// Source: "Software Design ...", John A Robinson, Newnes, 2004, page 98.
// Simple Roman number convertor: Doesn't handle subtraction prefices
#include <iostream>
#include <cstdlib>
#include <cctype>		// Needed for isdigit()
using namespace std;
struct convertor		// A convertor stores an arabic integer and its equivalent
				// Roman character. E.g. 10, 'X'
       {
       int value;
       char roman_symbol;
       };

// The array of convertors is declared globally and initialized to contain all relevant
// arabic/roman number pairs.
const convertor conv[7] =
          {
          1, 'I', 5, 'V', 10, 'X', 50, 'L', 100, 'C', 500, 'D', 1000, 'M'
          };

void arabic_to_roman(const char *in, char *out);
void roman_to_arabic(const char *in, long *value);

int main(int argc, char *argv[])
    {
    char outroman[20];
    long outarabic;
    if (argc != 2)
       {
       cerr << "Usage: romanize number (where number is in Arabic or Roman numerals)\n";
       return -1;
       }
    // Read the first character to see whether input is Arabic or Roman
    if (isdigit(argv[1][0]))	// First character is a digit, i.e. an arabic numeral
       {
       arabic_to_roman(argv[1], outroman);
       cout << outroman << '\n';
       }
    else
       {
       roman_to_arabic(argv[1], &outarabic);
       cout << outarabic << '\n';
       }
    return 0;
    }


void arabic_to_roman(const char *in, char *out)
    {
    long value;
    value = atoi(in);	// Convert input to an integer value
    int i = 6;         	// Start at largest numbers
			// (i indexes the convertor array from large values to small)
    while(value >0)
        {
        if ((value - conv[i].value) >= 0)	// Keep outputting current symbol so long as
						// its value can be subtracted from the residue
						// of the input value
           {
           *out++ = conv[i].roman_symbol;	// Output symbol
           value -= conv[i].value;		// Reduce "input" value appropriately
           }
        else
            i--;			// Move to next convertor down (next lowest value)
        }
    *out = '\0';     // Terminating null
    }
void roman_to_arabic(const char *in, long *value)
    {
    *value = 0;
    char c;
    int i = 6;		// Assume Roman number is correctly formed: Starts with highest
			// value symbol and moves down values monotonically through string
    c = *in++;
    while ((c != '\0')&&(i >= 0))
          {
          // Find character
          if (c == conv[i].roman_symbol)		// Matches current symbol
             {
             *value += conv[i].value;			// Add symbol value to output value
             c = *in++;
             }
          else
              i--;					// Move to next convertor down
          }
    }