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    use linked_hash_map::LinkedHashMap;
    use primitives::hash::H256;

    /// Maximal number of hashes to store in known-hashes filter
    pub const MAX_KNOWN_HASHES_LEN: usize = 2048;

    /// Hash-knowledge type
    #[derive(Debug, PartialEq, Clone, Copy)]
    pub enum KnownHashType {
    	/// Peer knows transaction with this hash
    	Transaction,
    	/// Peer knows block with this hash
    	Block,
    	/// Peer knows compact block with this hash
    	CompactBlock,
    }

    /// Known-hashes filter
    #[derive(Debug, Default)]
    pub struct KnownHashFilter {
    	/// Insertion-time ordered known hashes
    	known_hashes: LinkedHashMap<H256, KnownHashType>,
    }

    impl KnownHashFilter {
    	/// Insert known hash
    	pub fn insert(&mut self, hash: H256, hash_type: KnownHashType) {
    		if !self.known_hashes.contains_key(&hash) {
    			self.known_hashes.insert(hash, hash_type);
    			// remove oldest-known hash, if limits overflow
    			if self.known_hashes.len() > MAX_KNOWN_HASHES_LEN {
    				self.known_hashes.pop_front();
    			}
    		}
    	}

    	/// Returns number of known hashes
    	#[cfg(test)]
    	pub fn len(&self) -> usize {
    		self.known_hashes.len()
    	}

    	/// Returns true if peer knows about this hash with this type
    	pub fn contains(&self, hash: &H256, hash_type: KnownHashType) -> bool {
    		self.known_hashes.get(hash)
    			.map(|stored_hash_type| *stored_hash_type == hash_type)
    			.unwrap_or(false)
    	}

    	/// Filter block using its hash
    	pub fn filter_block(&self, hash: &H256) -> bool {
    		self.known_hashes.get(hash)
    			.map(|stored_hash_type| *stored_hash_type != KnownHashType::Block
    				&& *stored_hash_type != KnownHashType::CompactBlock)
    			.unwrap_or(true)
    	}

    	/// Filter transaction using its hash
    	pub fn filter_transaction(&self, hash: &H256) -> bool {
    		self.known_hashes.get(hash)
    			.map(|stored_hash_type| *stored_hash_type != KnownHashType::Transaction)
    			.unwrap_or(true)
    	}
    }

    #[cfg(test)]
    mod tests {
    	use primitives::hash::H256;
    	use super::{KnownHashFilter, KnownHashType, MAX_KNOWN_HASHES_LEN};

    	#[test]
    	fn known_hash_filter_empty() {
    		assert!(KnownHashFilter::default().filter_transaction(&H256::from(0)));
    		assert!(KnownHashFilter::default().filter_block(&H256::from(0)));
    	}

    	#[test]
    	fn known_hash_filter_block() {
    		let mut filter = KnownHashFilter::default();
    		filter.insert(H256::from(0), KnownHashType::Block);
    		filter.insert(H256::from(1), KnownHashType::CompactBlock);
    		filter.insert(H256::from(2), KnownHashType::Transaction);
    		assert!(!filter.filter_block(&H256::from(0)));
    		assert!(!filter.filter_block(&H256::from(1)));
    		assert!(filter.filter_block(&H256::from(2)));
    		assert!(filter.filter_block(&H256::from(3)));
    	}

    	#[test]
    	fn known_hash_filter_transaction() {
    		let mut filter = KnownHashFilter::default();
    		filter.insert(H256::from(0), KnownHashType::Block);
    		filter.insert(H256::from(1), KnownHashType::CompactBlock);
    		filter.insert(H256::from(2), KnownHashType::Transaction);
    		assert!(filter.filter_transaction(&H256::from(0)));
    		assert!(filter.filter_transaction(&H256::from(1)));
    		assert!(!filter.filter_transaction(&H256::from(2)));
    		assert!(filter.filter_transaction(&H256::from(3)));
    	}

    	#[test]
    	fn known_hash_filter_contains() {
    		let mut filter = KnownHashFilter::default();
    		filter.insert(H256::from(0), KnownHashType::Block);
    		filter.insert(H256::from(1), KnownHashType::CompactBlock);
    		filter.insert(H256::from(2), KnownHashType::Transaction);
    		assert!(filter.contains(&H256::from(0), KnownHashType::Block));
    		assert!(!filter.contains(&H256::from(0), KnownHashType::CompactBlock));
    		assert!(filter.contains(&H256::from(1), KnownHashType::CompactBlock));
    		assert!(!filter.contains(&H256::from(1), KnownHashType::Block));
    		assert!(filter.contains(&H256::from(2), KnownHashType::Transaction));
    		assert!(!filter.contains(&H256::from(2), KnownHashType::Block));
    		assert!(!filter.contains(&H256::from(3), KnownHashType::Block));
    		assert!(!filter.contains(&H256::from(3), KnownHashType::CompactBlock));
    		assert!(!filter.contains(&H256::from(3), KnownHashType::Transaction));
    	}

    	#[test]
    	fn known_hash_filter_insert() {
    		let mut hash_data = [0u8; 32];
    		let mut filter = KnownHashFilter::default();
    		assert_eq!(filter.len(), 0);
    		// insert new hash
    		filter.insert(H256::from(hash_data.clone()), KnownHashType::Block);
    		assert_eq!(filter.len(), 1);
    		// insert already known hash => nothing should change
    		filter.insert(H256::from(hash_data.clone()), KnownHashType::Block);
    		assert_eq!(filter.len(), 1);
    		// insert MAX_KNOWN_HASHES_LEN
    		for i in 1..MAX_KNOWN_HASHES_LEN {
    			hash_data[0] = (i % 255) as u8;
    			hash_data[1] = ((i / 255) % 255) as u8;
    			filter.insert(H256::from(hash_data.clone()), KnownHashType::Block);
    			assert_eq!(filter.len(), i + 1);
    		}
    		// insert new unknown hash => nothing should change as we already have max number of hashes
    		hash_data[0] = ((MAX_KNOWN_HASHES_LEN + 1) % 255) as u8;
    		hash_data[1] = (((MAX_KNOWN_HASHES_LEN + 1) / 255) % 255) as u8;
    		filter.insert(H256::from(hash_data.clone()), KnownHashType::Block);
    		assert_eq!(filter.len(), MAX_KNOWN_HASHES_LEN);
    		// check that oldest known hash has been removed
    		hash_data[0] = 0; hash_data[1] = 0;
    		assert!(!filter.contains(&H256::from(hash_data.clone()), KnownHashType::Block));
    		hash_data[0] = 1; hash_data[1] = 0;
    		assert!(filter.contains(&H256::from(hash_data.clone()), KnownHashType::Block));
    	}
    }

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