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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/>.

    //! Header download state machine.

    use std::cmp::Ordering;
    use std::collections::{BinaryHeap, HashMap, HashSet, VecDeque};
    use std::fmt;

    use ethcore::header::Header;

    use light::net::ReqId;
    use light::request::Headers as HeadersRequest;

    use network::PeerId;
    use util::{Bytes, H256};

    use super::response;

    // number of attempts to make to get a full scaffold for a sync round.
    const SCAFFOLD_ATTEMPTS: usize = 3;

    /// Context for a headers response.
    pub trait ResponseContext {
    	/// Get the peer who sent this response.
    	fn responder(&self) ->	PeerId;
    	/// Get the request ID this response corresponds to.
    	fn req_id(&self) -> &ReqId;
    	/// Get the (unverified) response data.
    	fn data(&self) -> &[Bytes];
    	/// Punish the responder.
    	fn punish_responder(&self);
    }

    /// Reasons for sync round abort.
    #[derive(Debug, Clone)]
    pub enum AbortReason {
    	/// Bad sparse header chain along with a list of peers who contributed to it.
    	BadScaffold(Vec<PeerId>),
    	/// No incoming data.
    	NoResponses,
    	/// Sync rounds completed.
    	TargetReached,
    }

    // A request for headers with a known starting header hash.
    // and a known parent hash for the last block.
    #[derive(PartialEq, Eq)]
    struct SubchainRequest {
    	subchain_parent: (u64, H256),
    	headers_request: HeadersRequest,
    	subchain_end: (u64, H256),
    	downloaded: VecDeque<Header>,
    }

    // ordered by subchain parent number so pending requests towards the
    // front of the round are dispatched first.
    impl PartialOrd for SubchainRequest {
    	fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
    		self.subchain_parent.0
    			.partial_cmp(&other.subchain_parent.0)
    			.map(Ordering::reverse)
    	}
    }

    impl Ord for SubchainRequest {
    	fn cmp(&self, other: &Self) -> Ordering {
    		self.subchain_parent.0.cmp(&other.subchain_parent.0).reverse()
    	}
    }

    /// Manages downloading of interior blocks of a sparse header chain.
    pub struct Fetcher {
    	sparse: VecDeque<Header>, // sparse header chain.
    	requests: BinaryHeap<SubchainRequest>,
    	complete_requests: HashMap<H256, SubchainRequest>,
    	pending: HashMap<ReqId, SubchainRequest>,
    	scaffold_contributors: Vec<PeerId>,
    	ready: VecDeque<Header>,
    	end: (u64, H256),
    	target: (u64, H256),
    }

    impl Fetcher {
    	// Produce a new fetcher given a sparse headerchain, in ascending order along
    	// with a list of peers who helped produce the chain.
    	// The headers must be valid RLP at this point and must have a consistent
    	// non-zero gap between them. Will abort the round if found wrong.
    	fn new(sparse_headers: Vec<Header>, contributors: Vec<PeerId>, target: (u64, H256)) -> SyncRound {
    		let mut requests = BinaryHeap::with_capacity(sparse_headers.len() - 1);

    		for pair in sparse_headers.windows(2) {
    			let low_rung = &pair[0];
    			let high_rung = &pair[1];

    			let diff = high_rung.number() - low_rung.number();

    			// should never happen as long as we verify the gaps
    			// gotten from SyncRound::Start
    			if diff < 2 { continue }

    			let needed_headers = HeadersRequest {
    				start: high_rung.parent_hash().clone().into(),
    				max: diff as usize - 1,
    				skip: 0,
    				reverse: true,
    			};

    			requests.push(SubchainRequest {
    				headers_request: needed_headers,
    				subchain_end: (high_rung.number() - 1, *high_rung.parent_hash()),
    				downloaded: VecDeque::new(),
    				subchain_parent: (low_rung.number(), low_rung.hash()),
    			});
    		}

    		let end = match sparse_headers.last().map(|h| (h.number(), h.hash())) {
    			Some(end) => end,
    			None => return SyncRound::abort(AbortReason::BadScaffold(contributors), VecDeque::new()),
    		};

    		SyncRound::Fetch(Fetcher {
    			sparse: sparse_headers.into(),
    			requests: requests,
    			complete_requests: HashMap::new(),
    			pending: HashMap::new(),
    			scaffold_contributors: contributors,
    			ready: VecDeque::new(),
    			end: end,
    			target: target,
    		})
    	}

    	// collect complete requests and their subchain from the sparse header chain
    	// into the ready set in order.
    	fn collect_ready(&mut self) {
    		loop {
    			let start_hash = match self.sparse.front() {
    				Some(first) => first.hash(),
    				None => break,
    			};

    			match self.complete_requests.remove(&start_hash) {
    				None => break,
    				Some(complete_req) => {
    					self.ready.push_back(self.sparse.pop_front().expect("first known to exist; qed"));
    					self.ready.extend(complete_req.downloaded);
    				}
    			}
    		}

