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    use primitives::compact::Compact;
    use primitives::hash::H256;
    use primitives::bigint::{Uint, U256};
    use chain::{IndexedBlockHeader, BlockHeader};
    use network::{Network, ConsensusParams, BitcoinCashConsensusParams};
    use storage::BlockHeaderProvider;
    use timestamp::median_timestamp_inclusive;
    use work::{is_retarget_height, work_required_testnet, work_required_retarget};

    use constants::{
    	DOUBLE_SPACING_SECONDS, TARGET_SPACING_SECONDS, RETARGETING_INTERVAL
    };

    /// Returns work required for given header for the post-HF Bitcoin Cash block
    pub fn work_required_bitcoin_cash(parent_header: IndexedBlockHeader, time: u32, height: u32, store: &BlockHeaderProvider, consensus: &ConsensusParams, fork: &BitcoinCashConsensusParams, max_bits: Compact) -> Compact {
    	// special processing of Bitcoin Cash difficulty adjustment hardfork (Nov 2017), where difficulty is adjusted after each block
    	// `height` is the height of the new block => comparison is shifted by one
    	if height.saturating_sub(1) >= fork.difficulty_adjustion_height {
    		return work_required_bitcoin_cash_adjusted(parent_header, time, height, store, consensus);
    	}

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

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

    	if parent_header.raw.bits == max_bits {
    		return parent_header.raw.bits;
    	}

    	// REQ-7 Difficulty adjustement in case of hashrate drop
    	// In case the MTP of the tip of the chain is 12h or more after the MTP 6 block before the tip,
    	// the proof of work target is increased by a quarter, or 25%, which corresponds to a difficulty
    	// reduction of 20%.
    	let ancient_block_ref = (height - 6 - 1).into();
    	let ancient_header = store.block_header(ancient_block_ref)
    		.expect("parent_header.bits != max_bits; difficulty is max_bits for first RETARGETING_INTERVAL height; RETARGETING_INTERVAL > 7; qed");

    	let ancient_timestamp = median_timestamp_inclusive(ancient_header.hash(), store);
    	let parent_timestamp = median_timestamp_inclusive(parent_header.hash.clone(), store);
    	let timestamp_diff = parent_timestamp.checked_sub(ancient_timestamp).unwrap_or_default();
    	if timestamp_diff < 43_200 {
    		// less than 12h => no difficulty change needed
    		return parent_header.raw.bits;
    	}

    	let mut new_bits: U256 = parent_header.raw.bits.into();
    	let max_bits: U256 = max_bits.into();
    	new_bits = new_bits + (new_bits >> 2);
    	if new_bits > max_bits {
    		new_bits = max_bits
    	}

    	new_bits.into()
    }

    /// Algorithm to adjust difficulty after each block. Implementation is based on Bitcoin ABC commit:
    /// https://github.com/Bitcoin-ABC/bitcoin-abc/commit/be51cf295c239ff6395a0aa67a3e13906aca9cb2
    fn work_required_bitcoin_cash_adjusted(parent_header: IndexedBlockHeader, time: u32, height: u32, store: &BlockHeaderProvider, consensus: &ConsensusParams) -> Compact {
    	/// To reduce the impact of timestamp manipulation, we select the block we are
    	/// basing our computation on via a median of 3.
    	fn suitable_block(mut header2: BlockHeader, store: &BlockHeaderProvider) -> BlockHeader {
    		let reason = "header.height >= RETARGETNG_INTERVAL; RETARGETING_INTERVAL > 2; qed";
    		let mut header1 = store.block_header(header2.previous_header_hash.clone().into()).expect(reason);
    		let mut header0 = store.block_header(header1.previous_header_hash.clone().into()).expect(reason);

    		if header0.time > header2.time {
    			::std::mem::swap(&mut header0, &mut header2);
    		}
    		if header0.time > header1.time {
    			::std::mem::swap(&mut header0, &mut header1);
    		}
    		if header1.time > header2.time {
    			::std::mem::swap(&mut header1, &mut header2);
    		}

    		header1
    	}

    	/// Get block proof.
    	fn block_proof(header: &BlockHeader) -> U256 {
    		let proof: U256 = header.bits.into();
    		// We need to compute 2**256 / (bnTarget+1), but we can't represent 2**256
    		// as it's too large for a arith_uint256. However, as 2**256 is at least as
    		// large as bnTarget+1, it is equal to ((2**256 - bnTarget - 1) /
    		// (bnTarget+1)) + 1, or ~bnTarget / (nTarget+1) + 1.
    		(!proof / (proof + U256::one())) + U256::one()
    	}

