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COMMON: Reduce overflow risk in Common::Rational += and -= operators

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svn-id: r55839
This commit is contained in:
Max Horn 2011-02-09 00:12:02 +00:00
parent 2184326dbc
commit 97bfd60e61
2 changed files with 35 additions and 7 deletions
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26
common/rational.cpp
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@ -57,7 +57,7 @@ void Rational::cancel() {
// Cancel the fraction by dividing both the num and the denom
// by their greatest common divisor.
int gcd = Common::gcd(_num, _denom);
const int gcd = Common::gcd(_num, _denom);
_num /= gcd;
_denom /= gcd;
@ -78,8 +78,14 @@ Rational &Rational::operator=(int right) {
}
Rational &Rational::operator+=(const Rational &right) {
_num = _num * right._denom + right._num * _denom;
_denom = _denom * right._denom;
// Cancel common factors to avoid unnecessary overflow.
// Note that the result is *not* always normalized.
const int gcd = Common::gcd(_denom, right._denom);
_num = _num * (right._denom / gcd);
_denom = _denom / gcd;
_num += right._num * _denom;
_denom *= right._denom;
cancel();
@ -87,8 +93,14 @@ Rational &Rational::operator+=(const Rational &right) {
}
Rational &Rational::operator-=(const Rational &right) {
_num = _num * right._denom - right._num * _denom;
_denom = _denom * right._denom;
// Cancel common factors to avoid unnecessary overflow.
// Note that the result is *not* always normalized.
const int gcd = Common::gcd(_denom, right._denom);
_num = _num * (right._denom / gcd);
_denom = _denom / gcd;
_num -= right._num * _denom;
_denom *= right._denom;
cancel();
@ -98,8 +110,8 @@ Rational &Rational::operator-=(const Rational &right) {
Rational &Rational::operator*=(const Rational &right) {
// Cross-cancel to avoid unnecessary overflow;
// the result then is automatically normalized
int gcd1 = Common::gcd(_num, right._denom);
int gcd2 = Common::gcd(right._num, _denom);
const int gcd1 = Common::gcd(_num, right._denom);
const int gcd2 = Common::gcd(right._num, _denom);
_num = (_num / gcd1) * (right._num / gcd2);
_denom = (_denom / gcd2) * (right._denom / gcd1);

16
test/common/rational.h
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@ -86,6 +86,22 @@ public:
TS_ASSERT_EQUALS(r1 - 1, Common::Rational(-1, 2));
}
void test_add_sub2() {
// Make sure cancelation works correctly
const Common::Rational r0(4, 15); // = 8 / 30
const Common::Rational r1(1, 6); // = 5 / 30
TS_ASSERT_EQUALS(r0 + r1, Common::Rational(13, 30));
TS_ASSERT_EQUALS(r1 + r0, Common::Rational(13, 30));
TS_ASSERT_EQUALS(r0 - r1, Common::Rational(1, 10));
TS_ASSERT_EQUALS(r1 - r0, Common::Rational(-1, 10));
TS_ASSERT_EQUALS(1 + r1, Common::Rational(7, 6));
TS_ASSERT_EQUALS(r1 + 1, Common::Rational(7, 6));
TS_ASSERT_EQUALS(1 - r1, Common::Rational(5, 6));
TS_ASSERT_EQUALS(r1 - 1, Common::Rational(-5, 6));
}
void test_mul() {
const Common::Rational r0(6, 3);
const Common::Rational r1(1, 2);