interface: Optimize the StakeHistory::get function

`StakeHistory::get` was performing a binary search for every requested
epoch.

Given that the stake history is contigous and ordered starting from the
latest epochs, we don't to binary search. We can derive the index by
looking up the latest element's epoch and subtracting the requested
epoch from it.

Author: vadorovsky

interface/src/stake_history.rs

@@ -66,10 +66,15 @@ impl std::ops::Add for StakeHistoryEntry {
 pub struct StakeHistory(Vec<(Epoch, StakeHistoryEntry)>);
 
 impl StakeHistory {
+    #[inline]
+    fn latest_epoch(&self) -> Option<&Epoch> {
+        self.first().map(|(epoch, _)| epoch)
+    }
+
     pub fn get(&self, epoch: Epoch) -> Option<&StakeHistoryEntry> {
-        self.binary_search_by(|probe| epoch.cmp(&probe.0))
-            .ok()
-            .map(|index| &self[index].1)
+        self.latest_epoch()
+            .and_then(|latest| latest.checked_sub(epoch))
+            .and_then(|index| self.0.get(index as usize).map(|(_, entry)| entry))
     }
 
     pub fn add(&mut self, epoch: Epoch, entry: StakeHistoryEntry) {
@@ -120,12 +125,15 @@ mod tests {
         assert_eq!(stake_history.len(), MAX_ENTRIES);
         assert_eq!(stake_history.iter().map(|entry| entry.0).min().unwrap(), 1);
         assert_eq!(stake_history.get(0), None);
-        assert_eq!(
-            stake_history.get(1),
-            Some(&StakeHistoryEntry {
-                activating: 1,
-                ..StakeHistoryEntry::default()
-            })
-        );
+        for i in 0..MAX_ENTRIES {
+            let epoch = (i + 1) as u64;
+            assert_eq!(
+                stake_history.get(epoch),
+                Some(&StakeHistoryEntry {
+                    activating: epoch,
+                    ..StakeHistoryEntry::default()
+                })
+            );
+        }
     }
 }