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    use std::collections::HashMap;
    use chain::{Transaction, TransactionOutput, OutPoint};
    use storage::TransactionOutputProvider;
    use miner::{DoubleSpendCheckResult, HashedOutPoint, NonFinalDoubleSpendSet};
    use verification::TransactionError;
    use super::super::types::{MemoryPoolRef, StorageRef};

    /// Transaction output observer, which looks into both storage && into memory pool.
    /// It also allows to replace non-final transactions in the memory pool.
    pub struct MemoryPoolTransactionOutputProvider {
    	/// Storage provider
    	storage_provider: StorageRef,
    	/// Transaction inputs from memory pool transactions
    	mempool_inputs: HashMap<HashedOutPoint, Option<TransactionOutput>>,
    	/// Previous outputs, for which we should return 'Not spent' value.
    	/// These are used when new version of transaction is received.
    	nonfinal_spends: Option<NonFinalDoubleSpendSet>,
    }

    impl MemoryPoolTransactionOutputProvider {
    	/// Create new provider for verifying given transaction
    	pub fn for_transaction(storage: StorageRef, memory_pool: &MemoryPoolRef, transaction: &Transaction) -> Result<Self, TransactionError> {
    		// we have to check if there are another in-mempool transactions which spent same outputs here
    		let memory_pool = memory_pool.read();
    		let check_result = memory_pool.check_double_spend(transaction);
    		match check_result {
    			// input of transaction is already spent by another final transaction from memory pool
    			DoubleSpendCheckResult::DoubleSpend(_, hash, index) => Err(TransactionError::UsingSpentOutput(hash, index)),
    			// there are no transactions, which are spending same inputs in memory pool
    			DoubleSpendCheckResult::NoDoubleSpend => Ok(MemoryPoolTransactionOutputProvider {
    				storage_provider: storage,
    				mempool_inputs: transaction.inputs.iter()
    					.map(|input| (
    						input.previous_output.clone().into(),
    						memory_pool.transaction_output(&input.previous_output, usize::max_value()),
    					)).collect(),
    				nonfinal_spends: None,
    			}),
    			// there are non-final transactions, which are spending same inputs in memory pool
    			DoubleSpendCheckResult::NonFinalDoubleSpend(nonfinal_spends) => Ok(MemoryPoolTransactionOutputProvider {
    				storage_provider: storage,
    				mempool_inputs: transaction.inputs.iter()
    					.map(|input| (
    						input.previous_output.clone().into(),
    						memory_pool.transaction_output(&input.previous_output, usize::max_value()),
    					)).collect(),
    				nonfinal_spends: Some(nonfinal_spends),
    			}),
    		}
    	}
    }

    impl TransactionOutputProvider for MemoryPoolTransactionOutputProvider {
    	fn transaction_output(&self, prevout: &OutPoint, transaction_index: usize) -> Option<TransactionOutput> {
    		let hashed_prevout: HashedOutPoint = prevout.clone().into();

    		// check if that is output of some transaction, which is vitually removed from memory pool
    		if let Some(ref nonfinal_spends) = self.nonfinal_spends {
    			if nonfinal_spends.dependent_spends.contains(&hashed_prevout) {
    				// transaction is trying to replace some nonfinal transaction
    				// + it is also depends on this transaction
    				// => this is definitely an error
    				return None;
    			}
    		}

    		// check if this is output from memory pool transaction
    		if let Some(output) = self.mempool_inputs.get(&hashed_prevout) {
    			if let Some(ref output) = *output {
    				return Some(output.clone());
    			}
    		}

    		// now check in storage
    		self.storage_provider.transaction_output(prevout, transaction_index)
    	}

    	fn is_spent(&self, prevout: &OutPoint) -> bool {
    		// check if this output is spent by some non-final mempool transaction
    		if let Some(ref nonfinal_spends) = self.nonfinal_spends {
    			if nonfinal_spends.double_spends.contains(&prevout.clone().into()) {
    				return false;
    			}
    		}

    		// we can omit memory_pool check here, because it has been completed in `for_transaction` method
    		// => just check spending in storage
    		self.storage_provider.is_spent(prevout)
    	}
    }

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

    	use std::sync::Arc;
    	use parking_lot::RwLock;
    	use chain::OutPoint;
    	use storage::{TransactionOutputProvider};
    	use db::BlockChainDatabase;
    	use miner::MemoryPool;
    	use super::MemoryPoolTransactionOutputProvider;

    	#[test]
    	fn when_transaction_depends_on_removed_nonfinal_transaction() {
    		let dchain = &mut test_data::ChainBuilder::new();

    		test_data::TransactionBuilder::with_output(10).store(dchain)					// t0
    			.reset().set_input(&dchain.at(0), 0).add_output(20).lock().store(dchain)	// nonfinal: t0[0] -> t1
    			.reset().set_input(&dchain.at(1), 0).add_output(30).store(dchain)			// dependent: t0[0] -> t1[0] -> t2
    			.reset().set_input(&dchain.at(0), 0).add_output(40).store(dchain);			// good replacement: t0[0] -> t3

    		let storage = Arc::new(BlockChainDatabase::init_test_chain(vec![test_data::genesis().into()]));
    		let memory_pool = Arc::new(RwLock::new(MemoryPool::new()));
    		{
    			memory_pool.write().insert_verified(dchain.at(0).into());
    			memory_pool.write().insert_verified(dchain.at(1).into());
    			memory_pool.write().insert_verified(dchain.at(2).into());
    		}

    		// when inserting t3:
    		// check that is_spent(t0[0]) == Some(false) (as it is spent by nonfinal t1)
    		// check that is_spent(t1[0]) == None (as t1 is virtually removed)
    		// check that is_spent(t2[0]) == None (as t2 is virtually removed)
    		// check that previous_transaction_output(t0[0]) = Some(_)
    		// check that previous_transaction_output(t1[0]) = None (as t1 is virtually removed)
    		// check that previous_transaction_output(t2[0]) = None (as t2 is virtually removed)
    		// =>
    		// if t3 is also depending on t1[0] || t2[0], it will be rejected by verification as missing inputs
    		let provider = MemoryPoolTransactionOutputProvider::for_transaction(storage, &memory_pool, &dchain.at(3)).unwrap();
    		assert_eq!(provider.is_spent(&OutPoint { hash: dchain.at(0).hash(), index: 0, }), false);
    		assert_eq!(provider.is_spent(&OutPoint { hash: dchain.at(1).hash(), index: 0, }), false);
    		assert_eq!(provider.is_spent(&OutPoint { hash: dchain.at(2).hash(), index: 0, }), false);
    		assert_eq!(provider.transaction_output(&OutPoint { hash: dchain.at(0).hash(), index: 0, }, 0), Some(dchain.at(0).outputs[0].clone()));
    		assert_eq!(provider.transaction_output(&OutPoint { hash: dchain.at(1).hash(), index: 0, }, 0), None);
    		assert_eq!(provider.transaction_output(&OutPoint { hash: dchain.at(2).hash(), index: 0, }, 0), None);
    	}
    }

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