simplified sparse checkout

This is the initial logic to support a simple sparse checkout where
directories to be included can be specified in CheckoutOptions.
This change doesn't fully support the sparse patterns, nor does this
change include the optimization to collapse flie entries in ithe index
that are excluded via the sparse checkout directory patterns included
under the parent directory.

options.go

@@ -291,6 +291,8 @@ type CheckoutOptions struct {
 	// target branch. Force and Keep are mutually exclusive, should not be both
 	// set to true.
 	Keep bool
+	// SparseCheckoutDirectories
+	SparseCheckoutDirectories []string
 }
 
 // Validate validates the fields and sets the default values.

plumbing/format/index/index.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"path/filepath"
+	"strings"
 	"time"
 
 	"github.com/go-git/go-git/v5/plumbing"
@@ -211,3 +212,20 @@ type EndOfIndexEntry struct {
 	//	their contents).
 	Hash plumbing.Hash
 }
+
+// SkipUnless applies patterns in the form of A, A/B, A/B/C
+// to the index to prevent the files from being checked out
+func (i *Index) SkipUnless(patterns []string) {
+	for _, e := range i.Entries {
+		var include bool
+		for _, pattern := range patterns {
+			if strings.HasPrefix(e.Name, pattern) {
+				include = true
+				break
+			}
+		}
+		if !include {
+			e.SkipWorktree = true
+		}
+	}
+}

plumbing/object/treenoder.go

@@ -38,6 +38,10 @@ func NewTreeRootNode(t *Tree) noder.Noder {
 	}
 }
 
+func (t *treeNoder) Skip() bool {
+	return false
+}
+
 func (t *treeNoder) isRoot() bool {
 	return t.name == ""
 }

repository_test.go

@@ -210,6 +210,37 @@ func (s *RepositorySuite) TestCloneWithTags(c *C) {
 	c.Assert(count, Equals, 3)
 }
 
+func (s *RepositorySuite) TestCloneSparse(c *C) {
+	fs := memfs.New()
+	r, err := Clone(memory.NewStorage(), fs, &CloneOptions{
+		URL: s.GetBasicLocalRepositoryURL(),
+	})
+	c.Assert(err, IsNil)
+
+	w, err := r.Worktree()
+	c.Assert(err, IsNil)
+
+	sparseCheckoutDirectories := []string{"go", "json", "php"}
+	c.Assert(w.Checkout(&CheckoutOptions{
+		Branch:                    "refs/heads/master",
+		SparseCheckoutDirectories: sparseCheckoutDirectories,
+	}), IsNil)
+
+	fis, err := fs.ReadDir(".")
+	c.Assert(err, IsNil)
+	for _, fi := range fis {
+		c.Assert(fi.IsDir(), Equals, true)
+		var oneOfSparseCheckoutDirs bool
+
+		for _, sparseCheckoutDirectory := range sparseCheckoutDirectories {
+			if strings.HasPrefix(fi.Name(), sparseCheckoutDirectory) {
+				oneOfSparseCheckoutDirs = true
+			}
+		}
+		c.Assert(oneOfSparseCheckoutDirs, Equals, true)
+	}
+}
+
 func (s *RepositorySuite) TestCreateRemoteAndRemote(c *C) {
 	r, _ := Init(memory.NewStorage(), nil)
 	remote, err := r.CreateRemote(&config.RemoteConfig{

utils/merkletrie/difftree.go

@@ -304,13 +304,38 @@ func DiffTreeContext(ctx context.Context, fromTree, toTree noder.Noder,
 				return nil, err
 			}
 		case onlyToRemains:
-			if err = ret.AddRecursiveInsert(to); err != nil {
-				return nil, err
+			if to.Skip() {
+				if err = ret.AddRecursiveDelete(to); err != nil {
+					return nil, err
+				}
+			} else {
+				if err = ret.AddRecursiveInsert(to); err != nil {
+					return nil, err
+				}
 			}
 			if err = ii.nextTo(); err != nil {
 				return nil, err
 			}
 		case bothHaveNodes:
+			if from.Skip() {
+				if err = ret.AddRecursiveDelete(from); err != nil {
+					return nil, err
+				}
+				if err := ii.nextBoth(); err != nil {
+					return nil, err
+				}
+				break
+			}
+			if to.Skip() {
+				if err = ret.AddRecursiveDelete(to); err != nil {
+					return nil, err
+				}
+				if err := ii.nextBoth(); err != nil {
+					return nil, err
+				}
+				break
+			}
+
 			if err = diffNodes(&ret, ii); err != nil {
 				return nil, err
 			}

