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    use std::collections::HashSet;
    use primitives::hash::H256;
    use primitives::compact::Compact;
    use chain::{OutPoint, TransactionOutput, IndexedTransaction};
    use storage::{SharedStore, TransactionOutputProvider};
    use network::{ConsensusParams, ConsensusFork, TransactionOrdering};
    use memory_pool::{MemoryPool, OrderingStrategy, Entry};
    use verification::{work_required, block_reward_satoshi, transaction_sigops, median_timestamp_inclusive};

    const BLOCK_VERSION: u32 = 0x20000000;
    const BLOCK_HEADER_SIZE: u32 = 4 + 32 + 32 + 4 + 4 + 4;

    /// Block template as described in [BIP0022](https://github.com/bitcoin/bips/blob/master/bip-0022.mediawiki#block-template-request)
    pub struct BlockTemplate {
    	/// Version
    	pub version: u32,
    	/// The hash of previous block
    	pub previous_header_hash: H256,
    	/// The current time as seen by the server
    	pub time: u32,
    	/// The compressed difficulty
    	pub bits: Compact,
    	/// Block height
    	pub height: u32,
    	/// Block transactions (excluding coinbase)
    	pub transactions: Vec<IndexedTransaction>,
    	/// Total funds available for the coinbase (in Satoshis)
    	pub coinbase_value: u64,
    	/// Number of bytes allowed in the block
    	pub size_limit: u32,
    	/// Number of sigops allowed in the block
    	pub sigop_limit: u32,
    }

    /// Block size and number of signatures opcodes is limited
    /// This structure should be used for storing this values.
    struct SizePolicy {
    	/// Current size
    	current_size: u32,
    	/// Max size
    	max_size: u32,
    	/// When current_size + size_buffer > max_size
    	/// we need to start finishing the block
    	size_buffer: u32,
    	/// Number of transactions checked since finishing started
    	finish_counter: u32,
    	/// Number of transactions to check when finishing the block
    	finish_limit: u32,
    }

    /// When appending transaction, opcode count and block size policies
    /// must agree on appending the transaction to the block
    #[derive(Debug, PartialEq, Copy, Clone)]
    enum NextStep {
    	/// Append the transaction, check the next one
    	Append,
    	/// Append the transaction, do not check the next one
    	FinishAndAppend,
    	/// Ignore transaction, check the next one
    	Ignore,
    	/// Ignore transaction, do not check the next one
    	FinishAndIgnore,
    }

    impl NextStep {
    	fn and(self, other: NextStep) -> Self {
    		match (self, other) {
    			(_, NextStep::FinishAndIgnore) |
    			(NextStep::FinishAndIgnore, _) |
    			(NextStep::FinishAndAppend, NextStep::Ignore) |
    			(NextStep::Ignore, NextStep::FinishAndAppend) => NextStep::FinishAndIgnore,

    			(NextStep::Ignore, _) |
    			(_, NextStep::Ignore) => NextStep::Ignore,

    			(_, NextStep::FinishAndAppend) |
    			(NextStep::FinishAndAppend, _) => NextStep::FinishAndAppend,

    			(NextStep::Append, NextStep::Append) => NextStep::Append,
    		}
    	}
    }

    impl SizePolicy {
    	fn new(current_size: u32, max_size: u32, size_buffer: u32, finish_limit: u32) -> Self {
    		SizePolicy {
    			current_size: current_size,
    			max_size: max_size,
    			size_buffer: size_buffer,
    			finish_counter: 0,
    			finish_limit: finish_limit,
    		}
    	}

    	fn decide(&mut self, size: u32) -> NextStep {
    		let finishing = self.current_size + self.size_buffer > self.max_size;
    		let fits = self.current_size + size <= self.max_size;
    		let finish = self.finish_counter + 1 >= self.finish_limit;

    		if finishing {
    			self.finish_counter += 1;
    		}

    		match (fits, finish) {
    			(true, true) => NextStep::FinishAndAppend,
    			(true, false) => NextStep::Append,
    			(false, true) => NextStep::FinishAndIgnore,
    			(false, false) => NextStep::Ignore,
    		}
    	}

    	fn apply(&mut self, size: u32) {
    		self.current_size += size;
    	}
    }

    /// Block assembler
    pub struct BlockAssembler {
    	/// Maximal block size.
    	pub max_block_size: u32,
    	/// Maximal # of sigops in the block.
    	pub max_block_sigops: u32,
    }

