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AnswerRe: Looking for an alternative to bit shifting Pin
PIEBALDconsult1-Aug-08 6:34
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AnswerRe: Looking for an alternative to bit shifting Pin
Guffa1-Aug-08 11:01
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GeneralRe: Looking for an alternative to bit shifting Pin
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GeneralRe: Looking for an alternative to bit shifting Pin
Guffa2-Aug-08 1:48
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GeneralRe: Looking for an alternative to bit shifting [modified] Pin
PIEBALDconsult2-Aug-08 5:41
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GeneralRe: Looking for an alternative to bit shifting Pin
Guffa2-Aug-08 10:13
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AnswerRe: Looking for an alternative to bit shifting Pin
PIEBALDconsult2-Aug-08 8:18
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AnswerRe: Looking for an alternative to bit shifting [modified] Pin
PIEBALDconsult2-Aug-08 8:52
professionalPIEBALDconsult2-Aug-08 8:52 
Ha ha ha! Here it is in C#

private static readonly int[] masks = unchecked ( new int[] { (int) 0xFFFFFFFF , 0x00FFFFFF , 0x0000FFFF , 0x000000FF } ) ;
 
public static unsafe int
ShiftAndSumArray
(
    int[] Array
,
    uint  Shift
)
{
    int result = 0 ;
           
    fixed ( int* b = Array )
    {
        int* a = (int*) ( ((byte*) b) + ( Shift %= sizeof(int) ) ) ;
        int  i ;
                 
        for ( i = 0 ; i < Array.Length-1 ; i++ )
        {
            result += a [ i ] ;
        }
 
        result += a [ i ] & masks [ Shift ] ;
    }
             
    return ( result ) ;
}


This appears to take about half the time of shifting each int separately, but it doesn't involve very much time either way;
100,000,000 (10,000 passes across an array of 10,000 ints) this way takes 507 milliseconds, the shifting technique takes 969 milliseconds.
(And 2665 milliseconds for the division technique.)

public static int
ShiftAndSumArray2
(
    int[] Array
,
    uint  Shift
)
{
    int result = 0 ;
 
    int shift = (int) Shift % sizeof(int) * 8 ;

    // Using foreach is not much slower 
    for ( int i = 0 ; i < Array.Length ; i++ )
    {
        result += Array [ i ] >> shift ;
    }            
 
    return ( result ) ;
}



Correction: Earlier I had forgotten to Reset the Stopwatch D'Oh! | :doh: , corrected results

457 milliseconds for altered pointer
459 milliseconds for shifted values
1768 milliseconds for division

I have also quickened up the implementations somewhat.
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