_to_stage(CoreStage::Last, World::clear_trackers.exclusive_system());

        #[cfg(feature = "bevy_ci_testing")]
        {
            crate::ci_testing::setup_app(&mut app);
        }

        app
    }
}

impl App {
    pub fn new() -> App {
        App::default()
    }

    pub fn empty() -> App {
        Self {
            world: Default::default(),
            schedule: Default::default(),
            runner: Box::new(run_once),
            sub_apps: Vec::new(),
        }
    }

    pub fn update(&mut self) {
        #[cfg(feature = "trace")]
        let bevy_frame_update_span = info_span!("frame");
        #[cfg(feature = "trace")]
        let _bevy_frame_update_guard = bevy_frame_update_span.enter();
        self.schedule.run(&mut self.world);
        for sub_app in self.sub_apps.iter_mut() {
            (sub_app.runner)(&mut self.world, &mut sub_app.app);
        }
    }

    /// Start the application (through main runner)
    ///
    /// Runs the application main loop.
    ///
    /// Usually the main loop is handled by Bevy integrated plugins (`winit`), but
    /// but one can also set the runner function through [`App::set_runner`].
    ///
    /// ## Example
    /// ```
    /// # use bevy_app::prelude::*;
    /// #
    /// App::new()
    ///     // all required plugin insertions, systems, etc inserted here
    ///     // finally, call:
    ///     .run();
    /// ```
    pub fn run(&mut self) {
        #[cfg(feature = "trace")]
        let bevy_app_run_span = info_span!("bevy_app");
        #[cfg(feature = "trace")]
        let _bevy_app_run_guard = bevy_app_run_span.enter();

        let mut app = std::mem::replace(self, App::empty());
        let runner = std::mem::replace(&mut app.runner, Box::new(run_once));
        (runner)(app);
    }

    pub fn add_stage<S: Stage>(&mut self, label: impl StageLabel, stage: S) -> &mut Self {
        self.schedule.add_stage(label, stage);
        self
    }

    pub fn add_stage_after<S: Stage>(
        &mut self,
        target: impl StageLabel,
        label: impl StageLabel,
        stage: S,
    ) -> &mut Self {
        self.schedule.add_stage_after(target, label, stage);
        self
    }

    pub fn add_stage_before<S: Stage>(
        &mut self,
        target: impl StageLabel,
        label: impl StageLabel,
        stage: S,
    ) -> &mut Self {
        self.schedule.add_stage_before(target, label, stage);
        self
    }

    pub fn add_startup_stage<S: Stage>(&mut self, label: impl StageLabel, stage: S) -> &mut Self {
        self.schedule
            .stage(CoreStage::Startup, |schedule: &mut Schedule| {
                schedule.add_stage(label, stage)
            });
        self
    }

    pub fn add_startup_stage_after<S: Stage>(
        &mut self,
        target: impl StageLabel,
        label: impl StageLabel,
        stage: S,
    ) -> &mut Self {
        self.schedule
            .stage(CoreStage::Startup, |schedule: &mut Schedule| {
                schedule.add_stage_after(target, label, stage)
            });
        self
    }

    pub fn add_startup_stage_before<S: Stage>(
        &mut self,
        target: impl StageLabel,
        label: impl StageLabel,
        stage: S,
    ) -> &mut Self {
        self.schedule
            .stage(CoreStage::Startup, |schedule: &mut Schedule| {
                schedule.add_stage_before(target, label, stage)
            });
        self
    }

    pub fn stage<T: Stage, F: FnOnce(&mut T) -> &mut T>(
        &mut self,
        label: impl StageLabel,
        func: F,
    ) -> &mut Self {
        self.schedule.stage(label, func);
        self
    }

    /// Adds a system that runs every time `app.update()` is called by the runner
    ///
    /// Systems are the main building block in the Bevy ECS app model. You can define
    /// normal rust functions, and call `.system()` to make them be Bevy systems.
    ///
    /// System functions can have parameters, through which one can query and
    /// mutate Bevy ECS states.
    /// See [The Bevy Book](https://bevyengine.org/learn/book/introduction/) for more information.
    ///
    /// Systems are run in parallel, and the execution order is not deterministic.
    /// If you want more fine-grained control for order, see [`App::add_system_to_stage`].
    ///
    /// For adding a system that runs only at app startup, see [`App::add_startup_system`].
    ///
    /// ## Example
    /// ```
    /// # use bevy_app::prelude::*;
    /// # use bevy_ecs::prelude::*;
    /// #
    /// fn my_system(_commands: Commands) {
    ///     println!("My system, triggered once per frame");
    /// }
    ///
    /// App::new()
    ///     .add_system(my_system.system());
    /// ```
    pub fn add_system<Params>(&mut self, system: impl IntoSystemDescriptor<Params>) -> &mut Self {
        self.add_system_to_stage(CoreStage::Update, system)
    }

