Adding inverse exponential strategy

.gitignore

@@ -1 +1,2 @@
 node_modules/
+.vscode/

index.js

@@ -4,6 +4,7 @@
 var Backoff = require('./lib/backoff');
 var ExponentialBackoffStrategy = require('./lib/strategy/exponential');
 var FibonacciBackoffStrategy = require('./lib/strategy/fibonacci');
+var InverseExponentialBackoffStrategy = require('./lib/strategy/inverse_exponential');
 var FunctionCall = require('./lib/function_call.js');
 
 module.exports.Backoff = Backoff;
@@ -21,6 +22,11 @@ module.exports.exponential = function(options) {
     return new Backoff(new ExponentialBackoffStrategy(options));
 };
 
+// Constructs an inverse exponential backoff.
+module.exports.inverseExponential = function(options) {
+    return new Backoff(new InverseExponentialBackoffStrategy(options));
+};
+
 // Constructs a FunctionCall for the given function and arguments.
 module.exports.call = function(fn, vargs, callback) {
     var args = Array.prototype.slice.call(arguments);

lib/strategy/inverse_exponential.js

@@ -0,0 +1,38 @@
+var util = require('util');
+var precond = require('precond');
+
+var BackoffStrategy = require('./strategy');
+
+// Inverse Exponential backoff strategy.
+function InverseExponentialBackoffStrategy (options) {
+  BackoffStrategy.call(this, options);
+  this.backoffDelay_ = 512;
+  this.nextBackoffDelay_ = this.getInitialDelay();
+  this.factor_ = InverseExponentialBackoffStrategy.DEFAULT_FACTOR;
+
+  if (options && options.factor !== undefined) {
+    precond.checkArgument(options.factor > 1,
+      'Exponential factor should be greater than 1 but got %s.',
+      options.factor);
+    this.factor_ = options.factor;
+  }
+}
+util.inherits(InverseExponentialBackoffStrategy, BackoffStrategy);
+
+// Default division factor used to compute the next backoff delay from
+// the current one. The value can be overridden by passing a custom factor as
+// part of the options.
+InverseExponentialBackoffStrategy.DEFAULT_FACTOR = 2;
+
+InverseExponentialBackoffStrategy.prototype.next_ = function () {
+  this.backoffDelay_ = Math.max(this.nextBackoffDelay_, 0);
+  this.nextBackoffDelay_ = parseInt(this.backoffDelay_ / this.factor_);
+  return this.backoffDelay_;
+};
+
+InverseExponentialBackoffStrategy.prototype.reset_ = function () {
+  this.backoffDelay_ = 512;
+  this.nextBackoffDelay_ = this.getInitialDelay();
+};
+
+module.exports = InverseExponentialBackoffStrategy;

package.json

@@ -1,32 +1,38 @@
 {
-    "name": "backoff",
-    "description": "Fibonacci and exponential backoffs.",
-    "version": "2.5.0",
-    "license": "MIT",
-    "author": "Mathieu Turcotte <[email protected]>",
-    "keywords": ["backoff", "retry", "fibonacci", "exponential"],
-    "repository": {
-        "type": "git",
-        "url": "https://github.com/MathieuTurcotte/node-backoff.git"
-    },
-    "dependencies": {
-        "precond": "0.2"
-    },
-    "devDependencies": {
-        "sinon": "1.10",
-        "nodeunit": "0.9"
-    },
-    "scripts": {
-        "docco" : "docco lib/*.js lib/strategy/* index.js",
-        "pretest": "jshint lib/ tests/ examples/ index.js",
-        "test": "node_modules/nodeunit/bin/nodeunit tests/"
-    },
-    "engines": {
-        "node": ">= 0.6"
-    },
-    "files": [
-        "index.js",
-        "lib",
-        "tests"
-    ]
+  "name": "backoff",
+  "description": "Fibonacci and exponential backoffs.",
+  "version": "2.5.0",
+  "license": "MIT",
+  "author": "Mathieu Turcotte <[email protected]>",
+  "keywords": [
+    "backoff",
+    "retry",
+    "fibonacci",
+    "exponential"
+  ],
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/MathieuTurcotte/node-backoff.git"
+  },
+  "dependencies": {
+    "precond": "0.2"
+  },
+  "devDependencies": {
+    "jshint": "2.9.5",
+    "nodeunit": "0.9",
+    "sinon": "1.10"
+  },
+  "scripts": {
+    "docco": "docco lib/*.js lib/strategy/* index.js",
+    "pretest": "jshint lib/ tests/ examples/ index.js",
+    "test": "node_modules/nodeunit/bin/nodeunit tests/"
+  },
+  "engines": {
+    "node": ">= 0.6"
+  },
+  "files": [
+    "index.js",
+    "lib",
+    "tests"
+  ]
 }

tests/inverse_exponential_backoff_strategy.js

@@ -0,0 +1,58 @@
+/*
+ * 
+ * Licensed under the MIT license.
+ */
+
+var sinon = require('sinon');
+
+var InverseExponentialBackoffStrategy = require('../lib/strategy/inverse_exponential');
+
+exports["InverseExponentialBackoffStrategy"] = {
+
+    "backoff delays should follow an inverse exponential sequence": function(test) {
+        var strategy = new InverseExponentialBackoffStrategy({
+            initialDelay: 512
+        });
+
+        // sequence: x[i] = x[i-1] / 2.
+        var expectedDelays = [512, 256, 128, 64, 32, 16, 8, 4, 2, 1, 0];
+        var actualDelays = expectedDelays.map(function () {
+            return strategy.next();
+        });
+
+        test.deepEqual(expectedDelays, actualDelays,
+            'Generated delays should follow an exponential sequence.');
+        test.done();
+    },
+
+    "backoff delay factor should be configurable": function (test) {
+        var strategy = new InverseExponentialBackoffStrategy({
+            initialDelay: 1024,
+            factor: 4
+        });
+
+        // Exponential sequence: x[i] = x[i-1] / 4.
+        var expectedDelays = [1024, 256, 64, 16, 4, 1, 0];
+        var actualDelays = expectedDelays.map(function () {
+            return strategy.next();
+        });
+
+        test.deepEqual(expectedDelays, actualDelays,
+            'Generated delays should follow a configurable exponential sequence.');
+        test.done();
+    },
+
+    "backoff delays should restart from the initial delay after reset": function(test) {
+        var strategy = new InverseExponentialBackoffStrategy({
+            initialDelay: 512
+        });
+
+        strategy.next();
+        strategy.reset();
+
+        var backoffDelay = strategy.next();
+        test.equals(backoffDelay, 512,
+            'Strategy should return the initial delay after reset.');
+        test.done();
+    }
+};