add simple difftree implementation (#63)

* add simple difftree implementation

* strings.Compare needs at least go1.5

1 files changed, 194 insertions, 0 deletions

diff --git a/utils/difftree/difftree.go b/utils/difftree/difftree.go
new file mode 100644
index 0000000..76bdb84
--- /dev/null
+++ b/utils/difftree/difftree.go
@@ -0,0 +1,194 @@
+package difftree
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"sort"
+	"strings"
+
+	"gopkg.in/src-d/go-git.v3"
+	"gopkg.in/src-d/go-git.v3/core"
+)
+
+type Action int
+
+func (a Action) String() string {
+	switch a {
+	case Insert:
+		return "Insert"
+	case Delete:
+		return "Delete"
+	case Modify:
+		return "Modify"
+	default:
+		panic(fmt.Sprintf("unsupported action: %d", a))
+	}
+}
+
+const (
+	Insert Action = iota
+	Delete
+	Modify
+)
+
+type Change struct {
+	Action
+	File *git.File
+}
+
+func (c *Change) String() string {
+	return fmt.Sprintf("<Action: %s, Path: %s, Size: %d>", c.Action, c.File.Name, c.File.Size)
+}
+
+type Changes []*Change
+
+func newEmpty() Changes {
+	return make([]*Change, 0, 0)
+}
+
+func New(a, b *git.Tree) (Changes, error) {
+	if a == b {
+		return newEmpty(), nil
+	}
+
+	if a == nil || b == nil {
+		return newWithEmpty(a, b)
+	}
+
+	return newDiffTree(a, b)
+}
+
+func (c Changes) Len() int {
+	return len(c)
+}
+
+func (c Changes) Swap(i, j int) {
+	c[i], c[j] = c[j], c[i]
+}
+
+func (c Changes) Less(i, j int) bool {
+	return strings.Compare(c[i].File.Name, c[j].File.Name) < 0
+}
+
+func (c Changes) String() string {
+	var buffer bytes.Buffer
+	buffer.WriteString("[")
+	comma := ""
+	for _, v := range c {
+		buffer.WriteString(comma)
+		buffer.WriteString(v.String())
+		comma = ", "
+	}
+	buffer.WriteString("]")
+
+	return buffer.String()
+}
+
+func newWithEmpty(a, b *git.Tree) (Changes, error) {
+	changes := newEmpty()
+
+	var action Action
+	var tree *git.Tree
+	if a == nil {
+		action = Insert
+		tree = b
+	} else {
+		action = Delete
+		tree = a
+	}
+
+	iter := tree.Files()
+	defer iter.Close()
+
+	for {
+		file, err := iter.Next()
+		if err == io.EOF {
+			break
+		} else if err != nil {
+			return nil, fmt.Errorf("cannot get next file: %s", err)
+		}
+
+		insert := &Change{
+			Action: action,
+			File:   file,
+		}
+		changes = append(changes, insert)
+	}
+
+	return changes, nil
+}
+
+// FIXME: this is very inefficient, but correct.
+// The proper way to do this is to implement a diff-tree algorithm,
+// while taking advantage of the tree hashes to avoid traversing
+// subtrees when the hash is equal in both inputs.
+func newDiffTree(a, b *git.Tree) (Changes, error) {
+	result := newEmpty()
+
+	aChanges, err := newWithEmpty(a, nil)
+	if err != nil {
+		return nil, fmt.Errorf("cannot create nil-diff of source tree: %s", err)
+	}
+	sort.Sort(aChanges)
+
+	bChanges, err := newWithEmpty(nil, b)
+	if err != nil {
+		return nil, fmt.Errorf("cannot create nil-diff of destination tree: %s", err)
+	}
+	sort.Sort(bChanges)
+
+	for len(aChanges) > 0 && len(bChanges) > 0 {
+		switch comp := strings.Compare(aChanges[0].File.Name, bChanges[0].File.Name); {
+		case comp == 0: // append as "Modify" or ignore if not changed
+			modified, err := hasChange(a, b, aChanges[0].File.Name)
+			if err != nil {
+				return nil, err
+			}
+			if modified {
+				bChanges[0].Action = Modify
+				result = append(result, bChanges[0])
+			} else {
+			}
+			aChanges = aChanges[1:]
+			bChanges = bChanges[1:]
+		case comp < 0: // delete first a change
+			result = append(result, aChanges[0])
+			aChanges = aChanges[1:]
+		case comp > 0: // insert first b change
+			result = append(result, bChanges[0])
+			bChanges = bChanges[1:]
+		}
+	}
+
+	// append all remaining changes in aChanges, if any, as deletes
+	// append all remaining changes in bChanges, if any, as inserts
+	result = append(result, aChanges...)
+	result = append(result, bChanges...)
+
+	return result, nil
+}
+
+func hasChange(a, b *git.Tree, path string) (bool, error) {
+	ha, err := hash(a, path)
+	if err != nil {
+		return false, err
+	}
+
+	hb, err := hash(b, path)
+	if err != nil {
+		return false, err
+	}
+
+	return ha != hb, nil
+}
+
+func hash(tree *git.Tree, path string) (core.Hash, error) {
+	file, err := tree.File(path)
+	if err != nil {
+		var empty core.Hash
+		return empty, fmt.Errorf("cannot find file %s in tree: %s", path, err)
+	}
+
+	return file.Hash, nil
+}