//! Very simple connection pool, to avoid an external dependency on r2d2 and other crates. If this
//! endup work it can be re-used in other parts of the project and may be promoted to its own
//! generic crate

use std::fmt::Debug;
use std::ops::{Deref, DerefMut};
use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
use std::sync::{Arc, Condvar, Mutex};
use std::time::Duration;

/// Pool error
#[derive(thiserror::Error, Debug)]
pub enum Error<E> {
    /// Mutex Poison Error
    #[error("Internal: PoisonError")]
    Poison,

    /// Timeout error
    #[error("Timed out waiting for a resource")]
    Timeout,

    /// Internal database error
    #[error(transparent)]
    Resource(#[from] E),
}

/// Trait to manage resources
pub trait ResourceManager: Debug {
    /// The resource to be pooled
    type Resource: Debug;

    /// The configuration that is needed in order to create the resource
    type Config: Clone + Debug;

    /// The error the resource may return when creating a new instance
    type Error: Debug;

    /// Creates a new resource with a given config
    fn new_resource(
        config: &Self::Config,
        still_valid: Arc<AtomicBool>,
        timeout: Duration,
    ) -> Result<Self::Resource, Error<Self::Error>>;

    /// The object is dropped
    fn drop(_resource: Self::Resource) {}
}

/// Generic connection pool of resources R
#[derive(Debug)]
pub struct Pool<RM>
where
    RM: ResourceManager,
{
    config: RM::Config,
    queue: Mutex<Vec<(Arc<AtomicBool>, RM::Resource)>>,
    in_use: AtomicUsize,
    max_size: usize,
    default_timeout: Duration,
    waiter: Condvar,
}

/// The pooled resource
pub struct PooledResource<RM>
where
    RM: ResourceManager,
{
    resource: Option<(Arc<AtomicBool>, RM::Resource)>,
    pool: Arc<Pool<RM>>,
}

impl<RM> Drop for PooledResource<RM>
where
    RM: ResourceManager,
{
    fn drop(&mut self) {
        if let Some(resource) = self.resource.take() {
            let mut active_resource = self.pool.queue.lock().expect("active_resource");
            active_resource.push(resource);
            self.pool.in_use.fetch_sub(1, Ordering::AcqRel);

            // Notify a waiting thread
            self.pool.waiter.notify_one();
        }
    }
}

impl<RM> Deref for PooledResource<RM>
where
    RM: ResourceManager,
{
    type Target = RM::Resource;

    fn deref(&self) -> &Self::Target {
        &self.resource.as_ref().expect("resource already dropped").1
    }
}

impl<RM> DerefMut for PooledResource<RM>
where
    RM: ResourceManager,
{
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.resource.as_mut().expect("resource already dropped").1
    }
}

impl<RM> Pool<RM>
where
    RM: ResourceManager,
{
    /// Creates a new pool
    pub fn new(config: RM::Config, max_size: usize, default_timeout: Duration) -> Arc<Self> {
        Arc::new(Self {
            config,
            queue: Default::default(),
            in_use: Default::default(),
            waiter: Default::default(),
            default_timeout,
            max_size,
        })
    }

    /// Similar to get_timeout but uses the default timeout value.
    #[inline(always)]
    pub fn get(self: &Arc<Self>) -> Result<PooledResource<RM>, Error<RM::Error>> {
        self.get_timeout(self.default_timeout)
    }

    /// Get a new resource or fail after timeout is reached.
    ///
    /// This function will return a free resource or create a new one if there is still room for it;
    /// otherwise, it will wait for a resource to be released for reuse.
    #[inline(always)]
    pub fn get_timeout(
        self: &Arc<Self>,
        timeout: Duration,
    ) -> Result<PooledResource<RM>, Error<RM::Error>> {
        let mut resources = self.queue.lock().map_err(|_| Error::Poison)?;

        loop {
            if let Some(resource) = resources.pop() {
                if resource.0.load(Ordering::SeqCst) {
                    drop(resources);
                    self.in_use.fetch_add(1, Ordering::AcqRel);

                    return Ok(PooledResource {
                        resource: Some(resource),
                        pool: self.clone(),
                    });
                }
            }

            if self.in_use.load(Ordering::Relaxed) < self.max_size {
                drop(resources);
                self.in_use.fetch_add(1, Ordering::AcqRel);
                let still_valid: Arc<AtomicBool> = Arc::new(true.into());

                return Ok(PooledResource {
                    resource: Some((
                        still_valid.clone(),
                        RM::new_resource(&self.config, still_valid, timeout)?,
                    )),
                    pool: self.clone(),
                });
            }

            resources = self
                .waiter
                .wait_timeout(resources, timeout)
                .map_err(|_| Error::Poison)
                .and_then(|(lock, timeout_result)| {
                    if timeout_result.timed_out() {
                        Err(Error::Timeout)
                    } else {
                        Ok(lock)
                    }
                })?;
        }
    }
}

impl<RM> Drop for Pool<RM>
where
    RM: ResourceManager,
{
    fn drop(&mut self) {
        if let Ok(mut resources) = self.queue.lock() {
            loop {
                while let Some(resource) = resources.pop() {
                    RM::drop(resource.1);
                }

                if self.in_use.load(Ordering::Relaxed) == 0 {
                    break;
                }

                resources = if let Ok(resources) = self.waiter.wait(resources) {
                    resources
                } else {
                    break;
                };
            }
        }
    }
}