Add simple fraction adder

leetcode/592.cpp

@@ -0,0 +1,86 @@
+#include <cstdlib>
+#include <numeric>
+#include <string>
+#include <string_view>
+#include <tuple>
+#include <utility>
+#include <vector>
+
+#include <gtest/gtest.h>
+
+using std::make_pair;
+using std::pair;
+using std::string_view;
+
+struct fraction {
+    constexpr fraction() : den(0), num(0), neg(false) {}
+    int den;
+    int num;
+    bool neg;
+};
+
+constexpr auto parse_number(string_view expr, const size_t i) -> pair<int, size_t> {
+    if (expr[i] == '1')
+        return i < expr.length() - 1 and expr[i + 1] == '0' ? make_pair(10, i + 2) : make_pair( 1, i + 1);
+    else
+        return {expr[i] - '0', i + 1};
+}
+
+constexpr auto parse_fraction(string_view expression, const size_t i = 0) -> pair<fraction, size_t> {
+    fraction f;
+    size_t j = i;
+    if (expression[j] == '+' or expression[j] == '-') {
+        f.neg = expression[j] == '-';
+        j++;
+    } 
+    std::tie(f.num, j) = parse_number(expression, j);
+    j++;
+    std::tie(f.den, j) = parse_number(expression, j);
+    return {f, j};
+}
+
+constexpr auto reduce(const std::vector<fraction>& fs) -> fraction {
+    fraction result;
+    int prod = 1;
+    for (const fraction& f : fs)
+        prod *= f.den;
+    result.den = prod;
+    for (const fraction& f : fs)
+        result.num += (f.neg ? -1 : 1) * f.num * (prod / f.den);
+    const int gcd = std::gcd(result.num, result.den);
+    result.num /= gcd;
+    result.den /= gcd;
+    if (result.num < 0) {
+        result.num *= -1;
+        result.neg = true;
+    }
+    return result;
+}
+
+constexpr auto fraction_addition(std::string expression) -> std::string {
+    std::vector<fraction> fractions;
+    auto [f, i] = parse_fraction(expression);
+    fractions.push_back(f);
+    while (i < expression.length()) {
+        std::tie(f, i) = parse_fraction(expression, i);
+        fractions.push_back(f);
+    }
+    const fraction result = reduce(fractions);
+    return (result.neg ? "-" : "") + std::to_string(result.num) + "/" + std::to_string(result.den);
+}
+
+// try with <charconv>'s to_chars instead
+//using namespace std::literals;
+//static_assert(fraction_addition("-1/2+1/2") == "0/1");
+
+
+TEST(FractionAddition, a) { EXPECT_EQ(fraction_addition("-1/2+1/2"),     "0/1"); }
+TEST(FractionAddition, b) { EXPECT_EQ(fraction_addition("-1/2+1/2+1/3"), "1/3"); }
+TEST(FractionAddition, c) { EXPECT_EQ(fraction_addition(" 1/3-1/2"),    "-1/6"); }
+
+TEST(FractionAddition, d__expression_with_a_10) { EXPECT_EQ(fraction_addition("-5/2+10/3+7/9"), "29/18"); }
+
+int main(int argc, char **argv) {
+    testing::InitGoogleTest(&argc, argv);
+    return RUN_ALL_TESTS();
+}