package backoff

import (
	"math"
	"time"
)

// Fibonacci grows interval by formula `value*F(N)`
// where `n` - number of attempts and `F` - Fibonacci function.
// Example (for 1 second interval):
// * 1st step - 1 second
// * 2nd step - 1 second
// * 3rd step - 2 second
// * 4th step - 3 second
// * 5th step - 5 second
// * ...
type Fibonacci struct{}

func (Fibonacci) Next(interval time.Duration, attempt int) time.Duration {
	if attempt > 93 {
		return math.MaxInt64
	}
	return interval * fibtable[attempt]
}

var fibtable = [93]time.Duration{
	0,
	1,
	1,
	2,
	3,
	5,
	8,
	13,
	21,
	34,
	55,
	89,
	144,
	233,
	377,
	610,
	987,
	1597,
	2584,
	4181,
	6765,
	10946,
	17711,
	28657,
	46368,
	75025,
	121393,
	196418,
	317811,
	514229,
	832040,
	1346269,
	2178309,
	3524578,
	5702887,
	9227465,
	14930352,
	24157817,
	39088169,
	63245986,
	102334155,
	165580141,
	267914296,
	433494437,
	701408733,
	1134903170,
	1836311903,
	2971215073,
	4807526976,
	7778742049,
	12586269025,
	20365011074,
	32951280099,
	53316291173,
	86267571272,
	139583862445,
	225851433717,
	365435296162,
	591286729879,
	956722026041,
	1548008755920,
	2504730781961,
	4052739537881,
	6557470319842,
	10610209857723,
	17167680177565,
	27777890035288,
	44945570212853,
	72723460248141,
	117669030460994,
	190392490709135,
	308061521170129,
	498454011879264,
	806515533049393,
	1304969544928657,
	2111485077978050,
	3416454622906707,
	5527939700884757,
	8944394323791464,
	14472334024676221,
	23416728348467685,
	37889062373143906,
	61305790721611591,
	99194853094755497,
	160500643816367088,
	259695496911122585,
	420196140727489673,
	679891637638612258,
	1100087778366101931,
	1779979416004714189,
	2880067194370816120,
	4660046610375530309,
	7540113804746346429,
	// Greater values overflows int64.
}