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    use std::collections::HashMap;
    use chain::IndexedBlockHeader;
    use primitives::hash::H256;
    use super::{HashQueue, HashPosition};

    /// Best headers chain information
    #[derive(Debug)]
    pub struct Information {
    	/// Number of headers in best chain
    	pub best: u32,
    	/// Total number of headers
    	pub total: u32,
    }

    // TODO: currently it supports first chain only (so whatever headers sequence came first, it is best)
    /// Builds the block-header-chain of in-memory blocks, for which only headers are currently known
    #[derive(Debug)]
    pub struct BestHeadersChain {
    	/// Best hash in storage
    	storage_best_hash: H256,
    	/// Headers by hash
    	headers: HashMap<H256, IndexedBlockHeader>,
    	/// Best chain
    	best: HashQueue,
    }

    impl BestHeadersChain {
    	/// Create new best headers chain
    	pub fn new(storage_best_hash: H256) -> Self {
    		BestHeadersChain {
    			storage_best_hash: storage_best_hash,
    			headers: HashMap::new(),
    			best: HashQueue::new(),
    		}
    	}

    	/// Get information on headers chain
    	pub fn information(&self) -> Information {
    		Information {
    			best: self.best.len(),
    			total: self.headers.len() as u32,
    		}
    	}

    	/// Get header from main chain at given position
    	pub fn at(&self, height: u32) -> Option<IndexedBlockHeader> {
    		self.best.at(height)
    			.and_then(|hash| self.headers.get(&hash).cloned())
    	}

    	/// Get geader by given hash
    	pub fn by_hash(&self, hash: &H256) -> Option<IndexedBlockHeader> {
    		self.headers.get(hash).cloned()
    	}

    	/// Get height of main chain
    	pub fn height(&self, hash: &H256) -> Option<u32> {
    		self.best.position(hash)
    	}

    	/// Get all direct child blocks hashes of given block hash
    	pub fn children(&self, hash: &H256) -> Vec<H256> {
    		self.best.position(hash)
    			.and_then(|pos| self.best.at(pos + 1))
    			.and_then(|child| Some(vec![child]))
    			.unwrap_or_default()
    	}

    	/// Get hash of best block
    	pub fn best_block_hash(&self) -> H256 {
    		self.best.back()
    			.or_else(|| Some(self.storage_best_hash.clone()))
    			.expect("storage_best_hash is always known")
    	}

    	/// Insert new block header
    	pub fn insert(&mut self, header: IndexedBlockHeader) {
    		// append to the best chain
    		if self.best_block_hash() == header.raw.previous_header_hash {
    			let header_hash = header.hash.clone();
    			self.headers.insert(header_hash.clone(), header);
    			self.best.push_back(header_hash);
    			return;
    		}
    	}

    	/// Insert new blocks headers
    	pub fn insert_n(&mut self, headers: Vec<IndexedBlockHeader>) {
    		for header in headers {
    			self.insert(header);
    		}
    	}

    	/// Remove block header with given hash and all its children
    	pub fn remove(&mut self, hash: &H256) {
    		if self.headers.remove(hash).is_some() {
    			match self.best.remove(hash) {
    				HashPosition::Front => self.clear(),
    				HashPosition::Inside(position) => self.clear_after(position),
    				_ => (),
    			}
    		}
    	}

    	/// Remove blocks headers with given hash and all its children
    	pub fn remove_n<I: IntoIterator<Item=H256>> (&mut self, hashes: I) {
    		for hash in hashes {
    			self.remove(&hash);
    		}
    	}

    	/// Called when new blocks is inserted to storage
    	pub fn block_inserted_to_storage(&mut self, hash: &H256, storage_best_hash: &H256) {
    		if self.best.front().map(|h| &h == hash).unwrap_or(false) {
    			self.best.pop_front();
    			self.headers.remove(hash);
    		}
    		self.storage_best_hash = storage_best_hash.clone();
    	}

    	/// Clears headers chain
    	pub fn clear(&mut self) {
    		self.headers.clear();
    		self.best.clear();
    	}

    	/// Remove headers after position
    	fn clear_after(&mut self, position: u32) {
    		if position == 0 {
    			self.clear()
    		} else {
    			while self.best.len() > position {
    				self.headers.remove(&self.best.pop_back().expect("len() > position; qed"));
    			}
    		}
    	}
    }