    		// frames are between two sparse headers and keyed by subchain parent, so the last
    		// remaining will be the last header.
    		if self.sparse.len() == 1 {
    			self.ready.push_back(self.sparse.pop_back().expect("sparse known to have one entry; qed"))
    		}

    		trace!(target: "sync", "{} headers ready to drain", self.ready.len());
    	}

    	fn process_response<R: ResponseContext>(mut self, ctx: &R) -> SyncRound {
    		let mut request = match self.pending.remove(ctx.req_id()) {
    			Some(request) => request,
    			None => return SyncRound::Fetch(self),
    		};

    		trace!(target: "sync", "Received response for subchain ({} -> {})",
    			request.subchain_parent.0 + 1, request.subchain_end.0);

    		let headers = ctx.data();

    		if headers.len() == 0 {
    			trace!(target: "sync", "Punishing peer {} for empty response", ctx.responder());
    			ctx.punish_responder();

    			self.requests.push(request);
    			return SyncRound::Fetch(self);
    		}

    		match response::decode_and_verify(headers, &request.headers_request) {
    			Err(e) => {
    				trace!(target: "sync", "Punishing peer {} for invalid response ({})", ctx.responder(), e);
    				ctx.punish_responder();

    				// TODO: track number of attempts per request,
    				// abort if failure rate too high.
    				self.requests.push(request);
    				SyncRound::Fetch(self)
    			}
    			Ok(headers) => {
    				let mut parent_hash = None;
    				for header in headers {
    					if parent_hash.as_ref().map_or(false, |h| h != &header.hash()) {
    						trace!(target: "sync", "Punishing peer {} for parent mismatch", ctx.responder());
    						ctx.punish_responder();

    						self.requests.push(request);
    						return SyncRound::Fetch(self);
    					}

    					// incrementally update the frame request as we go so we can
    					// return at any time in the loop.
    					parent_hash = Some(header.parent_hash().clone());
    					request.headers_request.start = header.parent_hash().clone().into();
    					request.headers_request.max -= 1;

    					request.downloaded.push_front(header);
    				}

    				let subchain_parent = request.subchain_parent.1;

    				// check if the subchain portion has been completely filled.
    				if request.headers_request.max == 0 {
    					if parent_hash.map_or(true, |hash| hash != subchain_parent) {
    						let abort = AbortReason::BadScaffold(self.scaffold_contributors);
    						return SyncRound::abort(abort, self.ready);
    					}

    					self.complete_requests.insert(subchain_parent, request);
    					self.collect_ready();
    				}

    				// state transition not triggered until drain is finished.
    				(SyncRound::Fetch(self))
    			}
    		}
    	}

    	fn requests_abandoned(mut self, abandoned: &[ReqId]) -> SyncRound {
    		for abandoned in abandoned {
    			match self.pending.remove(abandoned) {
    				None => {},
    				Some(req) => self.requests.push(req),
    			}
    		}

    		// TODO: track failure rate and potentially abort.
    		SyncRound::Fetch(self)
    	}

    	fn dispatch_requests<D>(mut self, mut dispatcher: D) -> SyncRound
    		where D: FnMut(HeadersRequest) -> Option<ReqId>
    	{
    		while let Some(pending_req) = self.requests.pop() {
    			match dispatcher(pending_req.headers_request.clone()) {
    				Some(req_id) => {
    					trace!(target: "sync", "Assigned request for subchain ({} -> {})",
    						pending_req.subchain_parent.0 + 1, pending_req.subchain_end.0);

    					self.pending.insert(req_id, pending_req);
    				}
    				None => {
    					self.requests.push(pending_req);
    					break;
    				}
    			}
    		}

    		SyncRound::Fetch(self)
    	}

    	fn drain(mut self, headers: &mut Vec<Header>, max: Option<usize>) -> SyncRound {
    		let max = ::std::cmp::min(max.unwrap_or(usize::max_value()), self.ready.len());
    		headers.extend(self.ready.drain(0..max));

    		if self.sparse.is_empty() && self.ready.is_empty() {
    			trace!(target: "sync", "sync round complete. Starting anew from {:?}", self.end);
    			SyncRound::begin(self.end, self.target)
    		} else {
    			SyncRound::Fetch(self)
    		}
    	}
    }