utils/merkletrie/filesystem/node.go

@@ -61,6 +61,10 @@ func (n *node) IsDir() bool {
 	return n.isDir
 }
 
+func (n *node) Skip() bool {
+	return false
+}
+
 func (n *node) Children() ([]noder.Noder, error) {
 	if err := n.calculateChildren(); err != nil {
 		return nil, err

utils/merkletrie/index/node.go

@@ -19,6 +19,7 @@ type node struct {
 	entry    *index.Entry
 	children []noder.Noder
 	isDir    bool
+	skip     bool
 }
 
 // NewRootNode returns the root node of a computed tree from a index.Index,
@@ -39,7 +40,7 @@ func NewRootNode(idx *index.Index) noder.Noder {
 				continue
 			}
 
-			n := &node{path: fullpath}
+			n := &node{path: fullpath, skip: e.SkipWorktree}
 			if fullpath == e.Name {
 				n.entry = e
 			} else {
@@ -58,6 +59,10 @@ func (n *node) String() string {
 	return n.path
 }
 
+func (n *node) Skip() bool {
+	return n.skip
+}
+
 // Hash the hash of a filesystem is a 24-byte slice, is the result of
 // concatenating the computed plumbing.Hash of the file as a Blob and its
 // plumbing.FileMode; that way the difftree algorithm will detect changes in the

utils/merkletrie/internal/fsnoder/dir.go

@@ -112,6 +112,10 @@ func (d *dir) NumChildren() (int, error) {
 	return len(d.children), nil
 }
 
+func (d *dir) Skip() bool {
+	return false
+}
+
 const (
 	dirStartMark  = '('
 	dirEndMark    = ')'

utils/merkletrie/internal/fsnoder/file.go

@@ -55,6 +55,10 @@ func (f *file) NumChildren() (int, error) {
 	return 0, nil
 }
 
+func (f *file) Skip() bool {
+	return false
+}
+
 const (
 	fileStartMark = '<'
 	fileEndMark   = '>'

utils/merkletrie/noder/noder.go

@@ -53,6 +53,7 @@ type Noder interface {
 	// implement NumChildren in O(1) while Children is usually more
 	// complex.
 	NumChildren() (int, error)
+	Skip()	bool
 }
 
 // NoChildren represents the children of a noder without children.

utils/merkletrie/noder/noder_test.go

@@ -25,6 +25,7 @@ func (n noderMock) Name() string               { return n.name }
 func (n noderMock) IsDir() bool                { return n.isDir }
 func (n noderMock) Children() ([]Noder, error) { return n.children, nil }
 func (n noderMock) NumChildren() (int, error)  { return len(n.children), nil }
+func (n noderMock) Skip() bool                 { return false }
 
 // Returns a sequence with the noders 3, 2, and 1 from the
 // following diagram:

utils/merkletrie/noder/path.go

@@ -15,6 +15,14 @@ import (
 // not be used.
 type Path []Noder
 
+func (p Path) Skip() bool {
+	if len(p) > 0 {
+		return p.Last().Skip()
+	}
+
+	return false
+}
+
 // String returns the full path of the final noder as a string, using
 // "/" as the separator.
 func (p Path) String() string {

worktree.go

@@ -11,6 +11,8 @@ import (
 	"strings"
 	"sync"
 
