Ensure that lex and parse methods are only called once per instance

core/src/main/java/it/cavallium/warppi/math/rules/dsl/frontend/Lexer.java

@@ -29,6 +29,7 @@ public class Lexer {
 	private final String source;
 	private final Consumer<? super DslError> errorReporter;
 
+	private boolean used = false;
 	private final List<Token> tokens = new ArrayList<>();
 	private int startOfLexeme = 0;
 	private int curPosition = 0;
@@ -55,8 +56,14 @@ public class Lexer {
 	 * 		   If any errors are reported, this list should not be considered to represent a valid set of DSL rules,
 	 * 		   but it can still be parsed to potentially find additional errors (which may allow the user to fix more
 	 * 		   errors before having to rerun the <code>Lexer</code>).
+	 * @throws IllegalStateException if called multiple times on the same instance.
 	 */
 	public List<Token> lex() {
+		if (used) {
+			throw new IllegalStateException("Lexer.lex can only be called once per instance");
+		}
+		used = true;
+
 		while (!atEnd()) {
 			startOfLexeme = curPosition;
 			lexAndHandleErrors();

core/src/main/java/it/cavallium/warppi/math/rules/dsl/frontend/Parser.java

@@ -29,6 +29,7 @@ public class Parser {
 
 	private final List<Token> tokens;
 	private final Consumer<? super DslError> errorReporter;
+	private boolean used = false;
 	private int currentIndex = 0;
 
 	// For error reporting
@@ -57,8 +58,14 @@ public class Parser {
 	 * 		   but each rule can still be analyzed to look for undefined sub-functions in replacement patterns and
 	 * 		   report them (which may allow the user to fix more errors before having to rerun the <code>Lexer</code>
 	 * 		   and <code>Parser</code>).
+	 * @throws IllegalStateException if called multiple times on the same instance.
 	 */
 	public List<PatternRule> parse() {
+		if (used) {
+			throw new IllegalStateException("Parser.parse can only be called once per instance");
+		}
+		used = true;
+
 		return rules();
 	}
 

core/src/test/java/it/cavallium/warppi/math/rules/dsl/frontend/LexerTest.java

@@ -29,6 +29,13 @@ class LexerTest {
 		assertEquals(expected, lexer.lex());
 	}
 
+	@Test
+	void multipleLexCalls() {
+		final Lexer lexer = new Lexer("", errors::add);
+		lexer.lex();
+		assertThrows(IllegalStateException.class, lexer::lex);
+	}
+
 	@Test
 	void validRule() {
 		final Lexer lexer = new Lexer(

core/src/test/java/it/cavallium/warppi/math/rules/dsl/frontend/ParserTest.java

@@ -38,6 +38,16 @@ class ParserTest {
 		assertEquals(Collections.emptyList(), parser.parse());
 	}
 
+	@Test
+	void multipleParseCalls() {
+		final List<Token> tokens = Collections.singletonList(
+			new Token(EOF, "", 0)
+		);
+		final Parser parser = new Parser(tokens, errors::add);
+		parser.parse();
+		assertThrows(IllegalStateException.class, parser::parse);
+	}
+
 	@Test
 	void validRuleMultipleReplacements() {
 		final List<Token> tokens = Arrays.asList(