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package packfile
import (
"encoding/binary"
"errors"
"fmt"
)
const (
// signature is the 4-byte "PACK" magic at the start of pack files.
signature = 0x5041434b
// version is the only pack version this package reads or writes.
version = 2
)
// HeaderLen is the size of a pack header:
// signature, version, and object count.
const HeaderLen = 12
// ErrMalformedHeader reports that
// a pack header is truncated,
// or has a bad signature or unsupported version.
var ErrMalformedHeader = errors.New("internal/format/packfile: malformed pack header")
// Header is one parsed pack header.
type Header struct {
// ObjectCount is the number of object entries
// declared to follow the header.
ObjectCount uint32
}
// ParseHeader parses and validates the pack header
// at the start of data.
func ParseHeader(data []byte) (Header, error) {
var zero Header
if len(data) < HeaderLen {
return zero, fmt.Errorf("%w: truncated", ErrMalformedHeader)
}
if binary.BigEndian.Uint32(data[0:4]) != signature {
return zero, fmt.Errorf("%w: bad signature", ErrMalformedHeader)
}
parsedVersion := binary.BigEndian.Uint32(data[4:8])
if parsedVersion != version {
return zero, fmt.Errorf("%w: unsupported version %d", ErrMalformedHeader, parsedVersion)
}
return Header{
ObjectCount: binary.BigEndian.Uint32(data[8:12]),
}, nil
}
// AppendHeader appends an encoded pack header for objectCount to dst.
func AppendHeader(dst []byte, objectCount uint32) []byte {
dst = binary.BigEndian.AppendUint32(dst, signature)
dst = binary.BigEndian.AppendUint32(dst, version)
dst = binary.BigEndian.AppendUint32(dst, objectCount)
return dst
}
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