onasty @ fa8001ef7b978b20df8dba33a04af109a2b5b18e

// thanks to https://www.alexedwards.net/blog/how-to-rate-limit-http-requests

package ratelimit

import (

	"net/http"

	"sync"

	"time"

	"github.com/gin-gonic/gin"

	"golang.org/x/time/rate"

)

type (

	rateLimiter struct {

		mu sync.RWMutex

		visitors map[visitorIP]*visitor

		// limit is the maximum number of requests per second

		limit rate.Limit

		// ttl is the time after which a visitor is forgotten

		ttl time.Duration

		// burst is the maximum number of requests that can be made in a short amount of time

		burst int

	}

	visitorIP string

	visitor   struct {

		limiter  *rate.Limiter

		lastSeen time.Time

	}

)

func newLimiter(rps, burst int, ttl time.Duration) *rateLimiter {

	return &rateLimiter{ //nolint:exhaustruct

		visitors: make(map[visitorIP]*visitor),

		limit:    rate.Limit(rps),

		burst:    burst,

		ttl:      ttl,

	}

}

// Retrieve and return the rate limiter for the current visitor if it

// already exists. Otherwise create a new rate limiter and add it to

// the visitors map, using the IP address as the key.

func (r *rateLimiter) getVisitor(ip visitorIP) *rate.Limiter {

	r.mu.RLock()

	v, exists := r.visitors[ip]

	r.mu.RUnlock()

	if !exists {

		limit := rate.NewLimiter(r.limit, r.burst)

		r.mu.Lock()

		r.visitors[ip] = &visitor{

			limiter:  limit,

			lastSeen: time.Now(),

		}

		r.mu.Unlock()

		return limit

	}

	r.mu.Lock()

	v.lastSeen = time.Now()

	r.mu.Unlock()

	return v.limiter

}

// Every minute check the map for visitors that haven't been seen for

// more than 3 minutes and delete the entries.

func (r *rateLimiter) cleanupVisitors() {

	for {

		time.Sleep(time.Minute)

		r.mu.Lock()

		for ip, v := range r.visitors {

			if time.Since(v.lastSeen) > r.ttl {

				delete(r.visitors, ip)

			}

		}

		r.mu.Unlock()

	}

}

type Config struct {

	// RPS is the maximum number of requests per second

	RPS int

	// TTL is the time after which a visitor is forgotten

	TTL time.Duration

	// Burst is the maximum number of requests that can be made in a short amount of time

	Burst int

}

// MiddlewareWithConfig returns a new rate limiting middleware with the given config

func MiddlewareWithConfig(c Config) gin.HandlerFunc {

	lmt := newLimiter(c.RPS, c.Burst, c.TTL)

	go lmt.cleanupVisitors()

	return func(c *gin.Context) {

		visitor := lmt.getVisitor(visitorIP(c.ClientIP()))

		if visitor == nil {

			c.AbortWithStatus(http.StatusInternalServerError)

			return

		}

		if !visitor.Allow() {

			c.AbortWithStatus(http.StatusTooManyRequests)

			return

		}

		c.Next()

	}

}