mirror of
https://github.com/pezkuwichain/pezkuwi-subxt.git
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Bastian Köcher
e3e651f72c
* Happy new year Updates the copyright years and fixes wrong license headers. * Fix the template * Split HEADER into HEADER-APACHE & HEADER-GPL
105 lines
3.5 KiB
Rust
105 lines
3.5 KiB
Rust
// This file is part of Substrate.
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// Copyright (C) 2017-2021 Parity Technologies (UK) Ltd.
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// SPDX-License-Identifier: Apache-2.0
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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//! A simple pseudo random number generator that allows a stream of random numbers to be efficiently
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//! created from a single initial seed hash.
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use codec::{Encode, Decode};
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use crate::traits::{Hash, TrailingZeroInput};
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/// Pseudo-random number streamer. This retains the state of the random number stream. It's as
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/// secure as the combination of the seed with which it is constructed and the hash function it uses
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/// to cycle elements.
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///
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/// It can be saved and later reloaded using the Codec traits.
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///
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/// Example:
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/// ```
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/// use sp_runtime::traits::{Hash, BlakeTwo256};
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/// use sp_runtime::RandomNumberGenerator;
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/// let random_seed = BlakeTwo256::hash(b"Sixty-nine");
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/// let mut rng = <RandomNumberGenerator<BlakeTwo256>>::new(random_seed);
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/// assert_eq!(rng.pick_u32(100), 59);
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/// assert_eq!(rng.pick_item(&[1, 2, 3]), Some(&1));
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/// ```
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///
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/// This can use any cryptographic `Hash` function as the means of entropy-extension, and avoids
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/// needless extensions of entropy.
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///
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/// If you're persisting it over blocks, be aware that the sequence will start to repeat. This won't
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/// be a practical issue unless you're using tiny hash types (e.g. 64-bit) and pulling hundred of
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/// megabytes of data from it.
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#[derive(Encode, Decode)]
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pub struct RandomNumberGenerator<Hashing: Hash> {
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current: Hashing::Output,
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offset: u32,
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}
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impl<Hashing: Hash> RandomNumberGenerator<Hashing> {
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/// A new source of random data.
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pub fn new(seed: Hashing::Output) -> Self {
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Self {
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current: seed,
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offset: 0,
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}
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}
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fn offset(&self) -> usize { self.offset as usize }
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/// Returns a number at least zero, at most `max`.
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pub fn pick_u32(&mut self, max: u32) -> u32 {
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let needed = (4 - max.leading_zeros() / 8) as usize;
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let top = ((1 << (needed as u64 * 8)) / ((max + 1) as u64) * ((max + 1) as u64) - 1) as u32;
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loop {
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if self.offset() + needed > self.current.as_ref().len() {
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// rehash
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self.current = <Hashing as Hash>::hash(self.current.as_ref());
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self.offset = 0;
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}
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let data = &self.current.as_ref()[self.offset()..self.offset() + needed];
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self.offset += needed as u32;
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let raw = u32::decode(&mut TrailingZeroInput::new(data)).unwrap_or(0);
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if raw <= top {
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break if max < u32::max_value() {
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raw % (max + 1)
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} else {
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raw
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}
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}
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}
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}
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/// Returns a number at least zero, at most `max`.
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///
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/// This returns a `usize`, but internally it only uses `u32` so avoid consensus problems.
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pub fn pick_usize(&mut self, max: usize) -> usize {
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self.pick_u32(max as u32) as usize
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}
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/// Pick a random element from an array of `items`.
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///
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/// This is guaranteed to return `Some` except in the case that the given array `items` is
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/// empty.
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pub fn pick_item<'a, T>(&mut self, items: &'a [T]) -> Option<&'a T> {
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if items.is_empty() {
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None
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} else {
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Some(&items[self.pick_usize(items.len() - 1)])
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}
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}
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}
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