Merge pull request #1563 from ipfs/dag-diff

implement a basic DAG diffing algorithm

utils/diff.go

+package dagutils
+
+import (
+	"bytes"
+	"fmt"
+	"path"
+
+	key "github.com/ipfs/go-ipfs/blocks/key"
+	dag "github.com/ipfs/go-ipfs/merkledag"
+	context "golang.org/x/net/context"
+)
+
+const (
+	Add = iota
+	Remove
+	Mod
+)
+
+type Change struct {
+	Type   int
+	Path   string
+	Before key.Key
+	After  key.Key
+}
+
+func (c *Change) String() string {
+	switch c.Type {
+	case Add:
+		return fmt.Sprintf("Added %s at %s", c.After.B58String()[:6], c.Path)
+	case Remove:
+		return fmt.Sprintf("Removed %s from %s", c.Before.B58String()[:6], c.Path)
+	case Mod:
+		return fmt.Sprintf("Changed %s to %s at %s", c.Before.B58String()[:6], c.After.B58String()[:6], c.Path)
+	default:
+		panic("nope")
+	}
+}
+
+func ApplyChange(ctx context.Context, ds dag.DAGService, nd *dag.Node, cs []*Change) (*dag.Node, error) {
+	e := NewDagEditor(ds, nd)
+	for _, c := range cs {
+		switch c.Type {
+		case Add:
+			err := e.InsertNodeAtPath(ctx, c.Path, c.After, nil)
+			if err != nil {
+				return nil, err
+			}
+
+		case Remove:
+			err := e.RmLink(ctx, c.Path)
+			if err != nil {
+				return nil, err
+			}
+
+		case Mod:
+			err := e.RmLink(ctx, c.Path)
+			if err != nil {
+				return nil, err
+			}
+			err = e.InsertNodeAtPath(ctx, c.Path, c.After, nil)
+			if err != nil {
+				return nil, err
+			}
+		}
+	}
+	return e.GetNode(), nil
+}
+
+func Diff(ctx context.Context, ds dag.DAGService, a, b *dag.Node) []*Change {
+	if len(a.Links) == 0 && len(b.Links) == 0 {
+		ak, _ := a.Key()
+		bk, _ := b.Key()
+		return []*Change{
+			&Change{
+				Type:   Mod,
+				Before: ak,
+				After:  bk,
+			},
+		}
+	}
+
+	var out []*Change
+	clean_a := a.Copy()
+	clean_b := b.Copy()
+
+	// strip out unchanged stuff
+	for _, lnk := range a.Links {
+		l, err := b.GetNodeLink(lnk.Name)
+		if err == nil {
+			if bytes.Equal(l.Hash, lnk.Hash) {
+				// no change... ignore it
+			} else {
+				anode, _ := lnk.GetNode(ctx, ds)
+				bnode, _ := l.GetNode(ctx, ds)
+				sub := Diff(ctx, ds, anode, bnode)
+
+				for _, subc := range sub {
+					subc.Path = path.Join(lnk.Name, subc.Path)
+					out = append(out, subc)
+				}
+			}
+			clean_a.RemoveNodeLink(l.Name)
+			clean_b.RemoveNodeLink(l.Name)
+		}
+	}
+
+	for _, lnk := range clean_a.Links {
+		out = append(out, &Change{
+			Type:   Remove,
+			Path:   lnk.Name,
+			Before: key.Key(lnk.Hash),
+		})
+	}
+	for _, lnk := range clean_b.Links {
+		out = append(out, &Change{
+			Type:  Add,
+			Path:  lnk.Name,
+			After: key.Key(lnk.Hash),
+		})
+	}
+
+	return out
+}
+
+type Conflict struct {
+	A *Change
+	B *Change
+}
+
+func MergeDiffs(a, b []*Change) ([]*Change, []Conflict) {
+	var out []*Change
+	var conflicts []Conflict
+	paths := make(map[string]*Change)
+	for _, c := range a {
+		paths[c.Path] = c
+	}
+
+	for _, c := range b {
+		if ca, ok := paths[c.Path]; ok {
+			conflicts = append(conflicts, Conflict{
+				A: ca,
+				B: c,
+			})
+		} else {
+			out = append(out, c)
+		}
+	}
+	for _, c := range paths {
+		out = append(out, c)
+	}
+	return out, conflicts
+}