/*
Program: ROMAN.CPP
Author : Kim Moser
Date   : 13 May, 1992
System : IBM PC / Borland C++ 3.0
Descrip: Convert Roman numerals to Arabic and vice versa

Usage  :
*/

#include <iostream.h>
#include <string.h>
#include <ctype.h>

static int roman_digit_value(int ch);
static int roman_digit_value(int ch)
{
	switch (ch) {
		case 'M':
			return (1000);
		case 'C':
			return (100);
		case 'L':
			return (50);
		case 'X':
			return (10);
		case 'V':
			return (5);
		case 'I':
			return (1);
		default:
			return (0);
	}
}

static int is_roman_digit(int ch);
static int is_roman_digit(int ch)
{
	return (roman_digit_value(ch));
}

/*
			I	1
			II	2
			III	3
			IV	4
			V	5
			VI	6
			VII	7
			VIII	8
			IX	9
			X	10
			XI	11
			XII	12
			XIII	13
			XIV	14
			XV	15
			XVI	16
*/

static int roman2arabic(char* s);
static int roman2arabic(char* s)
{
	int arabic=0;
	int ch;
	int ch_val, prev_val=0;

	for (int i=0; (ch = toupper(s[i])) != '\0'; i++) {
		if (is_roman_digit(ch)) {
			ch_val = roman_digit_value(ch);
			if (prev_val && (ch_val > prev_val)) {
				// XIV
				//   ^
				// Subtract 'I', add 'V':
				arabic -= 2*prev_val;
			}
			// XIV
			//   ^
			// Add 'V':
			arabic += ch_val;
		} else {
			arabic = 0;
			break;
		}
		prev_val = roman_digit_value(ch);
	}

	return (arabic);
}

static char* chrcat(char* s, int ch);
static char* chrcat(char* s, int ch)
{
	int len = strlen(s);

	s[len] = ch;
	s[len+1] = '\0';

	return (s);
}

static char* arabic2roman(int n);
static char* arabic2roman(int n)
{
	static char roman[20];
	int digit, prev_digit='\0';
	int value, prev_value=0;

	roman[0] = '\0';

	while (n) {
		for (int i=0; i < 6; i++) {
			switch (i) {
				case 0:
					digit = 'M';
					break;
				case 1:
					digit = 'C';
					break;
				case 2:
					digit = 'L';
					break;
				case 3:
					digit = 'X';
					break;
				case 4:
					digit = 'V';
					break;
				case 5:
					digit = 'I';
					break;
				default:
					// Should never happen
					roman[0] = '\0';
					return (roman);
			}

			// 1992 == MCMLXLII
			// 992     M

			value = roman_digit_value(digit);

			if ((digit != 'M') && (n / value > 3)) {
				while (n / value) {
					chrcat(roman, digit);
					n -= value;
				}
			} else {
				while (n && (n >= value)) {
					chrcat(roman, digit);
					n -= value;
				}
			}

			prev_digit = digit;
			prev_value = value;
		}
	}

	return (roman);
}

main()
{
	char roman[20];

	cout << "\n\nGive me a Roman number: ";
	cin.width(sizeof(roman)-1);
	cin >> roman;

	cout << roman << " = " << roman2arabic(roman) << '\n';

	return (0);
}