// This file is part of Substrate.

// Copyright (C) Parity Technologies (UK) Ltd.

// SPDX-License-Identifier: Apache-2.0

// Licensed under the Apache License, Version 2.0 (the "License");

// you may not use this file except in compliance with the License.

// You may obtain a copy of the License at

//

// 	http://www.apache.org/licenses/LICENSE-2.0

//

// Unless required by applicable law or agreed to in writing, software

// distributed under the License is distributed on an "AS IS" BASIS,

// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

// See the License for the specific language governing permissions and

// limitations under the License.

//! Common traits and types that are useful for describing offences for usage in environments

//! that use staking.

use alloc::vec::Vec;

use codec::{Decode, Encode};

use sp_core::Get;

use sp_runtime::{transaction_validity::TransactionValidityError, DispatchError, Perbill};

use crate::SessionIndex;

/// The kind of an offence, is a byte string representing some kind identifier

/// e.g. `b"im-online:offlin"`, `b"babe:equivocatio"`

pub type Kind = [u8; 16];

/// Number of times the offence of this authority was already reported in the past.

///

/// Note that we don't buffer offence reporting, so every time we see a new offence

/// of the same kind, we will report past authorities again.

/// This counter keeps track of how many times the authority was already reported in the past,

/// so that we can slash it accordingly.

pub type OffenceCount = u32;

/// A trait implemented by an offence report.

///

/// This trait assumes that the offence is legitimate and was validated already.

///

/// Examples of offences include: a BABE equivocation or a GRANDPA unjustified vote.

pub trait Offence<Offender> {

	/// Identifier which is unique for this kind of an offence.

	const ID: Kind;

	/// A type that represents a point in time on an abstract timescale.

	///

	/// See `Offence::time_slot` for details. The only requirement is that such timescale could be

	/// represented by a single `u128` value.

	type TimeSlot: Clone + codec::Codec + Ord;

	/// The list of all offenders involved in this incident.

	///

	/// The list has no duplicates, so it is rather a set.

	fn offenders(&self) -> Vec<Offender>;

	/// The session index that is used for querying the validator set for the `slash_fraction`

	/// function.

	///

	/// This is used for filtering historical sessions.

	fn session_index(&self) -> SessionIndex;

	/// Return a validator set count at the time when the offence took place.

	fn validator_set_count(&self) -> u32;

	/// A point in time when this offence happened.

	///

	/// This is used for looking up offences that happened at the "same time".

	///

	/// The timescale is abstract and doesn't have to be the same across different implementations

	/// of this trait. The value doesn't represent absolute timescale though since it is interpreted

	/// along with the `session_index`. Two offences are considered to happen at the same time iff

	/// both `session_index` and `time_slot` are equal.

	///

	/// As an example, for GRANDPA timescale could be a round number and for BABE it could be a slot

	/// number. Note that for GRANDPA the round number is reset each epoch.

	fn time_slot(&self) -> Self::TimeSlot;

	/// A slash fraction of the total exposure that should be slashed for this

	/// particular offence for the `offenders_count` that happened at a singular `TimeSlot`.

	///

	/// `offenders_count` - the count of unique offending authorities for this `TimeSlot`. It is >0.

	fn slash_fraction(&self, offenders_count: u32) -> Perbill;

}

/// Errors that may happen on offence reports.

#[derive(PartialEq, sp_runtime::RuntimeDebug)]

pub enum OffenceError {

	/// The report has already been submitted.

	DuplicateReport,

	/// Other error has happened.

	Other(u8),

}

impl sp_runtime::traits::Printable for OffenceError {

	fn print(&self) {

		"OffenceError".print();

		match self {

			Self::DuplicateReport => "DuplicateReport".print(),

			Self::Other(e) => {

				"Other".print();

				e.print();

			},

		}

	}

}

/// A trait for decoupling offence reporters from the actual handling of offence reports.

pub trait ReportOffence<Reporter, Offender, O: Offence<Offender>> {

	/// Report an `offence` and reward given `reporters`.

	fn report_offence(reporters: Vec<Reporter>, offence: O) -> Result<(), OffenceError>;

	/// Returns true iff all of the given offenders have been previously reported

	/// at the given time slot. This function is useful to prevent the sending of

	/// duplicate offence reports.

	fn is_known_offence(offenders: &[Offender], time_slot: &O::TimeSlot) -> bool;

}

impl<Reporter, Offender, O: Offence<Offender>> ReportOffence<Reporter, Offender, O> for () {

	fn report_offence(_reporters: Vec<Reporter>, _offence: O) -> Result<(), OffenceError> {

		Ok(())

	}

	fn is_known_offence(_offenders: &[Offender], _time_slot: &O::TimeSlot) -> bool {

		true

	}

}

/// A trait to take action on an offence.

