// Copyright 2019-2021 Parity Technologies (UK) Ltd.
// This file is part of Parity Bridges Common.

// Parity Bridges Common 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 Bridges Common 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 Bridges Common.  If not, see <http://www.gnu.org/licenses/>.

//! Support of different finality engines, available in Substrate.

use crate::error::Error;
use async_trait::async_trait;
use bp_header_chain::{
	find_grandpa_authorities_scheduled_change,
	justification::{verify_justification, GrandpaJustification},
	FinalityProof,
};
use codec::{Decode, Encode};
use finality_grandpa::voter_set::VoterSet;
use num_traits::{One, Zero};
use relay_substrate_client::{
	BlockNumberOf, Chain, ChainWithGrandpa, Client, Error as SubstrateError, HashOf, HeaderOf,
	Subscription,
};
use sp_core::{storage::StorageKey, Bytes};
use sp_finality_grandpa::AuthorityList as GrandpaAuthoritiesSet;
use sp_runtime::{traits::Header, ConsensusEngineId};
use std::marker::PhantomData;

/// Finality enfine, used by the Substrate chain.
#[async_trait]
pub trait Engine<C: Chain> {
	/// Unique consensus engine identifier.
	const ID: ConsensusEngineId;
	/// Type of finality proofs, used by consensus engine.
	type FinalityProof: FinalityProof<BlockNumberOf<C>> + Decode + Encode;
	/// Type of bridge pallet initialization data.
	type InitializationData: std::fmt::Debug + Send + Sync + 'static;

	/// Returns storage key at the bridged (target) chain that corresponds to the `bool` value,
	/// which is true when the bridge pallet is halted.
	fn is_halted_key() -> StorageKey;
	/// Returns storage at the bridged (target) chain that corresponds to some value that is
	/// missing from the storage until bridge pallet is initialized.
	///
	/// Note that we don't care about type of the value - just if it present or not.
	fn is_initialized_key() -> StorageKey;
	/// A method to subscribe to encoded finality proofs, given source client.
	async fn finality_proofs(client: Client<C>) -> Result<Subscription<Bytes>, SubstrateError>;
	/// Prepare initialization data for the finality bridge pallet.
	async fn prepare_initialization_data(
		client: Client<C>,
	) -> Result<Self::InitializationData, Error<HashOf<C>, BlockNumberOf<C>>>;
}

/// GRANDPA finality engine.
pub struct Grandpa<C>(PhantomData<C>);

impl<C: ChainWithGrandpa> Grandpa<C> {
	/// Read header by hash from the source client.
	async fn source_header(
		source_client: &Client<C>,
		header_hash: C::Hash,
	) -> Result<C::Header, Error<HashOf<C>, BlockNumberOf<C>>> {
		source_client
			.header_by_hash(header_hash)
			.await
			.map_err(|err| Error::RetrieveHeader(C::NAME, header_hash, err))
	}

	/// Read GRANDPA authorities set at given header.
	async fn source_authorities_set(
		source_client: &Client<C>,
		header_hash: C::Hash,
	) -> Result<GrandpaAuthoritiesSet, Error<HashOf<C>, BlockNumberOf<C>>> {
		let raw_authorities_set = source_client
			.grandpa_authorities_set(header_hash)
			.await
			.map_err(|err| Error::RetrieveAuthorities(C::NAME, header_hash, err))?;
		GrandpaAuthoritiesSet::decode(&mut &raw_authorities_set[..])
			.map_err(|err| Error::DecodeAuthorities(C::NAME, header_hash, err))
	}
}

#[async_trait]
impl<C: ChainWithGrandpa> Engine<C> for Grandpa<C> {
	const ID: ConsensusEngineId = sp_finality_grandpa::GRANDPA_ENGINE_ID;
	type FinalityProof = GrandpaJustification<HeaderOf<C>>;
	type InitializationData = bp_header_chain::InitializationData<C::Header>;

	fn is_halted_key() -> StorageKey {
		bp_header_chain::storage_keys::is_halted_key(C::WITH_CHAIN_GRANDPA_PALLET_NAME)
	}

	fn is_initialized_key() -> StorageKey {
		bp_header_chain::storage_keys::best_finalized_hash_key(C::WITH_CHAIN_GRANDPA_PALLET_NAME)
	}

	async fn finality_proofs(client: Client<C>) -> Result<Subscription<Bytes>, SubstrateError> {
		client.subscribe_grandpa_justifications().await
	}

