use std::collections::HashMap;
use std::mem::{replace, swap};

// No clone, no copy! That asserts that an LRUHandle exists only once.
type LRUHandle<T> = *mut LRUNode<T>;

struct LRUNode<T> {
    next: Option<Box<LRUNode<T>>>, // None in the list's last node
    prev: Option<*mut LRUNode<T>>,
    data: Option<T>, // if None, then we have reached the head node
}

struct LRUList<T> {
    head: LRUNode<T>,
    count: usize,
}

/// This is likely unstable; more investigation is needed into correct behavior!
impl<T> LRUList<T> {
    fn new() -> LRUList<T> {
        LRUList {
            head: LRUNode {
                data: None,
                next: None,
                prev: None,
            },
            count: 0,
        }
    }

    /// Inserts new element at front (least recently used element)
    fn insert(&mut self, elem: T) -> LRUHandle<T> {
        self.count += 1;
        // Not first element
        if self.head.next.is_some() {
            let mut new = Box::new(LRUNode {
                data: Some(elem),
                next: None,
                prev: Some(&mut self.head as *mut LRUNode<T>),
            });
            let newp = new.as_mut() as *mut LRUNode<T>;

            // Set up the node after the new one
            self.head.next.as_mut().unwrap().prev = Some(newp);
            // Replace head.next with None and set the new node's next to that
            new.next = replace(&mut self.head.next, None);
            self.head.next = Some(new);

            newp
        } else {
            // First node; the only node right now is an empty head node
            let mut new = Box::new(LRUNode {
                data: Some(elem),
                next: None,
                prev: Some(&mut self.head as *mut LRUNode<T>),
            });
            let newp = new.as_mut() as *mut LRUNode<T>;

            // Set tail
            self.head.prev = Some(newp);
            // Set first node
            self.head.next = Some(new);

            newp
        }
    }

    fn remove_last(&mut self) -> Option<T> {
        if self.head.prev.is_some() {
            let mut lasto = unsafe {
                replace(
                    &mut (*((*self.head.prev.unwrap()).prev.unwrap())).next,
                    None,
                )
            };

            if let Some(ref mut last) = lasto {
                self.head.prev = last.prev;
                self.count -= 1;
                return replace(&mut (*last).data, None);
            } else {
                None
            }
        } else {
            None
        }
    }

    fn remove(&mut self, node_handle: LRUHandle<T>) -> T {
        unsafe {
            // If has next
            if let Some(ref mut nextp) = (*node_handle).next {
                swap(&mut (**nextp).prev, &mut (*node_handle).prev);
            }
            // If has prev
            if let Some(ref mut prevp) = (*node_handle).prev {
                // swap prev.next
                // (node_handle will own itself now)
                swap(&mut (**prevp).next, &mut (*node_handle).next);
            }

            self.count -= 1;
            // node_handle now only has references/objects that point to itself,
            // so it's safe to drop
            replace(&mut (*node_handle).data, None).unwrap()
        }
    }

    /// Reinserts the referenced node at the front.
    fn reinsert_front(&mut self, node_handle: LRUHandle<T>) {
        unsafe {
            let prevp = (*node_handle).prev.unwrap();

            // If not last node, update following node's prev
            if let Some(next) = (*node_handle).next.as_mut() {
                next.prev = Some(prevp);
            } else {
                // If last node, update head
                self.head.prev = Some(prevp);
            }

            // Swap this.next with prev.next. After that, this.next refers to this (!)
            swap(&mut (*prevp).next, &mut (*node_handle).next);
            // To reinsert at head, swap head's next with this.next
            swap(&mut (*node_handle).next, &mut self.head.next);
            // Update this' prev reference to point to head.

