Update slot clock to use genesis slot

eth2/utils/slot_clock/src/system_time_slot_clock.rs

@@ -13,6 +13,7 @@ pub enum Error {
 /// Determines the present slot based upon the present system time.
 #[derive(Clone)]
 pub struct SystemTimeSlotClock {
+    genesis_slot: Slot,
     genesis_seconds: u64,
     slot_duration_seconds: u64,
 }
@@ -22,6 +23,7 @@ impl SystemTimeSlotClock {
     ///
     /// Returns an Error if `slot_duration_seconds == 0`.
     pub fn new(
+        genesis_slot: Slot,
         genesis_seconds: u64,
         slot_duration_seconds: u64,
     ) -> Result<SystemTimeSlotClock, Error> {
@@ -29,6 +31,7 @@ impl SystemTimeSlotClock {
             Err(Error::SlotDurationIsZero)
         } else {
             Ok(Self {
+                genesis_slot,
                 genesis_seconds,
                 slot_duration_seconds,
             })
@@ -44,9 +47,11 @@ impl SlotClock for SystemTimeSlotClock {
         let duration_since_epoch = syslot_time.duration_since(SystemTime::UNIX_EPOCH)?;
         let duration_since_genesis =
             duration_since_epoch.checked_sub(Duration::from_secs(self.genesis_seconds));
+
         match duration_since_genesis {
             None => Ok(None),
-            Some(d) => Ok(slot_from_duration(self.slot_duration_seconds, d)),
+            Some(d) => Ok(slot_from_duration(self.slot_duration_seconds, d)
+                .and_then(|s| Some(s + self.genesis_slot))),
         }
     }
 }
@@ -74,6 +79,7 @@ mod tests {
     #[test]
     fn test_slot_now() {
         let slot_time = 100;
+        let genesis_slot = Slot::new(0);
 
         let now = SystemTime::now();
         let since_epoch = now.duration_since(SystemTime::UNIX_EPOCH).unwrap();
@@ -81,18 +87,21 @@ mod tests {
         let genesis = since_epoch.as_secs() - slot_time * 89;
 
         let clock = SystemTimeSlotClock {
+            genesis_slot,
             genesis_seconds: genesis,
             slot_duration_seconds: slot_time,
         };
         assert_eq!(clock.present_slot().unwrap(), Some(Slot::new(89)));
 
         let clock = SystemTimeSlotClock {
+            genesis_slot,
             genesis_seconds: since_epoch.as_secs(),
             slot_duration_seconds: slot_time,
         };
         assert_eq!(clock.present_slot().unwrap(), Some(Slot::new(0)));
 
         let clock = SystemTimeSlotClock {
+            genesis_slot,
             genesis_seconds: since_epoch.as_secs() - slot_time * 42 - 5,
             slot_duration_seconds: slot_time,
         };