unsigned add( unsigned a, unsigned b )
{
return (unsigned)&((char*)a)[b]; // ignore compiler warnings
// (if pointers are bigger than unsigned). it still works.
}
unsigned umul( unsigned a, unsigned b )
{
unsigned res = 0;
while( a != 0 ){
if( a & 1) res = add( res, b );
b <<= 1;
a >>= 1;
}
return res;
}
int mul( int a, int b ){
return (int)umul( (unsigned)a, (unsigned)b );
}
Se consideri che l'hack a [b] sia un imbroglio (dato che si tratta davvero di un'aggiunta), allora funziona invece. Ma le ricerche delle tabelle implicano anche l'aggiunta di puntatori.
Vedi http://en.wikipedia.org/wiki/IBM_1620 - un computer che effettivamente ha fatto l'aggiunta utilizzando le tabelle di ricerca ...
Qualcosa di soddisfacente nell'uso di un meccanismo di tabella per "accelerare" un'operazione che potrebbe effettivamente essere eseguita in un'istruzione.
static unsigned sumtab[17][16]= {
{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,10,11,12,13,14,15},
{ 1, 2, 3, 4, 5, 6, 7, 8, 9,10,11,12,13,14,15,16},
{ 2, 3, 4, 5, 6, 7, 8, 9,10,11,12,13,14,15,16,17},
{ 3, 4, 5, 6, 7, 8, 9,10,11,12,13,14,15,16,17,18},
{ 4, 5, 6, 7, 8, 9,10,11,12,13,14,15,16,17,18,19},
{ 5, 6, 7, 8, 9,10,11,12,13,14,15,16,17,18,19,20},
{ 6, 7, 8, 9,10,11,12,13,14,15,16,17,18,19,20,21},
{ 7, 8, 9,10,11,12,13,14,15,16,17,18,19,20,21,22},
{ 8, 9,10,11,12,13,14,15,16,17,18,19,20,21,22,23},
{ 9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24},
{10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25},
{11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26},
{12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27},
{13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28},
{14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29},
{15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30},
{16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31}
};
unsigned add( unsigned a, unsigned b )
{
static const int add4hack[8] = {4,8,12,16,20,24,28,32};
unsigned carry = 0;
unsigned (*sumtab0)[16] = &sumtab[0];
unsigned (*sumtab1)[16] = &sumtab[1];
unsigned result = 0;
int nshift = 0;
while( (a|b) != 0 ){
unsigned psum = (carry?sumtab1:sumtab0)[ a & 0xF ][ b & 0xF ];
result = result | ((psum & 0xF)<<nshift);
carry = psum >> 4;
a = a >> 4
b = b >> 4;
nshift= add4hack[nshift>>2]; // add 4 to nshift.
}
return result;
}