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Google chart extended data encoding help

Hey, I need some help with an algorithm for the GoogleChart "wrappers" that I am working with. I'm in the process of writing the extended data encoding algorithm, I've come up with something that works, but I now there is a better solution to it using math. Here is the code that I came up with, it all works and should prove to be a good reference.

php Code:

public static function convert($nValue)
    {
        // This code will be slow, and should be replaced with a mathematical algorithm
        
        assert($nValue >= 0);
                            
        $aMask = array('A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L',
                        'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X',
                        'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 
                        'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 
                        'w', 'x', 'y', 'z', '0', '1', '2', '3', '4', '5', '6', '7', 
                        '8', '9', '-', '.');
        
        $iLength = count($aMask);                     
        $iCount = 0;        
            
        for ($i = 0; $i < $iLength; $i++)
        {         
            for ($j = 0; $j < $iLength; $j++)
            {
                if ($nValue == $iCount)
                {
                    return $aMask[$i] . $aMask[$j];
                }

                $iCount++;
            }
        }
                            
        return '__';
    }

Any help with this would be greatly appreciated.

Here's the link to Google's documentation on the extended encoding:
http://code.google.com/apis/chart/#extended

One last note, I'm not sure if I was on the right track, but I was thinking that the math algorithm would need to use a base64 style numbering system, replacing each of the 64 possible values with the values provided by Google and then somehow do the math (it just all got too much for me).

at the moment my brain is returning a fatal error, i think i need to reboot.

i can see what you mean, but its just how to do it, if i get my brain working ill have a go.

its a tough one xD

I think that I've come up with a function which will do this conversion properly. I've done a bunch of tests against answers which I know are correct (from the Google Charts docs) and everything appears ok. If there is something broken, at least you'll have a starting point to push forwards from.

So, below is the function and the tests used to see if it works or not.

php Code:

<?php


// This function converts an integer (0 < i <= 4095)
// into the 'extended' format for Google Charts data.
function convert($iValue)
{
    static $szBase, $iBaseLen;
    
    // Initialise static variables only once
    isset($szBase)   or $szBase   = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-.';
    isset($iBaseLen) or $iBaseLen = strlen($szBase);
    
    // If an invalid value is provided, return 'missing value'
    if (!is_int($iValue) or $iValue < 0 or $iValue > 4095)
    {
        return '__';
    }
    
    // Process the integer value into the new 'base' and return
    return $szBase[(int) ($iValue / $iBaseLen)] . $szBase[$iValue % $iBaseLen];
}


// Set up some test cases to which we know the expected results
$aExpected = array('AA', 'AZ', 'Aa', 'Az', 'A0', 'A9', 'A-', 'A.', 
                   'BA', 'BZ', 'Ba', 'Bz', 'B0', 'B9', 'B-', 'B.', 
                   '.A', '.Z', '.a', '.z', '.0', '.9', '.-', '..');
                   
$aTests    = array(0,    25,   26,   51,   52,   61,   62,   63,
                   64,   89,   90,   115,  116,  125,  126,  127,
                   4032, 4057, 4058, 4083, 4084, 4093, 4094, 4095);

// Run the test cases
$aResults = array_map('convert', $aTests);

// Output the result for each of the tests
header('Content-Type: text/plain');
foreach ($aExpected as $iKey => $szExpected)
{
    $szResult = ($aResults[$iKey] === $szExpected ? 'passed' : '***** FAILED, should be: ' . $szExpected);
    printf("%4d -> %s %s\n", $aTests[$iKey], $aResults[$iKey], $szResult); 
}

For me, that test page returns the following output:

Code:

   0 -> AA passed

  25 -> AZ passed

  26 -> Aa passed

  51 -> Az passed

  52 -> A0 passed

  61 -> A9 passed

  62 -> A- passed

  63 -> A. passed

  64 -> BA passed

  89 -> BZ passed

  90 -> Ba passed

 115 -> Bz passed

 116 -> B0 passed

 125 -> B9 passed

 126 -> B- passed

 127 -> B. passed

4032 -> .A passed

4057 -> .Z passed

4058 -> .a passed

4083 -> .z passed

4084 -> .0 passed

4093 -> .9 passed

4094 -> .- passed

4095 -> .. passed

perfect, cheers m8 :-)