abacus: bignum: add multiplication

This commit is contained in:
Bruno BELANYI 2021-08-20 22:19:20 +02:00
parent eb47889eef
commit 4a4b9f361c
3 changed files with 85 additions and 0 deletions

View file

@ -69,6 +69,23 @@ digits_type do_substraction(digits_type const& lhs, digits_type const& rhs) {
return complement; return complement;
} }
digits_type do_multiplication(digits_type const& lhs, digits_type const& rhs) {
digits_type res(lhs.size() + rhs.size());
for (std::size_t i = 0; i < lhs.size(); ++i) {
int carry = 0;
for (std::size_t j = 0; j < rhs.size(); ++j) {
int multiplication = lhs[i] * rhs[j];
res[i + j] += multiplication + carry;
carry = res[i + j] / 10;
res[i + j] %= 10;
}
res[i + rhs.size()] += carry;
}
return res;
}
} // namespace } // namespace
BigNum::BigNum(std::int64_t number) { BigNum::BigNum(std::int64_t number) {
@ -188,6 +205,21 @@ void BigNum::substract(BigNum const& rhs) {
assert(is_canonicalized()); assert(is_canonicalized());
} }
void BigNum::multiply(BigNum const& rhs) {
assert(is_canonicalized());
assert(rhs.is_canonicalized());
if (sign_ == 0 || rhs.sign_ == 0) {
*this = BigNum();
return;
}
digits_ = do_multiplication(digits_, rhs.digits_);
sign_ *= rhs.sign_;
canonicalize();
}
bool BigNum::equal(BigNum const& rhs) const { bool BigNum::equal(BigNum const& rhs) const {
assert(is_canonicalized()); assert(is_canonicalized());
assert(rhs.is_canonicalized()); assert(rhs.is_canonicalized());

View file

@ -51,6 +51,17 @@ public:
return ret; return ret;
} }
friend BigNum& operator*=(BigNum& lhs, BigNum const& rhs) {
lhs.multiply(rhs);
return lhs;
}
friend BigNum operator*(BigNum const& lhs, BigNum const& rhs) {
auto ret = lhs;
ret *= rhs;
return ret;
}
friend bool operator==(BigNum const& lhs, BigNum const& rhs) { friend bool operator==(BigNum const& lhs, BigNum const& rhs) {
return lhs.equal(rhs); return lhs.equal(rhs);
} }
@ -82,6 +93,7 @@ private:
void flip_sign(); void flip_sign();
void add(BigNum const& rhs); void add(BigNum const& rhs);
void substract(BigNum const& rhs); void substract(BigNum const& rhs);
void multiply(BigNum const& rhs);
bool equal(BigNum const& rhs) const; bool equal(BigNum const& rhs) const;
bool less_than(BigNum const& rhs) const; bool less_than(BigNum const& rhs) const;

View file

@ -156,3 +156,44 @@ TEST(BigNum, substraction_flips_sign) {
EXPECT_EQ(one - two, minus_one); EXPECT_EQ(one - two, minus_one);
EXPECT_EQ(minus_one - minus_two, one); EXPECT_EQ(minus_one - minus_two, one);
} }
TEST(BigNum, multiplication_zero) {
auto const zero = BigNum(0);
auto const one = BigNum(1);
auto const minus_one = BigNum(-1);
EXPECT_EQ(zero * zero, zero);
EXPECT_EQ(zero * one, zero);
EXPECT_EQ(one * zero, zero);
EXPECT_EQ(zero * minus_one, zero);
EXPECT_EQ(minus_one * zero, zero);
}
TEST(BigNum, multiplication_positive) {
auto const one = BigNum(1);
auto const two = BigNum(2);
auto const four = BigNum(4);
EXPECT_EQ(one * one, one);
EXPECT_EQ(two * one, two);
EXPECT_EQ(one * two, two);
EXPECT_EQ(two * two, four);
}
TEST(BigNum, multiplication_mixed_signs) {
auto const minus_one = BigNum(-1);
auto const one = BigNum(1);
auto const minus_two = BigNum(-2);
auto const two = BigNum(2);
auto const minus_four = BigNum(-4);
auto const four = BigNum(4);
EXPECT_EQ(one * minus_one, minus_one);
EXPECT_EQ(minus_one * one, minus_one);
EXPECT_EQ(minus_one * minus_one, one);
EXPECT_EQ(two * minus_one, minus_two);
EXPECT_EQ(minus_one * two, minus_two);
EXPECT_EQ(two * minus_two, minus_four);
EXPECT_EQ(minus_two * two, minus_four);
EXPECT_EQ(minus_two * minus_two, four);
}