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    // Copyright 2015-2017 Parity Technologies (UK) Ltd.
    // This file is part of Parity.

    // Parity is free software: you can redistribute it and/or modify
    // it under the terms of the GNU General Public License as published by
    // the Free Software Foundation, either version 3 of the License, or
    // (at your option) any later version.

    // Parity is distributed in the hope that it will be useful,
    // but WITHOUT ANY WARRANTY; without even the implied warranty of
    // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    // GNU General Public License for more details.

    // You should have received a copy of the GNU General Public License
    // along with Parity.  If not, see <http://www.gnu.org/licenses/>.

    //! Light client header chain.
    //!
    //! Unlike a full node's `BlockChain` this doesn't store much in the database.
    //! It stores candidates for the last 2048-4096 blocks as well as CHT roots for
    //! historical blocks all the way to the genesis.
    //!
    //! This is separate from the `BlockChain` for two reasons:
    //!   - It stores only headers (and a pruned subset of them)
    //!   - To allow for flexibility in the database layout once that's incorporated.
    // TODO: use DB instead of memory. DB Layout: just the contents of `candidates`/`headers`
    //

    use std::collections::{BTreeMap, HashMap};

    use cht;

    use ethcore::block_status::BlockStatus;
    use ethcore::error::BlockError;
    use ethcore::ids::BlockId;
    use ethcore::views::HeaderView;
    use util::{Bytes, H256, U256, HeapSizeOf, Mutex, RwLock};

    use smallvec::SmallVec;

    /// Store at least this many candidate headers at all times.
    /// Also functions as the delay for computing CHTs as they aren't
    /// relevant to any blocks we've got in memory.
    const HISTORY: u64 = 2048;

    /// Information about a block.
    #[derive(Debug, Clone)]
    pub struct BlockDescriptor {
    	/// The block's hash
    	pub hash: H256,
    	/// The block's number
    	pub number: u64,
    	/// The block's total difficulty.
    	pub total_difficulty: U256,
    }

    // candidate block description.
    struct Candidate {
    	hash: H256,
    	parent_hash: H256,
    	total_difficulty: U256,
    }

    struct Entry {
    	candidates: SmallVec<[Candidate; 3]>, // 3 arbitrarily chosen
    	canonical_hash: H256,
    }

    impl HeapSizeOf for Entry {
    	fn heap_size_of_children(&self) -> usize {
    		match self.candidates.spilled() {
    			false => 0,
    			true => self.candidates.capacity() * ::std::mem::size_of::<Candidate>(),
    		}
    	}
    }

    /// Header chain. See module docs for more details.
    pub struct HeaderChain {
    	genesis_header: Bytes, // special-case the genesis.
    	candidates: RwLock<BTreeMap<u64, Entry>>,
    	headers: RwLock<HashMap<H256, Bytes>>,
    	best_block: RwLock<BlockDescriptor>,
    	cht_roots: Mutex<Vec<H256>>,
    }

    impl HeaderChain {
    	/// Create a new header chain given this genesis block.
    	pub fn new(genesis: &[u8]) -> Self {
    		let g_view = HeaderView::new(genesis);

    		HeaderChain {
    			genesis_header: genesis.to_owned(),
    			best_block: RwLock::new(BlockDescriptor {
    				hash: g_view.hash(),
    				number: 0,
    				total_difficulty: g_view.difficulty(),
    			}),
    			candidates: RwLock::new(BTreeMap::new()),
    			headers: RwLock::new(HashMap::new()),
    			cht_roots: Mutex::new(Vec::new()),
    		}
    	}

    	/// Insert a pre-verified header.
    	///
    	/// This blindly trusts that the data given to it is
    	/// a) valid RLP encoding of a header and
    	/// b) has sensible data contained within it.
    	pub fn insert(&self, header: Bytes) -> Result<(), BlockError> {
    		let view = HeaderView::new(&header);
    		let hash = view.hash();
    		let number = view.number();
    		let parent_hash = view.parent_hash();

    		// hold candidates the whole time to guard import order.
    		let mut candidates = self.candidates.write();

    		// find parent details.
    		let parent_td =
    			if number == 1 {
    				let g_view = HeaderView::new(&self.genesis_header);
    				g_view.difficulty()
    			} else {
    				candidates.get(&(number - 1))
    					.and_then(|entry| entry.candidates.iter().find(|c| c.hash == parent_hash))
    					.map(|c| c.total_difficulty)
    					.ok_or_else(|| BlockError::UnknownParent(parent_hash))?
    			};

    		let total_difficulty = parent_td + view.difficulty();

