# jshashtable 2.1

## Introduction

jshashtable is a JavaScript implementation of a hash table. It associates objects ("keys") with other objects ("values"). Each key is associated with precisely one value.

Version 2.0 of jshashtable features significant performance improvements over version 1.0 (typically 200% faster for `put` and `get` operations) and adds a few new methods: `clone()`, `putAll()`, `entries()` and `each()`. Version 2.1 is minor upgrade that allows use of ActiveX objects as keys in IE.

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## Set-up

1. ### Download the code

   Download jshashtable. You can download a compressed or uncompressed version of `jshashtable.js` which are functionally identical or a zip containing this document and both JavaScript files.

2. ### Include jshashtable in your page

   Include `jshashtable.js` in a script tag in your page. This file creates one object in the global scope, which is `Hashtable`.

3. ### Create your hash table

   Create your hash table, as in the example below. Any non-null JavaScript object can be used as a key or a value.

       <script type="text/javascript" src="jshashtable.js"></script>
       <script type="text/javascript">
           var typesHash = new Hashtable();

           typesHash.put("A string", "string");
           typesHash.put(1, "number");

           var o = new Object();
           typesHash.put(o, "object");

           alert( typesHash.get(o) ); // "object"
       </script>

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## Usage

jshashtable is a JavaScript implementation of a hash table. It associates objects ("keys") with other objects ("values"). Each key is associated with precisely one value.

The code is contained in one JavaScript file, which creates a single constructor function called `Hashtable` in the global scope.

### Doesn't JavaScript already do this?

No, although JavaScript's built-in objects do provide this functionality using the square brackets notation for properties, provided your keys are strings or numbers:

    var key = "A key";
    var o = new Object();
    o[key] = 1;

However, it is often desirable to use other kinds of objects as keys. JavaScript doesn't complain if you try to use objects other than strings and numbers inside square brackets; indeed, it superficially looks like this kind of association works for any object:

    var key = new Object();
    var o = new Object();
    o[key] = "First";
    alert( o[key] ); // Alerts "First"

It doesn't take much effort to discover the flaw in this approach:

    var key1 = new Object();
    var key2 = new Object();
    var o = new Object();
    o[key1] = "First";
    o[key2] = "Second";
    alert( o[key1] ); // Alerts "Second", not "First"

The reason for this is that JavaScript silently calls `toString()` on the object being passed in as the key and uses the result as the property name. In the example above, `key1` and `key2` both become "[object Object]", hence the second assignment simply replaces the original value of the property "[object Object]".

With jshashtable, any object can be used as the key:

    var key1 = new Object();
    var key2 = new Object();
    var h = new Hashtable();
    h.put(key1, "First");
    h.put(key2, "Second");
    alert( h.get(key1) ); // Alerts "First"
    alert( h.get(key2) ); // Alerts "Second"

The above examples shows use of `put()` to add a key/value pair to the hash table and `get()` to retrieve a value. If a value already exists in the hash table for the key used, it is replaced with the new value.

### Equality

This is all very well but there is a drawback. Imagine you have a hash table of colour values at various different positions on the screen. Each position on the screen is represented by a `Point` object, which has properties `x` and `y`, representing the x and y coordinates of the point on the screen.

    function Point(x, y) {
        this.x = x;
        this.y = y;
    }

    var coloursForPoints = new Hashtable();

    function getColourAt(x, y) {
        var point = new Point(x, y);
        return coloursForPoints.get(point);
    }

    coloursForPoints.put( new Point(1, 2), "green" );

    alert( getColourAt(1, 2) ); // Alerts null

Why do we get `null`? Because the `Point` object that gets created in `getColourAt` is not the self same object as the original `Point` that got used as key in the `coloursForPoints` hash table. This is clearly not the ideal behaviour - any two `Point` objects with the same x and y values are to all intents and purposes the same thing. What we need is a way of defining when two objects are equal. By default, jshashtable uses the strict equals (`===`) operator in JavaScript. However, if one of the objects being compared has an `equals` method, it will use that instead. We can implement an `equals` method in the above example to get the behaviour we want:

    function Point(x, y) {
        this.x = x;
        this.y = y;
    }

    Point.prototype.equals = function(obj) {
        return (obj instanceof Point) &&
            (obj.x === this.x) &&
            (obj.y === this.y);
    };

    var coloursForPoints = new Hashtable();

    function getColourAt(x, y) {
        var point = new Point(x, y);
        return coloursForPoints.get(point);
    }

    coloursForPoints.put( new Point(1, 2), "green" );

    alert( getColourAt(1, 2) ); // Alerts "green"

### Hash codes

This works but is still not quite ideal. Internally, jshashtable stores key/values pairs in arrays, called buckets. When `put()` is called, jshashtable converts the key into a hash code, and stores the key/value pair in the bucket for that particular hash code. A hash code in jshashtable is a string so that the buckets themselves can be associated with hash codes using an object and JavaScript's built-in string property names. When `get()` is called, jshashtable finds the correct bucket for the key it's looking for and then searches the contents of that bucket for that key. Since the process of locating the correct bucket is massively faster than searching through a bucket's contents, it is most efficient to have as many buckets as possible containing the least possible number of items (ideally one). My tests have shown that for a hash table with 1000 elements, it is around 70 times faster to replace or retrieve a value if each key has a unique hash code than if each key had the same hash code (tested using objects with a very simple `hashCode` method). For 10000 elements, it's closer to 300 times faster. So, generating meaningful hash codes for keys makes the hash table much more efficient.

