Scaling 64 bit integers

//
// TestMulDiv64.cpp : Defines the entry point for the console application.
//
// This is a small test program that shows the use of MulDiv64.
// You can run it to see the three calculated results.
// The first result should be 0x0000000000000001, which is clearly wrong.
// The second result is 0x38E38E38E38E38E2, which is the correct result.
// The third result is the same as the second showing that for certain 
// denominators the faster MulShr64 can be used.
//
// Author: Richard van der Wal
// e-Mail: R.vdWal@xs4all.nl
//

#include "stdafx.h"
#include "..\MulDiv64.h"


// For printing the 64-bit values to the console
union{
	__int64 qword;
	DWORD	dword[2];
} rep;


// The program starts here
int _tmain(int argc, _TCHAR* argv[])
{
	__int64 r = 0;
	__int64 a = 0xaaaaaaaaaaaaaaaa;
	__int64 b = 0x5555555555555555;
	__int64 c = 0x1000000000000000;	// = 1 shl 60
	char s = 60;
	
	// This will return an incorrect result
	r = a * b / c;

	rep.qword = r;
	printf("Plain result: \t\t%08X %08X\n", rep.dword[1], rep.dword[0]);
	
	// This will return the correct result
	r = MulDiv64(a, b, c);

	rep.qword = r;
	printf("MulDiv64 result: \t%08X %08X\n", rep.dword[1], rep.dword[0]);
	
	// Because dividing by c can be expressed as a right shift
	// we can obtain the correct scaling faster with:
	r = MulShr64(a, b, s);

	rep.qword = r;
	printf("MulShr64 result: \t%08X %08X\n", rep.dword[1], rep.dword[0]);
	
	return 0;
}