    	/// Compute chain work between two blocks. Last block work is included. First block work is excluded.
    	fn compute_work_between_blocks(first: H256, last: &BlockHeader, store: &BlockHeaderProvider) -> U256 {
    		debug_assert!(last.hash() != first);
    		let mut chain_work: U256 = block_proof(last);
    		let mut prev_hash = last.previous_header_hash.clone();
    		loop {
    			let header = store.block_header(prev_hash.into())
    				.expect("last header is on main chain; first is at height last.height - 144; it is on main chain; qed");

    			chain_work = chain_work + block_proof(&header);
    			prev_hash = header.previous_header_hash;
    			if prev_hash == first {
    				return chain_work;
    			}
    		}
    	}

    	/// Compute the a target based on the work done between 2 blocks and the time
    	/// required to produce that work.
    	fn compute_target(first_header: BlockHeader, last_header: BlockHeader, store: &BlockHeaderProvider) -> U256 {
    		// From the total work done and the time it took to produce that much work,
    		// we can deduce how much work we expect to be produced in the targeted time
    		// between blocks.
    		let mut work = compute_work_between_blocks(first_header.hash(), &last_header, store);
    		work = work * TARGET_SPACING_SECONDS.into();

    		// In order to avoid difficulty cliffs, we bound the amplitude of the
    		// adjustement we are going to do.
    		debug_assert!(last_header.time > first_header.time);
    		let mut actual_timespan = last_header.time - first_header.time;
    		if actual_timespan > 288 * TARGET_SPACING_SECONDS {
    			actual_timespan = 288 * TARGET_SPACING_SECONDS;
    		} else if actual_timespan < 72 * TARGET_SPACING_SECONDS {
    			actual_timespan = 72 * TARGET_SPACING_SECONDS;
    		}

    		let work = work / actual_timespan.into();

    		// We need to compute T = (2^256 / W) - 1 but 2^256 doesn't fit in 256 bits.
    		// By expressing 1 as W / W, we get (2^256 - W) / W, and we can compute
    		// 2^256 - W as the complement of W.
    		(!work) / work
    	}

    	// This cannot handle the genesis block and early blocks in general.
    	debug_assert!(height > 0);

    	// Special difficulty rule for testnet:
    	// If the new block's timestamp is more than 2 * 10 minutes then allow
    	// mining of a min-difficulty block.
    	let max_bits = consensus.network.max_bits();
    	if consensus.network == Network::Testnet || consensus.network == Network::Unitest {
    		let max_time_gap = parent_header.raw.time + DOUBLE_SPACING_SECONDS;
    		if time > max_time_gap {
    			return max_bits.into();
    		}
    	}

    	// Compute the difficulty based on the full adjustement interval.
    	let last_height = height - 1;
    	debug_assert!(last_height >= RETARGETING_INTERVAL);

    	// Get the last suitable block of the difficulty interval.
    	let last_header = suitable_block(parent_header.raw, store);

    	// Get the first suitable block of the difficulty interval.
    	let first_height = last_height - 144;
    	let first_header = store.block_header(first_height.into())
    		.expect("last_height >= RETARGETING_INTERVAL; RETARGETING_INTERVAL - 144 > 0; qed");
    	let first_header = suitable_block(first_header, store);

    	// Compute the target based on time and work done during the interval.
    	let next_target = compute_target(first_header, last_header, store);
    	let max_bits = consensus.network.max_bits();
    	if next_target > max_bits {
    		return max_bits.into();
    	}

    	next_target.into()
    }

    #[cfg(test)]
    mod tests {
    	use std::collections::HashMap;
    	use primitives::bytes::Bytes;
    	use primitives::hash::H256;
    	use primitives::bigint::U256;
    	use network::{Network, ConsensusParams, BitcoinCashConsensusParams, ConsensusFork};
    	use storage::{BlockHeaderProvider, BlockRef};
    	use chain::BlockHeader;
    	use work::work_required;
    	use super::work_required_bitcoin_cash_adjusted;

