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</ul><div id="page_body"><h1><span class="toc_number">2.5.4</span>Class HashBase</h1><p class="brief">Base class for each hash algorithm, specialized for overloading. </p>
<pre class="prototype">Class HashBase</pre>
<p>The HashBase class provides a set of shared interfaces that are syntactically equivalent for each specialized hash. </p>
<p>Hashes are generated by creating an instance of a specialized class and putting data into it. When the result is requested, a hash is finalized, which means that no more data can be added; any attempts to do so will raise an exception. </p>
<p>Basic usage example: </p>
<pre>
crc = CRC32()
crc.update("abc")
> crc // prints "352441c2"
</pre><p class='note'><b>Note:</b> Instantiating HashBase directly and calling any method will raise an error. </p>
<h2><a name="hashbase_overload">Overloading HashBase</a></h2>
<p>To easily implement other hash algorithms in native falcon code, HashBase can be overloaded. For simplicity, only 2 methods have to be overloaded, and 2 new methods have to be added: </p>
<pre>
class MyHash from HashBase
state = nil // internal state
outp = nil
function bytes(): return 12 // must be overloaded and return a constant integer > 0
function toMemBuf(): return self.outp // must be overloaded and return a MemBuf with wordSize 1 and length equal to bytes()
function process(buf) // must be declared, as it is invoked by the module on update() calls
// *mangle MemBuf and update state*
end
function finalize() // must be declared, as it is invoked by the module to produce the actual digest
// *transform state and assign result MemBuf(1, bytes()) to outp*
end
end
</pre><p>How this works: <ul><li><b>bytes()</b> is internally invoked by bits() (once, the returned integer is cached by the module) </li><li><b>process()</b> is invoked by update() and updateInt(), beeing passed a MemBuf with word size 1 </li><li><b>toMemBuf()</b> is invoked by toString() and toInt() </li><li><b>finalize()</b> is called ONCE before toMemBuf() and is intended to do process remaining buffers, and produce the actual digest. Does not have to be called manually. </li></ul></p>
<p class='note'><b>Note:</b> You are strongly advised NOT to overload any other methods except the four above, unless you REALLY know what you're doing. </p>
<p>Advantages of doing it this way: <ul><li>It is not necessary to implement update() in native falcon code. </li><li>All value endian conversions, type mangling, and error checking is done by the module, so focus can be set on the algorithm itself. </li><li>The values returned by bytes(), toMemBuf() and toInt() are cached by the module, means less calls, less time. </li><li>The module ensures that finalize() is called only once, no explicit checking required. </li></ul></p>
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