abacus: bignum: add log2
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@ -468,4 +468,28 @@ BigNum sqrt(BigNum const& num) {
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return res;
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}
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BigNum log2(BigNum const& num) {
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assert(num.is_canonicalized());
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if (num.is_zero()) {
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throw std::invalid_argument("attempt to take the log2 of zero");
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} else if (num.is_negative()) {
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throw std::invalid_argument(
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"attempt to take the log2 of a negative number");
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}
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auto tmp = num;
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auto res = BigNum(0);
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auto one = BigNum(1);
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while (tmp > one) {
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tmp.digits_ = do_halve(tmp.digits_);
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res += one;
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}
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assert(res.is_canonicalized());
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return res;
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}
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} // namespace abacus::bignum
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@ -91,6 +91,8 @@ public:
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friend BigNum sqrt(BigNum const& num);
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friend BigNum log2(BigNum const& num);
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friend bool operator==(BigNum const& lhs, BigNum const& rhs) {
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return lhs.equal(rhs);
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}
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@ -376,3 +376,24 @@ TEST(BigNum, sqrt) {
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EXPECT_EQ(sqrt(ninety_nine), nine);
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EXPECT_EQ(sqrt(hundred), ten);
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}
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TEST(BigNum, log2) {
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auto const zero = BigNum(0);
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auto const one = BigNum(1);
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auto const two = BigNum(2);
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auto const three = BigNum(3);
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auto const four = BigNum(4);
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auto const five = BigNum(5);
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auto const seven = BigNum(7);
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auto const eight = BigNum(8);
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auto const nine = BigNum(9);
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EXPECT_EQ(log2(one), zero);
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EXPECT_EQ(log2(two), one);
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EXPECT_EQ(log2(three), one);
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EXPECT_EQ(log2(four), two);
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EXPECT_EQ(log2(five), two);
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EXPECT_EQ(log2(seven), two);
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EXPECT_EQ(log2(eight), three);
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EXPECT_EQ(log2(nine), three);
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}
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