Added RelFrom command and tests (also for RelTo)

Author: cmaglie

paths.go

@@ -128,7 +128,10 @@ func (p *Path) HasSuffix(suffixies ...string) bool {
 }
 
 // RelTo returns a relative Path that is lexically equivalent to r when
-// joined to the current Path
+// joined to the current Path.
+//
+// For example paths.New("/my/path/ab/cd").RelTo(paths.New("/my/path"))
+// returns "../..".
 func (p *Path) RelTo(r *Path) (*Path, error) {
 	rel, err := filepath.Rel(p.path, r.path)
 	if err != nil {
@@ -137,6 +140,19 @@ func (p *Path) RelTo(r *Path) (*Path, error) {
 	return New(rel), nil
 }
 
+// RelFrom returns a relative Path that when joined with r is lexically
+// equivalent to the current path.
+//
+// For example paths.New("/my/path/ab/cd").RelFrom(paths.New("/my/path"))
+// returns "ab/cd".
+func (p *Path) RelFrom(r *Path) (*Path, error) {
+	rel, err := filepath.Rel(r.path, p.path)
+	if err != nil {
+		return nil, err
+	}
+	return New(rel), nil
+}
+
 // Abs returns the absolute path of the current Path
 func (p *Path) Abs() (*Path, error) {
 	abs, err := filepath.Abs(p.path)

paths_test.go

@@ -311,3 +311,29 @@ func TestEquivalentPaths(t *testing.T) {
 	require.True(t, wd.Join("file1").EquivalentTo(New("file1")))
 	require.True(t, wd.Join("file1").EquivalentTo(New("file1", "abc", "..")))
 }
+
+func TestRelativeTo(t *testing.T) {
+	res, err := New("/my/abs/path/123/456").RelTo(New("/my/abs/path"))
+	require.NoError(t, err)
+	require.Equal(t, "../..", res.String())
+
+	res, err = New("/my/abs/path").RelTo(New("/my/abs/path/123/456"))
+	require.NoError(t, err)
+	require.Equal(t, "123/456", res.String())
+
+	res, err = New("my/path").RelTo(New("/other/path"))
+	require.Error(t, err)
+	require.Nil(t, res)
+
+	res, err = New("/my/abs/path/123/456").RelFrom(New("/my/abs/path"))
+	require.Equal(t, "123/456", res.String())
+	require.NoError(t, err)
+
+	res, err = New("/my/abs/path").RelFrom(New("/my/abs/path/123/456"))
+	require.NoError(t, err)
+	require.Equal(t, "../..", res.String())
+
+	res, err = New("my/path").RelFrom(New("/other/path"))
+	require.Error(t, err)
+	require.Nil(t, res)
+}