    	#[derive(Default)]
    	struct MemoryBlockHeaderProvider {
    		pub by_height: Vec<BlockHeader>,
    		pub by_hash: HashMap<H256, usize>,
    	}

    	impl MemoryBlockHeaderProvider {
    		pub fn insert(&mut self, header: BlockHeader) {
    			self.by_hash.insert(header.hash(), self.by_height.len());
    			self.by_height.push(header);
    		}
    	}

    	impl BlockHeaderProvider for MemoryBlockHeaderProvider {
    		fn block_header_bytes(&self, _block_ref: BlockRef) -> Option<Bytes> {
    			unimplemented!()
    		}

    		fn block_header(&self, block_ref: BlockRef) -> Option<BlockHeader> {
    			match block_ref {
    				BlockRef::Hash(ref hash) => self.by_hash.get(hash).map(|h| &self.by_height[*h]).cloned(),
    				BlockRef::Number(height) => self.by_height.get(height as usize).cloned(),
    			}
    		}
    	}

    	// original test link:
    	// https://github.com/bitcoinclassic/bitcoinclassic/blob/8bf1fb856df44d1b790b0b835e4c1969be736e25/src/test/pow_tests.cpp#L108
    	#[test]
    	fn bitcoin_cash_req7() {
    		let main_consensus = ConsensusParams::new(Network::Mainnet, ConsensusFork::BitcoinCore);
    		let uahf_consensus = ConsensusParams::new(Network::Mainnet, ConsensusFork::BitcoinCash(BitcoinCashConsensusParams {
    			height: 1000,
    			difficulty_adjustion_height: 0xffffffff,
    			monolith_time: 0xffffffff,
    			magnetic_anomaly_time: 0xffffffff,
    		}));
    		let mut header_provider = MemoryBlockHeaderProvider::default();
    		header_provider.insert(BlockHeader {
    				version: 0,
    				previous_header_hash: 0.into(),
    				merkle_root_hash: 0.into(),
    				time: 1269211443,
    				bits: 0x207fffff.into(),
    				nonce: 0,
    			});

    		// create x100 pre-HF blocks
    		for height in 1..1000 {
    			let mut header = header_provider.block_header((height - 1).into()).unwrap();
    			header.previous_header_hash = header.hash();
    			header.time = header.time + 10 * 60;
    			header_provider.insert(header);
    		}

    		// create x10 post-HF blocks every 2 hours
    		// MTP still less than 12h
    		for height in 1000..1010 {
    			let mut header = header_provider.block_header((height - 1).into()).unwrap();
    			header.previous_header_hash = header.hash();
    			header.time = header.time + 2 * 60 * 60;
    			header_provider.insert(header.clone());

    			let main_bits: u32 = work_required(header.hash(), 0, height as u32, &header_provider, &main_consensus).into();
    			assert_eq!(main_bits, 0x207fffff_u32);
    			let uahf_bits: u32 = work_required(header.hash(), 0, height as u32, &header_provider, &uahf_consensus).into();
    			assert_eq!(uahf_bits, 0x207fffff_u32);
    		}

    		// MTP becames greater than 12h
    		let mut header = header_provider.block_header(1009.into()).unwrap();
    		header.previous_header_hash = header.hash();
    		header.time = header.time + 2 * 60 * 60;
    		header_provider.insert(header.clone());

    		let main_bits: u32 = work_required(header.hash(), 0, 1010, &header_provider, &main_consensus).into();
    		assert_eq!(main_bits, 0x207fffff_u32);
    		let uahf_bits: u32 = work_required(header.hash(), 0, 1010, &header_provider, &uahf_consensus).into();
    		assert_eq!(uahf_bits, 0x1d00ffff_u32);
    	}

    	// original test link:
    	// https://github.com/Bitcoin-ABC/bitcoin-abc/blob/d8eac91f8d16716eed0ad11ccac420122280bb13/src/test/pow_tests.cpp#L193
    	#[test]
    	fn bitcoin_cash_adjusted_difficulty() {
    		let uahf_consensus = ConsensusParams::new(Network::Mainnet, ConsensusFork::BitcoinCash(BitcoinCashConsensusParams {
    			height: 1000,
    			difficulty_adjustion_height: 0xffffffff,
    			monolith_time: 0xffffffff,
    			magnetic_anomaly_time: 0xffffffff,
    		}));

    		let limit_bits = uahf_consensus.network.max_bits();
    		let initial_bits = limit_bits >> 4;
    		let mut header_provider = MemoryBlockHeaderProvider::default();

    		// Genesis block.
    		header_provider.insert(BlockHeader {
    				version: 0,
    				previous_header_hash: 0.into(),
    				merkle_root_hash: 0.into(),
    				time: 1269211443,
    				bits: initial_bits.into(),
    				nonce: 0,
    			});

