Add bounds checking with sensible error to UnderscoreFunctionNameSanitizer

Author: guyburton

core/src/main/java/cucumber/runtime/snippets/UnderscoreFunctionNameSanitizer.java

@@ -7,6 +7,9 @@ public String sanitizeFunctionName(String functionName) {
         StringBuilder sanitized = new StringBuilder();
 
         String trimmedFunctionName = functionName.trim();
+        if (trimmedFunctionName.isEmpty()) {
+            throw new IllegalArgumentException("Cannot have empty function name");
+        }
 
         sanitized.append(Character.isJavaIdentifierStart(trimmedFunctionName.charAt(0)) ? trimmedFunctionName.charAt(0) : SUBST);
         for (int i = 1; i < trimmedFunctionName.length(); i++) {

core/src/test/java/cucumber/runtime/snippets/UnderscoreFunctionNameSanitizerTest.java

@@ -0,0 +1,20 @@
+package cucumber.runtime.snippets;
+
+import org.junit.Test;
+import static org.junit.Assert.assertEquals;
+
+public class UnderscoreFunctionNameSanitizerTest {
+
+    private UnderscoreFunctionNameSanitizer sanitizer = new UnderscoreFunctionNameSanitizer();
+
+    @Test
+    public void testSanitizeFunctionName() {
+        assertEquals("_test_function_123", sanitizer.sanitizeFunctionName(".test function 123 "));
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testSanitizeEmptyFunctionName() {
+        sanitizer.sanitizeFunctionName("");
+    }
+
+}