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Convert 7bit text into octets (8bit)

Posted on 2004-08-18
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Last Modified: 2011-10-03
Hi,

I have been looking for a way to do this in php for a while and cant automate this....

I am looking for a php function that can take in normal 7bit text, like "hellohello" which is 10 characters and convert them to 8bit octets (ie. E8 32 9B FD 46 97 D9 EC 37). If you look at:

http://www.dreamfabric.com/sms/hello.html

Quote from the page:
-----------------------------
The first septet (h) is turned into an octet by adding the rightmost bit of the second septet. This bit is inserted to the left which yields 1 + 1101000 = 11101000 ("E8"). The rightmost bit of the second character is then consumed, so the second character (septet) needs two bits (yellow) of the third character to make an 8bit octet. This process goes on and on yielding the following octets:

The 9 octets from "hellohello" are E8 32 9B FD 46 97 D9 EC 37
-----------------------------

Thanks in advance!
Bill.
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Question by:billbrigden
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7 Comments
 

Author Comment

by:billbrigden
ID: 11829058
Further to my question - if anyone can do the reverse as well, ie. The octets, E8 32 9B FD 46 97 D9 EC 37 back into "hellohello" that would be great.

Cheers.
Bill.
0
 
LVL 6

Accepted Solution

by:
CosminB earned 2000 total points
ID: 11830205
Try this:
<?
function seven2eight($string, $param = 'hexa')
{
      $l = strlen($string);
      $a = array();
      for ($i=0;$i<$l; $i++)
      {
            $a[] = decbin(ord($string{$i})); //convert to bin
      }
      $step = 0;
      for ($i=0; $i<$l-1; $i++)
      {
            $step = (++$step)%8;
            $toAdd =  strrev(substr(strrev($a[$i+1]), 0, $step));
            $a[$i] = substr($toAdd.$a[$i], 0, 8);
      }
      //strip the extra values
      for ($i=0; $i<$l;$i++)
      {
            if (($i+1)%8 != 0)
                  $resultArray[] = $a[$i];
      }
      
      
      if ($param == 'array')
      {
            return $resultArray;
      }
      if ($param == 'hexa')
      {
            for ($i=0;$i<count($resultArray); $i++)
            {
                  $resultArray[$i] = dechex(bindec($resultArray[$i]));
            }
            return $resultArray;
      }
      if ($param == 'string')
      {
            $result = '';
            
            for ($i=0;$i<count($resultArray); $i++)
            {
                  $result.= chr(bindec($resultArray[$i]));
            }
            return $result;
      }
}

//possible value for the seconf parameter are 'array', 'hexa' and 'string'

print_r(seven2eight($string, 'array'));
print_r(seven2eight($string, 'hexa'));
print_r(seven2eight($string, 'string'));
?>

the code is a little messy but it will work. the reverse is very similar to this but I don't have time to do it right now ... maybe tomorrow
0
 
LVL 6

Expert Comment

by:CosminB
ID: 11830214
... of course you have to initialise $string with 'hellohello' or smth. before calling seven2eight()
0
 
LVL 10

Expert Comment

by:ThaSmartUno
ID: 11831673
so far this is what I have.

<?php

function convert7to8($str) {
    $newarr=array();
    $bins=array();
    $let='';
    for ($i=0;$i<strlen($str);$i++){
        $let = substr($str,$i,1);
        $bins[] = decbin(ord($let));
    }

    $carry='';
    for($i=0;$i<count($bins)-1;$i++) {
        $cnt = ($i+1) % 8;
        if ($bins[$i] != '') {
            $carry = substr($bins[$i+1],-1 * $cnt);
            if ($carry) {
                $bins[$i+1] = substr($bins[$i+1],0,-1 * $cnt);
            }
            $bins[$i] = $carry . $bins[$i];
            $newarr[] = dechex(bindec($bins[$i]));
        }
    }
    if ($bins[$i] != '') {
        $newarr[] = dechex(bindec($bins[$i]));
    }
    return $newarr;
}

// expecting array of hex characters
function convert8to7($arr) {
    if (!is_array($arr)) {
        echo "expecting array of hex characters\n";
    } else {
        $carry='';
        $curBin='';
        $oldCarry='';
        $newArr=array();
        $cnt=0;
        for($i=0;$i<count($arr)-1;$i++) {
            $cnt = (++$cnt) % 8;
            $curBin = decbin(hexdec($arr[$i]));
            while (strlen($curBin) < 8) { $curBin = '0'.$curBin; }
            $carry = substr($curBin, 0, $cnt);
            $curBin = substr($curBin, $cnt);
            $newArr[] = $curBin.$oldCarry;
            if ((($cnt+1) % 8) == 0) {
                $newArr[] = $carry;
                $carry='';
                $cnt = (++$cnt) % 8;
            }
            $oldCarry=$carry;
        }
        $newArr[] = decbin(hexdec($arr[$i])).$oldCarry;

        $newStr = '';
        foreach($newArr as $bin){
            $newStr .= chr(bindec($bin));
        }
        return $newStr;
    }
}

$newstr = convert7to8("hellohello");
echo "The " . count($newstr) . " octets are " . strtoupper(join(' ', $newstr)) . "<br/>\n";
print_r(convert8to7($newstr));

?>

The convert8to7 I have works for some, but for some reason it does not work for anything ending in p
ex. hellohelp
0
 
LVL 10

Expert Comment

by:ThaSmartUno
ID: 11833646
or rather ... it doesn't work with 'hellohellp' or 'hellohellohellop' (this chops the p off completely)

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