abacus: bignum: add output operator

src/bignum/bignum.cc

@@ -1,6 +1,8 @@
 #include "bignum.hh"
 
 #include <algorithm>
+#include <iostream>
+#include <iterator>
 
 #include <cassert>
 #include <cmath>
@@ -27,6 +29,20 @@ BigNum::BigNum(std::int64_t number) {
     assert(is_canonicalized());
 }
 
+std::ostream& BigNum::dump(std::ostream& out) const {
+    if (sign_ == 0) {
+        return out << '0';
+    }
+
+    if (sign_ < 0) {
+        out << '-';
+    }
+    std::copy(digits_.rbegin(), digits_.rend(),
+              std::ostream_iterator<int>(out));
+
+    return out;
+}
+
 void BigNum::flip_sign() {
     assert(is_canonicalized());
 

src/bignum/bignum.hh

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <iosfwd>
 #include <vector>
 
 #include <cstdint>
@@ -10,6 +11,10 @@ class BigNum {
 public:
     explicit BigNum(std::int64_t number);
 
+    friend std::ostream& operator<<(std::ostream& out, BigNum const& num) {
+        return num.dump(out);
+    }
+
     friend BigNum operator+(BigNum const& rhs) {
         return rhs;
     }
@@ -45,6 +50,8 @@ public:
     }
 
 private:
+    std::ostream& dump(std::ostream& out) const;
+
     void flip_sign();
 
     bool equal(BigNum const& rhs) const;