Fixed the evaluation of 'substring' built-in function

According to the XQuery specification, this function must graciously
handle out-of-range 'startPos' and 'length' argument values.

See http://www.w3.org/TR/xquery-operators/#func-substring

pmml-evaluator/src/main/java/org/jpmml/evaluator/Functions.java

@@ -404,13 +404,18 @@ public FieldValue evaluate(List<FieldValue> arguments){
 				throw new FunctionException(this, "Invalid position value " + position + ". Must be equal or greater than 1");
 			}
 
+			// "The first character of a string is located at position 1 (not position 0)"
+			int javaPosition = Math.min(position - 1, string.length());
+
 			int length = (arguments.get(2)).asInteger();
 			if(length < 0){
 				throw new FunctionException(this, "Invalid length value " + length);
 			}
 
-			// "The first character of a string is located at position 1 (not position 0)"
-			String result = string.substring(position - 1, (position + length) - 1);
+			int javaLength = Math.min(length, (string.length() - javaPosition));
+
+			// This expression must never throw a StringIndexOutOfBoundsException
+			String result = string.substring(javaPosition, javaPosition + javaLength);
 
 			return FieldValueUtil.create(result);
 		}

pmml-evaluator/src/test/java/org/jpmml/evaluator/FunctionTest.java

@@ -217,11 +217,21 @@ public void evaluateStringFunctions(){
 		assertEquals("VALUE", evaluate(Functions.UPPERCASE, "Value"));
 		assertEquals("value", evaluate(Functions.LOWERCASE, "Value"));
 
+		assertEquals("Bc9", evaluate(Functions.SUBSTRING, "aBc9x", 2, 3));
+
+		assertEquals("", evaluate(Functions.SUBSTRING, "", 1, 0));
+		assertEquals("", evaluate(Functions.SUBSTRING, "", 1, Integer.MAX_VALUE));
+		assertEquals("", evaluate(Functions.SUBSTRING, "", Integer.MAX_VALUE, 0));
+		assertEquals("", evaluate(Functions.SUBSTRING, "", Integer.MAX_VALUE, Integer.MAX_VALUE));
+
 		assertEquals("", evaluate(Functions.SUBSTRING, "value", 1, 0));
+		assertEquals("valu", evaluate(Functions.SUBSTRING, "value", 1, 4));
 		assertEquals("value", evaluate(Functions.SUBSTRING, "value", 1, 5));
-
+		assertEquals("value", evaluate(Functions.SUBSTRING, "value", 1, Integer.MAX_VALUE));
 		assertEquals("alue", evaluate(Functions.SUBSTRING, "value", 2, 4));
-		assertEquals("valu", evaluate(Functions.SUBSTRING, "value", 1, 4));
+		assertEquals("alue", evaluate(Functions.SUBSTRING, "value", 2, Integer.MAX_VALUE));
+		assertEquals("", evaluate(Functions.SUBSTRING, "value", 6, Integer.MAX_VALUE));
+		assertEquals("", evaluate(Functions.SUBSTRING, "value", Integer.MAX_VALUE, Integer.MAX_VALUE));
 
 		assertEquals("value", evaluate(Functions.TRIM_BLANKS, "\tvalue\t"));
 	}