Parachains runtime improvements

runtime/parachains/src/paras_inherent/misc.rs

@@ -0,0 +1,40 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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 Polkadot.  If not, see <http://www.gnu.org/licenses/>.
+
+use sp_std::vec::Vec;
+
+/// A helper trait to allow calling retain while getting access
+/// to the index of the item in the `vec`.
+pub trait IndexedRetain<T> {
+	/// Retains only the elements specified by the predicate.
+	///
+	/// In other words, remove all elements `e` residing at
+	/// index `i` such that `f(i, &e)` returns `false`. This method
+	/// operates in place, visiting each element exactly once in the
+	/// original order, and preserves the order of the retained elements.
+	fn indexed_retain(&mut self, f: impl FnMut(usize, &T) -> bool);
+}
+
+impl<T> IndexedRetain<T> for Vec<T> {
+	fn indexed_retain(&mut self, mut f: impl FnMut(usize, &T) -> bool) {
+		let mut idx = 0_usize;
+		self.retain(move |item| {
+			let ret = f(idx, item);
+			idx += 1_usize;
+			ret
+		})
+	}
+}