    pub fn add_system_set(&mut self, system_set: SystemSet) -> &mut Self {
        self.add_system_set_to_stage(CoreStage::Update, system_set)
    }

    pub fn add_system_to_stage<Params>(
        &mut self,
        stage_label: impl StageLabel,
        system: impl IntoSystemDescriptor<Params>,
    ) -> &mut Self {
        self.schedule.add_system_to_stage(stage_label, system);
        self
    }

    pub fn add_system_set_to_stage(
        &mut self,
        stage_label: impl StageLabel,
        system_set: SystemSet,
    ) -> &mut Self {
        self.schedule
            .add_system_set_to_stage(stage_label, system_set);
        self
    }

    /// Adds a system that is run once at application startup
    ///
    /// Startup systems run exactly once BEFORE all other systems. These are generally used for
    /// app initialization code (ex: adding entities and resources).
    ///
    /// * For adding a system that runs for every frame, see [`App::add_system`].
    /// * For adding a system to specific stage, see [`App::add_system_to_stage`].
    ///
    /// ## Example
    /// ```
    /// # use bevy_app::prelude::*;
    /// # use bevy_ecs::prelude::*;
    /// #
    /// fn my_startup_system(_commands: Commands) {
    ///     println!("My startup system");
    /// }
    ///
    /// App::new()
    ///     .add_startup_system(my_startup_system.system());
    /// ```
    pub fn add_startup_system<Params>(
        &mut self,
        system: impl IntoSystemDescriptor<Params>,
    ) -> &mut Self {
        self.add_startup_system_to_stage(StartupStage::Startup, system)
    }

    pub fn add_startup_system_set(&mut self, system_set: SystemSet) -> &mut Self {
        self.add_startup_system_set_to_stage(StartupStage::Startup, system_set)
    }

    pub fn add_startup_system_to_stage<Params>(
        &mut self,
        stage_label: impl StageLabel,
        system: impl IntoSystemDescriptor<Params>,
    ) -> &mut Self {
        self.schedule
            .stage(CoreStage::Startup, |schedule: &mut Schedule| {
                schedule.add_system_to_stage(stage_label, system)
            });
        self
    }

    pub fn add_startup_system_set_to_stage(
        &mut self,
        stage_label: impl StageLabel,
        system_set: SystemSet,
    ) -> &mut Self {
        self.schedule
            .stage(CoreStage::Startup, |schedule: &mut Schedule| {
                schedule.add_system_set_to_stage(stage_label, system_set)
            });
        self
    }

    /// Adds a new [State] with the given `initial` value.
    /// This inserts a new `State<T>` resource and adds a new "driver" to [CoreStage::Update].
    /// Each stage that uses `State<T>` for system run criteria needs a driver. If you need to use
    /// your state in a different stage, consider using [Self::add_state_to_stage] or manually
    /// adding [State::get_driver] to additional stages you need it in.
    pub fn add_state<T>(&mut self, initial: T) -> &mut Self
    where
        T: Component + Debug + Clone + Eq + Hash,
    {
        self.add_state_to_stage(CoreStage::Update, initial)
    }

    /// Adds a new [State] with the given `initial` value.
    /// This inserts a new `State<T>` resource and adds a new "driver" to the given stage.
    /// Each stage that uses `State<T>` for system run criteria needs a driver. If you need to use
    /// your state in more than one stage, consider manually adding [State::get_driver] to the
    /// stages you need it in.
    pub fn add_state_to_stage<T>(&mut self, stage: impl StageLabel, initial: T) -> &mut Self
    where
        T: Component + Debug + Clone + Eq + Hash,
    {
        self.insert_resource(State::new(initial))
            .add_system_set_to_stage(stage, State::<T>::get_driver())
    }

    pub fn add_default_stages(&mut self) -> &mut Self {
        self.add_stage(CoreStage::First, SystemStage::parallel())
            .add_stage(
                CoreStage::Startup,
                Schedule::default()
                    .with_run_criteria(RunOnce::default())
                    .with_stage(StartupStage::PreStartup, SystemStage::parallel())
                    .with_stage(StartupStage::Startup, SystemStage::parallel())
                    .with_stage(StartupStage::PostStartup, SystemStage::parallel()),
            )
            .add_stage(CoreStage::PreUpdate, SystemStage::parallel())
            .add_stage(CoreStage::Update, SystemStage::parallel())
            .add_stage(CoreStage::PostUpdate, SystemStage::parallel())
            .add_stage(CoreStage::Last, SystemStage::parallel())
    }

