Allow negative indexes in DecodePath

decoder.go

@@ -147,13 +147,21 @@ PATH:
  // XXX - use type names in errors.
  return fmt.Errorf("expected a slice for %d but found %d", v, typeNum)
  }
- if size < uint(v) {
- // Slice is smaller than index, not found
- return nil
+ var i uint
+ if v < 0 {
+ if size < uint(-v) {
+ // Slice is smaller than negative index, not found
+ return nil
+ }
+ i = size - uint(-v)
+ } else {
+ if size <= uint(v) {
+ // Slice is smaller than index, not found
+ return nil
+ }
+ i = uint(v)
  }
- // TODO: support negative indexes? Seems useful for subdivisions in
- // particular.
- offset, err = d.nextValueOffset(offset, uint(v))
+ offset, err = d.nextValueOffset(offset, i)
  if err != nil {
  return err
  }

reader_test.go

@@ -336,11 +336,33 @@ func TestDecodePath(t *testing.T) {
  require.NoError(t, result.DecodePath(&u, "array", 0))
  assert.Equal(t, uint(1), u)
 
- require.NoError(t, result.DecodePath(&u, "array", 2))
- assert.Equal(t, uint(3), u)
+ var u2 uint
+ require.NoError(t, result.DecodePath(&u2, "array", 2))
+ assert.Equal(t, uint(3), u2)
 
- require.NoError(t, result.DecodePath(&u, "map", "mapX", "arrayX", 1))
- assert.Equal(t, uint(8), u)
+ // This is past the end of the array
+ var u3 uint
+ require.NoError(t, result.DecodePath(&u3, "array", 3))
+ assert.Equal(t, uint(0), u3)
+
+ // Negative offsets
+
+ var n1 uint
+ require.NoError(t, result.DecodePath(&n1, "array", -1))
+ assert.Equal(t, uint(3), n1)
+
+ var n2 uint
+ require.NoError(t, result.DecodePath(&n2, "array", -3))
+ assert.Equal(t, uint(1), n2)
+
+ var u4 uint
+ require.NoError(t, result.DecodePath(&u4, "map", "mapX", "arrayX", 1))
+ assert.Equal(t, uint(8), u4)
+
+ // Does key not exist
+ var ne uint
+ require.NoError(t, result.DecodePath(&ne, "does-not-exist", 1))
+ assert.Equal(t, uint(0), ne)
 }
 
 type TestInterface interface {

result.go

@@ -62,7 +62,9 @@ func (r Result) Decode(v any) error {
 // value.
 //
 // For maps, string path elements are used as keys.
-// For arrays, int path elements are used as indices.
+// For arrays, int path elements are used as indices. A negative offset will
+// return values from the end of the array, e.g., -1 will return the last
+// element.
 //
 // If the path is empty, the entire data structure is decoded into v.
 //