Task: Long multiplication






PHP: <?php
function longMult($a, $b)
{
  $as = (string) $a;
  $bs = (string) $b;
  for($pi = 0, $ai = strlen($as) - 1; $ai >= 0; $pi++, $ai--)
    {
      for($p = 0; $p < $pi; $p++)
        {
          $regi[$ai][] = 0;
        }
      for($bi = strlen($bs) - 1; $bi >= 0; $bi--)
        {
          $regi[$ai][] = $as[$ai] * $bs[$bi];
        }
    }
  return $regi;
}

function longAdd($arr)
{
  $outer = count($arr);
  $inner = count($arr[$outer-1]) + $outer;
  for($i = 0; $i <= $inner; $i++)
    {
      for($o = 0; $o < $outer; $o++)
        {
          $val  = isset($arr[$o][$i]) ? $arr[$o][$i] : 0;
          @$sum[$i] += $val;
        }
    }
  return $sum;
}

function carry($arr)
{
  for($i = 0; $i < count($arr); $i++)
    {
      $s = (string) $arr[$i];
      switch(strlen($s))
        {
          case 2:
            $arr[$i] = $s{1};
            @$arr[$i+1] += $s{0};
            break;
          case 3:
            $arr[$i] = $s{2};
            @$arr[$i+1] += $s{0}.$s{1};
            break;
        }
    }
  return ltrim(implode('',array_reverse($arr)),'0');
}

function lm($a,$b)
{
  return carry(longAdd(longMult($a,$b)));
}

if(lm('18446744073709551616','18446744073709551616') == '340282366920938463463374607431768211456')
  {
    echo 'pass!';
  }; // 2^64 * 2^64



VB: Imports System
Imports System.Console
Imports BI = System.Numerics.BigInteger

Module Module1

    Dim a As Decimal, mx As Decimal = 1E28D, hm As Decimal = 1E14D

    
    Structure bd
        Public hi, lo As Decimal
    End Structure

    
    Function toStr(ByVal a As bd, ByVal Optional comma As Boolean = False) As String
        Dim r As String = If(a.hi = 0, String.Format("{0:0}", a.lo),
                                       String.Format("{0:0}{1:" & New String("0"c, 28) & "}", a.hi, a.lo))
        If Not comma Then Return r 
        Dim rc As String = ""
        For i As Integer = r.Length - 3 To 0 Step -3
            rc = "," & r.Substring(i, 3) & rc : Next
        toStr = r.Substring(0, r.Length Mod 3) & rc
            toStr = toStr.Substring(If(toStr.Chars(0) = "," , 1, 0))
    End Function

    
    Function Pow_dec(ByVal bas As Decimal, ByVal exp As UInteger) As Decimal
        If exp = 0 Then Pow_dec = 1D else Pow_dec = Pow_dec(bas, exp >> 1) : _
        Pow_dec *= Pow_dec : If (exp And 1) <> 0 Then Pow_dec *= bas
    End Function

    Sub Main(ByVal args As String())
         For p As UInteger = 64 To 95 - 1 Step 30                 
            Dim y As bd, x As bd : a = Pow_dec(2D, p)             
            WriteLine("The square of (2^{0}):                    {1,38:n0}", p, a)
            x.hi = Math.Floor(a / hm) : x.lo = a Mod hm           
            Dim BS As BI = BI.Pow(CType(a, BI), 2)                
            y.lo = x.lo * x.lo : y.hi = x.hi * x.hi               
            a = x.hi * x.lo * 2D                                  
            y.hi += Math.Floor(a / hm) : y.lo += (a Mod hm) * hm  
            While y.lo > mx : y.lo -= mx : y.hi += 1 : End While  
            WriteLine(" is {0,75} (which {1} match the BigInteger computation)" & vbLf,
                toStr(y, True), If(BS.ToString() = toStr(y), "does", "fails to"))
        Next
    End Sub

End Module