feat: limit placeholder generation to uppercase chars

src/main/java/de/dhbwstuttgart/typeinference/unify/model/PlaceholderType.java

@@ -24,17 +24,11 @@ public final class PlaceholderType extends UnifyType{
 	 * Used for generating fresh placeholders.
 	 */
 	public static final ArrayList<String> EXISTING_PLACEHOLDERS = new ArrayList<String>();
-	
-	/**
-	 * Prefix of auto-generated placeholder names.
-	 */
-	protected static String nextName = "gen_";
-	
-	/**
-	 * Random number generator used to generate fresh placeholder name.
-	 */
-	protected static Random rnd = new Random(43558747548978L);
-	
+
+
+	private static final char[] PLACEHOLDER_CHARS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".toCharArray();
+	private static int placeholderCount = 0;
+
 	/**
 	 * True if this object was auto-generated, false if this object was user-generated.
 	 */
@@ -100,16 +94,24 @@ public final class PlaceholderType extends UnifyType{
 	}
 	
 	/**
-	 * Creates a fresh placeholder type with a name that does so far not exist.
+	 * Creates a fresh placeholder type with a name that does so far not exist from the chars A-Z.
 	 * A user could later instantiate a type using the same name that is equivalent to this type.
 	 * @return A fresh placeholder type.
 	 */
 	public synchronized static PlaceholderType freshPlaceholder() {
-		String name = nextName + (char) (rnd.nextInt(22) + 97); // Returns random char between 'a' and 'z'
-		// Add random chars while the name is in use.
-		while(EXISTING_PLACEHOLDERS.contains(name)) {
-			name += (char) (rnd.nextInt(22) + 97); // Returns random char between 'a' and 'z'
+		String name;
+		do {
+			int pc = PlaceholderType.placeholderCount++;
+
+			StringBuilder sb = new StringBuilder();
+			while (pc >= 0) {
+				sb.insert(0, PLACEHOLDER_CHARS[pc % 26]);
+				pc = pc / 26 - 1;
+			}
+			name = sb.toString();
 		}
+		while (EXISTING_PLACEHOLDERS.contains(name));
+
 		return new PlaceholderType(name, true);
 	}