Add UsingQueue.

miner/src/miner.rs

@@ -39,7 +39,6 @@ pub struct Miner {
 	gas_floor_target: RwLock<U256>,
 	author: RwLock<Address>,
 	extra_data: RwLock<Bytes>,
-
 }
 
 impl Default for Miner {

util/src/lib.rs

@@ -124,6 +124,7 @@ pub mod hash;
 pub mod bytes;
 pub mod rlp;
 pub mod misc;
+pub mod using_queue;
 mod json_aid;
 pub mod vector;
 pub mod sha3;
@@ -149,6 +150,7 @@ pub mod table;
 
 pub use common::*;
 pub use misc::*;
+pub use using_queue::*;
 pub use json_aid::*;
 pub use rlp::*;
 pub use hashdb::*;

util/src/using_queue.rs

@@ -0,0 +1,207 @@
+// Copyright 2015, 2016 Ethcore (UK) Ltd.
+// This file is part of Parity.
+
+// Parity 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.
+
+// Parity 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 Parity.  If not, see <http://www.gnu.org/licenses/>.
+//! Queue-like datastructure including notion of usage.
+
+/// Special queue-like datastructure that includes the notion of
+/// usage to avoid items that were queued but never used from making it into
+/// the queue.
+pub struct UsingQueue<T> where T: Clone {
+	/// Not yet being sealed by a miner, but if one asks for work, we'd prefer they do this.
+	pending: Option<T>,
+	/// Currently being sealed by miners.
+	in_use: Vec<T>,
+	/// The maximum allowable number of items in_use.
+	max_size: usize,
+}
+
+impl<T> UsingQueue<T> where T: Clone {
+	/// Create a new struct with a maximum size of `max_size`.
+	pub fn new(max_size: usize) -> UsingQueue<T> {
+		UsingQueue {
+			pending: None,
+			in_use: vec![],
+			max_size: max_size,
+		}
+	}
+
+	/// Return a reference to the item at the top of the queue (or `None` if the queue is empty);
+	/// it doesn't constitute noting that the item is used.
+	pub fn peek_last_ref(&self) -> Option<&T> {
+		self.pending.as_ref().or(self.in_use.last())
+	}
+	
+	/// Return a reference to the item at the top of the queue (or `None` if the queue is empty);
+	/// this constitutes using the item and will remain in the queue for at least another
+	/// `max_size` invocations of `push()`.
+	pub fn use_last_ref(&mut self) -> Option<&T> {
+		if let Some(x) = self.pending.take() {
+			self.in_use.push(x);
+			if self.in_use.len() > self.max_size {
+				self.in_use.remove(0);
+			}
+		}
+		self.in_use.last()
+	}
+
+	/// Place an item on the end of the queue. The previously `push()`ed item will be removed
+	/// if `use_last_ref()` since it was `push()`ed.
+	pub fn push(&mut self, b: T) {
+		self.pending = Some(b);
+	}
+
+	/// Clears everything; the queue is entirely reset.
+	pub fn reset(&mut self) {
+		self.pending = None;
+		self.in_use.clear();
+	}
+
+	/// Returns `Some` reference to first block that `f` returns `true` with it as a parameter
+	/// or `None` if no such block exists in the queue.
+	pub fn find_if<P>(&self, predicate: P) -> Option<&T> where P: Fn(&T) -> bool {
+		if self.pending.as_ref().map(|r| predicate(r)).unwrap_or(false) {
+			self.pending.as_ref()
+		} else {
+			self.in_use.iter().find(|r| predicate(r))