+	"github.com/go-git/go-billy/v5"
+	"github.com/go-git/go-billy/v5/util"
 	"github.com/go-git/go-git/v5/config"
 	"github.com/go-git/go-git/v5/plumbing"
 	"github.com/go-git/go-git/v5/plumbing/filemode"
@@ -20,9 +22,6 @@ import (
 	"github.com/go-git/go-git/v5/plumbing/storer"
 	"github.com/go-git/go-git/v5/utils/ioutil"
 	"github.com/go-git/go-git/v5/utils/merkletrie"
-
-	"github.com/go-git/go-billy/v5"
-	"github.com/go-git/go-billy/v5/util"
 )
 
 var (
@@ -183,6 +182,10 @@ func (w *Worktree) Checkout(opts *CheckoutOptions) error {
 		return err
 	}
 
+	if len(opts.SparseCheckoutDirectories) > 0 {
+		return w.ResetSparsely(ro, opts.SparseCheckoutDirectories)
+	}
+
 	return w.Reset(ro)
 }
 func (w *Worktree) createBranch(opts *CheckoutOptions) error {
@@ -263,8 +266,7 @@ func (w *Worktree) setHEADToBranch(branch plumbing.ReferenceName, commit plumbin
 	return w.r.Storer.SetReference(head)
 }
 
-// Reset the worktree to a specified state.
-func (w *Worktree) Reset(opts *ResetOptions) error {
+func (w *Worktree) ResetSparsely(opts *ResetOptions, dirs []string) error {
 	if err := opts.Validate(w.r); err != nil {
 		return err
 	}
@@ -294,7 +296,7 @@ func (w *Worktree) Reset(opts *ResetOptions) error {
 	}
 
 	if opts.Mode == MixedReset || opts.Mode == MergeReset || opts.Mode == HardReset {
-		if err := w.resetIndex(t); err != nil {
+		if err := w.resetIndex(t, dirs); err != nil {
 			return err
 		}
 	}
@@ -308,8 +310,17 @@ func (w *Worktree) Reset(opts *ResetOptions) error {
 	return nil
 }
 
-func (w *Worktree) resetIndex(t *object.Tree) error {
+// Reset the worktree to a specified state.
+func (w *Worktree) Reset(opts *ResetOptions) error {
+	return w.ResetSparsely(opts, nil)
+}
+
+func (w *Worktree) resetIndex(t *object.Tree, dirs []string) error {
 	idx, err := w.r.Storer.Index()
+	if len(dirs) > 0 {
+		idx.SkipUnless(dirs)
+	}
+
 	if err != nil {
 		return err
 	}

worktree_test.go

@@ -10,6 +10,7 @@ import (
 	"path/filepath"
 	"regexp"
 	"runtime"
+	"strings"
 	"testing"
 	"time"
 
@@ -417,6 +418,37 @@ func (s *WorktreeSuite) TestCheckoutSymlink(c *C) {
 	c.Assert(err, IsNil)
 }
 
+func (s *WorktreeSuite) TestCheckoutSparse(c *C) {
+	fs := memfs.New()
+	r, err := Clone(memory.NewStorage(), fs, &CloneOptions{
+		URL: s.GetBasicLocalRepositoryURL(),
+	})
+	c.Assert(err, IsNil)
+
+	w, err := r.Worktree()
+	c.Assert(err, IsNil)
+
+	sparseCheckoutDirectories := []string{"go", "json", "php"}
+	c.Assert(w.Checkout(&CheckoutOptions{
+		SparseCheckoutDirectories: sparseCheckoutDirectories,
+	}), IsNil)
+
+	fis, err := fs.ReadDir("/")
+	c.Assert(err, IsNil)
+
+	for _, fi := range fis {
+		c.Assert(fi.IsDir(), Equals, true)
+		var oneOfSparseCheckoutDirs bool
+
+		for _, sparseCheckoutDirectory := range sparseCheckoutDirectories {
+			if strings.HasPrefix(fi.Name(), sparseCheckoutDirectory) {
+				oneOfSparseCheckoutDirs = true
+			}
+		}
+		c.Assert(oneOfSparseCheckoutDirs, Equals, true)
+	}
+}
+
 func (s *WorktreeSuite) TestFilenameNormalization(c *C) {
 	if runtime.GOOS == "windows" {
 		c.Skip("windows paths may contain non utf-8 sequences")