abacus: bignum: add log10

src/bignum/bignum.cc

@@ -492,4 +492,22 @@ BigNum log2(BigNum const& num) {
     return res;
 }
 
+BigNum log10(BigNum const& num) {
+    assert(num.is_canonicalized());
+    assert(BASE == 10);
+
+    if (num.is_zero()) {
+        throw std::invalid_argument("attempt to take the log10 of zero");
+    } else if (num.is_negative()) {
+        throw std::invalid_argument(
+            "attempt to take the log10 of a negative number");
+    }
+
+    auto res = BigNum(num.digits_.size() - 1);
+
+    assert(res.is_canonicalized());
+
+    return res;
+}
+
 } // namespace abacus::bignum

src/bignum/bignum.hh

@@ -93,6 +93,8 @@ public:
 
     friend BigNum log2(BigNum const& num);
 
+    friend BigNum log10(BigNum const& num);
+
     friend bool operator==(BigNum const& lhs, BigNum const& rhs) {
         return lhs.equal(rhs);
     }

tests/unit/bignum.cc

@@ -397,3 +397,23 @@ TEST(BigNum, log2) {
     EXPECT_EQ(log2(eight), three);
     EXPECT_EQ(log2(nine), three);
 }
+
+TEST(BigNum, log10) {
+    auto const zero = BigNum(0);
+    auto const one = BigNum(1);
+    auto const two = BigNum(2);
+    auto const nine = BigNum(9);
+    auto const ten = BigNum(10);
+    auto const eleven = BigNum(11);
+    auto const ninety_nine = BigNum(99);
+    auto const hundred = BigNum(100);
+    auto const hundred_one = BigNum(101);
+
+    EXPECT_EQ(log10(one), zero);
+    EXPECT_EQ(log10(nine), zero);
+    EXPECT_EQ(log10(ten), one);
+    EXPECT_EQ(log10(eleven), one);
+    EXPECT_EQ(log10(ninety_nine), one);
+    EXPECT_EQ(log10(hundred), two);
+    EXPECT_EQ(log10(hundred_one), two);
+}