void update (int a, int x, int y){
int ix;
x++; y++;
for(; y < (size+1); y += LOW_BIT(y))
for(ix = x; ix < (size+1); ix += LOW_BIT(ix))
cum[ix-1][y-1] += a;
}
int cumulative (int x, int y)
{
int res, ix;
res = 0;
x++; y++;
for(; y > 0; y -= LOW_BIT(y))
for(ix = x; ix > 0; ix -= LOW_BIT(ix))
res += cum[ix-1][y-1];
return res;
}
/* compute sum 0 < i < x, 0 < j < y */
int cumulative (int x, int y)
{
int res, ix;
res = 0;
x++; y++;
for(; y > 0; y -= LOW_BIT(y))
for(ix = x; ix > 0; ix -= LOW_BIT(ix))
res += cum[ix-1][y-1];
//printf ("cumul (%d, %d) = %d \n", x, y, res);
return res;
}
int sum (int x1, int y1, int x2, int y2)
{
int res, ix1, ix2, iy1, iy2;
res = 0;
for(iy2 = y2+1; iy2 > y1; iy2 -= LOW_BIT(iy2))
{
for (ix2 = x2+1; ix2 > x1; ix2 -= LOW_BIT(ix2))
res += cum[ix2-1][iy2-1];
for (ix1 = x1; ix1 > ix2; ix1 -= LOW_BIT(ix1))
res -= cum[ix1-1][iy2-1];
}
for(iy1 = y1; iy1 > iy2; iy1 -= LOW_BIT(iy1))
{
for (ix2 = x2+1; ix2 > x1; ix2 -= LOW_BIT(ix2))
res -= cum[ix2-1][iy1-1];
for (ix1 = x1; ix1 > ix2; ix1 -= LOW_BIT(ix1))
res += cum[ix1-1][iy1-1];
}
return res;
}