            // Update the second node's prev reference.
            if let Some(ref mut newnext) = (*node_handle).next {
                (*node_handle).prev = newnext.prev;
                newnext.prev = Some(node_handle);
            } else {
                // Only one node, being the last one; avoid head.prev pointing to head
                self.head.prev = Some(node_handle);
            }

            assert!(self.head.next.is_some());
            assert!(self.head.prev.is_some());
        }
    }

    fn count(&self) -> usize {
        self.count
    }

    fn _testing_head_ref(&self) -> Option<&T> {
        if let Some(ref first) = self.head.next {
            first.data.as_ref()
        } else {
            None
        }
    }
}


pub type CacheKey = [u8; 16];
pub type CacheID = u64;
type CacheEntry<T> = (T, LRUHandle<CacheKey>);

/// Implementation of `ShardedLRUCache`.
/// Based on a HashMap; the elements are linked in order to support the LRU ordering.
pub struct Cache<T> {
    // note: CacheKeys (Vec<u8>) are duplicated between list and map. If this turns out to be a
    // performance bottleneck, another layer of indirection™ can solve this by mapping the key
    // to a numeric handle that keys both list and map.
    list: LRUList<CacheKey>,
    map: HashMap<CacheKey, CacheEntry<T>>,
    cap: usize,
    id: u64,
}

impl<T> Cache<T> {
    pub fn new(capacity: usize) -> Cache<T> {
        assert!(capacity > 0);
        Cache {
            list: LRUList::new(),
            map: HashMap::with_capacity(1024),
            cap: capacity,
            id: 0,
        }
    }

    /// Returns an ID that is unique for this cache and that can be used to partition the cache
    /// among several users.
    pub fn new_cache_id(&mut self) -> CacheID {
        self.id += 1;
        return self.id;
    }

    /// How many the cache currently contains
    pub fn count(&self) -> usize {
        return self.list.count();
    }

    /// The capacity of this cache
    pub fn cap(&self) -> usize {
        return self.cap;
    }

    /// Insert a new element into the cache. The returned `CacheHandle` can be used for further
    /// operations on that element.
    /// If the capacity has been reached, the least recently used element is removed from the
    /// cache.
    pub fn insert(&mut self, key: &CacheKey, elem: T) {
        if self.list.count() >= self.cap {
            if let Some(removed_key) = self.list.remove_last() {
                assert!(self.map.remove(&removed_key).is_some());
            } else {
                panic!("could not remove_last(); bug!");
            }
        }

        let lru_handle = self.list.insert(key.clone());
        self.map.insert(key.clone(), (elem, lru_handle));
    }

    /// Retrieve an element from the cache.
    /// If the element has been preempted from the cache in the meantime, this returns None.
    pub fn get<'a>(&'a mut self, key: &CacheKey) -> Option<&'a T> {
        match self.map.get(key) {
            None => None,
            Some(&(ref elem, ref lru_handle)) => {
                self.list.reinsert_front(*lru_handle);
                Some(elem)
            }
        }
    }

    /// Remove an element from the cache (for invalidation).
    pub fn remove(&mut self, key: &CacheKey) -> Option<T> {
        match self.map.remove(key) {
            None => None,
            Some((elem, lru_handle)) => {
                self.list.remove(lru_handle);
                Some(elem)
            }
        }
    }
}


// The compiler does not automatically derive Send and Sync for Cache because it contains
// raw pointers.
// These raw pointers are only pointing to the elements hold in the same cache and insertion
// clones the values. It is therefore safe to implement Send for Cache.
// Since all functions that access these raw pointers are mutable member functions, it is also safe to implement Sync
// (Sync is defined as "if &T is Send-able")
unsafe impl<T: Send> Send for Cache<T> {}
unsafe impl<T: Sync> Sync for Cache<T> {}

#[cfg(test)]
mod tests {
    use super::LRUList;
    use super::*;

    fn make_key(a: u8, b: u8, c: u8) -> CacheKey {
        [a, b, c, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
    }

    #[test]
    fn test_blockcache_cache_add_rm() {
        let mut cache = Cache::new(128);

        let h_123 = make_key(1, 2, 3);
        let h_521 = make_key(1, 2, 4);
        let h_372 = make_key(3, 4, 5);
        let h_332 = make_key(6, 3, 1);
        let h_899 = make_key(8, 2, 1);

        cache.insert(&h_123, 123);
        cache.insert(&h_332, 332);
        cache.insert(&h_521, 521);
        cache.insert(&h_372, 372);
        cache.insert(&h_899, 899);

        assert_eq!(cache.count(), 5);

        assert_eq!(cache.get(&h_123), Some(&123));
        assert_eq!(cache.get(&h_372), Some(&372));

        assert_eq!(cache.remove(&h_521), Some(521));
        assert_eq!(cache.get(&h_521), None);
        assert_eq!(cache.remove(&h_521), None);