    /// Iterator iterating over mempool transactions and yielding only those which fit the block
    struct FittingTransactionsIterator<'a, T> {
    	/// Shared store is used to query previous transaction outputs from database
    	store: &'a TransactionOutputProvider,
    	/// Memory pool transactions iterator
    	iter: T,
    	/// New block height
    	block_height: u32,
    	/// New block time
    	block_time: u32,
    	/// Are OP_CHECKDATASIG && OP_CHECKDATASIGVERIFY enabled for this block.
    	checkdatasig_active: bool,
    	/// Size policy decides if transactions size fits the block
    	block_size: SizePolicy,
    	/// Sigops policy decides if transactions sigops fits the block
    	sigops: SizePolicy,
    	/// Previous entries are needed to get previous transaction outputs
    	previous_entries: Vec<&'a Entry>,
    	/// Hashes of ignored entries
    	ignored: HashSet<H256>,
    	/// True if block is already full
    	finished: bool,
    }

    impl<'a, T> FittingTransactionsIterator<'a, T> where T: Iterator<Item = &'a Entry> {
    	fn new(
    		store: &'a TransactionOutputProvider,
    		iter: T,
    		max_block_size: u32,
    		max_block_sigops: u32,
    		block_height: u32,
    		block_time: u32,
    		checkdatasig_active: bool,
    	) -> Self {
    		FittingTransactionsIterator {
    			store: store,
    			iter: iter,
    			block_height: block_height,
    			block_time: block_time,
    			checkdatasig_active,
    			// reserve some space for header and transations len field
    			block_size: SizePolicy::new(BLOCK_HEADER_SIZE + 4, max_block_size, 1_000, 50),
    			sigops: SizePolicy::new(0, max_block_sigops, 8, 50),
    			previous_entries: Vec::new(),
    			ignored: HashSet::new(),
    			finished: false,
    		}
    	}
    }

    impl<'a, T> TransactionOutputProvider for FittingTransactionsIterator<'a, T> where T: Send + Sync {
    	fn transaction_output(&self, prevout: &OutPoint, transaction_index: usize) -> Option<TransactionOutput> {
    		self.store.transaction_output(prevout, transaction_index)
    			.or_else(|| {
    				self.previous_entries.iter()
    					.find(|e| e.hash == prevout.hash)
    					.and_then(|e| e.transaction.outputs.iter().nth(prevout.index as usize))
    					.cloned()
    			})
    	}

    	fn is_spent(&self, _outpoint: &OutPoint) -> bool {
    		unimplemented!();
    	}
    }

    impl<'a, T> Iterator for FittingTransactionsIterator<'a, T> where T: Iterator<Item = &'a Entry> + Send + Sync {
    	type Item = &'a Entry;

    	fn next(&mut self) -> Option<Self::Item> {
    		while !self.finished {
    			let entry = match self.iter.next() {
    				Some(entry) => entry,
    				None => {
    					self.finished = true;
    					return None;
    				}
    			};

    			let transaction_size = entry.size as u32;
    			let bip16_active = true;
    			let sigops_count = transaction_sigops(&entry.transaction, self, bip16_active, self.checkdatasig_active) as u32;

    			let size_step = self.block_size.decide(transaction_size);
    			let sigops_step = self.sigops.decide(sigops_count);

