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package packfile
import (
"errors"
"fmt"
"math"
)
// ErrMalformedOfsDeltaDistance reports that
// an ofs-delta backward distance encoding
// is truncated, overlong, or overflows uint64.
var ErrMalformedOfsDeltaDistance = errors.New("internal/format/packfile: malformed ofs-delta distance")
// MaxOfsDeltaDistanceLen is the maximum encoded length
// of an ofs-delta backward distance.
// Every uint64 distance is encodable within this bound,
// and [ParseOfsDeltaDistance] rejects longer encodings.
const MaxOfsDeltaDistanceLen = 10
// ParseOfsDeltaDistance parses an ofs-delta backward distance.
func ParseOfsDeltaDistance(buf []byte) (dist uint64, consumed int, err error) {
if len(buf) == 0 {
return 0, 0, fmt.Errorf("%w: truncated", ErrMalformedOfsDeltaDistance)
}
b := buf[0]
dist = uint64(b & 0x7f)
consumed = 1
for b&0x80 != 0 {
if consumed >= MaxOfsDeltaDistanceLen {
return 0, 0, fmt.Errorf("%w: overlong encoding", ErrMalformedOfsDeltaDistance)
}
if consumed >= len(buf) {
return 0, 0, fmt.Errorf("%w: truncated", ErrMalformedOfsDeltaDistance)
}
if dist >= math.MaxUint64>>7 {
return 0, 0, fmt.Errorf("%w: overflows uint64", ErrMalformedOfsDeltaDistance)
}
b = buf[consumed]
consumed++
dist = ((dist + 1) << 7) | uint64(b&0x7f)
}
return dist, consumed, nil
}
// AppendOfsDeltaDistance appends the encoding of
// an ofs-delta backward distance to dst.
func AppendOfsDeltaDistance(dst []byte, dist uint64) []byte {
var buf [MaxOfsDeltaDistanceLen]byte
pos := len(buf) - 1
buf[pos] = byte(dist & 0x7f)
dist >>= 7
for dist != 0 {
dist--
pos--
buf[pos] = 0x80 | byte(dist&0x7f)
dist >>= 7
}
return append(dst, buf[pos:]...)
}
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