[SOLVED] help with big numbers and %

[bobbfwed]

I am creating a decimal to base-36 (and back) system. But I've found that while getting the remainder of certain large numbers, I get negative numbers, what does this mean? Maybe a system-specific problem (I've tried it on two of my computers).

 

Examples: 2655429305 % 36 = -35; 4128789450 % 36 = -10 ... but there is a long list, and it seems random, but as the number gets higher, there are more negative returns. Is there some arbitrary limit that operator? Try the above math on your PHP system. Just in case it may matter, here is my code:

<?PHP
$b10 = 4128789450; // decimal input here
dectobase36($b10);

echo $b10.'<sub>10</sub><br>';
echo '=<br>';
$b36 = dectobase36($b10);
echo $b36.'<sub>36</sub><br>'; // display base-36 output
echo base36todec($b36); // double check the results


function dectobase36($b10) {
  $b36 = NULL;
  $b102 = $b10;
  while ($b10 > 35) {
    if (($b10 % 36) < 0)
      die($b102.' '.$b10.' '.($b10 % 36).' Decimal value too high.'); // die, and show some values
    $b36 .= chr(ret_ord_value($b10 % 36));
    $b10 = floor($b10 / 36);
  }
  $b36 .= chr(ret_ord_value($b10 % 36));
  return strtoupper(strrev($b36));
}

function ret_ord_value ($num) {
  if ($num > 9)
    return $num + 87;
  else
    return $num + 48;
}



function base36todec($b36) {
  $value = 0;
  $b36 = strtolower($b36);

  for ($i = 0; $i < strlen($b36); $i++)
    $value += ret_let_value(substr($b36, -1 - $i, 1)) * pow(36, $i);
  
  return $value;
}

function ret_let_value ($let) {
  if (ord($let) >= 48 && ord($let) <= 57)
    return $let;
  elseif (ord($let) >= 97 && ord($let) <= 122)
    return ord($let) - 87;
  else
    die($let.'...Bad character.'); // if it is fed a invalid character
}
?>

[Mchl]

Manual says: Remainder $a % $b is negative for negative $a.

Which could mean that youre dealing with overflow (integers are signed 32 bit in most cases)

http://www.php.net/manual/en/language.types.integer.php

 

Both you're examples are larger than 2^31, which means they're overflowing

[bobbfwed]

ah, I didn't think of 31 bits....stupid signed variables. There's no way to unsign them for this operation is there (I want 32 bits!)? Is there even a way to handle variables in PHP as not signed (no negative value available)?

[Mchl]

Manual says: no

 

 

[Mchl]

I've come up with this freaky idea, that might help you, at least for modulo 36

 

Divisible by 36 means divisible by 4 and by nine at the same time.

 

So some pseudocode:

 

check 
if a number made of two last digits is divisible by 4
and
if a sum of all digits is divisible by 9
yes:
remainder is 0
no:
remainder++
decrease your number by one, and check again

[sasa]

or

<?php
function my_r($a, $d){
$l = ceil(strlen($a)/3) * 3;
$a= str_pad($a, $l, '0', STR_PAD_LEFT);
for ($i=0; $i<$l; $i+=3) $ar[] = substr($a,$i,3);
$r = 0;
foreach ($ar as $part) $r = ($r *1000 + $part) % $d;
return $r;
}
$test = '2655429305';
echo my_r($test, 36);
?>

[bobbfwed]

Or, smaller (and probably a good deal faster), but only works up to (around 2^57 -- maybe someone can explain why):

function remainder ($value, $div) {
  $tmp = floor($value / $div);
  return ($value - ($tmp * $div));
}

[Mchl]

Probably that's where double float precision ends (although I think it should've ended much sooner). Double float has precision of approx 14 significant digits.