[mapguide-users] Base64 decoding process

[Traian Stanev]

First of all, w = 48 = 110000, not 100000.

Then "piw" => 101001 100010 110000 => 1010 0110 0010 1100 => A6 2C => 2C A6 (little endian byte order) => 11430.


Traian


-----Original Message-----
From: mapguide-users-bounces at lists.osgeo.org [mailto:mapguide-users-bounces at lists.osgeo.org] On Behalf Of Jim O'Leary
Sent: Friday, December 28, 2007 5:56 PM
To: mapguide-users at lists.osgeo.org
Subject: [mapguide-users] Base64 decoding process


I'm trying to figure out the Base64 decoding process that MapGuide uses
to store the user's selections on the map.

When I select a given parcel in Sheboygan, the Properties pane tells me

FeatId = 11430

When I capture the selection XML for this feature, it gives me the
the ID tag as

<ID>piwAAA==</ID>

I break down this Base64-encoded string as follows, using a Base64
translation table:

p = 41 = 101001
i = 34 = 100010
w = 48 = 100000

Putting those three 6 bit binary numbers in one line yields

101001 100010 100000

I am assuming that the A characters represent 0, or no valid bits.

Recombining these three 6 bit binary numbers into 8 bit numbers yields

10100110 00101000 00

Turning these binary numbers in decimal yields

10100111 = 166
00101000 = 40

Here I am lost. How does 166 and 40 equate to 11430, which is the FeatId
that this Base64-encoded string evaluates to?

Thanks
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