    			// both next checks could be checked above, but then it will break finishing
    			// check if transaction is still not finalized in this block
    			if !entry.transaction.is_final_in_block(self.block_height, self.block_time) {
    				continue;
    			}
    			// check if any parent transaction has been ignored
    			if !self.ignored.is_empty() && entry.transaction.inputs.iter().any(|input| self.ignored.contains(&input.previous_output.hash)) {
    				continue;
    			}

    			match size_step.and(sigops_step) {
    				NextStep::Append => {
    					self.block_size.apply(transaction_size);
    					self.sigops.apply(transaction_size);
    					self.previous_entries.push(entry);
    					return Some(entry);
    				},
    				NextStep::FinishAndAppend => {
    					self.finished = true;
    					self.block_size.apply(transaction_size);
    					self.sigops.apply(transaction_size);
    					self.previous_entries.push(entry);
    					return Some(entry);
    				},
    				NextStep::Ignore => (),
    				NextStep::FinishAndIgnore => {
    					self.ignored.insert(entry.hash.clone());
    					self.finished = true;
    				},
    			}
    		}

    		None
    	}
    }

    impl BlockAssembler {
    	pub fn create_new_block(&self, store: &SharedStore, mempool: &MemoryPool, time: u32, median_timestamp: u32, consensus: &ConsensusParams) -> BlockTemplate {
    		// get best block
    		// take it's hash && height
    		let best_block = store.best_block();
    		let previous_header_hash = best_block.hash;
    		let height = best_block.number + 1;
    		let bits = work_required(previous_header_hash.clone(), time, height, store.as_block_header_provider(), consensus);
    		let version = BLOCK_VERSION;

    		let checkdatasig_active = match consensus.fork {
    			ConsensusFork::BitcoinCash(ref fork) => median_timestamp >= fork.magnetic_anomaly_time,
    			_ => false
    		};

    		let mut coinbase_value = block_reward_satoshi(height);
    		let mut transactions = Vec::new();

    		let mempool_iter = mempool.iter(OrderingStrategy::ByTransactionScore);
    		let tx_iter = FittingTransactionsIterator::new(
    			store.as_transaction_output_provider(),
    			mempool_iter,
    			self.max_block_size,
    			self.max_block_sigops,
    			height,
    			time,
    			checkdatasig_active);
    		for entry in tx_iter {
    			// miner_fee is i64, but we can safely cast it to u64
    			// memory pool should restrict miner fee to be positive
    			coinbase_value += entry.miner_fee as u64;
    			let tx = IndexedTransaction::new(entry.hash.clone(), entry.transaction.clone());
    			transactions.push(tx);
    		}

    		// sort block transactions
    		let median_time_past = median_timestamp_inclusive(previous_header_hash.clone(), store.as_block_header_provider());
    		match consensus.fork.transaction_ordering(median_time_past) {
    			TransactionOrdering::Canonical => transactions.sort_unstable_by(|tx1, tx2|
    				tx1.hash.cmp(&tx2.hash)),
    			// memory pool iter returns transactions in topological order
    			TransactionOrdering::Topological => (),
    		}

    		BlockTemplate {
    			version: version,
    			previous_header_hash: previous_header_hash,
    			time: time,
    			bits: bits,
    			height: height,
    			transactions: transactions,
    			coinbase_value: coinbase_value,
    			size_limit: self.max_block_size,
    			sigop_limit: self.max_block_sigops,
    		}
    	}
    }

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

    	use std::sync::Arc;
    	use db::BlockChainDatabase;
    	use primitives::hash::H256;
    	use storage::SharedStore;
    	use network::{ConsensusParams, ConsensusFork, Network, BitcoinCashConsensusParams};
    	use memory_pool::MemoryPool;
    	use self::test_data::{ChainBuilder, TransactionBuilder};
    	use super::{BlockAssembler, SizePolicy, NextStep, BlockTemplate};

