accept backoff function in retry option

Closes #138

index.js

@@ -19,11 +19,6 @@ var nodeStatusCodes = require('node-status-codes');
 var isPlainObj = require('is-plain-obj');
 var parseJson = require('parse-json');
 
-function backoff(iter) {
-	var noise = Math.random() * 100;
-	return (1 << iter) * 1000 + noise;
-}
-
 function requestAsEventEmitter(opts) {
 	opts = opts || {};
 
@@ -61,8 +56,9 @@ function requestAsEventEmitter(opts) {
 		});
 
 		req.once('error', function (err) {
-			if (retryCount < opts.retries) {
-				setTimeout(get, backoff(++retryCount), opts);
+			var backoff = opts.retries(++retryCount);
+			if (backoff) {
+				setTimeout(get, backoff, opts);
 				return;
 			}
 
@@ -270,6 +266,18 @@ function normalizeArguments(url, opts) {
 		}
 	}
 
+	if (typeof opts.retries !== 'function') {
+		var retries = opts.retries;
+		opts.retries = function backoff(iter) {
+			if (iter > retries) {
+				return 0;
+			}
+
+			var noise = Math.random() * 100;
+			return (1 << iter) * 1000 + noise;
+		};
+	}
+
 	return opts;
 }
 

readme.md

@@ -118,11 +118,12 @@ Milliseconds after which the request will be aborted and an error event with `ET
 
 ###### retries
 
-Type: `number`  
+Type: `number`, `function`  
 Default: `5`
 
-Number of request retries when network errors happens.
+Number of request retries when network errors happens. Delays between retries counts with function `Math.pow(2, retry) + Math.random() * 100`, where `retry` is attempt number (starts from 0).
 
+Option accepts `function` that accepts `retry` argument and returns delay in milliseconds. `0` return value cancels retry.
 
 ##### callback(error, data, response)
 

test/retry.js

@@ -5,6 +5,7 @@ import {createServer} from './_server';
 let s;
 let trys = 0;
 let knocks = 0;
+let fifth = 0;
 
 test.before('setup', async t => {
 	s = await createServer();
@@ -21,6 +22,12 @@ test.before('setup', async t => {
 		trys++;
 	});
 
+	s.on('/fifth', (req, res) => {
+		if (fifth++ === 5) {
+			res.end('who`s there?');
+		}
+	});
+
 	await s.listen(s.port);
 });
 
@@ -38,6 +45,19 @@ test('can be disabled with option', async t => {
 	t.is(trys, 1);
 });
 
+test('funcion gets iter count', async t => {
+	await got(`${s.url}/fifth`, {timeout: 10, retries: iter => iter < 10});
+	t.is(fifth, 6);
+});
+
+test('falsy value prevent retries', async t => {
+	try {
+		await got(`${s.url}/long`, {timeout: 1000, retries: () => 0});
+	} catch (err) {
+		t.ok(err);
+	}
+});
+
 test.after('cleanup', async t => {
 	await s.close();
 });