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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/>.

    //! LES buffer flow management.
    //!
    //! Every request in the LES protocol leads to a reduction
    //! of the requester's buffer value as a rate-limiting mechanism.
    //! This buffer value will recharge at a set rate.
    //!
    //! This module provides an interface for configuration of buffer
    //! flow costs and recharge rates.
    //!
    //! Current default costs are picked completely arbitrarily, not based
    //! on any empirical timings or mathematical models.

    use request;
    use super::packet;
    use super::error::Error;

    use rlp::*;
    use util::U256;
    use time::{Duration, SteadyTime};

    /// A request cost specification.
    #[derive(Debug, Clone, PartialEq, Eq)]
    pub struct Cost(pub U256, pub U256);

    /// Buffer value.
    ///
    /// Produced and recharged using `FlowParams`.
    /// Definitive updates can be made as well -- these will reset the recharge
    /// point to the time of the update.
    #[derive(Debug, Clone, PartialEq, Eq)]
    pub struct Buffer {
    	estimate: U256,
    	recharge_point: SteadyTime,
    }

    impl Buffer {
    	/// Get the current buffer value.
    	pub fn current(&self) -> U256 { self.estimate.clone() }

    	/// Make a definitive update.
    	/// This will be the value obtained after receiving
    	/// a response to a request.
    	pub fn update_to(&mut self, value: U256) {
    		self.estimate = value;
    		self.recharge_point = SteadyTime::now();
    	}

    	/// Attempt to apply the given cost to the buffer.
    	///
    	/// If successful, the cost will be deducted successfully.
    	///
    	/// If unsuccessful, the structure will be unaltered an an
    	/// error will be produced.
    	pub fn deduct_cost(&mut self, cost: U256) -> Result<(), Error> {
    		match cost > self.estimate {
    			true => Err(Error::BufferEmpty),
    			false => {
    				self.estimate = self.estimate - cost;
    				Ok(())
    			}
    		}
    	}
    }

    /// A cost table, mapping requests to base and per-request costs.
    #[derive(Debug, Clone, PartialEq, Eq)]
    pub struct CostTable {
    	headers: Cost,
    	bodies: Cost,
    	receipts: Cost,
    	state_proofs: Cost,
    	contract_codes: Cost,
    	header_proofs: Cost,
    }

    impl Default for CostTable {
    	fn default() -> Self {
    		// arbitrarily chosen constants.
    		CostTable {
    			headers: Cost(100000.into(), 10000.into()),
    			bodies: Cost(150000.into(), 15000.into()),
    			receipts: Cost(50000.into(), 5000.into()),
    			state_proofs: Cost(250000.into(), 25000.into()),
    			contract_codes: Cost(200000.into(), 20000.into()),
    			header_proofs: Cost(150000.into(), 15000.into()),
    		}
    	}
    }

    impl RlpEncodable for CostTable {
    	fn rlp_append(&self, s: &mut RlpStream) {
    		fn append_cost(s: &mut RlpStream, msg_id: u8, cost: &Cost) {
    			s.begin_list(3)
    				.append(&msg_id)
    				.append(&cost.0)
    				.append(&cost.1);
    		}

    		s.begin_list(6);

    		append_cost(s, packet::GET_BLOCK_HEADERS, &self.headers);
    		append_cost(s, packet::GET_BLOCK_BODIES, &self.bodies);
    		append_cost(s, packet::GET_RECEIPTS, &self.receipts);
    		append_cost(s, packet::GET_PROOFS, &self.state_proofs);
    		append_cost(s, packet::GET_CONTRACT_CODES, &self.contract_codes);
    		append_cost(s, packet::GET_HEADER_PROOFS, &self.header_proofs);
    	}
    }

    impl RlpDecodable for CostTable {
    	fn decode<D>(decoder: &D) -> Result<Self, DecoderError> where D: Decoder {
    		let rlp = decoder.as_rlp();

    		let mut headers = None;
    		let mut bodies = None;
    		let mut receipts = None;
    		let mut state_proofs = None;
    		let mut contract_codes = None;
    		let mut header_proofs = None;

    		for row in rlp.iter() {
    			let msg_id: u8 = row.val_at(0)?;
    			let cost = {
    				let base = row.val_at(1)?;
    				let per = row.val_at(2)?;

    				Cost(base, per)
    			};

    			match msg_id {
    				packet::GET_BLOCK_HEADERS => headers = Some(cost),
    				packet::GET_BLOCK_BODIES => bodies = Some(cost),
    				packet::GET_RECEIPTS => receipts = Some(cost),
    				packet::GET_PROOFS => state_proofs = Some(cost),
    				packet::GET_CONTRACT_CODES => contract_codes = Some(cost),
    				packet::GET_HEADER_PROOFS => header_proofs = Some(cost),
    				_ => return Err(DecoderError::Custom("Unrecognized message in cost table")),
    			}
    		}

    		Ok(CostTable {
    			headers: headers.ok_or(DecoderError::Custom("No headers cost specified"))?,
    			bodies: bodies.ok_or(DecoderError::Custom("No bodies cost specified"))?,
    			receipts: receipts.ok_or(DecoderError::Custom("No receipts cost specified"))?,
    			state_proofs: state_proofs.ok_or(DecoderError::Custom("No proofs cost specified"))?,
    			contract_codes: contract_codes.ok_or(DecoderError::Custom("No contract codes specified"))?,
    			header_proofs: header_proofs.ok_or(DecoderError::Custom("No header proofs cost specified"))?,
    		})
    	}
    }

