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.encodedDeltaObject(h)
if err != nil {
return nil, err
}
otp := newObjectToPack(o)
if _, ok := o.(plumbing.DeltaObject); ok {
otp.Original = nil
}
objectsToPack = append(objectsToPack, otp)
}
if err := dw.fixAndBreakChains(objectsToPack); err != nil {
return nil, err
}
return objectsToPack, nil
}
func (dw *deltaSelector) encodedDeltaObject(h plumbing.Hash) (plumbing.EncodedObject, error) {
edos, ok := dw.storer.(storer.DeltaObjectStorer)
if !ok {
return dw.encodedObject(h)
}
return edos.DeltaObject(plumbing.AnyObject, h)
}
func (dw *deltaSelector) encodedObject(h plumbing.Hash) (plumbing.EncodedObject, error) {
return dw.storer.EncodedObject(plumbing.AnyObject, h)
}
func (dw *deltaSelector) fixAndBreakChains(objectsToPack []*ObjectToPack) error {
m := make(map[plumbing.Hash]*ObjectToPack, len(objectsToPack))
for _, otp := range objectsToPack {
m[otp.Hash()] = otp
}
for _, otp := range objectsToPack {
if err := dw.fixAndBreakChainsOne(m, otp); err != nil {
return err
}
}
return nil
}
func (dw *deltaSelector) fixAndBreakChainsOne(objectsToPack map[plumbing.Hash]*ObjectToPack, otp *ObjectToPack) error {
if !otp.Object.Type().IsDelta() {
return nil
}
// Initial ObjectToPack instances might have a delta assigned to Object
// but no actual base initially. Once Base is assigned to a delta, it means
// we already fixed it.
if otp.Base != nil {
return nil
}
do, ok := otp.Object.(plumbing.DeltaObject)
if !ok {
// if this is not a DeltaObject, then we cannot retrieve its base,
// so we have to break the delta chain here.
return dw.undeltify(otp)
}
base, ok := objectsToPack[do.BaseHash()]
if !ok {
// The base of the delta is not in our list of objects to pack, so
// we break the chain.
return dw.undeltify(otp)
}
if base.Size() <= otp.Size() {
// Bases should be bigger
return dw.undeltify(otp)
}
if err := dw.fixAndBreakChainsOne(objectsToPack, base); err != nil {
return err
}
otp.SetDelta(base, otp.Object)
return nil
}
func (dw *deltaSelector) restoreOriginal(otp *ObjectToPack) error {
if otp.Original != nil {
return nil
}
if !otp.Object.Type().IsDelta() {
return nil
}
obj, err := dw.encodedObject(otp.Hash())
if err != nil {
return err
}
otp.Original = obj
return nil
}
// undeltify undeltifies an *ObjectToPack by retrieving the original object from
// the storer and resetting it.
func (dw *deltaSelector) undeltify(otp *ObjectToPack) error {
if err := dw.restoreOriginal(otp); err != nil {
return err
}
otp.Object = otp.Original
otp.Depth = 0
return 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]
// If we already have a delta, we don't try to find a new one for this
// object. This happens when a delta is set to be reused from an existing
// packfile.
if target.IsDelta() {
continue
}
// We only want to create deltas from specific types.
if !applyDelta[target.Type()] {
continue
}
for j := i - 1; j >= 0; j-- {
base := objectsToPack[j]
// Objects must use only the same type as their delta base.
// Since objectsToPack is sorted by type and size, once we find
// a different type, we know we won't find more of them.
if base.Type() != target.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.Size() < base.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.Size()-target.Size() > msz {
return nil
}
// Original object might not be present if we're reusing a delta, so we
// ensure it is restored.
if err := dw.restoreOriginal(target); err != nil {
return err
}
if err := dw.restoreOriginal(base); err != nil {
return err
}
// 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].Type() < a[j].Type() {
return false
}
if a[i].Type() > a[j].Type() {
return true
}
return a[i].Size() > a[j].Size()
}
