//! Borrowed from `bevy_core`.

use std::marker::PhantomData;

use bevy_app::{App, Last, Plugin};
use bevy_ecs::system::{NonSend, Resource};
use bevy_tasks::{
    tick_global_task_pools_on_main_thread, AsyncComputeTaskPool, ComputeTaskPool, IoTaskPool,
    TaskPoolBuilder,
};

/// Setup of default task pools: `AsyncComputeTaskPool`, `ComputeTaskPool`,
/// `IoTaskPool`.
#[derive(Default)]
pub struct TaskPoolPlugin {
    /// Options for the [`TaskPool`](bevy_tasks::TaskPool) created at
    /// application start.
    pub task_pool_options: TaskPoolOptions,
}

impl Plugin for TaskPoolPlugin {
    fn build(&self, app: &mut App) {
        // Setup the default bevy task pools
        self.task_pool_options.create_default_pools();

        #[cfg(not(target_arch = "wasm32"))]
        app.add_systems(Last, tick_global_task_pools);
    }
}

pub struct NonSendMarker(PhantomData<*mut ()>);
#[cfg(not(target_arch = "wasm32"))]
fn tick_global_task_pools(_main_thread_marker: Option<NonSend<NonSendMarker>>) {
    tick_global_task_pools_on_main_thread();
}

/// Helper for configuring and creating the default task pools. For end-users
/// who want full control, set up [`TaskPoolPlugin`]
#[derive(Clone, Resource)]
pub struct TaskPoolOptions {
    /// If the number of physical cores is less than min_total_threads, force
    /// using min_total_threads
    pub min_total_threads: usize,
    /// If the number of physical cores is greater than max_total_threads, force
    /// using max_total_threads
    pub max_total_threads: usize,

    /// Used to determine number of IO threads to allocate
    pub io: TaskPoolThreadAssignmentPolicy,
    /// Used to determine number of async compute threads to allocate
    pub async_compute: TaskPoolThreadAssignmentPolicy,
    /// Used to determine number of compute threads to allocate
    pub compute: TaskPoolThreadAssignmentPolicy,
}

impl Default for TaskPoolOptions {
    fn default() -> Self {
        TaskPoolOptions {
            // By default, use however many cores are available on the system
            min_total_threads: 1,
            max_total_threads: usize::MAX,

            // Use 25% of cores for IO, at least 1, no more than 4
            io: TaskPoolThreadAssignmentPolicy {
                min_threads: 1,
                max_threads: 4,
                percent: 0.25,
            },

            // Use 25% of cores for async compute, at least 1, no more than 4
            async_compute: TaskPoolThreadAssignmentPolicy {
                min_threads: 1,
                max_threads: 4,
                percent: 0.25,
            },

            // Use all remaining cores for compute (at least 1)
            compute: TaskPoolThreadAssignmentPolicy {
                min_threads: 1,
                max_threads: usize::MAX,
                percent: 1.0, // This 1.0 here means "whatever is left over"
            },
        }
    }
}

impl TaskPoolOptions {
    // /// Create a configuration that forces using the given number of threads.
    // pub fn with_num_threads(thread_count: usize) -> Self {
    //     TaskPoolOptions {
    //         min_total_threads: thread_count,
    //         max_total_threads: thread_count,
    //         ..Default::default()
    //     }
    // }

    /// Inserts the default thread pools into the given resource map based on
    /// the configured values
    pub fn create_default_pools(&self) {
        let total_threads = bevy_tasks::available_parallelism()
            .clamp(self.min_total_threads, self.max_total_threads);

        let mut remaining_threads = total_threads;

        {
            // Determine the number of IO threads we will use
            let io_threads = self
                .io
                .get_number_of_threads(remaining_threads, total_threads);

            remaining_threads = remaining_threads.saturating_sub(io_threads);

            IoTaskPool::get_or_init(|| {
                TaskPoolBuilder::default()
                    .num_threads(io_threads)
                    .thread_name("IO Task Pool".to_string())
                    .build()
            });
        }

        {
            // Determine the number of async compute threads we will use
            let async_compute_threads = self
                .async_compute
                .get_number_of_threads(remaining_threads, total_threads);

            remaining_threads = remaining_threads.saturating_sub(async_compute_threads);

            AsyncComputeTaskPool::get_or_init(|| {
                TaskPoolBuilder::default()
                    .num_threads(async_compute_threads)
                    .thread_name("Async Compute Task Pool".to_string())
                    .build()
            });
        }

        {
            // Determine the number of compute threads we will use
            // This is intentionally last so that an end user can specify 1.0 as the percent
            let compute_threads = self
                .compute
                .get_number_of_threads(remaining_threads, total_threads);

            ComputeTaskPool::get_or_init(|| {
                TaskPoolBuilder::default()
                    .num_threads(compute_threads)
                    .thread_name("Compute Task Pool".to_string())
                    .build()
            });
        }
    }
}

/// Defines a simple way to determine how many threads to use given the number
/// of remaining cores and number of total cores
#[derive(Clone)]
pub struct TaskPoolThreadAssignmentPolicy {
    /// Force using at least this many threads
    pub min_threads: usize,
    /// Under no circumstance use more than this many threads for this pool
    pub max_threads: usize,
    /// Target using this percentage of total cores, clamped by min_threads and
    /// max_threads. It is permitted to use 1.0 to try to use all remaining
    /// threads
    pub percent: f32,
}

impl TaskPoolThreadAssignmentPolicy {
    /// Determine the number of threads to use for this task pool
    fn get_number_of_threads(&self, remaining_threads: usize, total_threads: usize) -> usize {
        assert!(self.percent >= 0.0);
        let mut desired = (total_threads as f32 * self.percent).round() as usize;

        // Limit ourselves to the number of cores available
        desired = desired.min(remaining_threads);

        // Clamp by min_threads, max_threads. (This may result in us using more threads
        // than are available, this is intended. An example case where this
        // might happen is a device with <= 2 threads.
        desired.clamp(self.min_threads, self.max_threads)
    }
}