    #[cfg(test)]
    mod tests {
    	extern crate test_data;

    	use primitives::hash::H256;
    	use super::BestHeadersChain;

    	#[test]
    	fn best_chain_empty() {
    		let chain = BestHeadersChain::new(H256::default());
    		assert_eq!(chain.at(0), None);
    		assert_eq!(chain.by_hash(&H256::from(0)), None);
    		assert_eq!(chain.height(&H256::default()), None);
    		assert_eq!(chain.children(&H256::default()), vec![]);
    		assert_eq!(chain.best_block_hash(), H256::default());
    	}

    	#[test]
    	fn best_chain_insert() {
    		let mut chain = BestHeadersChain::new(test_data::genesis().hash());
    		let b1 = test_data::block_h1().block_header;
    		let b2 = test_data::block_h2().block_header;
    		let b181 = test_data::block_h181().block_header;
    		let b182 = test_data::block_h182().block_header;
    		chain.insert(b1.into());
    		chain.insert(b181.clone().into());
    		assert_eq!(chain.information().best, 1);
    		assert_eq!(chain.information().total, 1);
    		chain.insert(b2.into());
    		assert_eq!(chain.information().best, 2);
    		assert_eq!(chain.information().total, 2);
    		chain.clear();
    		assert_eq!(chain.information().best, 0);
    		assert_eq!(chain.information().total, 0);
    		chain.insert(b181.clone().into());
    		assert_eq!(chain.information().best, 0);
    		assert_eq!(chain.information().total, 0);
    		chain.block_inserted_to_storage(&b181.hash(), &b181.hash());
    		assert_eq!(chain.information().best, 0);
    		assert_eq!(chain.information().total, 0);
    		chain.insert(b182.into());
    		assert_eq!(chain.information().best, 1);
    		assert_eq!(chain.information().total, 1);
    	}

    	#[test]
    	fn best_chain_remove() {
    		let b0 = test_data::block_builder().header().build().build();
    		let b1 = test_data::block_builder().header().parent(b0.hash()).build().build().block_header;
    		let b2 = test_data::block_builder().header().parent(b1.hash()).build().build().block_header;
    		let b3 = test_data::block_builder().header().parent(b2.hash()).build().build().block_header;
    		let b4 = test_data::block_builder().header().parent(b3.hash()).build().build().block_header;
    		let mut chain = BestHeadersChain::new(b0.hash());

    		chain.insert_n(vec![b1.clone().into(), b2.clone().into(), b3.clone().into(), b4.clone().into()]);
    		assert_eq!(chain.information().best, 4);
    		assert_eq!(chain.information().total, 4);
    		chain.remove(&b2.hash());
    		assert_eq!(chain.information().best, 1);
    		assert_eq!(chain.information().total, 1);

    		chain.insert_n(vec![b2.clone().into(), b3.clone().into(), b4.clone().into()]);
    		assert_eq!(chain.information().best, 4);
    		assert_eq!(chain.information().total, 4);
    		chain.remove(&H256::default());
    		assert_eq!(chain.information().best, 4);
    		assert_eq!(chain.information().total, 4);

    		chain.remove(&b1.hash());
    		assert_eq!(chain.information().best, 0);
    		assert_eq!(chain.information().total, 0);
    	}

    	#[test]
    	fn best_chain_insert_to_db_no_reorg() {
    		let mut chain = BestHeadersChain::new(test_data::genesis().hash());
    		let b1 = test_data::block_h1().block_header;
    		chain.insert(b1.clone().into());
    		assert_eq!(chain.at(0), Some(b1.clone().into()));
    		let b2 = test_data::block_h2().block_header;
    		chain.insert(b2.clone().into());
    		assert_eq!(chain.at(0), Some(b1.clone().into()));
    		assert_eq!(chain.at(1), Some(b2.clone().into()));

    		chain.block_inserted_to_storage(&b1.hash(), &b1.hash());

    		assert_eq!(chain.at(0), Some(b2.into()));
    		assert_eq!(chain.at(1), None);

    		assert_eq!(chain.information().best, 1);
    		assert_eq!(chain.information().total, 1);
    	}
    }

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