C program to convert Roman numerals to integer values

Makefile

rn: rn.c

test: rn
	./rn I IV V VI IX X XI XIV XIX XCIX CI MCMLXVII MD MDC MCD MM
	
.PHONY: test

rn.c

#include <stdio.h>
#include <stdlib.h>
#include <limits.h>

static int romanLetterValue(char letter)
{
    switch (letter)
    {
        case 'M': return 1000;
        case 'D': return 500;
        case 'C': return 100;
        case 'L': return 50;
        case 'X': return 10;
        case 'V': return 5;
        case 'I': return 1;
        default:
            fprintf(stderr, "error: invalid Roman numeral letter '%c'\n", letter);
            exit(1);
    }
}

int intFromRoman(const char *s)
{
    int result = 0;
    int lastValue = INT_MAX;
    for (char ch = *s; ch != 0; ch = *(++s))
    {
        int value = romanLetterValue(ch);
        if (value > lastValue) {
            // We've seen something like "IX" or "XC".
            // Need to undo the addition of the last value,
            // and then add (value - lastValue).
            result += (value - 2 * lastValue);
        }
        else {
            result += value;
        }
        lastValue = value;
    }
    return result;
}

int main(int argc, const char **argv)
{
    for (int i = 1; i < argc; ++i) {
        printf("%s = %d\n", argv[i], intFromRoman(argv[i]));
    }
    exit(0);
}