static VALUE
bigmul_low_digits(VALUE x, VALUE y, int digits)
{
int xn = RBIGNUM_LEN(x);
if (FIXNUM_P(y)) y = INTVALUE2BIG(y);
int yn = RBIGNUM_LEN(y);
if (digits > xn + yn)
{
digits = xn + yn;
}
VALUE z = rb_big_new(digits, 1);
BDIGIT* xds = RBIGNUM_DIGITS(x);
BDIGIT* yds = RBIGNUM_DIGITS(y);
BDIGIT* zds = RBIGNUM_DIGITS(z);
BDIGIT c0,c1,c2; // carry
int i, j, k, beg, end;
for (k = c0 = c1 = c2 = 0; k < digits; k++)
{
beg = k >= yn - 1 ? k - yn + 1 : 0;
end = k >= xn ? xn - 1 : k;
for (i = beg, j = k - beg; i <= end; ++i, --j)
MULADD(xds[i], yds[j]);
zds[k] = c0;
c0 = c1;
c1 = c2;
c2 = 0;
}
return rb_big_norm(z);
}
static VALUE
bug_big_zero(VALUE self, VALUE length)
{
long len = NUM2ULONG(length);
VALUE z = rb_big_new(len, 1);
MEMZERO(BIGNUM_DIGITS(z), BDIGIT, len);
return z;
}
static VALUE
bigmul_high_digits(VALUE x, VALUE y, int dontcare, int x_right_shift)
{
int xn = RBIGNUM_LEN(x);
if (FIXNUM_P(y)) y = INTVALUE2BIG(y);
int yn = RBIGNUM_LEN(y);
BDIGIT* xds = RBIGNUM_DIGITS(x);
BDIGIT* yds = RBIGNUM_DIGITS(y);
/* in order to avoid rb_big_clone call,
let's virtually "shift" x instead of actual shifting */
if (x_right_shift >= xn) {
return INT2BIG(0);
} else {
xds += x_right_shift;
xn -= x_right_shift;
}
int digits = xn + yn;
int i, j, k;
VALUE z = rb_big_new(digits, 1);
BDIGIT* zds = RBIGNUM_DIGITS(z);
for (i = 0; i < digits; zds[i++] = 0);
int beg, end;
BDIGIT c0, c1, c2;
for (k = dontcare, c0 = c1 = c2 = 0; k < digits; k++)
{
beg = k >= yn - 1 ? k - yn + 1 : 0;
end = k >= xn ? xn - 1 : k;
for (i = beg, j = k - beg; i <= end; ++i, --j)
MULADD(xds[i], yds[j]);
zds[k] = c0;
c0 = c1;
c1 = c2;
c2 = 0;
}
return z;
}