    	#[test]
    	fn test_size_policy() {
    		let mut size_policy = SizePolicy::new(0, 1000, 200, 3);
    		assert_eq!(size_policy.decide(100), NextStep::Append); size_policy.apply(100);
    		assert_eq!(size_policy.decide(500), NextStep::Append); size_policy.apply(500);
    		assert_eq!(size_policy.decide(600), NextStep::Ignore);
    		assert_eq!(size_policy.decide(200), NextStep::Append); size_policy.apply(200);
    		assert_eq!(size_policy.decide(300), NextStep::Ignore);
    		assert_eq!(size_policy.decide(300), NextStep::Ignore);
    		// this transaction will make counter + buffer > max size
    		assert_eq!(size_policy.decide(1), NextStep::Append); size_policy.apply(1);
    		// so now only 3 more transactions may accepted / ignored
    		assert_eq!(size_policy.decide(1), NextStep::Append); size_policy.apply(1);
    		assert_eq!(size_policy.decide(1000), NextStep::Ignore);
    		assert_eq!(size_policy.decide(1), NextStep::FinishAndAppend); size_policy.apply(1);
    		// we should not call decide again after it returned finish...
    		// but we can, let's check if result is ok
    		assert_eq!(size_policy.decide(1000), NextStep::FinishAndIgnore);
    	}

    	#[test]
    	fn test_next_step_and() {
    		assert_eq!(NextStep::Append.and(NextStep::Append), NextStep::Append);
    		assert_eq!(NextStep::Ignore.and(NextStep::Append), NextStep::Ignore);
    		assert_eq!(NextStep::FinishAndIgnore.and(NextStep::Append), NextStep::FinishAndIgnore);
    		assert_eq!(NextStep::Ignore.and(NextStep::FinishAndIgnore), NextStep::FinishAndIgnore);
    		assert_eq!(NextStep::FinishAndAppend.and(NextStep::FinishAndIgnore), NextStep::FinishAndIgnore);
    		assert_eq!(NextStep::FinishAndAppend.and(NextStep::Ignore), NextStep::FinishAndIgnore);
    		assert_eq!(NextStep::FinishAndAppend.and(NextStep::Append), NextStep::FinishAndAppend);
    	}

    	#[test]
    	fn test_fitting_transactions_iterator_max_block_size_reached() {
    	}

    	#[test]
    	fn test_fitting_transactions_iterator_ignored_parent() {
    		// TODO
    	}

    	#[test]
    	fn test_fitting_transactions_iterator_locked_transaction() {
    		// TODO
    	}

    	#[test]
    	fn block_assembler_transaction_order() {
    		fn construct_block(consensus: ConsensusParams) -> (BlockTemplate, H256, H256) {
    			let chain = &mut ChainBuilder::new();
    			TransactionBuilder::with_default_input(0).set_output(30).store(chain)	// transaction0
    				.into_input(0).set_output(50).store(chain);							// transaction0 -> transaction1
    			let hash0 = chain.at(0).hash();
    			let hash1 = chain.at(1).hash();

    			let mut pool = MemoryPool::new();
    			let storage: SharedStore = Arc::new(BlockChainDatabase::init_test_chain(vec![test_data::genesis().into()]));
    			pool.insert_verified(chain.at(0).into());
    			pool.insert_verified(chain.at(1).into());

    			(BlockAssembler {
    				max_block_size: 0xffffffff,
    				max_block_sigops: 0xffffffff,
    			}.create_new_block(&storage, &pool, 0, 0, &consensus), hash0, hash1)
    		}

    		// when topological consensus is used
    		let topological_consensus = ConsensusParams::new(Network::Mainnet, ConsensusFork::BitcoinCore);
    		let (block, hash0, hash1) = construct_block(topological_consensus);
    		assert!(hash1 < hash0);
    		assert_eq!(block.transactions[0].hash, hash0);
    		assert_eq!(block.transactions[1].hash, hash1);

    		// when canonocal consensus is used
    		let mut canonical_fork = BitcoinCashConsensusParams::new(Network::Mainnet);
    		canonical_fork.magnetic_anomaly_time = 0;
    		let canonical_consensus = ConsensusParams::new(Network::Mainnet, ConsensusFork::BitcoinCash(canonical_fork));
    		let (block, hash0, hash1) = construct_block(canonical_consensus);
    		assert!(hash1 < hash0);
    		assert_eq!(block.transactions[0].hash, hash1);
    		assert_eq!(block.transactions[1].hash, hash0);
    	}
    }

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