add delta tree (#4692)

introduces the delta tree in drive collections

---

#### Does this PR need a docs update or release note?

- [x] ⛔ No

#### Issue(s)

* #4689

#### Test Plan

- [x] ⚡ Unit test

src/internal/m365/collection/drive/delta_tree.go

@@ -0,0 +1,80 @@
+package drive
+
+import (
+	"time"
+
+	"github.com/alcionai/corso/src/pkg/path"
+)
+
+// folderyMcFolderFace owns our delta processing tree.
+type folderyMcFolderFace struct {
+	// tenant/service/resource/category/driveID
+	// (or whatever variant the service defines)
+	// allows the tree to focus only on folder structure,
+	// and minimizes the possibility of multi-prefix path bugs.
+	prefix path.Path
+
+	// the root of the tree;
+	// new, moved, and notMoved collections
+	collections *nodeyMcNodeFace
+
+	// the majority of operations we perform can be handled with
+	// a folder ID lookup instead of re-walking the entire tree.
+	// Ex: adding a new file to its parent folder.
+	folderIDToNode map[string]*nodeyMcNodeFace
+
+	// tombstones don't need to form a tree.
+	// we only need the folder ID and their previous path.
+	tombstones map[string]path.Path
+
+	// it's just a sensible place to store the data, since we're
+	// already pushing file additions through the api.
+	excludeFileIDs map[string]struct{}
+}
+
+func newFolderyMcFolderFace(
+	prefix path.Path,
+) *folderyMcFolderFace {
+	return &folderyMcFolderFace{
+		prefix:         prefix,
+		folderIDToNode: map[string]*nodeyMcNodeFace{},
+		tombstones:     map[string]path.Path{},
+		excludeFileIDs: map[string]struct{}{},
+	}
+}
+
+type nodeyMcNodeFace struct {
+	// required for mid-enumeration folder moves, else we have to walk
+	// the tree completely to remove the node from its old parent.
+	parent *nodeyMcNodeFace
+	// the microsoft item ID.  Mostly because we might as well
+	// attach that to the node if we're also attaching the dir.
+	id string
+	// single directory name, not a path
+	name string
+	// only contains the folders starting at and including '/root:'
+	prev path.Path
+	// map folderID -> node
+	childDirs map[string]*nodeyMcNodeFace
+	// items are keyed by item ID
+	items map[string]time.Time
+	// for special handling protocols around packages
+	isPackage bool
+}
+
+func newNodeyMcNodeFace(
+	parent *nodeyMcNodeFace,
+	id, name string,
+	prev path.Path,
+	isPackage bool,
+) *nodeyMcNodeFace {
+	return &nodeyMcNodeFace{
+		parent:    parent,
+		id:        id,
+		name:      name,
+		prev:      prev,
+		childDirs: map[string]*nodeyMcNodeFace{},
+		items:     map[string]time.Time{},
+		isPackage: isPackage,
+	}
+}

src/internal/m365/collection/drive/delta_tree_test.go

@@ -0,0 +1,56 @@
+package drive
+
+import (
+	"testing"
+
+	"github.com/alcionai/clues"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"github.com/stretchr/testify/suite"
+
+	"github.com/alcionai/corso/src/internal/tester"
+	"github.com/alcionai/corso/src/pkg/path"
+)
+
+type DeltaTreeUnitSuite struct {
+	tester.Suite
+}
+
+func TestDeltaTreeUnitSuite(t *testing.T) {
+	suite.Run(t, &DeltaTreeUnitSuite{Suite: tester.NewUnitSuite(t)})
+}
+
+func (suite *DeltaTreeUnitSuite) TestNewFolderyMcFolderFace() {
+	var (
+		t      = suite.T()
+		p, err = path.BuildPrefix("t", "r", path.OneDriveService, path.FilesCategory)
+	)
+
+	require.NoError(t, err, clues.ToCore(err))
+
+	folderFace := newFolderyMcFolderFace(p)
+	assert.Equal(t, p, folderFace.prefix)
+	assert.Nil(t, folderFace.collections)
+	assert.NotNil(t, folderFace.folderIDToNode)
+	assert.NotNil(t, folderFace.tombstones)
+	assert.NotNil(t, folderFace.excludeFileIDs)
+}
+
+func (suite *DeltaTreeUnitSuite) TestNewNodeyMcNodeFace() {
+	var (
+		t      = suite.T()
+		parent = &nodeyMcNodeFace{}
+		p, err = path.Build("t", "r", path.SharePointService, path.LibrariesCategory, false, "drive-id", "root:")
+	)
+
+	require.NoError(t, err, clues.ToCore(err))
+
+	nodeFace := newNodeyMcNodeFace(parent, "id", "name", p, true)
+	assert.Equal(t, parent, nodeFace.parent)
+	assert.Equal(t, "id", nodeFace.id)
+	assert.Equal(t, "name", nodeFace.name)
+	assert.Equal(t, p, nodeFace.prev)
+	assert.True(t, nodeFace.isPackage)
+	assert.NotNil(t, nodeFace.childDirs)
+	assert.NotNil(t, nodeFace.items)
+}