    /// Setup the application to manage events of type `T`.
    ///
    /// This is done by adding a `Resource` of type `Events::<T>`,
    /// and inserting a `Events::<T>::update_system` system into `CoreStage::First`.
    pub fn add_event<T>(&mut self) -> &mut Self
    where
        T: Component,
    {
        self.insert_resource(Events::<T>::default())
            .add_system_to_stage(CoreStage::First, Events::<T>::update_system.system())
    }

    /// Inserts a resource to the current [App] and overwrites any resource previously added of the same type.
    ///
    /// A resource in Bevy represents globally unique data. Resources must be added to Bevy Apps
    /// before using them. This happens with [`App::insert_resource`].
    ///
    /// See also `init_resource` for resources that implement `Default` or [`FromResources`].
    ///
    /// ## Example
    /// ```
    /// # use bevy_app::prelude::*;
    /// #
    /// struct MyCounter {
    ///     counter: usize,
    /// }
    ///
    /// App::new()
    ///    .insert_resource(MyCounter { counter: 0 });
    /// ```
    pub fn insert_resource<T>(&mut self, resource: T) -> &mut Self
    where
        T: Component,
    {
        self.world.insert_resource(resource);
        self
    }

    /// Inserts a non-send resource to the app
    ///
    /// You usually want to use `insert_resource`, but there are some special cases when a resource must
    /// be non-send.
    ///
    /// ## Example
    /// ```
    /// # use bevy_app::prelude::*;
    /// #
    /// struct MyCounter {
    ///     counter: usize,
    /// }
    ///
    /// App::new()
    ///     .insert_non_send_resource(MyCounter { counter: 0 });
    /// ```
    pub fn insert_non_send_resource<T>(&mut self, resource: T) -> &mut Self
    where
        T: 'static,
    {
        self.world.insert_non_send(resource);
        self
    }

    /// Initialize a resource in the current [App], if it does not exist yet
    ///
    /// Adds a resource that implements `Default` or [`FromResources`] trait.
    /// If the resource already exists, `init_resource` does nothing.
    ///
    /// ## Example
    /// ```
    /// # use bevy_app::prelude::*;
    /// #
    /// struct MyCounter {
    ///     counter: usize,
    /// }
    ///
    /// impl Default for MyCounter {
    ///     fn default() -> MyCounter {
    ///         MyCounter {
    ///             counter: 100
    ///         }
    ///     }
    /// }
    ///
    /// App::new()
    ///     .init_resource::<MyCounter>();
    /// ```
    pub fn init_resource<R>(&mut self) -> &mut Self
    where
        R: FromWorld + Send + Sync + 'static,
    {
        // PERF: We could avoid double hashing here, since the `from_resources` call is guaranteed
        // not to modify the map. However, we would need to be borrowing resources both
        // mutably and immutably, so we would need to be extremely certain this is correct
        if !self.world.contains_resource::<R>() {
            let resource = R::from_world(&mut self.world);
            self.insert_resource(resource);
        }
        self
    }

    pub fn init_non_send_resource<R>(&mut self) -> &mut Self
    where
        R: FromWorld + 'static,
    {
        // See perf comment in init_resource
        if self.world.get_non_send_resource::<R>().is_none() {
            let resource = R::from_world(&mut self.world);
            self.world.insert_non_send(resource);
        }
        self
    }

    /// Sets the main runner loop function for this Bevy App
    ///
    /// Usually the main loop is handled by Bevy integrated plugins ([`WinitPlugin`]), but
    /// in some cases one might wish to implement their own main loop.
    ///
    /// This method sets the main loop function, overwriting a previous runner if any.
    ///
    /// ## Example
    /// ```
    /// # use bevy_app::prelude::*;
    /// #
    /// fn my_runner(mut app: App) {
    ///     loop {
    ///         println!("In main loop");
    ///         app.update();
    ///     }
    /// }
    ///
    /// App::new()
    ///     .set_runner(my_runner);
    /// ```
    pub fn set_runner(&mut self, run_fn: impl Fn(App) + 'static) -> &mut Self {
        self.runner = Box::new(run_fn);
        self
    }

