// 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.

//! # DO NOT USE IN PRODUCTION
//!
//! The produced values do not fulfill the cryptographic requirements for random numbers.
//! Should not be used for high-stake production use-cases.
//!
//! # Randomness Pallet
//!
//! The Randomness Collective Flip pallet provides a [`random`](./struct.Module.html#method.random)
//! function that generates low-influence random values based on the block hashes from the previous
//! `81` blocks. Low-influence randomness can be useful when defending against relatively weak
//! adversaries. Using this pallet as a randomness source is advisable primarily in low-security
//! situations like testing.
//!
//! ## Public Functions
//!
//! See the [`Module`] struct for details of publicly available functions.
//!
//! ## Usage
//!
//! ### Prerequisites
//!
//! Import the Randomness Collective Flip pallet and derive your pallet's configuration trait from
//! the system trait.
//!
//! ### Example - Get random seed for the current block
//!
//! ```
//! use frame_support::traits::Randomness;
//!
//! #[frame_support::pallet]
//! pub mod pallet {
//!     use super::*;
//!     use frame_support::pallet_prelude::*;
//!     use frame_system::pallet_prelude::*;
//!
//!     #[pallet::pallet]
//!     pub struct Pallet<T>(_);
//!
//!     #[pallet::config]
//!     pub trait Config: frame_system::Config + pallet_insecure_randomness_collective_flip::Config {}
//!
//!     #[pallet::call]
//!     impl<T: Config> Pallet<T> {
//!         #[pallet::weight(0)]
//!         pub fn random_module_example(origin: OriginFor<T>) -> DispatchResult {
//!             let _random_value = <pallet_insecure_randomness_collective_flip::Pallet<T>>::random(&b"my context"[..]);
//!             Ok(())
//!         }
//!     }
//! }
//! # fn main() { }
//! ```

#![cfg_attr(not(feature = "std"), no_std)]

use safe_mix::TripletMix;

use codec::Encode;
use frame_support::{pallet_prelude::Weight, traits::Randomness};
use frame_system::pallet_prelude::BlockNumberFor;
use sp_runtime::traits::{Hash, Saturating};

const RANDOM_MATERIAL_LEN: u32 = 81;

fn block_number_to_index<T: Config>(block_number: BlockNumberFor<T>) -> usize {
	// on_initialize is called on the first block after genesis
	let index = (block_number - 1u32.into()) % RANDOM_MATERIAL_LEN.into();
	index.try_into().ok().expect("Something % 81 is always smaller than usize; qed")
}

pub use pallet::*;

#[frame_support::pallet]
pub mod pallet {
	use super::*;
	use frame_support::pallet_prelude::*;

	#[pallet::pallet]
	pub struct Pallet<T>(_);

	#[pallet::config]
	pub trait Config: frame_system::Config {}

	#[pallet::hooks]
	impl<T: Config> Hooks<BlockNumberFor<T>> for Pallet<T> {
		fn on_initialize(block_number: BlockNumberFor<T>) -> Weight {
			let parent_hash = <frame_system::Pallet<T>>::parent_hash();

			<RandomMaterial<T>>::mutate(|ref mut values| {
				if values.try_push(parent_hash).is_err() {
					let index = block_number_to_index::<T>(block_number);
					values[index] = parent_hash;
				}
			});

			T::DbWeight::get().reads_writes(1, 1)
		}
	}

	/// Series of block headers from the last 81 blocks that acts as random seed material. This
	/// is arranged as a ring buffer with `block_number % 81` being the index into the `Vec` of
	/// the oldest hash.
	#[pallet::storage]
	#[pallet::getter(fn random_material)]
	pub(super) type RandomMaterial<T: Config> =
		StorageValue<_, BoundedVec<T::Hash, ConstU32<RANDOM_MATERIAL_LEN>>, ValueQuery>;
}

impl<T: Config> Randomness<T::Hash, BlockNumberFor<T>> for Pallet<T> {
	/// This randomness uses a low-influence function, drawing upon the block hashes from the
	/// previous 81 blocks. Its result for any given subject will be known far in advance by anyone
	/// observing the chain. Any block producer has significant influence over their block hashes
	/// bounded only by their computational resources. Our low-influence function reduces the actual
	/// block producer's influence over the randomness, but increases the influence of small
	/// colluding groups of recent block producers.
	///
	/// WARNING: Hashing the result of this function will remove any low-influence properties it has
	/// and mean that all bits of the resulting value are entirely manipulatable by the author of
	/// the parent block, who can determine the value of `parent_hash`.
	fn random(subject: &[u8]) -> (T::Hash, BlockNumberFor<T>) {
		let block_number = <frame_system::Pallet<T>>::block_number();
		let index = block_number_to_index::<T>(block_number);

