Allow slot clock to handle clock disparity (#929)

* Add PH & MS slot clock changes

* Account for genesis time

* Use checked mul

* Account for genesis slot

* Change API

* Refactor "duration to..." functions

eth2/utils/slot_clock/src/lib.rs

@@ -1,14 +1,15 @@
 #[macro_use]
 extern crate lazy_static;
 
+mod manual_slot_clock;
 mod metrics;
 mod system_time_slot_clock;
-mod testing_slot_clock;
 
 use std::time::Duration;
 
+pub use crate::manual_slot_clock::ManualSlotClock;
+pub use crate::manual_slot_clock::ManualSlotClock as TestingSlotClock;
 pub use crate::system_time_slot_clock::SystemTimeSlotClock;
-pub use crate::testing_slot_clock::TestingSlotClock;
 pub use metrics::scrape_for_metrics;
 pub use types::Slot;
 
@@ -16,13 +17,21 @@ pub use types::Slot;
 ///
 /// The clock is not required to be monotonically increasing and may go backwards.
 pub trait SlotClock: Send + Sync + Sized {
-    /// Creates a new slot clock where the first slot is `genesis_slot`, genesis occured
+    /// Creates a new slot clock where the first slot is `genesis_slot`, genesis occurred
     /// `genesis_duration` after the `UNIX_EPOCH` and each slot is `slot_duration` apart.
     fn new(genesis_slot: Slot, genesis_duration: Duration, slot_duration: Duration) -> Self;
 
     /// Returns the slot at this present time.
     fn now(&self) -> Option<Slot>;
 
+    /// Returns the present time as a duration since the UNIX epoch.
+    ///
+    /// Returns `None` if the present time is before the UNIX epoch (unlikely).
+    fn now_duration(&self) -> Option<Duration>;
+
+    /// Returns the slot of the given duration since the UNIX epoch.
+    fn slot_of(&self, now: Duration) -> Option<Slot>;
+
     /// Returns the duration between slots
     fn slot_duration(&self) -> Duration;
 
@@ -31,4 +40,18 @@ pub trait SlotClock: Send + Sync + Sized {
 
     /// Returns the duration until the first slot of the next epoch.
     fn duration_to_next_epoch(&self, slots_per_epoch: u64) -> Option<Duration>;
+
+    /// Returns the first slot to be returned at the genesis time.
+    fn genesis_slot(&self) -> Slot;
+
+    /// Returns the slot if the internal clock were advanced by `duration`.
+    fn now_with_future_tolerance(&self, tolerance: Duration) -> Option<Slot> {
+        self.slot_of(self.now_duration()?.checked_add(tolerance)?)
+    }
+
+    /// Returns the slot if the internal clock were reversed by `duration`.
+    fn now_with_past_tolerance(&self, tolerance: Duration) -> Option<Slot> {
+        self.slot_of(self.now_duration()?.checked_sub(tolerance)?)
+            .or_else(|| Some(self.genesis_slot()))
+    }
 }