    /// A buffer-flow manager handles costs, recharge, limits
    #[derive(Debug, Clone, PartialEq)]
    pub struct FlowParams {
    	costs: CostTable,
    	limit: U256,
    	recharge: U256,
    }

    impl FlowParams {
    	/// Create new flow parameters from a request cost table,
    	/// buffer limit, and (minimum) rate of recharge.
    	pub fn new(limit: U256, costs: CostTable, recharge: U256) -> Self {
    		FlowParams {
    			costs: costs,
    			limit: limit,
    			recharge: recharge,
    		}
    	}

    	/// Create effectively infinite flow params.
    	pub fn free() -> Self {
    		let free_cost = Cost(0.into(), 0.into());
    		FlowParams {
    			limit: (!0u64).into(),
    			recharge: 1.into(),
    			costs: CostTable {
    				headers: free_cost.clone(),
    				bodies: free_cost.clone(),
    				receipts: free_cost.clone(),
    				state_proofs: free_cost.clone(),
    				contract_codes: free_cost.clone(),
    				header_proofs: free_cost.clone(),
    			}
    		}
    	}

    	/// Get a reference to the buffer limit.
    	pub fn limit(&self) -> &U256 { &self.limit }

    	/// Get a reference to the cost table.
    	pub fn cost_table(&self) -> &CostTable { &self.costs }

    	/// Get a reference to the recharge rate.
    	pub fn recharge_rate(&self) -> &U256 { &self.recharge }

    	/// Compute the actual cost of a request, given the kind of request
    	/// and number of requests made.
    	pub fn compute_cost(&self, kind: request::Kind, amount: usize) -> U256 {
    		let cost = match kind {
    			request::Kind::Headers => &self.costs.headers,
    			request::Kind::Bodies => &self.costs.bodies,
    			request::Kind::Receipts => &self.costs.receipts,
    			request::Kind::StateProofs => &self.costs.state_proofs,
    			request::Kind::Codes => &self.costs.contract_codes,
    			request::Kind::HeaderProofs => &self.costs.header_proofs,
    		};

    		let amount: U256 = amount.into();
    		cost.0 + (amount * cost.1)
    	}

    	/// Compute the maximum number of costs of a specific kind which can be made
    	/// with the given buffer.
    	/// Saturates at `usize::max()`. This is not a problem in practice because
    	/// this amount of requests is already prohibitively large.
    	pub fn max_amount(&self, buffer: &Buffer, kind: request::Kind) -> usize {
    		use util::Uint;
    		use std::usize;

    		let cost = match kind {
    			request::Kind::Headers => &self.costs.headers,
    			request::Kind::Bodies => &self.costs.bodies,
    			request::Kind::Receipts => &self.costs.receipts,
    			request::Kind::StateProofs => &self.costs.state_proofs,
    			request::Kind::Codes => &self.costs.contract_codes,
    			request::Kind::HeaderProofs => &self.costs.header_proofs,
    		};

    		let start = buffer.current();

    		if start <= cost.0 {
    			return 0;
    		} else if cost.1 == U256::zero() {
    			return usize::MAX;
    		}

    		let max = (start - cost.0) / cost.1;
    		if max >= usize::MAX.into() {
    			usize::MAX
    		} else {
    			max.as_u64() as usize
    		}
    	}

    	/// Create initial buffer parameter.
    	pub fn create_buffer(&self) -> Buffer {
    		Buffer {
    			estimate: self.limit,
    			recharge_point: SteadyTime::now(),
    		}
    	}

    	/// Recharge the buffer based on time passed since last
    	/// update.
    	pub fn recharge(&self, buf: &mut Buffer) {
    		let now = SteadyTime::now();

    		// recompute and update only in terms of full seconds elapsed
    		// in order to keep the estimate as an underestimate.
    		let elapsed = (now - buf.recharge_point).num_seconds();
    		buf.recharge_point = buf.recharge_point + Duration::seconds(elapsed);

    		let elapsed: U256 = elapsed.into();

    		buf.estimate = ::std::cmp::min(self.limit, buf.estimate + (elapsed * self.recharge));
    	}

    	/// Refund some buffer which was previously deducted.
    	/// Does not update the recharge timestamp.
    	pub fn refund(&self, buf: &mut Buffer, refund_amount: U256) {
    		buf.estimate = buf.estimate + refund_amount;

    		if buf.estimate > self.limit {
    			buf.estimate = self.limit
    		}
    	}
    }

    impl Default for FlowParams {
    	fn default() -> Self {
    		FlowParams {
    			limit: 50_000_000.into(),
    			costs: CostTable::default(),
    			recharge: 100_000.into(),
    		}
    	}
    }

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

    	#[test]
    	fn should_serialize_cost_table() {
    		let costs = CostTable::default();
    		let serialized = ::rlp::encode(&costs);

    		let new_costs: CostTable = ::rlp::decode(&*serialized);

    		assert_eq!(costs, new_costs);
    	}

    	#[test]
    	fn buffer_mechanism() {
    		use std::thread;
    		use std::time::Duration;

    		let flow_params = FlowParams::new(100.into(), Default::default(), 20.into());
    		let mut buffer =  flow_params.create_buffer();

    		assert!(buffer.deduct_cost(101.into()).is_err());
    		assert!(buffer.deduct_cost(10.into()).is_ok());

    		thread::sleep(Duration::from_secs(1));

    		flow_params.recharge(&mut buffer);

    		assert_eq!(buffer.estimate, 100.into());
    	}
    }

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