		let hash_series = <RandomMaterial<T>>::get();
		let seed = if !hash_series.is_empty() {
			// Always the case after block 1 is initialized.
			hash_series
				.iter()
				.cycle()
				.skip(index)
				.take(RANDOM_MATERIAL_LEN as usize)
				.enumerate()
				.map(|(i, h)| (i as i8, subject, h).using_encoded(T::Hashing::hash))
				.triplet_mix()
		} else {
			T::Hash::default()
		};

		(seed, block_number.saturating_sub(RANDOM_MATERIAL_LEN.into()))
	}
}

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

	use sp_core::H256;
	use sp_runtime::{traits::Header as _, BuildStorage};

	use frame_support::{
		derive_impl, parameter_types,
		traits::{OnInitialize, Randomness},
	};
	use frame_system::limits;

	type Block = frame_system::mocking::MockBlock<Test>;

	frame_support::construct_runtime!(
		pub enum Test
		{
			System: frame_system,
			CollectiveFlip: pallet_insecure_randomness_collective_flip,
		}
	);

	parameter_types! {
		pub BlockLength: limits::BlockLength = limits::BlockLength
			::max(2 * 1024);
	}

	#[derive_impl(frame_system::config_preludes::TestDefaultConfig)]
	impl frame_system::Config for Test {
		type Block = Block;
	}

	impl pallet_insecure_randomness_collective_flip::Config for Test {}

	fn new_test_ext() -> sp_io::TestExternalities {
		let t = frame_system::GenesisConfig::<Test>::default().build_storage().unwrap();
		t.into()
	}

	#[test]
	fn test_block_number_to_index() {
		for i in 1..1000 {
			assert_eq!((i - 1) as usize % 81, block_number_to_index::<Test>(i));
		}
	}

	fn setup_blocks(blocks: u64) {
		let mut parent_hash = System::parent_hash();

		for i in 1..(blocks + 1) {
			System::reset_events();
			System::initialize(&i, &parent_hash, &Default::default());
			CollectiveFlip::on_initialize(i);

			let header = System::finalize();
			parent_hash = header.hash();
			System::set_block_number(*header.number());
		}
	}

	#[test]
	fn test_random_material_partial() {
		new_test_ext().execute_with(|| {
			let genesis_hash = System::parent_hash();

			setup_blocks(38);

			let random_material = CollectiveFlip::random_material();

			assert_eq!(random_material.len(), 38);
			assert_eq!(random_material[0], genesis_hash);
		});
	}

	#[test]
	fn test_random_material_filled() {
		new_test_ext().execute_with(|| {
			let genesis_hash = System::parent_hash();

			setup_blocks(81);

			let random_material = CollectiveFlip::random_material();

			assert_eq!(random_material.len(), 81);
			assert_ne!(random_material[0], random_material[1]);
			assert_eq!(random_material[0], genesis_hash);
		});
	}

	#[test]
	fn test_random_material_filled_twice() {
		new_test_ext().execute_with(|| {
			let genesis_hash = System::parent_hash();

			setup_blocks(162);

			let random_material = CollectiveFlip::random_material();

			assert_eq!(random_material.len(), 81);
			assert_ne!(random_material[0], random_material[1]);
			assert_ne!(random_material[0], genesis_hash);
		});
	}

	#[test]
	fn test_random() {
		new_test_ext().execute_with(|| {
			setup_blocks(162);

			assert_eq!(System::block_number(), 162);
			assert_eq!(CollectiveFlip::random_seed(), CollectiveFlip::random_seed());
			assert_ne!(CollectiveFlip::random(b"random_1"), CollectiveFlip::random(b"random_2"));

			let (random, known_since) = CollectiveFlip::random_seed();

			assert_eq!(known_since, 162 - RANDOM_MATERIAL_LEN as u64);
			assert_ne!(random, H256::zero());
			assert!(!CollectiveFlip::random_material().contains(&random));
		});
	}
}
