package packfile import ( "sort" "gopkg.in/src-d/go-git.v4/plumbing" "gopkg.in/src-d/go-git.v4/plumbing/storer" ) const ( // deltas based on deltas, how many steps we can do. // 50 is the default value used in JGit maxDepth = int64(50) ) // applyDelta is the set of object types that we should apply deltas var applyDelta = map[plumbing.ObjectType]bool{ plumbing.BlobObject: true, plumbing.TreeObject: true, } type deltaSelector struct { storer storer.EncodedObjectStorer } func newDeltaSelector(s storer.EncodedObjectStorer) *deltaSelector { return &deltaSelector{s} } // ObjectsToPack creates a list of ObjectToPack from the hashes provided, // creating deltas if it's suitable, using an specific internal logic func (dw *deltaSelector) ObjectsToPack(hashes []plumbing.Hash) ([]*ObjectToPack, error) { otp, err := dw.objectsToPack(hashes) if err != nil { return nil, err } dw.sort(otp) if err := dw.walk(otp); err != nil { return nil, err } return otp, nil } func (dw *deltaSelector) objectsToPack(hashes []plumbing.Hash) ([]*ObjectToPack, error) { var objectsToPack []*ObjectToPack for _, h := range hashes { o, err := dw.storer.EncodedObject(plumbing.AnyObject, h) if err != nil { return nil, err } objectsToPack = append(objectsToPack, newObjectToPack(o)) } return objectsToPack, nil } func (dw *deltaSelector) sort(objectsToPack []*ObjectToPack) { sort.Sort(byTypeAndSize(objectsToPack)) } func (dw *deltaSelector) walk(objectsToPack []*ObjectToPack) error { for i := 0; i < len(objectsToPack); i++ { target := objectsToPack[i] // We only want to create deltas from specific types if !applyDelta[target.Original.Type()] { continue } for j := i - 1; j >= 0; j-- { base := objectsToPack[j] // Objects must use only the same type as their delta base. if base.Original.Type() != target.Original.Type() { break } if err := dw.tryToDeltify(base, target); err != nil { return err } } } return nil } func (dw *deltaSelector) tryToDeltify(base, target *ObjectToPack) error { // If the sizes are radically different, this is a bad pairing. if target.Original.Size() < base.Original.Size()>>4 { return nil } msz := dw.deltaSizeLimit( target.Object.Size(), base.Depth, target.Depth, target.IsDelta(), ) // Nearly impossible to fit useful delta. if msz <= 8 { return nil } // If we have to insert a lot to make this work, find another. if base.Original.Size()-target.Object.Size() > msz { return nil } // Now we can generate the delta using originals delta, err := GetDelta(base.Original, target.Original) if err != nil { return err } // if delta better than target if delta.Size() < msz { target.SetDelta(base, delta) } return nil } func (dw *deltaSelector) deltaSizeLimit(targetSize int64, baseDepth int, targetDepth int, targetDelta bool) int64 { if !targetDelta { // Any delta should be no more than 50% of the original size // (for text files deflate of whole form should shrink 50%). n := targetSize >> 1 // Evenly distribute delta size limits over allowed depth. // If src is non-delta (depth = 0), delta <= 50% of original. // If src is almost at limit (9/10), delta <= 10% of original. return n * (maxDepth - int64(baseDepth)) / maxDepth } // With a delta base chosen any new delta must be "better". // Retain the distribution described above. d := int64(targetDepth) n := targetSize // If target depth is bigger than maxDepth, this delta is not suitable to be used. if d >= maxDepth { return 0 } // If src is whole (depth=0) and base is near limit (depth=9/10) // any delta using src can be 10x larger and still be better. // // If src is near limit (depth=9/10) and base is whole (depth=0) // a new delta dependent on src must be 1/10th the size. return n * (maxDepth - int64(baseDepth)) / (maxDepth - d) } type byTypeAndSize []*ObjectToPack func (a byTypeAndSize) Len() int { return len(a) } func (a byTypeAndSize) Swap(i, j int) { a[i], a[j] = a[j], a[i] } func (a byTypeAndSize) Less(i, j int) bool { if a[i].Object.Type() < a[j].Object.Type() { return false } if a[i].Object.Type() > a[j].Object.Type() { return true } return a[i].Object.Size() > a[j].Object.Size() }