use core::cell::Cell;
use thread_local::ThreadLocal;

/// A cohesive set of thread-local values of a given type.
///
/// Mutable references can be fetched if `T: Default` via [`Parallel::scope`].
#[derive(Default)]
pub struct Parallel<T: Send> {
    locals: ThreadLocal<Cell<T>>,
}

impl<T: Send> Parallel<T> {
    /// Gets a mutable iterator over all of the per-thread queues.
    pub fn iter_mut(&mut self) -> impl Iterator<Item = &'_ mut T> {
        self.locals.iter_mut().map(|cell| cell.get_mut())
    }

    /// Clears all of the stored thread local values.
    pub fn clear(&mut self) {
        self.locals.clear();
    }
}

impl<T: Default + Send> Parallel<T> {
    /// Retrieves the thread-local value for the current thread and runs `f` on it.
    ///
    /// If there is no thread-local value, it will be initialized to it's default.
    pub fn scope<R>(&self, f: impl FnOnce(&mut T) -> R) -> R {
        let cell = self.locals.get_or_default();
        let mut value = cell.take();
        let ret = f(&mut value);
        cell.set(value);
        ret
    }
}

impl<T, I> Parallel<I>
where
    I: IntoIterator<Item = T> + Default + Send + 'static,
{
    /// Drains all enqueued items from all threads and returns an iterator over them.
    ///
    /// Unlike [`Vec::drain`], this will piecemeal remove chunks of the data stored.
    /// If iteration is terminated part way, the rest of the enqueued items in the same
    /// chunk will be dropped, and the rest of the undrained elements will remain.
    ///
    /// The ordering is not guaranteed.
    pub fn drain<B>(&mut self) -> impl Iterator<Item = T> + '_
    where
        B: FromIterator<T>,
    {
        self.locals.iter_mut().flat_map(|item| item.take())
    }
}

impl<T: Send> Parallel<Vec<T>> {
    /// Collect all enqueued items from all threads and appends them to the end of a
    /// single Vec.
    ///
    /// The ordering is not guarenteed.
    pub fn drain_into(&mut self, out: &mut Vec<T>) {
        let size = self
            .locals
            .iter_mut()
            .map(|queue| queue.get_mut().len())
            .sum();
        out.reserve(size);
        for queue in self.locals.iter_mut() {
            out.append(queue.get_mut());
        }
    }
}