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    use std::cmp;
    use primitives::compact::Compact;
    use primitives::hash::H256;
    use primitives::bigint::U256;
    use chain::{IndexedBlockHeader, BlockHeader};
    use network::{Network, ConsensusParams, ConsensusFork};
    use storage::{BlockHeaderProvider, BlockRef};
    use work_bch::work_required_bitcoin_cash;

    use constants::{
    	DOUBLE_SPACING_SECONDS, TARGET_TIMESPAN_SECONDS,
    	MIN_TIMESPAN, MAX_TIMESPAN, RETARGETING_INTERVAL
    };

    pub fn is_retarget_height(height: u32) -> bool {
    	height % RETARGETING_INTERVAL == 0
    }

    fn range_constrain(value: i64, min: i64, max: i64) -> i64 {
    	cmp::min(cmp::max(value, min), max)
    }

    /// Returns true if hash is lower or equal than target represented by compact bits
    pub fn is_valid_proof_of_work_hash(bits: Compact, hash: &H256) -> bool {
    	let target = match bits.to_u256() {
    		Ok(target) => target,
    		_err => return false,
    	};

    	let value = U256::from(&*hash.reversed() as &[u8]);
    	value <= target
    }

    /// Returns true if hash is lower or equal than target and target is lower or equal
    /// than current network maximum
    pub fn is_valid_proof_of_work(max_work_bits: Compact, bits: Compact, hash: &H256) -> bool {
    	let maximum = match max_work_bits.to_u256() {
    		Ok(max) => max,
    		_err => return false,
    	};

    	let target = match bits.to_u256() {
    		Ok(target) => target,
    		_err => return false,
    	};

    	let value = U256::from(&*hash.reversed() as &[u8]);
    	target <= maximum && value <= target
    }

    /// Returns constrained number of seconds since last retarget
    pub fn retarget_timespan(retarget_timestamp: u32, last_timestamp: u32) -> u32 {
    	// subtract unsigned 32 bit numbers in signed 64 bit space in
    	// order to prevent underflow before applying the range constraint.
    	let timespan = last_timestamp as i64 - retarget_timestamp as i64;
    	range_constrain(timespan, MIN_TIMESPAN as i64, MAX_TIMESPAN as i64) as u32
    }

    /// Returns work required for given header
    pub fn work_required(parent_hash: H256, time: u32, height: u32, store: &BlockHeaderProvider, consensus: &ConsensusParams) -> Compact {
    	let max_bits = consensus.network.max_bits().into();
    	if height == 0 {
    		return max_bits;
    	}

    	let parent_header = store.block_header(parent_hash.clone().into()).expect("self.height != 0; qed");

    	match consensus.fork {
    		ConsensusFork::BitcoinCash(ref fork) if height >= fork.height =>
    			return work_required_bitcoin_cash(IndexedBlockHeader {
    				hash: parent_hash,
    				raw: parent_header
    			}, time, height, store, consensus, fork, max_bits),
    		_ => (),
    	}

    	if is_retarget_height(height) {
    		return work_required_retarget(parent_header, height, store, max_bits);
    	}

    	if consensus.network == Network::Testnet {
    		return work_required_testnet(parent_hash, time, height, store, Network::Testnet)
    	}

    	parent_header.bits
    }

    pub fn work_required_testnet(parent_hash: H256, time: u32, height: u32, store: &BlockHeaderProvider, network: Network) -> Compact {
    	assert!(height != 0, "cannot calculate required work for genesis block");

    	let mut bits = Vec::new();
    	let mut block_ref: BlockRef = parent_hash.into();

    	let parent_header = store.block_header(block_ref.clone()).expect("height != 0; qed");
    	let max_time_gap = parent_header.time + DOUBLE_SPACING_SECONDS;
    	let max_bits = network.max_bits().into();
    	if time > max_time_gap {
    		return max_bits;
    	}

