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Improve performance of bignum[beg, len] (#14007)

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Implement rb_big_aref2.
Taking a small slice from large bignum was slow in rb_int_aref2.
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tomoya ishida 2025-07-30 01:34:13 +09:00 • committed by GitHub
parent 46d106f7ab
commit a66e4f2154
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5 changed files with 131 additions and 18 deletions
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67
bignum.c
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@ -6757,6 +6757,73 @@ rb_big_aref(VALUE x, VALUE y)
return (xds[s1] & bit) ? INT2FIX(1) : INT2FIX(0);
}
VALUE
rb_big_aref2(VALUE x, VALUE beg, VALUE len)
{
BDIGIT *xds, *vds;
VALUE v;
size_t copy_begin, xn, shift;
ssize_t begin, length, end;
bool negative_add_one;
beg = rb_to_int(beg);
len = rb_to_int(len);
length = NUM2SSIZET(len);
begin = NUM2SSIZET(beg);
end = NUM2SSIZET(rb_int_plus(beg, len));
shift = begin < 0 ? -begin : 0;
xn = BIGNUM_LEN(x);
xds = BDIGITS(x);
if (length < 0) return rb_big_rshift(x, beg);
if (length == 0 || end <= 0) return INT2FIX(0);
if (begin < 0) begin = 0;
if ((size_t)(end - 1) / BITSPERDIG >= xn) {
/* end > xn * BITSPERDIG */
end = xn * BITSPERDIG;
}
if ((size_t)begin / BITSPERDIG < xn) {
/* begin < xn * BITSPERDIG */
size_t shift_bits, copy_end;
copy_begin = begin / BITSPERDIG;
shift_bits = begin % BITSPERDIG;
copy_end = (end - 1) / BITSPERDIG + 1;
v = bignew(copy_end - copy_begin, 1);
vds = BDIGITS(v);
MEMCPY(vds, xds + copy_begin, BDIGIT, copy_end - copy_begin);
negative_add_one = (vds[0] & ((1 << shift_bits) - 1)) == 0;
v = bignorm(v);
if (shift_bits) v = rb_int_rshift(v, SIZET2NUM(shift_bits));
}
else {
/* Out of range */
v = INT2FIX(0);
negative_add_one = false;
copy_begin = begin = end = 0;
}
if (BIGNUM_NEGATIVE_P(x)) {
size_t mask_size = length - shift;
VALUE mask = rb_int_minus(rb_int_lshift(INT2FIX(1), SIZET2NUM(mask_size)), INT2FIX(1));
v = rb_int_xor(v, mask);
for (size_t i = 0; negative_add_one && i < copy_begin; i++) {
if (xds[i]) negative_add_one = false;
}
if (negative_add_one) v = rb_int_plus(v, INT2FIX(1));
v = rb_int_and(v, mask);
}
else {
size_t mask_size = (size_t)end - begin;
VALUE mask = rb_int_minus(rb_int_lshift(INT2FIX(1), SIZET2NUM(mask_size)), INT2FIX(1));
v = rb_int_and(v, mask);
}
RB_GC_GUARD(x);
if (shift) v = rb_int_lshift(v, SSIZET2NUM(shift));
return v;
}
VALUE
rb_big_hash(VALUE x)
{