	/// Prepare initialization data for the GRANDPA verifier pallet.
	async fn prepare_initialization_data(
		source_client: Client<C>,
	) -> Result<Self::InitializationData, Error<HashOf<C>, BlockNumberOf<C>>> {
		// In ideal world we just need to get best finalized header and then to read GRANDPA
		// authorities set (`pallet_grandpa::CurrentSetId` + `GrandpaApi::grandpa_authorities()`) at
		// this header.
		//
		// But now there are problems with this approach - `CurrentSetId` may return invalid value.
		// So here we're waiting for the next justification, read the authorities set and then try
		// to figure out the set id with bruteforce.
		let justifications = source_client
			.subscribe_grandpa_justifications()
			.await
			.map_err(|err| Error::Subscribe(C::NAME, err))?;
		// Read next justification - the header that it finalizes will be used as initial header.
		let justification = justifications
			.next()
			.await
			.map_err(|e| Error::ReadJustification(C::NAME, e))
			.and_then(|justification| {
				justification.ok_or(Error::ReadJustificationStreamEnded(C::NAME))
			})?;

		// Read initial header.
		let justification: GrandpaJustification<C::Header> =
			Decode::decode(&mut &justification.0[..])
				.map_err(|err| Error::DecodeJustification(C::NAME, err))?;

		let (initial_header_hash, initial_header_number) =
			(justification.commit.target_hash, justification.commit.target_number);

		let initial_header = Self::source_header(&source_client, initial_header_hash).await?;
		log::trace!(target: "bridge", "Selected {} initial header: {}/{}",
			C::NAME,
			initial_header_number,
			initial_header_hash,
		);

		// Read GRANDPA authorities set at initial header.
		let initial_authorities_set =
			Self::source_authorities_set(&source_client, initial_header_hash).await?;
		log::trace!(target: "bridge", "Selected {} initial authorities set: {:?}",
			C::NAME,
			initial_authorities_set,
		);

		// If initial header changes the GRANDPA authorities set, then we need previous authorities
		// to verify justification.
		let mut authorities_for_verification = initial_authorities_set.clone();
		let scheduled_change = find_grandpa_authorities_scheduled_change(&initial_header);
		assert!(
			scheduled_change.as_ref().map(|c| c.delay.is_zero()).unwrap_or(true),
			"GRANDPA authorities change at {} scheduled to happen in {:?} blocks. We expect\
			regular hange to have zero delay",
			initial_header_hash,
			scheduled_change.as_ref().map(|c| c.delay),
		);
		let schedules_change = scheduled_change.is_some();
		if schedules_change {
			authorities_for_verification =
				Self::source_authorities_set(&source_client, *initial_header.parent_hash()).await?;
			log::trace!(
				target: "bridge",
				"Selected {} header is scheduling GRANDPA authorities set changes. Using previous set: {:?}",
				C::NAME,
				authorities_for_verification,
			);
		}

		// Now let's try to guess authorities set id by verifying justification.
		let mut initial_authorities_set_id = 0;
		let mut min_possible_block_number = C::BlockNumber::zero();
		let authorities_for_verification = VoterSet::new(authorities_for_verification.clone())
			.ok_or(Error::ReadInvalidAuthorities(C::NAME, authorities_for_verification))?;
		loop {
			log::trace!(
				target: "bridge", "Trying {} GRANDPA authorities set id: {}",
				C::NAME,
				initial_authorities_set_id,
			);

			let is_valid_set_id = verify_justification::<C::Header>(
				(initial_header_hash, initial_header_number),
				initial_authorities_set_id,
				&authorities_for_verification,
				&justification,
			)
			.is_ok();

			if is_valid_set_id {
				break
			}

			initial_authorities_set_id += 1;
			min_possible_block_number += One::one();
			if min_possible_block_number > initial_header_number {
				// there can't be more authorities set changes than headers => if we have reached
				// `initial_block_number` and still have not found correct value of
				// `initial_authorities_set_id`, then something else is broken => fail
				return Err(Error::GuessInitialAuthorities(C::NAME, initial_header_number))
			}
		}

		Ok(bp_header_chain::InitializationData {
			header: Box::new(initial_header),
			authority_list: initial_authorities_set,
			set_id: if schedules_change {
				initial_authorities_set_id + 1
			} else {
				initial_authorities_set_id
			},
			is_halted: false,
		})
	}
}