From eca6f180a3f882578e989f864bf4ff6a5c58199f Mon Sep 17 00:00:00 2001
From: Ian Gulliver <ian@flamingcow.tv>
Date: Sun, 26 Sep 2021 03:41:40 +0000
Subject: [PATCH] Add RateLimit

---
 client/ratelimit.go | 75 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 75 insertions(+)
 create mode 100644 client/ratelimit.go

diff --git a/client/ratelimit.go b/client/ratelimit.go
new file mode 100644
index 0000000..c7adf4c
--- /dev/null
+++ b/client/ratelimit.go
@@ -0,0 +1,75 @@
+package client
+
+import "sync"
+import "time"
+
+type RateLimit struct {
+	perSecond float64
+	balance   float64
+	limit     float64
+	updated   time.Time
+	mu        sync.Mutex
+}
+
+func NewRateLimit(perSecond, limit float64) *RateLimit {
+	return &RateLimit{
+		perSecond: perSecond,
+		balance:   limit,
+		limit:     limit,
+		updated:   time.Now(),
+	}
+}
+
+func NewRateLimitPerMinute(perMinute, limit float64) *RateLimit {
+	return NewRateLimit(perMinute/60, limit)
+}
+
+// Acquire sufficient rate quota to execute 1 operation
+func (rl *RateLimit) Acquire1() {
+	rl.AcquireN(1.0)
+}
+
+// Acquire sufficient rate quota to execute /cost/ operations
+func (rl *RateLimit) AcquireN(cost float64) {
+	for {
+		rl.mu.Lock()
+
+		rl.replenish()
+
+		if rl.balance >= cost {
+			rl.balance -= cost
+			rl.mu.Unlock()
+			return
+		}
+
+		costDelta := cost - rl.balance
+		sleep := time.Duration(costDelta / rl.perSecond * float64(time.Second))
+		rl.mu.Unlock()
+		time.Sleep(sleep)
+	}
+}
+
+// Handle a Retry-After server response
+func (rl *RateLimit) RetryAfter(seconds int64) {
+	rl.mu.Lock()
+	defer rl.mu.Unlock()
+
+	target := 1.0 - (float64(seconds) * rl.perSecond)
+	if target < rl.balance {
+		rl.balance = target
+	}
+}
+
+// Must be called with rl.mu already locked
+func (rl *RateLimit) replenish() {
+	now := time.Now()
+	timeDelta := now.Sub(rl.updated)
+	balanceDelta := timeDelta.Seconds() * rl.perSecond
+
+	rl.balance += balanceDelta
+	if rl.balance > rl.limit {
+		rl.balance = rl.limit
+	}
+
+	rl.updated = now
+}