    // Compute scaffold parameters from non-zero distance between start and target block: (skip, pivots).
    fn scaffold_params(diff: u64) -> (u64, usize) {
    	// default parameters.
    	// amount of blocks between each scaffold pivot.
    	const ROUND_SKIP: u64 = 255;
    	// amount of scaffold pivots: these are the Xs in "X___X___X"
    	const ROUND_PIVOTS: usize = 256;

    	let rem = diff % (ROUND_SKIP + 1);
    	if diff <= ROUND_SKIP {
    		// just request headers from the start to the target.
    		(0, rem as usize)
    	} else {
    		// the number of pivots necessary to exactly hit or overshoot the target.
    		let pivots_to_target = (diff / (ROUND_SKIP + 1)) + if rem == 0 { 0 } else { 1 };
    		let num_pivots = ::std::cmp::min(pivots_to_target, ROUND_PIVOTS as u64) as usize;
    		(ROUND_SKIP, num_pivots)
    	}
    }

    /// Round started: get stepped header chain.
    /// from a start block with number X we request ROUND_PIVOTS headers stepped by ROUND_SKIP from
    /// block X + 1 to a target >= X + 1.
    /// If the sync target is within ROUND_SKIP of the start, we request
    /// only those blocks. If the sync target is within (ROUND_SKIP + 1) * (ROUND_PIVOTS - 1) of
    /// the start, we reduce the number of pivots so the target is outside it.
    pub struct RoundStart {
    	start_block: (u64, H256),
    	target: (u64, H256),
    	pending_req: Option<(ReqId, HeadersRequest)>,
    	sparse_headers: Vec<Header>,
    	contributors: HashSet<PeerId>,
    	attempt: usize,
    	skip: u64,
    	pivots: usize,
    }

    impl RoundStart {
    	fn new(start: (u64, H256), target: (u64, H256)) -> Self {
    		let (skip, pivots) = scaffold_params(target.0 - start.0);

    		trace!(target: "sync", "Beginning sync round: {} pivots and {} skip from block {}",
    			pivots, skip, start.0);

    		RoundStart {
    			start_block: start,
    			target: target,
    			pending_req: None,
    			sparse_headers: Vec::new(),
    			contributors: HashSet::new(),
    			attempt: 0,
    			skip: skip,
    			pivots: pivots,
    		}
    	}

    	// called on failed attempt. may trigger a transition after a number of attempts.
    	// a failed attempt is defined as any time a peer returns invalid or incomplete response
    	fn failed_attempt(mut self) -> SyncRound {
    		self.attempt += 1;

    		if self.attempt >= SCAFFOLD_ATTEMPTS {
    			return if self.sparse_headers.len() > 1 {
    				Fetcher::new(self.sparse_headers, self.contributors.into_iter().collect(), self.target)
    			} else {
    				let fetched_headers = if self.skip == 0 {
    					self.sparse_headers.into()
    				} else {
    					VecDeque::new()
    				};

    				SyncRound::abort(AbortReason::NoResponses, fetched_headers)
    			}
    		} else {
    			SyncRound::Start(self)
    		}
    	}

    	fn process_response<R: ResponseContext>(mut self, ctx: &R) -> SyncRound {
    		let req = match self.pending_req.take() {
    			Some((id, ref req)) if ctx.req_id() == &id => { req.clone() }
    			other => {
    				self.pending_req = other;
    				return SyncRound::Start(self);
    			}
    		};

    		match response::decode_and_verify(ctx.data(), &req) {
    			Ok(headers) => {
    				if self.sparse_headers.len() == 0
    					&& headers.get(0).map_or(false, |x| x.parent_hash() != &self.start_block.1) {
    					trace!(target: "sync", "Wrong parent for first header in round");
    					ctx.punish_responder(); // or should we reset?
    				}

    				self.contributors.insert(ctx.responder());
    				self.sparse_headers.extend(headers);

    				if self.sparse_headers.len() == self.pivots {
    					return if self.skip == 0 {
    						SyncRound::abort(AbortReason::TargetReached, self.sparse_headers.into())
    					} else {
    						trace!(target: "sync", "Beginning fetch of blocks between {} sparse headers",
    							self.sparse_headers.len());
    						Fetcher::new(
    							self.sparse_headers,
    							self.contributors.into_iter().collect(),
    							self.target
    						)
    					}
    				}
    			}
    			Err(e) => {
    				trace!(target: "sync", "Punishing peer {} for malformed response ({})", ctx.responder(), e);
    				ctx.punish_responder();
    			}
    		};

    		self.failed_attempt()
    	}

    	fn requests_abandoned(mut self, abandoned: &[ReqId]) -> SyncRound {
    		match self.pending_req.take() {
    			Some((id, req)) => {
    				if abandoned.iter().any(|r| r == &id) {
    					self.pending_req = None;
    					self.failed_attempt()
    				} else {
    					self.pending_req = Some((id, req));
    					SyncRound::Start(self)
    				}
    			}
    			None => SyncRound::Start(self),
    		}
    	}

    	fn dispatch_requests<D>(mut self, mut dispatcher: D) -> SyncRound
    		where D: FnMut(HeadersRequest) -> Option<ReqId>
    	{
    		if self.pending_req.is_none() {
    			// beginning offset + first block expected after last header we have.
    			let start = (self.start_block.0 + 1)
    				+ self.sparse_headers.len() as u64 * (self.skip + 1);