    		// Pile up some blocks every 10 mins to establish some history.
    		for height in 1..2050 {
    			let mut header = header_provider.block_header((height - 1).into()).unwrap();
    			header.previous_header_hash = header.hash();
    			header.time = header.time + 600;
    			header_provider.insert(header);
    		}

    		// Difficulty stays the same as long as we produce a block every 10 mins.
    		let current_bits = work_required_bitcoin_cash_adjusted(header_provider.block_header(2049.into()).unwrap().into(),
    			0, 2050, &header_provider, &uahf_consensus);
    		for height in 2050..2060 {
    			let mut header = header_provider.block_header((height - 1).into()).unwrap();
    			header.previous_header_hash = header.hash();
    			header.time = header.time + 600;
    			header.bits = current_bits;
    			header_provider.insert(header);

    			let calculated_bits = work_required_bitcoin_cash_adjusted(header_provider.block_header(height.into()).unwrap().into(),
    				0, height + 1, &header_provider, &uahf_consensus);
    			debug_assert_eq!(calculated_bits, current_bits);
    		}

    		// Make sure we skip over blocks that are out of wack. To do so, we produce
    		// a block that is far in the future
    		let mut header = header_provider.block_header(2059.into()).unwrap();
    		header.previous_header_hash = header.hash();
    		header.time = header.time + 6000;
    		header.bits = current_bits;
    		header_provider.insert(header);
    		let calculated_bits = work_required_bitcoin_cash_adjusted(header_provider.block_header(2060.into()).unwrap().into(),
    			0, 2061, &header_provider, &uahf_consensus);
    		debug_assert_eq!(calculated_bits, current_bits);

    		// .. and then produce a block with the expected timestamp.
    		let mut header = header_provider.block_header(2060.into()).unwrap();
    		header.previous_header_hash = header.hash();
    		header.time = header.time + 2 * 600 - 6000;
    		header.bits = current_bits;
    		header_provider.insert(header);
    		let calculated_bits = work_required_bitcoin_cash_adjusted(header_provider.block_header(2061.into()).unwrap().into(),
    			0, 2062, &header_provider, &uahf_consensus);
    		debug_assert_eq!(calculated_bits, current_bits);

    		// The system should continue unaffected by the block with a bogous timestamps.
    		for height in 2062..2082 {
    			let mut header = header_provider.block_header((height - 1).into()).unwrap();
    			header.previous_header_hash = header.hash();
    			header.time = header.time + 600;
    			header.bits = current_bits;
    			header_provider.insert(header);

    			let calculated_bits = work_required_bitcoin_cash_adjusted(header_provider.block_header(height.into()).unwrap().into(),
    				0, height + 1, &header_provider, &uahf_consensus);
    			debug_assert_eq!(calculated_bits, current_bits);
    		}

    		// We start emitting blocks slightly faster. The first block has no impact.
    		let mut header = header_provider.block_header(2081.into()).unwrap();
    		header.previous_header_hash = header.hash();
    		header.time = header.time + 550;
    		header.bits = current_bits;
    		header_provider.insert(header);
    		let calculated_bits = work_required_bitcoin_cash_adjusted(header_provider.block_header(2082.into()).unwrap().into(),
    			0, 2083, &header_provider, &uahf_consensus);
    		debug_assert_eq!(calculated_bits, current_bits);

    		// Now we should see difficulty increase slowly.
    		let mut current_bits = current_bits;
    		for height in 2083..2093 {
    			let mut header = header_provider.block_header((height - 1).into()).unwrap();
    			header.previous_header_hash = header.hash();
    			header.time = header.time + 550;
    			header.bits = current_bits;
    			header_provider.insert(header);

    			let calculated_bits = work_required_bitcoin_cash_adjusted(header_provider.block_header(height.into()).unwrap().into(),
    				0, height + 1, &header_provider, &uahf_consensus);

    			let current_work: U256 = current_bits.into();
    			let calculated_work: U256 = calculated_bits.into();
    			debug_assert!(calculated_work < current_work);
    			debug_assert!((current_work - calculated_work) < (current_work >> 10));

    			current_bits = calculated_bits;
    		}

    		// Check the actual value.
    		debug_assert_eq!(current_bits, 0x1c0fe7b1.into());

