abacus: bignum: add log2

src/bignum/bignum.cc

@@ -468,4 +468,28 @@ BigNum sqrt(BigNum const& num) {
     return res;
 }
 
+BigNum log2(BigNum const& num) {
+    assert(num.is_canonicalized());
+
+    if (num.is_zero()) {
+        throw std::invalid_argument("attempt to take the log2 of zero");
+    } else if (num.is_negative()) {
+        throw std::invalid_argument(
+            "attempt to take the log2 of a negative number");
+    }
+
+    auto tmp = num;
+    auto res = BigNum(0);
+    auto one = BigNum(1);
+
+    while (tmp > one) {
+        tmp.digits_ = do_halve(tmp.digits_);
+        res += one;
+    }
+
+    assert(res.is_canonicalized());
+
+    return res;
+}
+
 } // namespace abacus::bignum

src/bignum/bignum.hh

@@ -91,6 +91,8 @@ public:
 
     friend BigNum sqrt(BigNum const& num);
 
+    friend BigNum log2(BigNum const& num);
+
     friend bool operator==(BigNum const& lhs, BigNum const& rhs) {
         return lhs.equal(rhs);
     }

tests/unit/bignum.cc

@@ -376,3 +376,24 @@ TEST(BigNum, sqrt) {
     EXPECT_EQ(sqrt(ninety_nine), nine);
     EXPECT_EQ(sqrt(hundred), ten);
 }
+
+TEST(BigNum, log2) {
+    auto const zero = BigNum(0);
+    auto const one = BigNum(1);
+    auto const two = BigNum(2);
+    auto const three = BigNum(3);
+    auto const four = BigNum(4);
+    auto const five = BigNum(5);
+    auto const seven = BigNum(7);
+    auto const eight = BigNum(8);
+    auto const nine = BigNum(9);
+
+    EXPECT_EQ(log2(one), zero);
+    EXPECT_EQ(log2(two), one);
+    EXPECT_EQ(log2(three), one);
+    EXPECT_EQ(log2(four), two);
+    EXPECT_EQ(log2(five), two);
+    EXPECT_EQ(log2(seven), two);
+    EXPECT_EQ(log2(eight), three);
+    EXPECT_EQ(log2(nine), three);
+}