allow iterating over a fieldpath's ShapeRefs (#250)

Adds a few utility methods to `pkg/fieldpath.Path` to make it easier to
inspect associated ShapeRefs at different parts of the path:

* `Path.At(int)` returns the path part at the supplied index.
* `Path.CopyAt(int)` returns a new Path up to the supplied index.
* `Path.ShapeRefAt(*shapeRef, int)` returns a ShapeRef corresponding to
  the path part at the supplied index.
* `Path.IterShapeRefs(*shapeRef)` returns a slice of *ShapeRef that can
  be used to iterate over the ShapeRefs associated with each of the
  Path's parts.

Signed-off-by: Jay Pipes <jaypipes@gmail.com>

By submitting this pull request, I confirm that my contribution is made under
the terms of the Apache 2.0 license.

Author: jaypipes

pkg/fieldpath/path.go

@@ -55,6 +55,15 @@ func (p *Path) Pop() (part string) {
 	return part
 }
 
+// At returns the part of the Path at the supplied index, or empty string if
+// index exceeds boundary.
+func (p *Path) At(index int) string {
+	if index < 0 || len(p.parts) == 0 || index > len(p.parts)-1 {
+		return ""
+	}
+	return p.parts[index]
+}
+
 // Front returns the first part of the Path or empty string if the Path has no
 // parts.
 func (p *Path) Front() string {
@@ -92,6 +101,18 @@ func (p *Path) Copy() *Path {
 	return &Path{p.parts}
 }
 
+// CopyAt returns a new Path that is a copy of this Path up to the supplied
+// index.
+//
+// e.g. given Path $A containing "X.Y", $A.CopyAt(0) would return a new Path
+// containing just "X". $A.CopyAt(1) would return a new Path containing "X.Y".
+func (p *Path) CopyAt(index int) *Path {
+	if index < 0 || len(p.parts) == 0 || index > len(p.parts)-1 {
+		return nil
+	}
+	return &Path{p.parts[0 : index+1]}
+}
+
 // Empty returns true if there are no parts to the Path
 func (p *Path) Empty() bool {
 	return len(p.parts) == 0
@@ -175,6 +196,36 @@ func (p *Path) ShapeRef(
 	return compare
 }
 
+// ShapeRefAt returns an aws-sdk-go ShapeRef within the supplied ShapeRef that
+// matches the Path at the supplied index. Returns nil if no matching ShapeRef
+// could be found or index out of bounds.
+func (p *Path) ShapeRefAt(
+	subject *awssdkmodel.ShapeRef,
+	index int,
+) *awssdkmodel.ShapeRef {
+	if subject == nil || p == nil || len(p.parts) == 0 {
+		return nil
+	}
+
+	cp := p.CopyAt(index)
+	if cp == nil {
+		return nil
+	}
+	return cp.ShapeRef(subject)
+}
+
+// IterShapeRefs returns a slice of ShapeRef pointers representing each part of
+// the path
+func (p *Path) IterShapeRefs(
+	subject *awssdkmodel.ShapeRef,
+) []*awssdkmodel.ShapeRef {
+	res := make([]*awssdkmodel.ShapeRef, len(p.parts))
+	for idx, _ := range p.parts {
+		res[idx] = p.ShapeRefAt(subject, idx)
+	}
+	return res
+}
+
 // memberShapeRef returns the named member ShapeRef of the supplied
 // ShapeRef
 func memberShapeRef(

pkg/fieldpath/path_test.go

@@ -34,6 +34,14 @@ func TestBasics(t *testing.T) {
 	require.Equal("Author", pstate.Front())
 	require.Equal("State", pstate.Back())
 
+	require.Equal("Author", pstate.At(0))
+	require.Equal("Address", pstate.At(1))
+	require.Equal("State", pstate.At(2))
+	require.Equal("", pstate.At(3))
+
+	pauth := pstate.CopyAt(0)
+	require.Equal("Author", pauth.String())
+
 	last := pstate.Pop()
 	require.Equal("State", last)
 	require.Equal("Address", pstate.Back())
@@ -148,6 +156,30 @@ func TestShapeRef(t *testing.T) {
 	require.Equal("Name", ref.ShapeName)
 	require.Equal("string", ref.Shape.Type)
 
+	p = fieldpath.FromString("Author")
+	ref = p.ShapeRefAt(authShapeRef, 0)
+	require.NotNil(ref)
+	require.Equal("Author", ref.ShapeName)
+	ref = p.ShapeRefAt(authShapeRef, 1)
+	require.Nil(ref)
+
+	p = fieldpath.FromString("Author.Address")
+	ref = p.ShapeRefAt(authShapeRef, 0)
+	require.NotNil(ref)
+	require.Equal("Author", ref.ShapeName)
+	ref = p.ShapeRefAt(authShapeRef, 1)
+	require.NotNil(ref)
+	require.Equal("Address", ref.ShapeName)
+	ref = p.ShapeRefAt(authShapeRef, 2)
+	require.Nil(ref)
+
+	for idx, shapeRef := range p.IterShapeRefs(authShapeRef) {
+		ref = p.ShapeRefAt(authShapeRef, idx)
+		require.NotNil(shapeRef)
+		require.NotNil(ref)
+		require.Equal(ref.ShapeName, shapeRef.ShapeName)
+	}
+
 	// Path needs to match on outer-most shape first before going into member
 	// refs
 	p = fieldpath.FromString("Address")