Create SignedDuration struct

for signed `Duration` values.

Suggested-by: Bastian Köcher

core/consensus/aura/src/lib.rs

@@ -63,7 +63,8 @@ use srml_aura::{
 };
 use substrate_telemetry::{telemetry, CONSENSUS_TRACE, CONSENSUS_DEBUG, CONSENSUS_WARN, CONSENSUS_INFO};
 
-use slots::{CheckedHeader, SlotData, SlotWorker, SlotInfo, SlotCompatible, slot_now, check_equivocation};
+use slots::{CheckedHeader, SlotData, SlotWorker, SlotInfo, SlotCompatible};
+use slots::{SignedDuration, check_equivocation};
 
 pub use aura_primitives::*;
 pub use consensus_common::SyncOracle;
@@ -283,7 +284,7 @@ impl<H, B, C, E, I, P, Error, SO> SlotWorker<B> for AuraWorker<C, E, I, P, SO> w
 		Box::new(proposal_work.map(move |b| {
 			// minor hack since we don't have access to the timestamp
 			// that is actually set by the proposer.
-			let slot_after_building = slot_now(slot_duration, Default::default(), true);
+			let slot_after_building = SignedDuration::default().slot_now(slot_duration);
 			if slot_after_building != slot_num {
 				info!(
 					"Discarding proposal for slot {}; block production took too long",

core/consensus/babe/src/lib.rs

@@ -82,7 +82,7 @@ use futures::{Future, IntoFuture, future};
 use tokio_timer::Timeout;
 use log::{error, warn, debug, info, trace};
 
-use slots::{SlotWorker, SlotData, SlotInfo, SlotCompatible, slot_now};
+use slots::{SlotWorker, SlotData, SlotInfo, SlotCompatible, SignedDuration};
 
 pub use babe_primitives::AuthorityId;
 
@@ -337,7 +337,7 @@ impl<Hash, H, B, C, E, I, Error, SO> SlotWorker<B> for BabeWorker<C, E, I, SO> w
 		Box::new(proposal_work.map(move |b| {
 			// minor hack since we don't have access to the timestamp
 			// that is actually set by the proposer.
-			let slot_after_building = slot_now(slot_duration, Default::default(), true);
+			let slot_after_building = SignedDuration::default().slot_now(slot_duration);
 			if slot_after_building != slot_num {
 				info!(
 					target: "babe",
@@ -639,7 +639,7 @@ impl<B: Block, C> Verifier<B> for BabeVerifier<C> where
 				let mut timestamps = self.timestamps.lock();
 				// Remainder of this block is a critical section.
 				let num_timestamps = timestamps.1.len();
-				let { median_required_blocks, .. } = self.config.0;
+				let median_required_blocks = self.config.0.median_required_blocks;
 				if num_timestamps as u64 >= median_required_blocks && median_required_blocks > 0 {
 					let mut new_list: Vec<_> = timestamps.1.iter().map(|&(t, sl)| {
 							let offset: u128 = u128::from(self.config.get())
@@ -655,7 +655,7 @@ impl<B: Block, C> Verifier<B> for BabeVerifier<C> where
 					let &median = new_list
 						.get(num_timestamps / 2)
 						.expect("we have at least one timestamp, so this is a valid index; qed");
-					timestamps.1.clear()
+					timestamps.1.clear();
 					// Compute the (relative!) start time of the blockchain ―
 					// that is, the issue time of the genesis block that would
 					// give the values we observed.

core/consensus/slots/src/lib.rs

@@ -25,7 +25,7 @@
 mod slots;
 mod aux_schema;
 
-pub use slots::{slot_now, SlotInfo, Slots};
+pub use slots::{SignedDuration, SlotInfo, Slots};
 pub use aux_schema::{check_equivocation, MAX_SLOT_CAPACITY, PRUNING_BOUND};
 
 use codec::{Decode, Encode};

core/consensus/slots/src/slots.rs

@@ -39,13 +39,28 @@ pub fn duration_now() -> Duration {
 	))
 }
 
-/// Get the slot for now.
-pub fn slot_now(slot_duration: u64, offset: Duration, is_positive: bool) -> u64 {
-	if is_positive {
-		duration_now() + offset
-	} else {
-		duration_now() - offset
-	}.as_secs() / slot_duration
+
+/// A `Duration` with a sign (before or after).  Immutable.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Default)]
+pub struct SignedDuration {
+	offset: Duration,
+	is_positive: bool,
+}
+
+impl SignedDuration {
+	/// Construct a `SignedDuration`
+	pub fn new(offset: Duration, is_positive: bool) -> Self {
+		Self { offset, is_positive }
+	}
+
+	/// Get the slot for now.  Panics if `slot_duration` is 0.
+	pub fn slot_now(&self, slot_duration: u64) -> u64 {
+		if self.is_positive {
+			duration_now() + self.offset
+		} else {
+			duration_now() - self.offset
+		}.as_secs() / slot_duration
+	}
 }
 
 /// Returns the duration until the next slot, based on current duration since