Implement token bucket algo

Author: Pavan-Rana

internal/algorithm/token_bucket.go

@@ -1 +1,47 @@
-// internal/algorithm/token_bucket.go
+package algorithm
+
+import (
+	"sync"
+	"time"
+)
+
+// TokenBucket kept alongside SlidingWindow to highlight the tradeoff discussion in docs/tradeoffs.md.
+type TokenBucket struct {
+	mu			 sync.Mutex
+	tokens		 float64
+	capacity	 float64
+	rate		 float64 // tokens per second
+	lastFillTime time.Time
+}
+
+func NewTokenBucet(capacity float64, ratePerSec float64) *TokenBucket {
+	return &TokenBucket{
+		tokens:		 capacity,
+		capacity:	 capacity,
+		rate:		 ratePerSec,
+		lastFillTime: time.Now(),
+	}
+}
+
+func (tb *TokenBucket) Allow() bool {
+	tb.mu.Lock()
+	defer tb.mu.Unlock()
+
+	now := time.Now()
+	elapsed := now.Sub(tb.lastFillTime).Seconds()
+	tb.tokens = minF(tb.capacity, tb.tokens+elapsed*tb.rate)
+	tb.lastFill = now
+
+	if tb.tokens < 1 {
+		return false
+	}
+	tb.tokens--
+	return true
+}
+
+func minF(a, b float64) float64 {
+	if a < b {
+		return a
+	}
+	return b
+}