Version update

- Error handling of time moving backward
- Documentation update

README.md

@@ -98,6 +98,12 @@ It would give something like:
 <Buffer 50 dd d6 49 f0 00 00 00>
 ```
 
+### Counter overflow ###
+Flake ID Generator can generate up to 4096 unique identifiers within a millisecond. When generator tries to generate more than 4096 identifiers within a millisecond, the following things will happen:
+* When using `next()` without a callback function, an error is thrown.
+* When using `next(cb)` with a callback function, the callback function is called in the following millisecond without any error.
+
+### Additional generator setup parameters ###
 Flake Id generator constructor takes optional parameter (generator configuration options) with the following properties:
 * `datacenter` (5 bit) - datacenter identifier. It can have values from 0 to 31.
 * `worker` (5 bit) - worker identifier. It can have values from 0 to 31.
@@ -182,7 +188,12 @@ undefined
 0
 ```
 
-### Formatting ###
+### Clock moving backward ###
+From time to time Node.js clock may move backward. In most cases it is only a few millisecond. However, as the generator relies on current timestamp, it won't be able to generate conflict-free identifiers (i.e. without duplicates) until the clock catches up with the last timestamp value. In case of clock move backward the following things will happen:
+* When using `next()` without a callback function, an error is thrown.
+* When using `next(cb)` with a callback function, the callback function is called with a new identifier generated once the clock catches up with the last timestamp.
+
+## Formatting ##
 
 Flake Id generator returns node Buffer representing 64-bit number for the sake of future extensions or returned buffer modifications. Node Buffer can also be very easily converted to string format. There is a NPM [biguint-format](https://npmjs.org/package/biguint-format) module which provides Buffer to string conversion functionality e.g.
 

flake-id-gen.js

@@ -64,6 +64,13 @@
             var time = Date.now() - this.epoch;
 
             // Generates id in the same millisecond as the previous id
+            if (time < this.lastTime) {
+                if (cb) {
+                    setTimeout(self.next.bind(self, cb), this.lastTime - time);
+                    return;
+                }
+                throw new Error(`Clock moved backwards. Refusing to generate id for ${this.lastTime - time} milliseconds`);
+            }
             if (time === this.lastTime) {
 
                 // If all sequence values (4096 unique values including 0) have been used

package.json

@@ -1,6 +1,6 @@
 {
   "name": "flake-idgen",
-  "version": "1.3.0",
+  "version": "1.4.0",
   "description": "Flake ID generator yields k-ordered, conflict-free ids in a distributed environment",
   "main": "flake-id-gen.js",
   "scripts": {