HackRank

spartan.cpp

#include <string>
#include <algorithm>
#include <map>
#include <cassert>

namespace detail {
    std::map<char, unsigned int> numerals = { {'I', 1}, {'E', 3}, {'V', 5}, {'X', 10}, {'L', 50} };
    
    unsigned int numeral_rank(char c) {
        return numerals[c];
    }
    
    bool isValidSpartanNumeralString(std::string const& sn) {
        static std::string cs = "IEVXL";
        return std::all_of(sn.begin(), sn.end(), [&] (char c) { return cs.find(c) != std::string::npos; });
    }
}

int ConvertSpartanNumeral(std::string const& sn) {
    using detail::numeral_rank;
    using detail::isValidSpartanNumeralString;
    
    if (!sn.size() || !isValidSpartanNumeralString(sn)) {
        // invalid Roman numeral
        return -1;
    }
    
    int total = numeral_rank(sn[0]);
    if (sn.size() == 1) {
        return total;
    }
    
    char prev = sn[0];
    bool lesser = false;
    int rank = 1;
    
    for (int i = 1; i < sn.size(); ++i) {
        if (numeral_rank(sn[i]) < numeral_rank(prev)) {
            lesser = true;
            rank = -1;
        } else if (lesser && (numeral_rank(sn[i]) == numeral_rank(prev))) {
            rank = -1;
        }
        else {
            lesser = false;
            rank = 1;
        }
        
        total += rank * numeral_rank(sn[i]);
        prev = sn[i];
    }
    
    return total;
}

int main() {
    assert(ConvertSpartanNumeral("II") == 2);
    assert(ConvertSpartanNumeral("III") == 3);
    assert(ConvertSpartanNumeral("VII") == 3);
    assert(ConvertSpartanNumeral("VX") == 15);
    assert(ConvertSpartanNumeral("VVV") == 15);
    assert(ConvertSpartanNumeral("LVVV") == 35);
    assert(ConvertSpartanNumeral("LIX") == 59);
    assert(ConvertSpartanNumeral("VIIXL") == 63);
    assert(ConvertSpartanNumeral("EXL") == 63);
}