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extract initial sqrt estimation [Feature #13219]

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git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57708 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
This commit is contained in:
nobu 2017-02-24 23:31:07 +00:00
parent a0acd82f2a
commit a67c94cf40
1 changed files with 28 additions and 18 deletions
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46
bignum.c
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@ -6768,6 +6768,32 @@ BDIGIT rb_bdigit_dbl_isqrt(BDIGIT_DBL);
# define rb_bdigit_dbl_isqrt(x) (BDIGIT)rb_ulong_isqrt(x) # define rb_bdigit_dbl_isqrt(x) (BDIGIT)rb_ulong_isqrt(x)
#endif #endif
static BDIGIT *
estimate_initial_sqrt(VALUE *xp, const size_t xn, const BDIGIT *nds, size_t len)
{
const int zbits = nlz(nds[len-1]);
const int shift_bits = (len&1 ? BITSPERDIG/2 : BITSPERDIG) - (zbits+1)/2 + 1;
VALUE x = bignew_1(0, xn, 1); /* division may release the GVL */
BDIGIT *xds = BDIGITS(x);
*xp = x;
/* x = (n >> (b/2+1)) */
if (shift_bits == BITSPERDIG) {
MEMCPY(xds, nds+len-xn, BDIGIT, xn);
}
else if (shift_bits > BITSPERDIG) {
bary_small_rshift(xds, nds+len-xn, xn, shift_bits-BITSPERDIG, 0);
}
else {
bary_small_rshift(xds, nds+len-xn-1, xn, shift_bits, nds[len-1]);
}
/* x |= (1 << (b-1)/2) */
xds[xn-1] |= (BDIGIT)1u <<
((len&1 ? 0 : BITSPERDIG/2) + (BITSPERDIG-zbits-1)/2);
return xds;
}
VALUE VALUE
rb_big_isqrt(VALUE n) rb_big_isqrt(VALUE n)
{ {
@ -6783,25 +6809,9 @@ rb_big_isqrt(VALUE n)
#endif #endif
} }
else { else {
int zbits = nlz(nds[len-1]);
int shift_bits = (len&1 ? BITSPERDIG/2 : BITSPERDIG) - (zbits+1)/2 + 1;
size_t tn = (len+1) / 2, xn = tn; size_t tn = (len+1) / 2, xn = tn;
VALUE t, x = bignew_1(0, xn, 1); /* division may release the GVL */ VALUE t, x;
BDIGIT *tds, *xds = BDIGITS(x); BDIGIT *tds, *xds = estimate_initial_sqrt(&x, xn, nds, len);
/* x = (n >> (b/2+1)) */
if (shift_bits == BITSPERDIG) {
MEMCPY(xds, nds+tn, BDIGIT, xn);
}
else if (shift_bits > BITSPERDIG) {
bary_small_rshift(xds, nds+len-xn, xn, shift_bits-BITSPERDIG, 0);
}
else {
bary_small_rshift(xds, nds+len-xn-1, xn, shift_bits, nds[len-1]);
}
/* x |= (1 << (b-1)/2) */
xds[xn-1] |= (BDIGIT)1u <<
((len&1 ? 0 : BITSPERDIG/2) + (BITSPERDIG-zbits-1)/2);
/* t = n/x */ /* t = n/x */
tn += BIGDIVREM_EXTRA_WORDS; tn += BIGDIVREM_EXTRA_WORDS;