use frame::{self, Frame};

use slab::Slab;

use std::marker::PhantomData;

/// Buffers frames for multiple streams.
#[derive(Debug)]
pub struct Buffer<B> {
    slab: Slab<Slot<B>>,
}

/// A sequence of frames in a `Buffer`
#[derive(Debug)]
pub struct Deque<B> {
    indices: Option<Indices>,
    _p: PhantomData<B>,
}

/// Tracks the head & tail for a sequence of frames in a `Buffer`.
#[derive(Debug, Default, Copy, Clone)]
struct Indices {
    head: usize,
    tail: usize,
}

#[derive(Debug)]
struct Slot<B> {
    frame: Frame<B>,
    next: Option<usize>,
}

impl<B> Buffer<B> {
    pub fn new() -> Self {
        Buffer {
            slab: Slab::new(),
        }
    }
}

impl<B> Deque<B> {
    pub fn new() -> Self {
        Deque {
            indices: None,
            _p: PhantomData,
        }
    }

    pub fn is_empty(&self) -> bool {
        self.indices.is_none()
    }

    pub fn push_back(&mut self, buf: &mut Buffer<B>, frame: Frame<B>) {
        let key = buf.slab.insert(Slot {
            frame,
            next: None,
        });

        match self.indices {
            Some(ref mut idxs) => {
                buf.slab[idxs.tail].next = Some(key);
                idxs.tail = key;
            }
            None => {
                self.indices = Some(Indices {
                    head: key,
                    tail: key,
                });
            }
        }
    }

    pub fn push_front(&mut self, buf: &mut Buffer<B>, frame: Frame<B>) {
        let key = buf.slab.insert(Slot {
            frame,
            next: None,
        });

        match self.indices {
            Some(ref mut idxs) => {
                buf.slab[key].next = Some(idxs.head);
                idxs.head = key;
            }
            None => {
                self.indices = Some(Indices {
                    head: key,
                    tail: key,
                });
            }
        }
    }

    pub fn pop_front(&mut self, buf: &mut Buffer<B>) -> Option<Frame<B>> {
        match self.indices {
            Some(mut idxs) => {
                let mut slot = buf.slab.remove(idxs.head);

                if idxs.head == idxs.tail {
                    assert!(slot.next.is_none());
                    self.indices = None;
                } else {
                    idxs.head = slot.next.take().unwrap();
                    self.indices = Some(idxs);
                }

                return Some(slot.frame);
            }
            None => None,
        }
    }

    pub fn peek_front<'a>(&self, buf: &'a Buffer<B>) -> Option<&'a Frame<B>> {
        match self.indices {
            Some(idxs) => {
                Some(&buf.slab[idxs.head].frame)
            }
            None => None,
        }
    }
}