    		// insert headers and candidates entries.
    		candidates.entry(number).or_insert_with(|| Entry { candidates: SmallVec::new(), canonical_hash: hash })
    			.candidates.push(Candidate {
    				hash: hash,
    				parent_hash: parent_hash,
    				total_difficulty: total_difficulty,
    		});

    		self.headers.write().insert(hash, header.clone());

    		// reorganize ancestors so canonical entries are first in their
    		// respective candidates vectors.
    		if self.best_block.read().total_difficulty < total_difficulty {
    			let mut canon_hash = hash;
    			for (&height, entry) in candidates.iter_mut().rev().skip_while(|&(height, _)| *height > number) {
    				if height != number && entry.canonical_hash == canon_hash { break; }

    				trace!(target: "chain", "Setting new canonical block {} for block height {}",
    					canon_hash, height);

    				let canon_pos = entry.candidates.iter().position(|x| x.hash == canon_hash)
    					.expect("blocks are only inserted if parent is present; or this is the block we just added; qed");

    				// move the new canonical entry to the front and set the
    				// era's canonical hash.
    				entry.candidates.swap(0, canon_pos);
    				entry.canonical_hash = canon_hash;

    				// what about reorgs > cht::SIZE + HISTORY?
    				// resetting to the last block of a given CHT should be possible.
    				canon_hash = entry.candidates[0].parent_hash;
    			}

    			trace!(target: "chain", "New best block: ({}, {}), TD {}", number, hash, total_difficulty);
    			*self.best_block.write() = BlockDescriptor {
    				hash: hash,
    				number: number,
    				total_difficulty: total_difficulty,
    			};

    			// produce next CHT root if it's time.
    			let earliest_era = *candidates.keys().next().expect("at least one era just created; qed");
    			if earliest_era + HISTORY + cht::SIZE <= number {
    				let mut values = Vec::with_capacity(cht::SIZE as usize);
    				{
    					let mut headers = self.headers.write();
    					for i in (0..cht::SIZE).map(|x| x + earliest_era) {
    						let era_entry = candidates.remove(&i)
    							.expect("all eras are sequential with no gaps; qed");

    						for ancient in &era_entry.candidates {
    							headers.remove(&ancient.hash);
    						}

    						values.push((
    							::rlp::encode(&i).to_vec(),
    							::rlp::encode(&era_entry.canonical_hash).to_vec(),
    						));
    					}
    				}

    				let cht_root = ::util::triehash::trie_root(values);
    				debug!(target: "chain", "Produced CHT {} root: {:?}", (earliest_era - 1) % cht::SIZE, cht_root);

    				self.cht_roots.lock().push(cht_root);
    			}
    		}

    		Ok(())
    	}

    	/// Get a block header. In the case of query by number, only canonical blocks
    	/// will be returned.
    	pub fn get_header(&self, id: BlockId) -> Option<Bytes> {
    		match id {
    			BlockId::Earliest | BlockId::Number(0) => Some(self.genesis_header.clone()),
    			BlockId::Hash(hash) => self.headers.read().get(&hash).map(|x| x.to_vec()),
    			BlockId::Number(num) => {
    				if self.best_block.read().number < num { return None }

    				self.candidates.read().get(&num).map(|entry| entry.canonical_hash)
    					.and_then(|hash| self.headers.read().get(&hash).map(|x| x.to_vec()))
    			}
    			BlockId::Latest | BlockId::Pending => {
    				let hash = self.best_block.read().hash;
    				self.headers.read().get(&hash).map(|x| x.to_vec())
    			}
    		}
    	}

    	/// Get the nth CHT root, if it's been computed.
    	///
    	/// CHT root 0 is from block `1..2048`.
    	/// CHT root 1 is from block `2049..4096`
    	/// and so on.
    	///
    	/// This is because it's assumed that the genesis hash is known,
    	/// so including it within a CHT would be redundant.
    	pub fn cht_root(&self, n: usize) -> Option<H256> {
    		self.cht_roots.lock().get(n).map(|h| h.clone())
    	}

    	/// Get the genesis hash.
    	pub fn genesis_hash(&self) -> H256 {
    		::util::Hashable::sha3(&self.genesis_header)
    	}

    	/// Get the best block's data.
    	pub fn best_block(&self) -> BlockDescriptor {
    		self.best_block.read().clone()
    	}