    	// TODO: optimize it, so it does not make 2016!!! redundant queries each time
    	for _ in 0..RETARGETING_INTERVAL {
    		let previous_header = match store.block_header(block_ref) {
    			Some(h) => h,
    			None => { break; }
    		};
    		bits.push(previous_header.bits);
    		block_ref = previous_header.previous_header_hash.into();
    	}

    	for (index, bit) in bits.into_iter().enumerate() {
    		if bit != max_bits || is_retarget_height(height - index as u32 - 1) {
    			return bit;
    		}
    	}

    	max_bits
    }

    /// Algorithm used for retargeting work every 2 weeks
    pub fn work_required_retarget(parent_header: BlockHeader, height: u32, store: &BlockHeaderProvider, max_work_bits: Compact) -> Compact {
    	let retarget_ref = (height - RETARGETING_INTERVAL).into();
    	let retarget_header = store.block_header(retarget_ref).expect("self.height != 0 && self.height % RETARGETING_INTERVAL == 0; qed");

    	// timestamp of block(height - RETARGETING_INTERVAL)
    	let retarget_timestamp = retarget_header.time;
    	// timestamp of parent block
    	let last_timestamp = parent_header.time;
    	// bits of last block
    	let last_bits = parent_header.bits;

    	let mut retarget: U256 = last_bits.into();
    	let maximum: U256 = max_work_bits.into();

    	retarget = retarget * retarget_timespan(retarget_timestamp, last_timestamp).into();
    	retarget = retarget / TARGET_TIMESPAN_SECONDS.into();

    	if retarget > maximum {
    		max_work_bits
    	} else {
    		retarget.into()
    	}
    }

    pub fn block_reward_satoshi(block_height: u32) -> u64 {
    	let mut res = 50 * 100 * 1000 * 1000;
    	for _ in 0..block_height / 210000 { res /= 2 }
    	res
    }

    #[cfg(test)]
    mod tests {
    	use primitives::hash::H256;
    	use primitives::compact::Compact;
    	use network::Network;
    	use super::{is_valid_proof_of_work_hash, is_valid_proof_of_work, block_reward_satoshi};

    	fn is_valid_pow(max: Compact, bits: u32, hash: &'static str) -> bool {
    		is_valid_proof_of_work_hash(bits.into(), &H256::from_reversed_str(hash)) &&
    		is_valid_proof_of_work(max.into(), bits.into(), &H256::from_reversed_str(hash))
    	}

    	#[test]
    	fn test_is_valid_proof_of_work() {
    		// block 2
    		assert!(is_valid_pow(Network::Mainnet.max_bits().into(), 486604799u32, "000000006a625f06636b8bb6ac7b960a8d03705d1ace08b1a19da3fdcc99ddbd"));
    		// block 400_000
    		assert!(is_valid_pow(Network::Mainnet.max_bits().into(), 403093919u32, "000000000000000004ec466ce4732fe6f1ed1cddc2ed4b328fff5224276e3f6f"));

    		// other random tests
    		assert!(is_valid_pow(Network::Regtest.max_bits().into(), 0x181bc330u32, "00000000000000001bc330000000000000000000000000000000000000000000"));
    		assert!(!is_valid_pow(Network::Regtest.max_bits().into(), 0x181bc330u32, "00000000000000001bc330000000000000000000000000000000000000000001"));
    		assert!(!is_valid_pow(Network::Regtest.max_bits().into(), 0x181bc330u32, "ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff"));
    	}

    	#[test]
    	fn reward() {
    		assert_eq!(block_reward_satoshi(0), 5000000000);
    		assert_eq!(block_reward_satoshi(209999), 5000000000);
    		assert_eq!(block_reward_satoshi(210000), 2500000000);
    		assert_eq!(block_reward_satoshi(420000), 1250000000);
    		assert_eq!(block_reward_satoshi(420001), 1250000000);
    		assert_eq!(block_reward_satoshi(629999), 1250000000);
    		assert_eq!(block_reward_satoshi(630000), 625000000);
    		assert_eq!(block_reward_satoshi(630001), 625000000);
    	}
    }

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