negative index #77

happened generally when the index is less than (-size) - i.e. counting
in to the array still yields a negative index

Author: aeberhart

src/main/java/com/dashjoin/jsonata/Jsonata.java

@@ -500,7 +500,7 @@ Frame createFrameFromTuple(Frame environment, Map<String, Object> tuple) {
                 // count in from end of array
                 index = ((List)input).size() + index;
             }
-            var item = index<((List)input).size() ? ((List)input).get(index) : null;
+            var item = 0<=index && index<((List)input).size() ? ((List)input).get(index) : null;
             if(item != null) {
                 if(item instanceof List) {
                     results = (List)item;

src/test/java/com/dashjoin/jsonata/ArrayTest.java

@@ -13,6 +13,14 @@
 
 public class ArrayTest {
 
+  @Test
+  public void testNegativeIndex() {
+    Jsonata expr = jsonata("item[-1]");
+    Assertions.assertNull(expr.evaluate(Map.of("item", List.of())));
+    expr = jsonata("$[-1]");
+    Assertions.assertNull(expr.evaluate(List.of()));
+  }
+  
   @Test
   public void testArray() {
     Jsonata expr1 = jsonata("{'key': $append($.[{'x': 'y'}],$.[{'a': 'b'}])}");