    	/// If there is a gap between the genesis and the rest
    	/// of the stored blocks, return the first post-gap block.
    	pub fn first_block(&self) -> Option<BlockDescriptor> {
    		let candidates = self.candidates.read();
    		match candidates.iter().next() {
    			None | Some((&1, _)) => None,
    			Some((&height, entry)) => Some(BlockDescriptor {
    				number: height,
    				hash: entry.canonical_hash,
    				total_difficulty: entry.candidates.iter().find(|x| x.hash == entry.canonical_hash)
    					.expect("entry always stores canonical candidate; qed").total_difficulty,
    			})
    		}
    	}

    	/// Get block status.
    	pub fn status(&self, hash: &H256) -> BlockStatus {
    		match self.headers.read().contains_key(hash) {
    			true => BlockStatus::InChain,
    			false => BlockStatus::Unknown,
    		}
    	}
    }

    impl HeapSizeOf for HeaderChain {
    	fn heap_size_of_children(&self) -> usize {
    		self.candidates.read().heap_size_of_children() +
    			self.headers.read().heap_size_of_children() +
    			self.cht_roots.lock().heap_size_of_children()
    	}
    }

    #[cfg(test)]
    mod tests {
    	use super::HeaderChain;
    	use ethcore::ids::BlockId;
    	use ethcore::header::Header;
    	use ethcore::spec::Spec;

    	#[test]
    	fn basic_chain() {
    		let spec = Spec::new_test();
    		let genesis_header = spec.genesis_header();

    		let chain = HeaderChain::new(&::rlp::encode(&genesis_header));

    		let mut parent_hash = genesis_header.hash();
    		let mut rolling_timestamp = genesis_header.timestamp();
    		for i in 1..10000 {
    			let mut header = Header::new();
    			header.set_parent_hash(parent_hash);
    			header.set_number(i);
    			header.set_timestamp(rolling_timestamp);
    			header.set_difficulty(*genesis_header.difficulty() * i.into());

    			chain.insert(::rlp::encode(&header).to_vec()).unwrap();

    			parent_hash = header.hash();
    			rolling_timestamp += 10;
    		}

    		assert!(chain.get_header(BlockId::Number(10)).is_none());
    		assert!(chain.get_header(BlockId::Number(9000)).is_some());
    		assert!(chain.cht_root(2).is_some());
    		assert!(chain.cht_root(3).is_none());
    	}

    	#[test]
    	fn reorganize() {
    		let spec = Spec::new_test();
    		let genesis_header = spec.genesis_header();

    		let chain = HeaderChain::new(&::rlp::encode(&genesis_header));

    		let mut parent_hash = genesis_header.hash();
    		let mut rolling_timestamp = genesis_header.timestamp();
    		for i in 1..6 {
    			let mut header = Header::new();
    			header.set_parent_hash(parent_hash);
    			header.set_number(i);
    			header.set_timestamp(rolling_timestamp);
    			header.set_difficulty(*genesis_header.difficulty() * i.into());

    			chain.insert(::rlp::encode(&header).to_vec()).unwrap();

    			parent_hash = header.hash();
    			rolling_timestamp += 10;
    		}

    		{
    			let mut rolling_timestamp = rolling_timestamp;
    			let mut parent_hash = parent_hash;
    			for i in 6..16 {
    				let mut header = Header::new();
    				header.set_parent_hash(parent_hash);
    				header.set_number(i);
    				header.set_timestamp(rolling_timestamp);
    				header.set_difficulty(*genesis_header.difficulty() * i.into());

    				chain.insert(::rlp::encode(&header).to_vec()).unwrap();

    				parent_hash = header.hash();
    				rolling_timestamp += 10;
    			}
    		}

    		assert_eq!(chain.best_block().number, 15);

    		{
    			let mut rolling_timestamp = rolling_timestamp;
    			let mut parent_hash = parent_hash;

    			// import a shorter chain which has better TD.
    			for i in 6..13 {
    				let mut header = Header::new();
    				header.set_parent_hash(parent_hash);
    				header.set_number(i);
    				header.set_timestamp(rolling_timestamp);
    				header.set_difficulty(*genesis_header.difficulty() * (i * i).into());

    				chain.insert(::rlp::encode(&header).to_vec()).unwrap();

    				parent_hash = header.hash();
    				rolling_timestamp += 11;
    			}
    		}

    		let (mut num, mut canon_hash) = (chain.best_block().number, chain.best_block().hash);
    		assert_eq!(num, 12);

    		while num > 0 {
    			let header: Header = ::rlp::decode(&chain.get_header(BlockId::Number(num)).unwrap());
    			assert_eq!(header.hash(), canon_hash);

    			canon_hash = *header.parent_hash();
    			num -= 1;
    		}
    	}
    }

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