Support negative indexing

Author: cburgmer

src/json_path/walker.clj

@@ -65,8 +65,11 @@
                                  (if (= "*" sel)
                                    (select-all (:current context))
                                    (if (sequential? obj)
-                                     (let [index (Integer/parseInt sel)]
-                                       (m/with-context index (nth obj index) (:current context)))
+                                     (let [index (Integer/parseInt sel)
+                                           effective-index (if (< index 0)
+                                                             (+ (count obj) index)
+                                                             index)]
+                                       (m/with-context index (nth obj effective-index) (:current context)))
                                      (throw (Exception. "object must be an array.")))))
    (= :filter (first sel-expr)) (let [obj (:value (:current context))
                                       children (if (map? obj)

test/json_path/test/json_path_test.clj

@@ -17,6 +17,7 @@
   (at-path "$.foo[*]" {:foo ["a", "b", "c"]}) => ["a", "b", "c"]
   (at-path "$[*]" {:foo 1 :bar [2 3]}) => [1 [2 3]]
   (at-path "$..*" {:foo 1 :bar [2 3]}) => [1 [2 3] 2 3]
+  (at-path "$[-2]" [1 2 3]) => 2
   (at-path "$.foo[?(@.bar=\"baz\")].hello"
            {:foo [{:bar "wrong" :hello "goodbye"}
                   {:bar "baz" :hello "world"}]}) => ["world"]

test/json_path/test/walker_test.clj

@@ -43,6 +43,7 @@
 
 (facts
   (walk-selector [:index "1"] {:current (m/root ["foo", "bar", "baz"])}) => (m/create "bar" [1])
+  (walk-selector [:index "-1"] {:current (m/root ["foo", "bar", "baz"])}) => (m/create "baz" [-1])
   (walk-selector [:index "*"] {:current (m/root [:a :b])}) => (list (m/create :a [0]) (m/create :b [1]))
   (walk-selector [:index "*"] {:current (m/root {:foo "bar"})}) => (list (m/create "bar" [:foo]))
   (walk-selector [:index "*"] {:current (m/root 1)}) => '()