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    use std::ops::Index;
    use std::collections::{VecDeque, HashSet};
    use std::iter::repeat;
    use primitives::hash::H256;

    /// Block position
    #[derive(Debug, Copy, Clone, Eq, PartialEq)]
    pub enum HashPosition {
    	/// Block is not in the queue
    	Missing,
    	/// Block is at the front of the queue
    	Front,
    	/// Block is somewhere inside in the queue
    	Inside(u32),
    }

    /// Ordered queue with O(1) contains() && random access operations cost.
    #[derive(Debug, Clone)]
    pub struct HashQueue {
    	queue: VecDeque<H256>,
    	set: HashSet<H256>,
    }

    /// Chain of linked queues. First queue has index zero.
    #[derive(Debug)]
    pub struct HashQueueChain {
    	chain: Vec<HashQueue>,
    }

    impl HashQueue {
    	pub fn new() -> Self {
    		HashQueue {
    			queue: VecDeque::new(),
    			set: HashSet::new(),
    		}
    	}

    	/// Clears the queue
    	pub fn clear(&mut self) {
    		self.set.clear();
    		self.queue.clear();
    	}

    	/// Returns len of the given queue.
    	pub fn len(&self) -> u32 {
    		self.queue.len() as u32
    	}

    	/// Returns front element from the given queue.
    	pub fn front(&self) -> Option<H256> {
    		self.queue.front().cloned()
    	}

    	/// Returns back element from the given queue.
    	pub fn back(&self) -> Option<H256> {
    		self.queue.back().cloned()
    	}

    	/// Returns position of the element in the queue
    	pub fn position(&self, hash: &H256) -> Option<u32> {
    		self.queue.iter().enumerate()
    			.filter_map(|(pos, h)| if hash == h { Some(pos as u32) } else { None })
    			.nth(0)
    	}

    	/// Returns element at position
    	pub fn at(&self, position: u32) -> Option<H256> {
    		self.queue.get(position as usize).cloned()
    	}

    	/// Returns previous-to back element from the given queue.
    	pub fn pre_back(&self) -> Option<H256> {
    		let queue_len = self.queue.len();
    		if queue_len <= 1 {
    			return None;
    		}
    		Some(self.queue[queue_len - 2].clone())
    	}

    	/// Returns true if queue contains element.
    	pub fn contains(&self, hash: &H256) -> bool {
    		self.set.contains(hash)
    	}

    	/// Returns n elements from the front of the queue
    	pub fn front_n(&self, n: u32) -> Vec<H256> {
    		self.queue.iter().cloned().take(n as usize).collect()
    	}

    	/// Removes element from the front of the queue.
    	pub fn pop_front(&mut self) -> Option<H256> {
    		match self.queue.pop_front() {
    			Some(hash) => {
    				self.set.remove(&hash);
    				Some(hash)
    			},
    			None => None,
    		}
    	}

    	/// Removes n elements from the front of the queue.
    	pub fn pop_front_n(&mut self, n: u32) -> Vec<H256> {
    		let mut result: Vec<H256> = Vec::new();
    		for _ in 0..n {
    			match self.pop_front() {
    				Some(hash) => result.push(hash),
    				None => return result,
    			}
    		}
    		result
    	}

    	/// Removes element from the back of the queue.
    	pub fn pop_back(&mut self) -> Option<H256> {
    		match self.queue.pop_back() {
    			Some(hash) => {
    				self.set.remove(&hash);
    				Some(hash)
    			},
    			None => None,
    		}
    	}

    	/// Adds element to the back of the queue.
    	pub fn push_back(&mut self, hash: H256) {
    		if !self.set.insert(hash.clone()) {
    			panic!("must be checked by caller");
    		}
    		self.queue.push_back(hash);
    	}

    	/// Adds elements to the back of the queue.
    	pub fn push_back_n(&mut self, hashes: Vec<H256>) {
    		for hash in hashes {
    			self.push_back(hash);
    		}
    	}

    	/// Removes element from the queue, returning its position.
    	pub fn remove(&mut self, hash: &H256) -> HashPosition {
    		if !self.set.remove(hash) {
    			return HashPosition::Missing;
    		}

    		if self.queue.front().expect("checked one line above") == hash {
    			self.queue.pop_front();
    			return HashPosition::Front;
    		}

    		for i in 0..self.queue.len() {
    			if self.queue[i] == *hash {
    				self.queue.remove(i);
    				return HashPosition::Inside(i as u32);
    			}
    		}