    		// If we dramatically shorten block production, difficulty increases faster.
    		for height in 2093..2113 {
    			let mut header = header_provider.block_header((height - 1).into()).unwrap();
    			header.previous_header_hash = header.hash();
    			header.time = header.time + 10;
    			header.bits = current_bits;
    			header_provider.insert(header);

    			let calculated_bits = work_required_bitcoin_cash_adjusted(header_provider.block_header(height.into()).unwrap().into(),
    				0, height + 1, &header_provider, &uahf_consensus);

    			let current_work: U256 = current_bits.into();
    			let calculated_work: U256 = calculated_bits.into();
    			debug_assert!(calculated_work < current_work);
    			debug_assert!((current_work - calculated_work) < (current_work >> 4));

    			current_bits = calculated_bits;
    		}

    		// Check the actual value.
    		debug_assert_eq!(current_bits, 0x1c0db19f.into());

    		// We start to emit blocks significantly slower. The first block has no
    		// impact.
    		let mut header = header_provider.block_header(2112.into()).unwrap();
    		header.previous_header_hash = header.hash();
    		header.time = header.time + 6000;
    		header.bits = current_bits;
    		header_provider.insert(header);
    		let mut current_bits = work_required_bitcoin_cash_adjusted(header_provider.block_header(2113.into()).unwrap().into(),
    			0, 2114, &header_provider, &uahf_consensus);

    		// Check the actual value.
    		debug_assert_eq!(current_bits, 0x1c0d9222.into());

    		// If we dramatically slow down block production, difficulty decreases.
    		for height in 2114..2207 {
    			let mut header = header_provider.block_header((height - 1).into()).unwrap();
    			header.previous_header_hash = header.hash();
    			header.time = header.time + 6000;
    			header.bits = current_bits;
    			header_provider.insert(header);

    			let calculated_bits = work_required_bitcoin_cash_adjusted(header_provider.block_header(height.into()).unwrap().into(),
    				0, height + 1, &header_provider, &uahf_consensus);

    			let current_work: U256 = current_bits.into();
    			let calculated_work: U256 = calculated_bits.into();
    			debug_assert!(calculated_work < limit_bits);
    			debug_assert!(calculated_work > current_work);
    			debug_assert!((calculated_work - current_work) < (current_work >> 3));

    			current_bits = calculated_bits;
    		}

    		// Check the actual value.
    		debug_assert_eq!(current_bits, 0x1c2f13b9.into());

    		// Due to the window of time being bounded, next block's difficulty actually
    		// gets harder.
    		let mut header = header_provider.block_header(2206.into()).unwrap();
    		header.previous_header_hash = header.hash();
    		header.time = header.time + 6000;
    		header.bits = current_bits;
    		header_provider.insert(header);
    		let mut current_bits = work_required_bitcoin_cash_adjusted(header_provider.block_header(2207.into()).unwrap().into(),
    			0, 2208, &header_provider, &uahf_consensus);
    		debug_assert_eq!(current_bits, 0x1c2ee9bf.into());

    		// And goes down again. It takes a while due to the window being bounded and
    		// the skewed block causes 2 blocks to get out of the window.
    		for height in 2208..2400 {
    			let mut header = header_provider.block_header((height - 1).into()).unwrap();
    			header.previous_header_hash = header.hash();
    			header.time = header.time + 6000;
    			header.bits = current_bits;
    			header_provider.insert(header);

    			let calculated_bits = work_required_bitcoin_cash_adjusted(header_provider.block_header(height.into()).unwrap().into(),
    				0, height + 1, &header_provider, &uahf_consensus);

    			let current_work: U256 = current_bits.into();
    			let calculated_work: U256 = calculated_bits.into();
    			debug_assert!(calculated_work <= limit_bits);
    			debug_assert!(calculated_work > current_work);
    			debug_assert!((calculated_work - current_work) < (current_work >> 3));

    			current_bits = calculated_bits;
    		}

    		// Check the actual value.
    		debug_assert_eq!(current_bits, 0x1d00ffff.into());

    		// Once the difficulty reached the minimum allowed level, it doesn't get any
    		// easier.
    		for height in 2400..2405 {
    			let mut header = header_provider.block_header((height - 1).into()).unwrap();
    			header.previous_header_hash = header.hash();
    			header.time = header.time + 6000;
    			header.bits = current_bits;
    			header_provider.insert(header);

    			let calculated_bits = work_required_bitcoin_cash_adjusted(header_provider.block_header(height.into()).unwrap().into(),
    				0, height + 1, &header_provider, &uahf_consensus);
    			debug_assert_eq!(calculated_bits, limit_bits.into());

    			current_bits = calculated_bits;
    		}
    	}
    }

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