refactor: Enhance docs, add more tests in RomanToInteger

src/main/java/com/thealgorithms/conversions/RomanToInteger.java

@@ -3,9 +3,27 @@
 import java.util.HashMap;
 import java.util.Map;
 
+/**
+ * A utility class to convert Roman numerals into integers.
+ *
+ * <p>Roman numerals are based on seven symbols given below:
+ * <ul>
+ *   <li>I = 1</li>
+ *   <li>V = 5</li>
+ *   <li>X = 10</li>
+ *   <li>L = 50</li>
+ *   <li>C = 100</li>
+ *   <li>D = 500</li>
+ *   <li>M = 1000</li>
+ * </ul>
+ *
+ * <p>If a smaller numeral appears before a larger numeral, it is subtracted.
+ * Otherwise, it is added. For example:
+ * <pre>
+ *   MCMXCIV = 1000 + (1000 - 100) + (100 - 10) + (5 - 1) = 1994
+ * </pre>
+ */
 public final class RomanToInteger {
-    private RomanToInteger() {
-    }
 
     private static final Map<Character, Integer> ROMAN_TO_INT = new HashMap<>() {
         {
@@ -19,44 +37,53 @@ private RomanToInteger() {
         }
     };
 
+    private RomanToInteger() {
+    }
+
+    /**
+     * Converts a single Roman numeral character to its integer value.
+     *
+     * @param symbol the Roman numeral character
+     * @return the corresponding integer value
+     * @throws IllegalArgumentException if the symbol is not a valid Roman numeral
+     */
     private static int romanSymbolToInt(final char symbol) {
         return ROMAN_TO_INT.computeIfAbsent(symbol, c -> { throw new IllegalArgumentException("Unknown Roman symbol: " + c); });
     }
 
-    // Roman Number = Roman Numerals
-
     /**
-     * This function convert Roman number into Integer
+     * Converts a Roman numeral string to its integer equivalent.
+     * Steps:
+     * <ol>
+     *     <li>Iterate over the string from right to left.</li>
+     *     <li>For each character, convert it to an integer value.</li>
+     *     <li>If the current value is greater than or equal to the max previous value, add it.</li>
+     *     <li>Otherwise, subtract it from the sum.</li>
+     *     <li>Update the max previous value.</li>
+     *     <li>Return the sum.</li>
+     * </ol>
      *
-     * @param a Roman number string
-     * @return integer
+     * @param roman the Roman numeral string
+     * @return the integer value of the Roman numeral
+     * @throws IllegalArgumentException if the input contains invalid Roman characters
+     * @throws NullPointerException if the input is {@code null}
      */
-    public static int romanToInt(String a) {
-        a = a.toUpperCase();
-        char prev = ' ';
+    public static int romanToInt(String roman) {
+        if (roman == null) {
+            throw new NullPointerException("Input cannot be null");
+        }
 
+        roman = roman.toUpperCase();
         int sum = 0;
-
-        int newPrev = 0;
-        for (int i = a.length() - 1; i >= 0; i--) {
-            char c = a.charAt(i);
-
-            if (prev != ' ') {
-                // checking current Number greater than previous or not
-                newPrev = romanSymbolToInt(prev) > newPrev ? romanSymbolToInt(prev) : newPrev;
-            }
-
-            int currentNum = romanSymbolToInt(c);
-
-            // if current number greater than prev max previous then add
-            if (currentNum >= newPrev) {
-                sum += currentNum;
+        int maxPrevValue = 0;
+        for (int i = roman.length() - 1; i >= 0; i--) {
+            int currentValue = romanSymbolToInt(roman.charAt(i));
+            if (currentValue >= maxPrevValue) {
+                sum += currentValue;
+                maxPrevValue = currentValue;
             } else {
-                // subtract upcoming number until upcoming number not greater than prev max
-                sum -= currentNum;
+                sum -= currentValue;
             }
-
-            prev = c;
         }
 
         return sum;

src/test/java/com/thealgorithms/conversions/RomanToIntegerTest.java

@@ -8,16 +8,31 @@
 public class RomanToIntegerTest {
 
     @Test
-    public void testRomanToInteger() {
+    public void testValidRomanToInteger() {
         assertEquals(1994, RomanToInteger.romanToInt("MCMXCIV"));
         assertEquals(58, RomanToInteger.romanToInt("LVIII"));
         assertEquals(1804, RomanToInteger.romanToInt("MDCCCIV"));
+        assertEquals(9, RomanToInteger.romanToInt("IX"));
+        assertEquals(4, RomanToInteger.romanToInt("IV"));
+        assertEquals(3000, RomanToInteger.romanToInt("MMM"));
     }
 
     @Test
-    void testRomanToIntegerThrows() {
+    public void testLowercaseInput() {
+        assertEquals(1994, RomanToInteger.romanToInt("mcmxciv"));
+        assertEquals(58, RomanToInteger.romanToInt("lviii"));
+    }
+
+    @Test
+    public void testInvalidRomanNumerals() {
         assertThrows(IllegalArgumentException.class, () -> RomanToInteger.romanToInt("Z"));
         assertThrows(IllegalArgumentException.class, () -> RomanToInteger.romanToInt("MZI"));
         assertThrows(IllegalArgumentException.class, () -> RomanToInteger.romanToInt("MMMO"));
     }
+
+    @Test
+    public void testEmptyAndNullInput() {
+        assertEquals(0, RomanToInteger.romanToInt("")); // Empty string case
+        assertThrows(NullPointerException.class, () -> RomanToInteger.romanToInt(null)); // Null input case
+    }
 }