ref: b2ca764959a7f213b0d1d8b534c3f4d66e34285e
dir: /delta_write_encode.go/
package furgit
const deltaCopyDefaultLen = 0x10000
func deltaEncode(baseSize, resultSize int, instr []deltaInstruction) ([]byte, error) {
if baseSize < 0 || resultSize < 0 {
return nil, ErrInvalidObject
}
out := make([]byte, 0, 64)
baseHdr, err := packVarintEncode(baseSize)
if err != nil {
return nil, err
}
out = append(out, baseHdr...)
resultHdr, err := packVarintEncode(resultSize)
if err != nil {
return nil, err
}
out = append(out, resultHdr...)
for _, ins := range instr {
if ins.copy {
if ins.offset < 0 || ins.length <= 0 {
return nil, ErrInvalidObject
}
if ins.offset > 0xffffffff {
return nil, ErrInvalidObject
}
if ins.length > 0xffffff {
return nil, ErrInvalidObject
}
op := byte(0x80)
var tmp [7]byte
pos := 0
off := ins.offset
for i := 0; i < 4; i++ {
op |= 1 << i
tmp[pos] = byte(off & 0xff)
pos++
off >>= 8
if off == 0 {
break
}
}
if off != 0 {
return nil, ErrInvalidObject
}
n := ins.length
if n != deltaCopyDefaultLen {
for i := 0; i < 3 && n > 0; i++ {
op |= 1 << (i + 4)
tmp[pos] = byte(n & 0xff)
pos++
n >>= 8
}
if n > 0 {
return nil, ErrInvalidObject
}
}
out = append(out, op)
out = append(out, tmp[:pos]...)
continue
}
data := ins.data
if ins.length > 0 {
if len(data) < ins.length {
return nil, ErrInvalidObject
}
data = data[:ins.length]
}
if len(data) == 0 {
return nil, ErrInvalidObject
}
for len(data) > 0 {
n := len(data)
if n > 127 {
n = 127
}
out = append(out, byte(n))
out = append(out, data[:n]...)
data = data[n:]
}
}
return out, nil
}
func deltaTry(base, target []byte, seed uint32, minSavings int) ([]byte, bool) {
if minSavings < 0 {
minSavings = 0
}
instr, err := deltify(base, target, seed)
if err != nil {
return nil, false
}
delta, err := deltaEncode(len(base), len(target), instr)
if err != nil {
return nil, false
}
if len(delta)+minSavings >= len(target) {
return nil, false
}
return delta, true
}