// MinIO Cloud Storage, (C) 2020 MinIO, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//    http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package crypto

import (
	"math/rand"
	"time"
)

// default retry configuration
const (
	retryWaitMin = 500 * time.Millisecond // minimum retry limit.
	retryWaitMax = 3 * time.Second        // 3 secs worth of max retry.
)

// LinearJitterBackoff provides the time.Duration for a caller to
// perform linear backoff based on the attempt number and with jitter to
// prevent a thundering herd.
//
// min and max here are *not* absolute values. The number to be multiplied by
// the attempt number will be chosen at random from between them, thus they are
// bounding the jitter.
//
// For instance:
// * To get strictly linear backoff of one second increasing each retry, set
// both to one second (1s, 2s, 3s, 4s, ...)
// * To get a small amount of jitter centered around one second increasing each
// retry, set to around one second, such as a min of 800ms and max of 1200ms
// (892ms, 2102ms, 2945ms, 4312ms, ...)
// * To get extreme jitter, set to a very wide spread, such as a min of 100ms
// and a max of 20s (15382ms, 292ms, 51321ms, 35234ms, ...)
func LinearJitterBackoff(min, max time.Duration, attemptNum int) time.Duration {
	// attemptNum always starts at zero but we want to start at 1 for multiplication
	attemptNum++

	if max <= min {
		// Unclear what to do here, or they are the same, so return min *
		// attemptNum
		return min * time.Duration(attemptNum)
	}

	// Seed rand; doing this every time is fine
	rand := rand.New(rand.NewSource(int64(time.Now().Nanosecond())))

	// Pick a random number that lies somewhere between the min and max and
	// multiply by the attemptNum. attemptNum starts at zero so we always
	// increment here. We first get a random percentage, then apply that to the
	// difference between min and max, and add to min.
	jitter := rand.Float64() * float64(max-min)
	jitterMin := int64(jitter) + int64(min)
	return time.Duration(jitterMin * int64(attemptNum))
}