///

/// Used to decouple the module that handles offences and

/// the one that should punish for those offences.

pub trait OnOffenceHandler<Reporter, Offender, Res> {

	/// A handler for an offence of a particular kind.

	///

	/// Note that this contains a list of all previous offenders

	/// as well. The implementer should cater for a case, where

	/// the same authorities were reported for the same offence

	/// in the past (see `OffenceCount`).

	///

	/// The vector of `slash_fraction` contains `Perbill`s

	/// the authorities should be slashed and is computed

	/// according to the `OffenceCount` already. This is of the same length as `offenders.`

	/// Zero is a valid value for a fraction.

	///

	/// The `session` parameter is the session index of the offence.

	///

	/// The receiver might decide to not accept this offence. In this case, the call site is

	/// responsible for queuing the report and re-submitting again.

	fn on_offence(

		offenders: &[OffenceDetails<Reporter, Offender>],

		slash_fraction: &[Perbill],

		session: SessionIndex,

	) -> Res;

}

impl<Reporter, Offender, Res: Default> OnOffenceHandler<Reporter, Offender, Res> for () {

	fn on_offence(

		_offenders: &[OffenceDetails<Reporter, Offender>],

		_slash_fraction: &[Perbill],

		_session: SessionIndex,

	) -> Res {

		Default::default()

	}

}

/// A details about an offending authority for a particular kind of offence.

pub struct OffenceDetails<Reporter, Offender> {

	/// The offending authority id

	pub offender: Offender,

	/// A list of reporters of offences of this authority ID. Possibly empty where there are no

	/// particular reporters.

	pub reporters: Vec<Reporter>,

}

/// An abstract system to publish, check and process offence evidences.

///

/// Implementation details are left opaque and we don't assume any specific usage

/// scenario for this trait at this level. The main goal is to group together some

/// common actions required during a typical offence report flow.

///

/// Even though this trait doesn't assume too much, this is a general guideline

/// for a typical usage scenario:

///

/// 1. An offence is detected and an evidence is submitted on-chain via the

///    [`OffenceReportSystem::publish_evidence`] method. This will construct and submit an extrinsic

///    transaction containing the offence evidence.

///

/// 2. If the extrinsic is unsigned then the transaction receiver may want to perform some

///    preliminary checks before further processing. This is a good place to call the

///    [`OffenceReportSystem::check_evidence`] method.

///

/// 3. Finally the report extrinsic is executed on-chain. This is where the user calls the

///    [`OffenceReportSystem::process_evidence`] to consume the offence report and enact any

///    required action.

pub trait OffenceReportSystem<Reporter, Evidence> {

	/// Longevity, in blocks, for the evidence report validity.

	///

	/// For example, when using the staking pallet this should be set equal

	/// to the bonding duration in blocks, not eras.

	type Longevity: Get<u64>;

	/// Publish an offence evidence.

	///

	/// Common usage: submit the evidence on-chain via some kind of extrinsic.

	fn publish_evidence(evidence: Evidence) -> Result<(), ()>;

	/// Check an offence evidence.

	///

	/// Common usage: preliminary validity check before execution

	/// (e.g. for unsigned extrinsic quick checks).

	fn check_evidence(evidence: Evidence) -> Result<(), TransactionValidityError>;

	/// Process an offence evidence.

	///

	/// Common usage: enact some form of slashing directly or by forwarding

	/// the evidence to a lower level specialized subsystem (e.g. a handler

	/// implementing `ReportOffence` trait).

	fn process_evidence(reporter: Reporter, evidence: Evidence) -> Result<(), DispatchError>;

}

/// Dummy offence report system.

///

/// Doesn't do anything special and returns `Ok(())` for all the actions.

impl<Reporter, Evidence> OffenceReportSystem<Reporter, Evidence> for () {

	type Longevity = ();

	fn publish_evidence(_evidence: Evidence) -> Result<(), ()> {

		Ok(())

	}

	fn check_evidence(_evidence: Evidence) -> Result<(), TransactionValidityError> {

		Ok(())

	}

	fn process_evidence(_reporter: Reporter, _evidence: Evidence) -> Result<(), DispatchError> {

		Ok(())

	}

}