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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/>.

    //! Watcher for snapshot-related chain events.

    use util::Mutex;
    use client::{BlockChainClient, Client, ChainNotify};
    use ids::BlockId;
    use service::ClientIoMessage;

    use io::IoChannel;
    use util::{H256, Bytes};

    use std::sync::Arc;

    // helper trait for transforming hashes to numbers and checking if syncing.
    trait Oracle: Send + Sync {
    	fn to_number(&self, hash: H256) -> Option<u64>;

    	fn is_major_importing(&self) -> bool;
    }

    struct StandardOracle<F> where F: 'static + Send + Sync + Fn() -> bool {
    	client: Arc<Client>,
    	sync_status: F,
    }

    impl<F> Oracle for StandardOracle<F>
    	where F: Send + Sync + Fn() -> bool
    {
    	fn to_number(&self, hash: H256) -> Option<u64> {
    		self.client.block_header(BlockId::Hash(hash)).map(|h| h.number())
    	}

    	fn is_major_importing(&self) -> bool {
    		(self.sync_status)()
    	}
    }

    // helper trait for broadcasting a block to take a snapshot at.
    trait Broadcast: Send + Sync {
    	fn take_at(&self, num: Option<u64>);
    }

    impl Broadcast for Mutex<IoChannel<ClientIoMessage>> {
    	fn take_at(&self, num: Option<u64>) {
    		let num = match num {
    			Some(n) => n,
    			None => return,
    		};

    		trace!(target: "snapshot_watcher", "broadcast: {}", num);

    		if let Err(e) = self.lock().send(ClientIoMessage::TakeSnapshot(num)) {
    			warn!("Snapshot watcher disconnected from IoService: {}", e);
    		}
    	}
    }

    /// A `ChainNotify` implementation which will trigger a snapshot event
    /// at certain block numbers.
    pub struct Watcher {
    	oracle: Box<Oracle>,
    	broadcast: Box<Broadcast>,
    	period: u64,
    	history: u64,
    }

    impl Watcher {
    	/// Create a new `Watcher` which will trigger a snapshot event
    	/// once every `period` blocks, but only after that block is
    	/// `history` blocks old.
    	pub fn new<F>(client: Arc<Client>, sync_status: F, channel: IoChannel<ClientIoMessage>, period: u64, history: u64) -> Self
    		where F: 'static + Send + Sync + Fn() -> bool
    	{
    		Watcher {
    			oracle: Box::new(StandardOracle {
    				client: client,
    				sync_status: sync_status,
    			}),
    			broadcast: Box::new(Mutex::new(channel)),
    			period: period,
    			history: history,
    		}
    	}
    }

    impl ChainNotify for Watcher {
    	fn new_blocks(
    		&self,
    		imported: Vec<H256>,
    		_: Vec<H256>,
    		_: Vec<H256>,
    		_: Vec<H256>,
    		_: Vec<H256>,
    		_: Vec<Bytes>,
    		_duration: u64)
    	{
    		if self.oracle.is_major_importing() { return }

    		trace!(target: "snapshot_watcher", "{} imported", imported.len());

    		let highest = imported.into_iter()
    			.filter_map(|h| self.oracle.to_number(h))
    			.filter(|&num| num >= self.period + self.history)
    			.map(|num| num - self.history)
    			.filter(|num| num % self.period == 0)
    			.fold(0, ::std::cmp::max);

    		match highest {
    			0 => self.broadcast.take_at(None),
    			_ => self.broadcast.take_at(Some(highest)),
    		}
    	}
    }

    #[cfg(test)]
    mod tests {
    	use super::{Broadcast, Oracle, Watcher};

    	use client::ChainNotify;

    	use util::{H256, U256};

    	use std::collections::HashMap;

    	struct TestOracle(HashMap<H256, u64>);

    	impl Oracle for TestOracle {
    		fn to_number(&self, hash: H256) -> Option<u64> {
    			self.0.get(&hash).cloned()
    		}

    		fn is_major_importing(&self) -> bool { false }
    	}

    	struct TestBroadcast(Option<u64>);
    	impl Broadcast for TestBroadcast {
    		fn take_at(&self, num: Option<u64>) {
    			if num != self.0 {
    				panic!("Watcher broadcast wrong number. Expected {:?}, found {:?}", self.0, num);
    			}
    		}
    	}

    	// helper harness for tests which expect a notification.
    	fn harness(numbers: Vec<u64>, period: u64, history: u64, expected: Option<u64>) {
    		let hashes: Vec<_> = numbers.clone().into_iter().map(|x| H256::from(U256::from(x))).collect();
    		let map = hashes.clone().into_iter().zip(numbers).collect();

    		let watcher = Watcher {
    			oracle: Box::new(TestOracle(map)),
    			broadcast: Box::new(TestBroadcast(expected)),
    			period: period,
    			history: history,
    		};

    		watcher.new_blocks(
    			hashes,
    			vec![],
    			vec![],
    			vec![],
    			vec![],
    			vec![],
    			0,
    		);
    	}

    	// helper

    	#[test]
    	fn should_not_fire() {
    		harness(vec![0], 5, 0, None);
    	}

    	#[test]
    	fn fires_once_for_two() {
    		harness(vec![14, 15], 10, 5, Some(10));
    	}

    	#[test]
    	fn finds_highest() {
    		harness(vec![15, 25], 10, 5, Some(20));
    	}

    	#[test]
    	fn doesnt_fire_before_history() {
    		harness(vec![10, 11], 10, 5, None);
    	}
    }

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