abacus: add initial bignum library

src/bignum/bignum.cc

@@ -0,0 +1,37 @@
+#include "bignum.hh"
+
+#include <algorithm>
+
+#include <cmath>
+
+namespace abacus::bignum {
+
+BigNum::BigNum(std::int64_t number) {
+    if (number == 0) {
+        return;
+    }
+
+    if (number < 0) {
+        sign_ = -1;
+    } else {
+        sign_ = 1;
+    }
+
+    auto abs = static_cast<std::uint64_t>(std::abs(number));
+    do {
+        digits_.push_back(abs % 10);
+        abs /= 10;
+    } while (abs);
+}
+
+void BigNum::canonicalize() {
+    auto const it = std::find_if(digits_.rbegin(), digits_.rend(),
+                                 [](auto v) { return v != 0; });
+    digits_.erase(it.base(), digits_.end());
+
+    if (digits_.size() == 0) {
+        sign_ = 0;
+    }
+}
+
+} // namespace abacus::bignum