abacus: bignum: add input operator

src/bignum/bignum.cc

@@ -105,6 +105,41 @@ std::ostream& BigNum::dump(std::ostream& out) const {
     return out;
 }
 
+std::istream& BigNum::read(std::istream& in) {
+    bool parsed = false;
+    bool leading = true;
+
+    if (in.peek() == '-') {
+        in.get();
+        sign_ = -1;
+    } else {
+        sign_ = 1;
+    }
+
+    digits_type digits;
+    while (std::isdigit(in.peek())) {
+        parsed = true;
+        int digit = in.get() - '0';
+        if (digit != 0 || !leading) {
+            digits.push_back(digit);
+            leading = false;
+        }
+    }
+
+    if (leading) {
+        sign_ = 0;
+    }
+
+    if (!parsed) {
+        in.setstate(std::ios::failbit);
+    } else {
+        std::reverse(digits.begin(), digits.end());
+        digits_ = std::move(digits);
+    }
+
+    return in;
+}
+
 void BigNum::flip_sign() {
     assert(is_canonicalized());
 

src/bignum/bignum.hh

@@ -15,6 +15,10 @@ public:
         return num.dump(out);
     }
 
+    friend std::istream& operator>>(std::istream& in, BigNum& num) {
+        return num.read(in);
+    }
+
     friend BigNum operator+(BigNum const& rhs) {
         return rhs;
     }
@@ -73,6 +77,7 @@ public:
 
 private:
     std::ostream& dump(std::ostream& out) const;
+    std::istream& read(std::istream& in);
 
     void flip_sign();
     void add(BigNum const& rhs);

tests/unit/bignum.cc

@@ -24,6 +24,20 @@ TEST(BigNum, dump) {
     EXPECT_EQ(to_str(forty_two), "42");
 }
 
+TEST(BigNum, read) {
+    auto const from_str = [](auto num) -> BigNum {
+        std::stringstream str(num);
+        BigNum res;
+        EXPECT_TRUE(str >> res);
+        return res;
+    };
+
+    EXPECT_EQ(from_str("0"), BigNum(0));
+    EXPECT_EQ(from_str("1"), BigNum(1));
+    EXPECT_EQ(from_str("-1"), BigNum(-1));
+    EXPECT_EQ(from_str("42"), BigNum(42));
+}
+
 TEST(BigNum, equality) {
     auto const zero = BigNum(0);
     auto const one = BigNum(1);