Introduce efficient Hash-based RNG streamer (#4169)

* Introduce efficient Hash-based RNG streamer

* Loop in the case of being in bias space.

* Fix tests

* Minor cleanup

primitives/sr-primitives/src/lib.rs

@@ -52,6 +52,7 @@ pub mod offchain;
 pub mod testing;
 pub mod traits;
 pub mod transaction_validity;
+pub mod random_number_generator;
 
 /// Re-export these since they're only "kind of" generic.
 pub use generic::{DigestItem, Digest};
@@ -70,6 +71,8 @@ pub use arithmetic::helpers_128bit;
 /// Re-export big_uint stuff.
 pub use arithmetic::biguint;
 
+pub use random_number_generator::RandomNumberGenerator;
+
 /// An abstraction over justification for a block's validity under a consensus algorithm.
 ///
 /// Essentially a finality proof. The exact formulation will vary between consensus

primitives/sr-primitives/src/random_number_generator.rs

@@ -0,0 +1,103 @@
+// Copyright 2017-2019 Parity Technologies (UK) Ltd.
+// This file is part of Substrate.
+
+// Substrate 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.
+
+// Substrate 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 Substrate.  If not, see <http://www.gnu.org/licenses/>.
+
+//! A simple pseudo random number generator that allows a stream of random numbers to be efficiently
+//! created from a single initial seed hash.
+
+use codec::{Encode, Decode};
+use crate::traits::{Hash, TrailingZeroInput};
+
+/// Pseudo-random number streamer. This retains the state of the random number stream. It's as
+/// secure as the combination of the seed with which it is constructed and the hash function it uses
+/// to cycle elements.
+///
+/// It can be saved and later reloaded using the Codec traits.
+///
+/// Example:
+/// ```
+/// use sr_primitives::traits::{Hash, BlakeTwo256};
+/// use sr_primitives::RandomNumberGenerator;
+/// let random_seed = BlakeTwo256::hash(b"Sixty-nine");
+/// let mut rng = <RandomNumberGenerator<BlakeTwo256>>::new(random_seed);
+/// assert_eq!(rng.pick_u32(100), 59);
+/// assert_eq!(rng.pick_item(&[1, 2, 3]), Some(&1));
+/// ```
+///
+/// This can use any cryptographic `Hash` function as the means of entropy-extension, and avoids
+/// needless extensions of entropy.
+///
+/// If you're persisting it over blocks, be aware that the sequence will start to repeat. This won't
+/// be a practical issue unless you're using tiny hash types (e.g. 64-bit) and pulling hundred of
+/// megabytes of data from it.
+#[derive(Encode, Decode)]
+pub struct RandomNumberGenerator<Hashing: Hash> {
+	current: Hashing::Output,
+	offset: u32,
+}
+
+impl<Hashing: Hash> RandomNumberGenerator<Hashing> {
+	/// A new source of random data.
+	pub fn new(seed: Hashing::Output) -> Self {
+		Self {
+			current: seed,
+			offset: 0,
+		}
+	}
+
+	fn offset(&self) -> usize { self.offset as usize }
+
+	/// Returns a number at least zero, at most `max`.
+	pub fn pick_u32(&mut self, max: u32) -> u32 {
+		let needed = (4 - max.leading_zeros() / 8) as usize;
+		let top = ((1 << (needed as u64 * 8)) / ((max + 1) as u64) * ((max + 1) as u64) - 1) as u32;
+		loop {
+			if self.offset() + needed > self.current.as_ref().len() {
+				// rehash
+				self.current = Hashing::hash(self.current.as_ref());
+				self.offset = 0;
+			}
+			let data = &self.current.as_ref()[self.offset()..self.offset() + needed];
+			self.offset += needed as u32;
+			let raw = u32::decode(&mut TrailingZeroInput::new(data)).unwrap_or(0);
+			if raw <= top {
+				break if max < u32::max_value() {
+					raw % (max + 1)
+				} else {
+					raw
+				}
+			}
+		}
+	}
+
+	/// Returns a number at least zero, at most `max`.
+	///
+	/// This returns a `usize`, but internally it only uses `u32` so avoid consensus problems.
+	pub fn pick_usize(&mut self, max: usize) -> usize {
+		self.pick_u32(max as u32) as usize
+	}
+
+	/// Pick a random element from an array of `items`.
+	///
+	/// This is guaranteed to return `Some` except in the case that the given array `items` is
+	/// empty.
+	pub fn pick_item<'a, T>(&mut self, items: &'a [T]) -> Option<&'a T> {
+		if items.is_empty() {
+			None
+		} else {
+			Some(&items[self.pick_usize(items.len() - 1)])
+		}
+	}
+}

primitives/sr-primitives/src/traits.rs

@@ -1026,7 +1026,14 @@ pub trait OpaqueKeys: Clone {
 }
 
 /// Input that adds infinite number of zero after wrapped input.
-struct TrailingZeroInput<'a>(&'a [u8]);
+pub struct TrailingZeroInput<'a>(&'a [u8]);
+
+impl<'a> TrailingZeroInput<'a> {
+	/// Create a new instance from the given byte array.
+	pub fn new(data: &'a [u8]) -> Self {
+		Self(data)
+	}
+}
 
 impl<'a> codec::Input for TrailingZeroInput<'a> {
 	fn remaining_len(&mut self) -> Result<Option<usize>, codec::Error> {