bcpowmod()

(PHP 5, PHP 7)

Raise an arbitrary precision number to another, reduced by a specified modulus

说明

Use the fast-exponentiation method to raise$baseto the power$exponentwith respect to the modulus$modulus.

参数

The base, as an integral string(i.e. the scale has to be zero).

The exponent, as an non-negative, integral string(i.e. the scale has to be zero).

The modulus, as an integral string(i.e. the scale has to be zero).

此可选参数用于设置结果中小数点后的小数位数。也可通过使用bcscale()来设置全局默认的小数位数，用于所有函数。

返回值

Returns the result as a string, or NULL if$modulusis0or$exponentis negative.

注释

Because this method uses the modulus operation, numbers which are not positive integers may give unexpected results.

范例

The following two statements are functionally identical. The bcpowmod() version however, executes in less time and can accept larger parameters.

参见

• bcpow()任意精度数字的乘方
• bcmod()对一个任意精度数字取模

I found a better way to emulate bcpowmod on PHP 4, which works with very big numbers too:
function powmod($m,$e,$n) {
  if (intval(PHP_VERSION)>4) {
    return(bcpowmod($m,$e,$n));
  } else {
    $r="";
    while ($e!="0") {
      $t=bcmod($e,"4096");
      $r=substr("000000000000".decbin(intval($t)),-12).$r;
      $e=bcdiv($e,"4096");
    }
    $r=preg_replace("!^0+!","",$r);
    if ($r=="") $r="0";
    $m=bcmod($m,$n);
    $erb=strrev($r);
    $q="1";
    $a[0]=$m;
    for ($i=1;$i

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