feat: conversion from roman to decimal numbers (#1104)

* feat: added roman_numerals_to_decimal.c

* added newline at end of file

* updating DIRECTORY.md

* Update conversions/roman_numerals_to_decimal.c

Co-authored-by: David Leal <halfpacho@gmail.com>

* Update conversions/roman_numerals_to_decimal.c

Co-authored-by: David Leal <halfpacho@gmail.com>

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: David Leal <halfpacho@gmail.com>

conversions/roman_numerals_to_decimal.c

@@ -0,0 +1,121 @@
+/**
+ * @file
+ * @brief Conversion of [roman numerals](https://en.wikipedia.org/wiki/Roman_numerals) to decimal
+ * @details Roman numerals are an ancient Roman numeral system consisting of the symbols I, V, X, L, C, D, and M
+ * 
+ * @author [Focusucof](https://github.com/Focusucof)
+ */
+
+#include <assert.h> /// for assert
+#include <stdio.h>  /// for IO operations
+#include <string.h> /// for strlen()
+
+/**
+ * @brief Convert roman numeral symbol to a decimal value helper function
+ * @param symbol Roman numeral char
+ * @returns Integer of decimal value for given symbol
+ */
+int symbol(char symbol) {
+    int value = 0;
+    switch(symbol) {
+        case 'I':
+            value = 1;
+            break;
+        case 'V':
+            value = 5;
+            break;
+        case 'X':
+            value = 10;
+            break;
+        case 'L':
+            value = 50;
+            break;
+        case 'C':
+            value = 100;
+            break;
+        case 'D':
+            value = 500;
+            break;
+        case 'M':
+            value = 1000;
+            break;
+    }
+    return value;    
+}
+
+/**
+ * @brief Converts roman numerals into a decimal number
+ * @param input Input roman numeral as a C-string
+ * @returns The converted number in decimal form
+ */
+int roman_to_decimal(char input[]) {
+    int result = 0; // result in decimal
+
+    for(int i = 0; i < strlen(input); i++) {
+        if(strlen(input) > i + 1) {
+            if(symbol(input[i]) >= symbol(input[i + 1])) {
+                result += symbol(input[i]); // add value to sum
+            } else {
+                result += symbol(input[i + 1]) - symbol(input[i]); // if the current symbol is smaller than the next (ex. IV), subtract it from the next symbol
+                i++; // skip over an extra symbol 
+            }
+        } else {
+            result += symbol(input[i]); // add value to sum
+        }
+    }
+    return result;
+}
+
+/**
+ * @brief Self-test implementations
+ * @returns void
+ */
+static void test() {
+    // 1st test
+    char input[] = "MCMIV";
+    int expected = 1904;
+    
+    int output = roman_to_decimal(input);
+
+    printf("TEST 1\n");
+    printf("Input: %s\n", input);
+    printf("Expected Output: %d\n", expected);
+    printf("Output: %d\n", output);
+    assert(output == expected);
+    printf("== TEST PASSED ==\n\n");
+
+    // 2nd test
+    char input2[] = "MMMDCCXXIV";
+    expected = 3724;
+    
+    output = roman_to_decimal(input2);
+
+    printf("TEST 2\n");
+    printf("Input: %s\n", input2);
+    printf("Expected Output: %d\n", expected);
+    printf("Output: %d\n", output);
+    assert(output == expected);
+    printf("== TEST PASSED ==\n\n");
+
+    // 3rd test
+    char input3[] = "III";
+    expected = 3;
+    
+    output = roman_to_decimal(input3);
+
+    printf("TEST 3\n");
+    printf("Input: %s\n", input3);
+    printf("Expected Output: %d\n", expected);
+    printf("Output: %d\n", output);
+    assert(output == expected);
+    printf("== TEST PASSED ==\n\n");
+}
+
+/**
+ * @brief Main function
+ * @returns 0 on exit
+ */
+int main() {
+    test();  // run self-test implementations
+    return 0;
+}