difftree for git.Trees (#273)

Last PR to fix #82:

This PR modifies the difftree package itself. The old version extracted the files in both trees and compare them by hand. The new version turn the trees into merkletrie.Noders and call the merkletrie.Difftree function on them.

How to review this PR:

treenoder.go: defines the treeNoder type that wraps a git.Tree and implements merkletrie.Noder.
change.go: defines the type of the output of a difftree operation. The type is the same as before, but I have moved it into its own file to keep the package organized. The old package defines the Action type too (insert, delete, modify), now, we reuse merkletrie.Action and it is no longer a field, but a method.
change_adaptor.go: defines functions to turn merkletrie.Changes into difftree.Changes.
difftree.go: before this patch this file holds all the logic to do a difftree, now it just turns the git.Trees into treeNoders, call merkletrie.difftree on them, and turns the resulting merkletrie.Changes into difftree.Changes.
The only interesting piece of code here is that noders don't have the concept of mode (file permissions). The treenoder type codifies git.Tree modes into the merkletrie.Noder hash, so changes in the mode of a file are detected as modifications, just as the original git diff-tree command does.

1 files changed, 121 insertions, 0 deletions

diff --git a/plumbing/difftree/change.go b/plumbing/difftree/change.go
new file mode 100644
index 0000000..c636577
--- /dev/null
+++ b/plumbing/difftree/change.go
@@ -0,0 +1,121 @@
+package difftree
+
+import (
+	"bytes"
+	"fmt"
+	"strings"
+
+	"srcd.works/go-git.v4/plumbing/object"
+	"srcd.works/go-git.v4/utils/merkletrie"
+)
+
+// Change values represent a detected change between two git trees.  For
+// modifications, From is the original status of the node and To is its
+// final status.  For insertions, From is the zero value and for
+// deletions To is the zero value.
+type Change struct {
+	From ChangeEntry
+	To   ChangeEntry
+}
+
+var empty = ChangeEntry{}
+
+// Action returns the kind of action represented by the change, an
+// insertion, a deletion or a modification.
+func (c *Change) Action() (merkletrie.Action, error) {
+	if c.From == empty && c.To == empty {
+		return merkletrie.Action(0),
+			fmt.Errorf("malformed change: empty from and to")
+	}
+	if c.From == empty {
+		return merkletrie.Insert, nil
+	}
+	if c.To == empty {
+		return merkletrie.Delete, nil
+	}
+
+	return merkletrie.Modify, nil
+}
+
+// Files return the files before and after a change.
+// For insertions from will be nil. For deletions to will be nil.
+func (c *Change) Files() (from, to *object.File, err error) {
+	action, err := c.Action()
+	if err != nil {
+		return
+	}
+
+	if action == merkletrie.Insert || action == merkletrie.Modify {
+		to, err = c.To.Tree.TreeEntryFile(&c.To.TreeEntry)
+		if err != nil {
+			return
+		}
+	}
+
+	if action == merkletrie.Delete || action == merkletrie.Modify {
+		from, err = c.From.Tree.TreeEntryFile(&c.From.TreeEntry)
+		if err != nil {
+			return
+		}
+	}
+
+	return
+}
+
+func (c *Change) String() string {
+	action, err := c.Action()
+	if err != nil {
+		return fmt.Sprintf("malformed change")
+	}
+
+	return fmt.Sprintf("<Action: %s, Path: %s>", action, c.name())
+}
+
+func (c *Change) name() string {
+	if c.From != empty {
+		return c.From.Name
+	}
+
+	return c.To.Name
+}
+
+// ChangeEntry values represent a node that has suffered a change.
+type ChangeEntry struct {
+	// Full path of the node using "/" as separator.
+	Name string
+	// Parent tree of the node that has changed.
+	Tree *object.Tree
+	// The entry of the node.
+	TreeEntry object.TreeEntry
+}
+
+// Changes represents a collection of changes between two git trees.
+// Implements sort.Interface lexicographically over the path of the
+// changed files.
+type Changes []*Change
+
+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].name(), c[j].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()
+}