        assert_eq!(cache.count(), 4);
    }

    #[test]
    fn test_blockcache_cache_capacity() {
        let mut cache = Cache::new(3);

        let h_123 = make_key(1, 2, 3);
        let h_521 = make_key(1, 2, 4);
        let h_372 = make_key(3, 4, 5);
        let h_332 = make_key(6, 3, 1);
        let h_899 = make_key(8, 2, 1);

        cache.insert(&h_123, 123);
        cache.insert(&h_332, 332);
        cache.insert(&h_521, 521);
        cache.insert(&h_372, 372);
        cache.insert(&h_899, 899);

        assert_eq!(cache.count(), 3);

        assert_eq!(cache.get(&h_123), None);
        assert_eq!(cache.get(&h_332), None);
        assert_eq!(cache.get(&h_521), Some(&521));
        assert_eq!(cache.get(&h_372), Some(&372));
        assert_eq!(cache.get(&h_899), Some(&899));
    }

    #[test]
    fn test_blockcache_lru_remove() {
        let mut lru = LRUList::<usize>::new();

        let h_56 = lru.insert(56);
        lru.insert(22);
        lru.insert(223);
        let h_244 = lru.insert(244);
        lru.insert(1111);
        let h_12 = lru.insert(12);

        assert_eq!(lru.count(), 6);
        assert_eq!(244, lru.remove(h_244));
        assert_eq!(lru.count(), 5);
        assert_eq!(12, lru.remove(h_12));
        assert_eq!(lru.count(), 4);
        assert_eq!(56, lru.remove(h_56));
        assert_eq!(lru.count(), 3);
    }

    #[test]
    fn test_blockcache_lru_1() {
        let mut lru = LRUList::<usize>::new();

        lru.insert(56);
        lru.insert(22);
        lru.insert(244);
        lru.insert(12);

        assert_eq!(lru.count(), 4);

        assert_eq!(Some(56), lru.remove_last());
        assert_eq!(Some(22), lru.remove_last());
        assert_eq!(Some(244), lru.remove_last());

        assert_eq!(lru.count(), 1);

        assert_eq!(Some(12), lru.remove_last());

        assert_eq!(lru.count(), 0);

        assert_eq!(None, lru.remove_last());
    }

    #[test]
    fn test_blockcache_lru_reinsert() {
        let mut lru = LRUList::<usize>::new();

        let handle1 = lru.insert(56);
        let handle2 = lru.insert(22);
        let handle3 = lru.insert(244);

        assert_eq!(lru._testing_head_ref().map(|r| (*r)).unwrap(), 244);

        lru.reinsert_front(handle1);

        assert_eq!(lru._testing_head_ref().map(|r| (*r)).unwrap(), 56);

        lru.reinsert_front(handle3);

        assert_eq!(lru._testing_head_ref().map(|r| (*r)).unwrap(), 244);

        lru.reinsert_front(handle2);

        assert_eq!(lru._testing_head_ref().map(|r| (*r)).unwrap(), 22);

        assert_eq!(lru.remove_last(), Some(56));
        assert_eq!(lru.remove_last(), Some(244));
        assert_eq!(lru.remove_last(), Some(22));
    }

    #[test]
    fn test_blockcache_lru_reinsert_2() {
        let mut lru = LRUList::<usize>::new();

        let handles = vec![
            lru.insert(0),
            lru.insert(1),
            lru.insert(2),
            lru.insert(3),
            lru.insert(4),
            lru.insert(5),
            lru.insert(6),
            lru.insert(7),
            lru.insert(8),
        ];

        for i in 0..9 {
            lru.reinsert_front(handles[i]);
            assert_eq!(lru._testing_head_ref().map(|x| *x), Some(i));
        }
    }

    #[test]
    fn test_blockcache_lru_edge_cases() {
        let mut lru = LRUList::<usize>::new();

        let handle = lru.insert(3);

        lru.reinsert_front(handle);
        assert_eq!(lru._testing_head_ref().map(|x| *x), Some(3));
        assert_eq!(lru.remove_last(), Some(3));
        assert_eq!(lru.remove_last(), None);
        assert_eq!(lru.remove_last(), None);
    }
}