Handle negative array indices without panicking

simplejson.go

@@ -149,7 +149,7 @@ func (j *Json) GetPath(branch ...string) *Json {
 func (j *Json) GetIndex(index int) *Json {
 	a, err := j.Array()
 	if err == nil {
-		if len(a) > index {
+		if index >= 0 && index < len(a) {
 			return &Json{a[index]}
 		}
 	}

simplejson_index_test.go

@@ -0,0 +1,29 @@
+package simplejson
+
+import "testing"
+
+func TestGetIndexBounds(t *testing.T) {
+	for _, body := range []string{`[]`, `["first", "last"]`, `null`, `{}`} {
+		j, err := NewJson([]byte(body))
+		if err != nil {
+			t.Fatal(err)
+		}
+		for _, index := range []int{-int(^uint(0)>>1) - 1, -1, 2, int(^uint(0) >> 1)} {
+			if got := j.GetIndex(index); got == nil || got.Interface() != nil {
+				t.Errorf("GetIndex(%d) on %s = %#v, want a null Json", index, body, got)
+			}
+		}
+	}
+	j, err := NewJson([]byte(`["first", "last"]`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i, want := range []string{"first", "last"} {
+		if got := j.GetIndex(i).MustString(); got != want {
+			t.Errorf("GetIndex(%d) = %q, want %q", i, got, want)
+		}
+	}
+	if got := j.GetIndex(-1).Get("missing").MustString("fallback"); got != "fallback" {
+		t.Fatalf("chained missing index = %q", got)
+	}
+}