    		// unreachable because hash is not missing, not at the front and not inside
    		unreachable!()
    	}

    	/// Removes all elements from the queue.
    	pub fn remove_all(&mut self) -> VecDeque<H256> {
    		use std::mem::replace;

    		self.set.clear();
    		replace(&mut self.queue, VecDeque::new())
    	}
    }

    impl Index<u32> for HashQueue {
    	type Output = H256;

    	fn index(&self, index: u32) -> &Self::Output {
    		&self.queue[index as usize]
    	}
    }

    impl HashQueueChain {
    	/// Creates chain with given number of queues.
    	pub fn with_number_of_queues(number_of_queues: usize) -> Self {
    		assert!(number_of_queues != 0);
    		HashQueueChain {
    			chain: repeat(HashQueue::new()).take(number_of_queues).collect(),
    		}
    	}

    	/// Returns length of the whole chain.
    	pub fn len(&self) -> u32 {
    		self.chain.iter().fold(0, |total, chain| total + chain.len())
    	}

    	/// Returns length of the given queue.
    	pub fn len_of(&self, queue_index: usize) -> u32 {
    		self.chain[queue_index].len()
    	}

    	/// Returns element at the given position
    	pub fn at(&self, mut index: u32) -> Option<H256> {
    		for queue in &self.chain {
    			let queue_len = queue.len();
    			if index < queue_len {
    				return queue.at(index);
    			}

    			index -= queue_len;
    		}

    		None
    	}

    	/// Returns element at the front of the given queue.
    	pub fn front_at(&self, queue_index: usize) -> Option<H256> {
    		let queue = &self.chain[queue_index];
    		queue.front()
    	}

    	/// Returns element at the front of the given queue.
    	pub fn back_at(&self, queue_index: usize) -> Option<H256> {
    		let queue = &self.chain[queue_index];
    		queue.back()
    	}

    	/// Returns previous-to back element from the given queue.
    	pub fn pre_back_at(&self, chain_index: usize) -> Option<H256> {
    		let queue = &self.chain[chain_index];
    		queue.pre_back()
    	}

    	/// Returns the back of the whole chain.
    	pub fn back(&self) -> Option<H256> {
    		let mut queue_index = self.chain.len() - 1;
    		loop {
    			let queue = &self.chain[queue_index];
    			let queue_back = queue.back();
    			if queue_back.is_some() {
    				return queue_back;
    			}

    			queue_index -= 1;
    			if queue_index == 0 {
    				return None;
    			}
    		}
    	}

    	/// Checks if hash is contained in given queue.
    	#[cfg(test)]
    	pub fn is_contained_in(&self, queue_index: usize, hash: &H256) -> bool {
    		self.chain[queue_index].contains(hash)
    	}

    	/// Returns the index of queue, hash is contained in.
    	pub fn contains_in(&self, hash: &H256) -> Option<usize> {
    		for i in 0..self.chain.len() {
    			if self.chain[i].contains(hash) {
    				return Some(i);
    			}
    		}
    		None
    	}

    	/// Returns n elements from the front of the given queue
    	pub fn front_n_at(&self, queue_index: usize, n: u32) -> Vec<H256> {
    		self.chain[queue_index].front_n(n)
    	}

    	/// Remove a number of hashes from the front of the given queue.
    	pub fn pop_front_n_at(&mut self, queue_index: usize, n: u32) -> Vec<H256> {
    		self.chain[queue_index].pop_front_n(n)
    	}

    	/// Push hash onto the back of the given queue.
    	pub fn push_back_at(&mut self, queue_index: usize, hash: H256) {
    		self.chain[queue_index].push_back(hash)
    	}

    	/// Push a number of hashes onto the back of the given queue.
    	pub fn push_back_n_at(&mut self, queue_index: usize, hashes: Vec<H256>) {
    		self.chain[queue_index].push_back_n(hashes)
    	}

    	/// Remove hash from given queue.
    	pub fn remove_at(&mut self, queue_index: usize, hash: &H256) -> HashPosition {
    		self.chain[queue_index].remove(hash)
    	}

    	/// Remove all items from given queue.
    	pub fn remove_all_at(&mut self, queue_index: usize) -> VecDeque<H256> {
    		self.chain[queue_index].remove_all()
    	}
    }

    impl Index<u32> for HashQueueChain {
    	type Output = H256;

    	fn index(&self, mut index: u32) -> &Self::Output {
    		for queue in &self.chain {
    			let queue_len = queue.len();
    			if index < queue_len {
    				return &queue[index];
    			}

    			index -= queue_len;
    		}

    		panic!("invalid index");
    	}
    }

    #[cfg(test)]
    mod tests {
    	use super::{HashQueue, HashQueueChain, HashPosition};
    	use primitives::hash::H256;

