Refactor rate limit functions for improved performance (#189)

Author: spring1843

dnc/rate_limit.go

@@ -1,15 +1,19 @@
 package dnc
 
 import (
+	"sort"
 	"time"
 )
 
 var rateLimitEvents []int64
 
-// IsAllowed solves the problem in O(1) time and O(n) space.
+// nanoSecond represents one second in nanoseconds.
+const nanoSecond = 1e9
+
+// IsAllowed solves the problem in O(log n) time and O(n) space.
 func IsAllowed(limitPerSecond int) bool {
-	now := time.Now().Unix()
-	removeOldERateLimitEvents(now)
+	now := time.Now().UnixNano()
+	removeOldRateLimitEvents(now)
 	if len(rateLimitEvents) >= limitPerSecond {
 		return false
 	}
@@ -18,10 +22,16 @@ func IsAllowed(limitPerSecond int) bool {
 	return true
 }
 
-func removeOldERateLimitEvents(now int64) {
-	if len(rateLimitEvents) == 0 || now <= rateLimitEvents[0] {
+// removeOldRateLimitEvents uses binary search to remove events older than 1 second.
+func removeOldRateLimitEvents(now int64) {
+	if len(rateLimitEvents) == 0 {
 		return
 	}
 
-	rateLimitEvents = []int64{}
+	cutoff := now - nanoSecond
+	idx := sort.Search(len(rateLimitEvents), func(i int) bool {
+		return rateLimitEvents[i] >= cutoff
+	})
+
+	rateLimitEvents = rateLimitEvents[idx:]
 }