    			let max = self.pivots - self.sparse_headers.len();

    			let headers_request = HeadersRequest {
    				start: start.into(),
    				max: max,
    				skip: self.skip,
    				reverse: false,
    			};

    			if let Some(req_id) = dispatcher(headers_request.clone()) {
    				trace!(target: "sync", "Requesting scaffold: {} headers forward from {}, skip={}",
    					max, start, self.skip);

    				self.pending_req = Some((req_id, headers_request));
    			}
    		}

    		SyncRound::Start(self)
    	}
    }

    /// Sync round state machine.
    pub enum SyncRound {
    	/// Beginning a sync round.
    	Start(RoundStart),
    	/// Fetching intermediate blocks during a sync round.
    	Fetch(Fetcher),
    	/// Aborted + Sequential headers
    	Abort(AbortReason, VecDeque<Header>),
    }

    impl SyncRound {
    	fn abort(reason: AbortReason, remaining: VecDeque<Header>) -> Self {
    		trace!(target: "sync", "Aborting sync round: {:?}. To drain: {}", reason, remaining.len());

    		SyncRound::Abort(reason, remaining)
    	}

    	/// Begin sync rounds from a starting block, but not to go past a given target
    	pub fn begin(start: (u64, H256), target: (u64, H256)) -> Self {
    		if target.0 <= start.0 {
    			SyncRound::abort(AbortReason::TargetReached, VecDeque::new())
    		} else {
    			SyncRound::Start(RoundStart::new(start, target))
    		}
    	}

    	/// Process an answer to a request. Unknown requests will be ignored.
    	pub fn process_response<R: ResponseContext>(self, ctx: &R) -> Self {
    		match self {
    			SyncRound::Start(round_start) => round_start.process_response(ctx),
    			SyncRound::Fetch(fetcher) => fetcher.process_response(ctx),
    			other => other,
    		}
    	}

    	/// Return unfulfilled requests from disconnected peer. Unknown requests will be ignored.
    	pub fn requests_abandoned(self, abandoned: &[ReqId]) -> Self {
    		match self {
    			SyncRound::Start(round_start) => round_start.requests_abandoned(abandoned),
    			SyncRound::Fetch(fetcher) => fetcher.requests_abandoned(abandoned),
    			other => other,
    		}
    	}

    	/// Dispatch pending requests. The dispatcher provided will attempt to
    	/// find a suitable peer to serve the request.
    	// TODO: have dispatcher take capabilities argument? and return an error as
    	// to why no suitable peer can be found? (no buffer, no chain heads that high, etc)
    	pub fn dispatch_requests<D>(self, dispatcher: D) -> Self
    		where D: FnMut(HeadersRequest) -> Option<ReqId>
    	{
    		match self {
    			SyncRound::Start(round_start) => round_start.dispatch_requests(dispatcher),
    			SyncRound::Fetch(fetcher) => fetcher.dispatch_requests(dispatcher),
    			other => other,
    		}
    	}

    	/// Drain up to a maximum number (None -> all) of headers (continuous, starting with a child of
    	/// the round start block) from the round, starting a new one once finished.
    	pub fn drain(self, v: &mut Vec<Header>, max: Option<usize>) -> Self {
    		match self {
    			SyncRound::Fetch(fetcher) => fetcher.drain(v, max),
    			SyncRound::Abort(reason, mut remaining) => {
    				let len = ::std::cmp::min(max.unwrap_or(usize::max_value()), remaining.len());
    				v.extend(remaining.drain(..len));
    				SyncRound::Abort(reason, remaining)
    			}
    			other => other,
    		}
    	}
    }

    impl fmt::Debug for SyncRound {
    	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
    		match *self {
    			SyncRound::Start(ref state) => write!(f, "Scaffolding from {:?}", state.start_block),
    			SyncRound::Fetch(ref fetcher) => write!(f, "Filling scaffold up to {:?}", fetcher.end),
    			SyncRound::Abort(ref reason, ref remaining) =>
    				write!(f, "Aborted: {:?}, {} remain", reason, remaining.len()),
    		}
    	}
    }

    #[cfg(test)]
    mod tests {
    	use super::scaffold_params;

    	#[test]
    	fn scaffold_config() {
    		// within a certain distance of the head, we download
    		// sequentially.
    		assert_eq!(scaffold_params(1), (0, 1));
    		assert_eq!(scaffold_params(6), (0, 6));

    		// when scaffolds are useful, download enough frames to get
    		// within a close distance of the goal.
    		assert_eq!(scaffold_params(1000), (255, 4));
    		assert_eq!(scaffold_params(1024), (255, 4));
    	}
    }

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