ref: a1cfb8953022bead6dae8399fdee30a01e267cf6
dir: /delta_write_select.go/
package furgit
const defaultDeltaWindow = 64
type objectToPack struct {
id Hash
ty ObjectType
body []byte
size int
offset uint64
deltaDepth int
inPack bool
preferred bool
}
type deltaContext struct {
window int
candidates []*objectToPack
}
func (ctx *deltaContext) addCandidate(obj *objectToPack) {
if ctx.window <= 0 {
return
}
ctx.candidates = append(ctx.candidates, obj)
if len(ctx.candidates) > ctx.window {
over := len(ctx.candidates) - ctx.window
ctx.candidates = ctx.candidates[over:]
}
}
func pickDeltaBase(ctx *deltaContext, obj *objectToPack, seed uint64, minSavings, maxDepth int) (*objectToPack, []byte) {
if ctx == nil || len(ctx.candidates) == 0 {
return nil, nil
}
if maxDepth <= 0 {
maxDepth = 1
}
var bestBase *objectToPack
var bestDelta []byte
var bestPreferred *objectToPack
var bestPreferredDelta []byte
for i := len(ctx.candidates) - 1; i >= 0; i-- {
base := ctx.candidates[i]
if base.ty != ObjectTypeBlob {
continue
}
if base.deltaDepth >= maxDepth {
continue
}
if !deltaSizeOk(base, obj, maxDepth) {
continue
}
delta, ok := deltaTry(base.body, obj.body, seed, minSavings)
if base.preferred {
delta, ok = deltaTry(base.body, obj.body, seed, 0)
}
if !ok {
continue
}
if base.preferred {
if bestPreferredDelta == nil || len(delta) < len(bestPreferredDelta) {
bestPreferredDelta = delta
bestPreferred = base
}
continue
}
if bestDelta == nil || len(delta) < len(bestDelta) {
bestDelta = delta
bestBase = base
}
}
if bestPreferred != nil {
return bestPreferred, bestPreferredDelta
}
return bestBase, bestDelta
}
func deltaSizeOk(base, target *objectToPack, maxDepth int) bool {
if base == nil || target == nil {
return false
}
if base.size <= 0 || target.size <= 0 {
return false
}
if maxDepth <= 0 {
maxDepth = 1
}
if base.deltaDepth >= maxDepth {
return false
}
if target.size < base.size/32 {
return false
}
maxSize := target.size/2 - 32
if maxSize <= 0 {
return false
}
sizediff := 0
if base.size < target.size {
sizediff = target.size - base.size
}
if sizediff >= maxSize {
return false
}
return true
}