    /// Adds a single plugin
    ///
    /// One of Bevy's core principles is modularity. All Bevy engine features are implemented
    /// as plugins. This includes internal features like the renderer.
    ///
    /// Bevy also provides a few sets of default plugins. See [`App::add_plugins`].
    ///
    /// ## Example
    /// ```
    /// # use bevy_app::prelude::*;
    /// #
    /// App::new().add_plugin(bevy_log::LogPlugin::default());
    /// ```
    pub fn add_plugin<T>(&mut self, plugin: T) -> &mut Self
    where
        T: Plugin,
    {
        debug!("added plugin: {}", plugin.name());
        plugin.build(self);
        self
    }

    /// Adds a group of plugins
    ///
    /// Bevy plugins can be grouped into a set of plugins. Bevy provides
    /// built-in PluginGroups that provide core engine functionality.
    ///
    /// The plugin groups available by default are [`DefaultPlugins`] and [`MinimalPlugins`].
    ///
    /// ## Example
    /// ```
    /// # use bevy_app::{prelude::*, PluginGroupBuilder};
    /// #
    /// # // Dummy created to avoid using bevy_internal, which pulls in to many dependencies.
    /// # struct MinimalPlugins;
    /// # impl PluginGroup for MinimalPlugins {
    /// #     fn build(&mut self, group: &mut PluginGroupBuilder){;}
    /// # }
    /// #
    /// App::new()
    ///     .add_plugins(MinimalPlugins);
    /// ```
    pub fn add_plugins<T: PluginGroup>(&mut self, mut group: T) -> &mut Self {
        let mut plugin_group_builder = PluginGroupBuilder::default();
        group.build(&mut plugin_group_builder);
        plugin_group_builder.finish(self);
        self
    }

    /// Adds a group of plugins with an initializer method
    ///
    /// Can be used to add a group of plugins, where the group is modified
    /// before insertion into Bevy application. For example, you can add
    /// extra plugins at a specific place in the plugin group, or deactivate
    /// specific plugins while keeping the rest.
    ///
    /// ## Example
    /// ```
    /// # use bevy_app::{prelude::*, PluginGroupBuilder};
    /// #
    /// # // Dummies created to avoid using bevy_internal which pulls in to many dependencies.
    /// # struct DefaultPlugins;
    /// # impl PluginGroup for DefaultPlugins {
    /// #     fn build(&mut self, group: &mut PluginGroupBuilder){
    /// #         group.add(bevy_log::LogPlugin::default());
    /// #     }
    /// # }
    /// #
    /// # struct MyOwnPlugin;
    /// # impl Plugin for MyOwnPlugin {
    /// #     fn build(&self, app: &mut AppBuilder){;}
    /// # }
    /// #
    /// App::new()
    ///      .add_plugins_with(DefaultPlugins, |group| {
    ///             group.add_before::<bevy_log::LogPlugin, _>(MyOwnPlugin)
    ///         });
    /// ```
    pub fn add_plugins_with<T, F>(&mut self, mut group: T, func: F) -> &mut Self
    where
        T: PluginGroup,
        F: FnOnce(&mut PluginGroupBuilder) -> &mut PluginGroupBuilder,
    {
        let mut plugin_group_builder = PluginGroupBuilder::default();
        group.build(&mut plugin_group_builder);
        func(&mut plugin_group_builder);
        plugin_group_builder.finish(self);
        self
    }

    /// Registers a new component using the given [ComponentDescriptor]. Components do not need to
    /// be manually registered. This just provides a way to override default configuration.
    /// Attempting to register a component with a type that has already been used by [World]
    /// will result in an error.
    ///
    /// See [World::register_component]
    pub fn register_component(&mut self, descriptor: ComponentDescriptor) -> &mut Self {
        self.world.register_component(descriptor).unwrap();
        self
    }

    #[cfg(feature = "bevy_reflect")]
    pub fn register_type<T: bevy_reflect::GetTypeRegistration>(&mut self) -> &mut Self {
        {
            let registry = self
                .world
                .get_resource_mut::<bevy_reflect::TypeRegistryArc>()
                .unwrap();
            registry.write().register::<T>();
        }
        self
    }

    pub fn add_sub_app(
        &mut self,
        app: App,
        f: impl Fn(&mut World, &mut App) + 'static,
    ) -> &mut Self {
        self.sub_apps.push(SubApp {
            app,
            runner: Box::new(f),
        });
        self
    }

    // TODO: use labels instead of indices
    pub fn sub_app_mut(&mut self, index: usize) -> &mut App {
        &mut self.sub_apps[index].app
    }
}

fn run_once(mut app: App) {
    app.update();
}

/// An event that indicates the app should exit. This will fully exit the app process.
#[derive(Debug, Clone)]
pub struct AppExit;