the integer to be encoded.  The encoding characters are:
</p>

<blockquote><pre>
0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz~
</pre></blockquote>

<p>The least significant 6 bits of the integer are encoded by
the first character, followed by
the next 6 bits, and so on until all non-zero bits of the integer
are encoded.  The minimum number of encoding characters is used.
Note that for integers less than 10, the base-64 coding is a
ASCII decimal rendering of the number itself.
</p>