Fix parameter start for mid function (indexed from 1)

Author: PetrPytelka

src/main/java/com/scriptbasic/utility/functions/StringFunctions.java

@@ -1,6 +1,7 @@
 package com.scriptbasic.utility.functions;
 
 import com.scriptbasic.api.BasicFunction;
+import com.scriptbasic.interfaces.BasicRuntimeException;
 
 /**
  * <p>
@@ -94,11 +95,15 @@ static public String left(final String s, final int len) {
      * @param start parameter
      * @param len   parameter
      * @return return value
+     * @throws BasicRuntimeException incorrect parameter
      */
     @BasicFunction(classification = {com.scriptbasic.classification.String.class,
             com.scriptbasic.classification.Utility.class})
-    static public String mid(final String s, final int start, final int len) {
-        return s.substring(start, start + len);
+    static public String mid(final String s, final int start, final int len) throws BasicRuntimeException {
+        if (start < 1) {
+            throw new BasicRuntimeException("Incorrect value in parameter start: " + start);
+        }
+        return s.substring(start - 1, start - 1 + len);
     }
 
     /**

src/test/java/com/scriptbasic/testprograms/TestPrograms.java

@@ -162,7 +162,7 @@ public void testStringConcatenation() throws Exception {
 
     @Test
     public void testStringFunctions() throws Exception {
-        codeTest("TestStringFunctions.bas", "0189");
+        codeTest("TestStringFunctions.bas", "0189123");
     }
 
     @Test

src/test/resources/com/scriptbasic/testprograms/TestStringFunctions.bas

@@ -1,3 +1,4 @@
 v = "0123456789"
 PRINT left(v,2)
 PRINT right(v,2)
+PRINT mid(v,2,3)