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.
 //