+		}
+	}
+
+	/// Returns the most recently pushed block if `f` returns `true` with a reference to it as
+	/// a parameter, otherwise `None`.
+	/// Will not destroy a block if a reference to it has previously been returned by `use_last_ref`,
+	/// but rather clone it.
+	pub fn pop_if<P>(&mut self, predicate: P) -> Option<T> where P: Fn(&T) -> bool {
+		// a bit clumsy - TODO: think about a nicer way of expressing this.
+		if let Some(x) = self.pending.take() {
+			if predicate(&x) {
+				Some(x)
+			} else {
+				self.pending = Some(x);
+				None
+			}
+		} else {
+			self.in_use.last().into_iter().filter(|x| predicate(x)).next().cloned()
+		}
+	}
+}
+
+#[test]
+fn should_find_when_pushed() {
+	let mut q = UsingQueue::new(2);
+	q.push(1);
+	assert!(q.find_if(|i| i == &1).is_some());
+}
+
+#[test]
+fn should_find_when_pushed_and_used() {
+	let mut q = UsingQueue::new(2);
+	q.push(1);
+	q.use_last_ref();
+	assert!(q.find_if(|i| i == &1).is_some());
+}
+
+#[test]
+fn should_find_when_others_used() {
+	let mut q = UsingQueue::new(2);
+	q.push(1);
+	q.use_last_ref();
+	q.push(2);
+	q.use_last_ref();
+	assert!(q.find_if(|i| i == &1).is_some());
+}
+
+#[test]
+fn should_not_find_when_too_many_used() {
+	let mut q = UsingQueue::new(1);
+	q.push(1);
+	q.use_last_ref();
+	q.push(2);
+	q.use_last_ref();
+	assert!(q.find_if(|i| i == &1).is_none());
+}
+
+#[test]
+fn should_not_find_when_not_used_and_then_pushed() {
+	let mut q = UsingQueue::new(3);
+	q.push(1);
+	q.push(2);
+	q.use_last_ref();
+	assert!(q.find_if(|i| i == &1).is_none());
+}
+
+#[test]
+fn should_peek_correctly_after_push() {
+	let mut q = UsingQueue::new(3);
+	q.push(1);
+	assert_eq!(q.peek_last_ref(), Some(&1));
+	q.push(2);
+	assert_eq!(q.peek_last_ref(), Some(&2));
+}
+
+#[test]
+fn should_inspect_correctly() {
+	let mut q = UsingQueue::new(3);
+	q.push(1);
+	assert_eq!(q.use_last_ref(), Some(&1));
+	assert_eq!(q.peek_last_ref(), Some(&1));
+	q.push(2);
+	assert_eq!(q.use_last_ref(), Some(&2));
+	assert_eq!(q.peek_last_ref(), Some(&2));
+}
+
+#[test]
+fn should_not_find_when_not_used_peeked_and_then_pushed() {
+	let mut q = UsingQueue::new(3);
+	q.push(1);
+	q.peek_last_ref();
+	q.push(2);
+	q.use_last_ref();
+	assert!(q.find_if(|i| i == &1).is_none());
+}
+
+#[test]
+fn should_pop_used() {
+	let mut q = UsingQueue::new(3);
+	q.push(1);
+	q.use_last_ref();
+	let popped = q.pop_if(|i| i == &1);
+	assert_eq!(popped, Some(1));
+}
+
+#[test]
+fn should_pop_unused() {
+	let mut q = UsingQueue::new(3);
+	q.push(1);
+	assert_eq!(q.pop_if(|i| i == &1), Some(1));
+	assert_eq!(q.pop_if(|i| i == &1), None);
+}
+
+#[test]
+fn should_not_pop_unused_before_used() {
+	let mut q = UsingQueue::new(3);
+	q.push(1);
+	q.push(2);
+	let popped = q.pop_if(|i| i == &1);
+	assert_eq!(popped, None);
+}
+
+#[test]
+fn should_not_remove_used_popped() {
+	let mut q = UsingQueue::new(3);
+	q.push(1);
+	q.use_last_ref();
+	assert_eq!(q.pop_if(|i| i == &1), Some(1));
+	assert_eq!(q.pop_if(|i| i == &1), Some(1));
+}