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Simplify power by integer logic #322

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Simplify power by integer logic #322

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19 changes: 11 additions & 8 deletions ext/bigdecimal/bigdecimal.c
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Original file line number Diff line number Diff line change
Expand Up @@ -7655,7 +7655,7 @@ VpFrac(Real *y, Real *x)
VP_EXPORT int
VpPowerByInt(Real *y, Real *x, SIGNED_VALUE n)
{
size_t s, ss;
size_t s;
ssize_t sign;
Real *w1 = NULL;
Real *w2 = NULL;
Expand Down Expand Up @@ -7721,17 +7721,20 @@ VpPowerByInt(Real *y, Real *x, SIGNED_VALUE n)
/* calculation start */

VpAsgn(y, x, 1);
VpAsgn(w1, x, 1);
--n;
while (n > 0) {
VpAsgn(w1, x, 1);
s = 1;
while (ss = s, (s += s) <= (size_t)n) {
s = 1;

while (s <= (size_t)n) {
if (s != 1) {
VpMult(w2, w1, w1);
VpAsgn(w1, w2, 1);
}
n -= (SIGNED_VALUE)ss;
VpMult(w2, y, w1);
VpAsgn(y, w2, 1);
if (s & n) {
VpMult(w2, y, w1);
VpAsgn(y, w2, 1);
}
s <<= 1;
}
if (sign < 0) {
VpDivd(w1, w2, VpConstOne, y);
Expand Down
3 changes: 1 addition & 2 deletions test/bigdecimal/test_bigdecimal.rb
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Expand Up @@ -1660,8 +1660,7 @@ def test_limit
BigDecimal.save_limit do
BigDecimal.limit(1)
x = BigDecimal("3")
assert_equal(90, x ** 4) # OK? must it be 80?
# 3 * 3 * 3 * 3 = 10 * 3 * 3 = 30 * 3 = 90 ???

@tompng tompng May 17, 2025

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Before: 3**4 → (3 * (3**2)) * 3 → (3 * 9) * 3 → round(27) * 3 → 90
After: 3**4 → (3 * 3) * (3**2) → 9 * 9 → round(81) → 80

Although this test case seems to be improved, just a coincidence.
This pull request is not a bug fix. Large error still exist.

BigDecimal.limit 1
(BigDecimal('2')**100).to_i / 2**100 #=> 1577

assert_equal(80, x ** 4)
assert_raise(ArgumentError) { BigDecimal.limit(-1) }

bug7458 = '[ruby-core:50269] [#7458]'
Expand Down