how to convert a character to 7 bit even parity in php

Question

I want to convert a Character to a 7 bit even parity. Can you please suggest me, how to implement this?

Answer 1

At the risk of downvotes:

function to7BitEven($byte) {
   if( $byte > 0x7f ) return 0;
   $binary = decbin($byte);
   $one = 0;
   $ret = 0;

   for($x=0;$x<strlen($binary);$x++) {
      if( $binary[$x] == '1' ) $one++;
   }

   if( $one % 2 != 0 ) $ret = "1";
   else $ret = "0";

   for($x=0;$x<(7-strlen($binary));$x++)
      $ret .= "0";

   $ret .= $binary;
   return $ret;
}

echo to7BitEven(0x7f) . "\n";
echo to7BitEven(0x41) . "\n";
echo to7BitEven(0x3a) . "\n";
echo to7BitEven(0x3b) . "\n";

This will count the 1's, and add an extra 1 (if odd) or a 0 (if even) to the first bit. Then copy the other 7 bits into the return. This will leave you with a string representation of the 7 bit, even parity of the byte.

To those more experienced then I, does this look right? Example output:

11111111

01000001

00111010

10111011

Answer 2

Too bad you can't use the x86 JPO instruction (Jump if Parity Odd) ;-)

Depending on the amount of data you want to handle it might be faster if you first set up a translation table than to check/handle character by character.

$map = array();
for($char=0; $char<128; $char++) {
  $parity = 0;
  for($bit=0; $bit<8; $bit++) {
    if($char & (1<<$bit)) {
      $parity ^= 128;
    }
  }
  $map[chr($char)] = chr($char|$parity);
}

(you might want to test this code thoroughly, I haven't)
and then use strtr() to translate from ascii7 to ascii7-evenbit.

$input = 'mary had a little lamb'; // all characters must be within the ascii7 range
$evenbit = strtr($input, $map);
// test output
for($i=0; $i<strlen($evenbit); $i++) {
  printf("%08s\n", decbin(ord($evenbit[$i])));
}

Answer 3

Here's a C version:

uint8_t even_parity (uint8_t b)
{
    return b ^ ((3459840 >> ((b ^ (b >> 4)) & 15)) & 128);
}

I'm sure translation to PHP would be easy, but I'll avoid embarrassing myself.

Inspired by this Bit Twiddling Hack.