    	#[test]
    	fn hash_queue_empty() {
    		let mut queue = HashQueue::new();
    		assert_eq!(queue.len(), 0);
    		assert_eq!(queue.front(), None);
    		assert_eq!(queue.back(), None);
    		assert_eq!(queue.pre_back(), None);
    		assert_eq!(queue.contains(&"000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f".into()), false);
    		assert_eq!(queue.pop_front(), None);
    		assert_eq!(queue.pop_front_n(100), vec![]);
    		assert_eq!(queue.remove(&"000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f".into()), HashPosition::Missing);
    	}

    	#[test]
    	fn hash_queue_chain_empty() {
    		let mut chain = HashQueueChain::with_number_of_queues(3);
    		assert_eq!(chain.len(), 0);
    		assert_eq!(chain.len_of(0), 0);
    		assert_eq!(chain.front_at(0), None);
    		assert_eq!(chain.back_at(0), None);
    		assert_eq!(chain.pre_back_at(0), None);
    		assert_eq!(chain.back(), None);
    		assert_eq!(chain.is_contained_in(0, &"000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f".into()), false);
    		assert_eq!(chain.contains_in(&"000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f".into()), None);
    		assert_eq!(chain.pop_front_n_at(0, 100), vec![]);
    		assert_eq!(chain.remove_at(0, &"000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f".into()), HashPosition::Missing);
    	}

    	#[test]
    	fn hash_queue_chain_not_empty() {
    		let mut chain = HashQueueChain::with_number_of_queues(4);
    		chain.push_back_n_at(0, vec![
    			H256::from(0),
    			H256::from(1),
    			H256::from(2),
    		]);
    		chain.push_back_n_at(1, vec![
    			H256::from(3),
    			H256::from(4),
    		]);
    		chain.push_back_n_at(2, vec![
    			H256::from(5),
    		]);

    		assert_eq!(chain.len(), 6);
    		assert_eq!(chain.len_of(0), 3);
    		assert_eq!(chain.len_of(1), 2);
    		assert_eq!(chain.len_of(2), 1);
    		assert_eq!(chain.len_of(3), 0);
    		assert_eq!(chain.front_at(0), Some(H256::from(0)));
    		assert_eq!(chain.front_at(1), Some(H256::from(3)));
    		assert_eq!(chain.front_at(2), Some(H256::from(5)));
    		assert_eq!(chain.front_at(3), None);
    		assert_eq!(chain.back_at(0), Some(H256::from(2)));
    		assert_eq!(chain.back_at(1), Some(H256::from(4)));
    		assert_eq!(chain.back_at(2), Some(H256::from(5)));
    		assert_eq!(chain.back_at(3), None);
    		assert_eq!(chain.pre_back_at(0), Some(H256::from(1)));
    		assert_eq!(chain.pre_back_at(1), Some(H256::from(3)));
    		assert_eq!(chain.pre_back_at(2), None);
    		assert_eq!(chain.pre_back_at(3), None);
    		assert_eq!(chain.back(), Some(H256::from(5)));
    		assert_eq!(chain.is_contained_in(0, &H256::from(2)), true);
    		assert_eq!(chain.is_contained_in(1, &H256::from(2)), false);
    		assert_eq!(chain.is_contained_in(2, &H256::from(2)), false);
    		assert_eq!(chain.is_contained_in(3, &H256::from(2)), false);
    		assert_eq!(chain.contains_in(&H256::from(2)), Some(0));
    		assert_eq!(chain.contains_in(&H256::from(5)), Some(2));
    		assert_eq!(chain.contains_in(&H256::from(9)), None);
    	}

    	#[test]
    	fn hash_queue_front_n() {
    		let mut queue = HashQueue::new();
    		queue.push_back_n(vec![H256::from(0), H256::from(1)]);
    		assert_eq!(queue.front_n(3), vec![H256::from(0), H256::from(1)]);
    		assert_eq!(queue.front_n(3), vec![H256::from(0), H256::from(1)]);
    		assert_eq!(queue.pop_front_n(3), vec![H256::from(0), H256::from(1)]);
    		assert_eq!(queue.pop_front_n(3), vec![]);
    	}
    }

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