jshashtable generates a hash code for an object by checking to see if it has a `hashCode()` method and using that if it exists. Otherwise it calls `toString()` on the object, like JavaScript's built-in square bracket property accessor does. In the above example, `Point` has no `hashCode` method, so every point placed in `coloursForPoints` goes into the "[object Object]" bucket, which is very inefficient, particularly with a large number of points. What would be better would be to implement a `hashCode()` method on `Point` that returns a different value for every distinct point but the same value for equal points (this is very important: objects that are considered equal according to their `equals` method must return the same hash code). Something like the following would be good:

    Point.prototype.hashCode = function(obj) {
        return "Point:" + this.x + "," + this.y;
    };

So for our example point, this returns "Point:1,2", for any `Point` object with x and y coordinates of 1 and 2 respectively. Every point will therefore go in its own bucket and the hash table is efficient.

The "Point" at the start of the hash code is there to distinguish points from any other similar object that may have x and y coordinates and potentially have the same hash code. Note that even if a non-Point object did return a hash code of "Point:1,2", the hash table would still work fine, since the non-Point object would fail the equality test when searching through the "Point:1,2" bucket.

If you don't want to violate the keys of your hash table by giving them extra methods like `equals` and `hashCode`, you can instead pass in functions into the `Hashtable` constructor to generate hash codes and test key equality. The hash code generator function is passed an object and should return a hash code for that object, while the equality function is passed two objects and should return a boolean value representing whether the two objects are equal. This also has the advantage of improving performance, since the hash table does not have to check for the existence of `equals` and `hashCode` methods on every key object.

For a hash table where you knew in advance that all the keys would be `Point` objects, the previous example could be rewritten to be more efficient as follows:

    function Point(x, y) {
        this.x = x;
        this.y = y;
    }

    function hashPoint(p) {
        return "Point:" + p.x + "," + p.y;
    }

    function pointsEqual(p1, p2) {
        return p1.x === p2.x && p1.y === p2.y;
    }

    var coloursForPoints = new Hashtable(hashPoint, pointsEqual);

    function getColourAt(x, y) {
        var point = new Point(x, y);
        return coloursForPoints.get(point);
    }

    coloursForPoints.put( new Point(1, 2), "green" );

    alert( getColourAt(1, 2) ); // Alerts green

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## Public API

#### Constructors

- `Hashtable()`

  Creates a new, empty hash table.

- `Hashtable(Function hashingFunction, Function equalityFunction)`
  Parameters:
  - `hashingFunction`
    A function that provides hash codes for keys placed in the hash table. It is passed the object to be hashed as its only parameter. If not provided, the hash table checks whether the object has a `hashCode` method, and if not, calls `toString` on the object.
  - `equalityFunction`
    A function that checks for equality between two keys with the same hash code. Two keys that are considered equal will map to the same value in the hash table. This function is passed the two objects to be compared as its parameters. If not provided, the hash table checks whether either object being compared has an `equals` method, and if not, compares the objects using the `===`operator.

#### Methods

- `void put(Object key, Object value)`

  Updated in version 2.0: now returns previous value associated with the key

  Sets the value associated with the key supplied. If the hash table already contains the key then the old value is overwritten and the old value is returned, otherwise `null` is returned.

- `void putAll(Hashtable hashtable[, Function conflictCallback])`

  New in version 2.0

  Adds all entries from the supplied hash table to this hash table. For any key in the supplied hash table for which an entry already exists in this hash table, the optional callback function `conflictCallback` is called to resolve the conflict. This function should accept three parameters:

  - `key`: the key for the conflicting entry;
  - `thisValue`: the current value for this key in the current hash table;
  - `value`: the value for this key in the hash table supplied.

  The value returned by the callback function will be used as the value for the new entry in the current hash table. If no callback function is supplied, the existing value in the current hash table will be overwritten by the value in the hash table supplied.

- `void get(Object key)`

  Returns the value associated with the key supplied, or `null` if no value is found for that key.

- `Boolean containsKey(Object key)`

  Returns whether the hash table contains the specified key.

- `Boolean containsValue(Object value)`

  Returns whether the hash table contains the specified value.

- `void clear()`

  Removes all entries from the hash table.

- `Boolean isEmpty()`

  Returns true if the hash table contains no key/value pairs.

- `Array keys()`

  Returns an array containing all the keys contained in the hash table.

- `Array values()`

  Returns an array containing all the values contained in the hash table.

- `Array entries()`

  New in version 2.0

  Returns an array containing all the entries contained in the hash table. Each entry is a two element array containing the key and value respectively for that entry.

- `void remove(Object key)`

  Updated in version 2.0: now returns the value associated with the removed key

  Removes the key and its corresponding value from the hash table.

- `Number size()`

  Returns the number of key/value pairs contained in the hash table.

- `Hashtable clone()`

  New in version 2.0

  Creates and returns a shallow copy of the hash table. If hashing and equality functions were provided to the hash table when it was constructed, they are passed into the new hash table.

- `void each(Function callback)`

  New in version 2.0

  Iterates over all the entries in the hash table, calling the `callback` function for each entry. This function is passed two parameters:

  - `key`: the key for the current entry;
  - `value`: the value for the current entry.

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