Add qol generic selector Reduce func (#1356)

## Type of change

- [x] 🌻 Feature
- [x] 🐹 Trivial/Minor

## Issue(s)

* #1133

## Test Plan

- [x] 💪 Manual

src/internal/operations/restore.go

@@ -256,35 +256,9 @@ func formatDetailsForRestoration(
 	sel selectors.Selector,
 	deets *details.Details,
 ) ([]path.Path, error) {
-	var fds *details.Details
+	fds, err := sel.Reduce(ctx, deets)
-
+	if err != nil {
-	switch sel.Service {
+		return nil, err
-	case selectors.ServiceExchange:
-		er, err := sel.ToExchangeRestore()
-		if err != nil {
-			return nil, err
-		}
-
-		// format the details and retrieve the items from kopia
-		fds = er.Reduce(ctx, deets)
-		if len(fds.Entries) == 0 {
-			return nil, selectors.ErrorNoMatchingItems
-		}
-
-	case selectors.ServiceOneDrive:
-		odr, err := sel.ToOneDriveRestore()
-		if err != nil {
-			return nil, err
-		}
-
-		// format the details and retrieve the items from kopia
-		fds = odr.Reduce(ctx, deets)
-		if len(fds.Entries) == 0 {
-			return nil, selectors.ErrorNoMatchingItems
-		}
-
-	default:
-		return nil, errors.Errorf("Service %s not supported", sel.Service)
 	}
 
 	var (

src/pkg/selectors/onedrive.go

@@ -35,6 +35,8 @@ type (
 	}
 )
 
+var _ Reducer = &OneDriveRestore{}
+
 // NewOneDriveBackup produces a new Selector with the service set to ServiceOneDrive.
 func NewOneDriveBackup() *OneDriveBackup {
 	src := OneDriveBackup{

src/pkg/selectors/scopes.go

@@ -10,6 +10,19 @@ import (
 	"github.com/alcionai/corso/src/pkg/path"
 )
 
+// Any returns the set matching any value.
+func Any() []string {
+	return []string{AnyTgt}
+}
+
+// None returns the set matching None of the values.
+// This is primarily a fallback for empty values.  Adding None()
+// to any selector will force all matches() checks on that selector
+// to fail.
+func None() []string {
+	return []string{NoneTgt}
+}
+
 // ---------------------------------------------------------------------------
 // types & interfaces
 // ---------------------------------------------------------------------------

src/pkg/selectors/selectors.go

@@ -97,19 +97,6 @@ func newSelector(s service) Selector {
 	}
 }
 
-// Any returns the set matching any value.
-func Any() []string {
-	return []string{AnyTgt}
-}
-
-// None returns the set matching None of the values.
-// This is primarily a fallback for empty values.  Adding None()
-// to any selector will force all matches() checks on that selector
-// to fail.
-func None() []string {
-	return []string{NoneTgt}
-}
-
 func (s Selector) String() string {
 	bs, err := json.Marshal(s)
 	if err != nil {
@@ -190,6 +177,32 @@ func (s Selector) PathService() path.ServiceType {
 	return serviceToPathType[s.Service]
 }
 
+// Reduce is a quality-of-life interpreter that allows Reduce to be called
+// from the generic selector by interpreting the selector service type rather
+// than have the caller make that interpretation.  Returns an error if the
+// service is unsupported.
+func (s Selector) Reduce(ctx context.Context, deets *details.Details) (*details.Details, error) {
+	var (
+		r   Reducer
+		err error
+	)
+
+	switch s.Service {
+	case ServiceExchange:
+		r, err = s.ToExchangeRestore()
+	case ServiceOneDrive:
+		r, err = s.ToOneDriveRestore()
+	default:
+		return nil, errors.New("service not supported: " + s.Service.String())
+	}
+
+	if err != nil {
+		return nil, err
+	}
+
+	return r.Reduce(ctx, deets), nil
+}
+
 // ---------------------------------------------------------------------------
 // Printing Selectors for Human Reading
 // ---------------------------------------------------------------------------