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hashAlgorithm: An Abstract Base Class

hashAlgorithm is a generic helper class whose primary purpose is to manage hash algorithms. Its design was losely inspired by that of Microsoft's abstract .Net class HashAlgorithm. It works as an abstract base class from which concrete hash algorithms are derived via prototype inheritance. Instances of a concrete hash algorithm class should not be created directly using the new operator - use hashAlgorithm as a class factory and call hashAlgorithm.create() instead passing the name of the hash algorithm as parameter. This ensures that the new instance will be initialized properly. Furthermore, a loader function can be installed which requests the JavaScript code of an algorithm from a server if it has not yet been installed (load on demand).

Any hash algorithm to be managed by this class is required to do the following:

publish the class properties iInputBlockSize and iOutputBlockSize
insert the following statements immediately after the definition of the constructor function
```
       core.derive(xxx, hashAlgorithm);
```
where "
```
xxx
```
" denotes the algorithm class
provide for implementations of the instance methods hashCore() and hashFinal() (see the source code page for the signatures of these functions).
insert the statement
```
this.buildHash(); 
```
into the body of its hashFinal() method as last statement before successfully returning
if it implements an init() method, then insert the statement
```
this.sup.init(); 
```
at the end of its body
call hashAlgorithm.install() with the algorithm class as parameter at the end of its source file

These requirements can be easily satisfied by using code templates. For an example, see MD5 JavaScript source code.

Replacing the protoype provides instances of the installed hash algorithm with the following:

two buffers, buffer and hashValue, and an init() method to initialize or clear them
a sup property which refers to the hashAlgorithm base class
an overridden toString() method which returns its class name
a hash string property and a hashFinal() method which is to be overriden and should be called from inside the overriding method on the instantiated class to set the calculated digest as value of hash
a computeHash() method which re-initializes the algorithm instance and computes the hash digest from its input