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mruby-bigint: let mpz_get_int() answer the smallest Integer #7212

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Merged
matz merged 1 commit into from
Aug 16, 2026
+29 −4
Merged

mruby-bigint: let mpz_get_int() answer the smallest Integer #7212

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14 changes: 10 additions & 4 deletions mrbgems/mruby-bigint/core/bigint.c
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Original file line number Diff line number Diff line change
Expand Up @@ -4091,20 +4091,24 @@ mpz_get_int(mpz_t *y, mrb_int *v)
return TRUE;
}

/* The negative range is one wider than the positive one, so MRB_INT_MIN
fits as an absolute value of MRB_INT_MAX + 1. */
mrb_uint limit = (mrb_uint)MRB_INT_MAX + (y->sn < 0 ? 1 : 0);

#ifdef MRB_NO_MPZ64BIT
/* When using 16-bit limbs, we need to handle larger accumulation */
mrb_uint i = 0;
mp_limb *d = y->p + y->sz;

while (d-- > y->p) {
/* Check for overflow before shifting */
if (i > (mrb_uint)(MRB_INT_MAX >> DIG_SIZE)) {
if (i > (limit >> DIG_SIZE)) {
return FALSE;
}
i = (i << DIG_SIZE) | *d;
}

if (i > (mrb_uint)MRB_INT_MAX) {
if (i > limit) {
return FALSE;
}
#else
Expand All @@ -4119,14 +4123,16 @@ mpz_get_int(mpz_t *y, mrb_int *v)
}
i = (i << DIG_SIZE) | *d;
}
if (i > MRB_INT_MAX) {
if (i > limit) {
/* overflow */
return FALSE;
}
#endif

if (y->sn < 0) {
*v = -(mrb_int)i;
/* On this branch `limit` is the absolute value of MRB_INT_MIN, which has
no positive counterpart to negate, so it is spelled out instead. */
*v = (i == limit) ? MRB_INT_MIN : -(mrb_int)i;
}
else {
*v = (mrb_int)i;
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19 changes: 19 additions & 0 deletions mrbgems/mruby-bigint/test/bigint.rb
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Expand Up @@ -50,6 +50,25 @@
end
end

assert 'Bigint normalizes the smallest Integer' do
# An Integer's negative range is one wider than its positive one: where
# mrb_int is 64 bits the smallest Integer is -(2**63), and it cannot stay a
# big integer, since the same value built out of fixnums alone has to be
# indistinguishable from it. The exponent that is not this build's is asked
# too, where both spellings already share one representation, two fixnums on
# a 64 bit build and two big integers on a 32 bit one, and the rows hold for
# that reason.
[31, 63].each do |e|
from_bigint = -(2 ** e)
from_fixnum = -(2 ** e - 1) - 1

assert_equal from_fixnum, from_bigint
assert_true from_bigint.eql?(from_fixnum)
assert_true from_fixnum.eql?(from_bigint)
assert_equal from_fixnum.hash, from_bigint.hash
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end
end

assert 'Bigint +' do
n = 1<<65
assert_equal 36893488147419103232, n + 0
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