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package pack
import (
"encoding/binary"
"hash"
)
// V2 implements IndexVersion for v2 packfiles.
type V2 struct {
hash hash.Hash
}
// Name implements IndexVersion.Name by returning the 20 byte SHA-1 object name
// for the given entry at offset "at" in the v2 index file "idx".
func (v *V2) Name(idx *Index, at int64) ([]byte, error) {
var sha [MaxHashSize]byte
hashlen := v.hash.Size()
if _, err := idx.readAt(sha[:hashlen], v2ShaOffset(at, int64(hashlen))); err != nil {
return nil, err
}
return sha[:hashlen], nil
}
// Entry implements IndexVersion.Entry for v2 packfiles by parsing and returning
// the IndexEntry specified at the offset "at" in the given index file.
func (v *V2) Entry(idx *Index, at int64) (*IndexEntry, error) {
var offs [4]byte
hashlen := v.hash.Size()
if _, err := idx.readAt(offs[:], v2SmallOffsetOffset(at, int64(idx.Count()), int64(hashlen))); err != nil {
return nil, err
}
loc := uint64(binary.BigEndian.Uint32(offs[:]))
if loc&0x80000000 > 0 {
// If the most significant bit (MSB) of the offset is set, then
// the offset encodes the indexed location for an 8-byte offset.
//
// Mask away (offs&0x7fffffff) the MSB to use as an index to
// find the offset of the 8-byte pack offset.
lo := v2LargeOffsetOffset(int64(loc&0x7fffffff), int64(idx.Count()), int64(hashlen))
var offs [8]byte
if _, err := idx.readAt(offs[:], lo); err != nil {
return nil, err
}
loc = binary.BigEndian.Uint64(offs[:])
}
return &IndexEntry{PackOffset: loc}, nil
}
// Width implements IndexVersion.Width() by returning the number of bytes that
// v2 packfile index header occupy.
func (v *V2) Width() int64 {
return indexV2Width
}
// v2ShaOffset returns the offset of a SHA1 given at "at" in the V2 index file.
func v2ShaOffset(at int64, hashlen int64) int64 {
// Skip the packfile index header and the L1 fanout table.
return indexOffsetV2Start +
// Skip until the desired name in the sorted names table.
(hashlen * at)
}
// v2SmallOffsetOffset returns the offset of an object's small (4-byte) offset
// given by "at".
func v2SmallOffsetOffset(at, total, hashlen int64) int64 {
// Skip the packfile index header and the L1 fanout table.
return indexOffsetV2Start +
// Skip the name table.
(hashlen * total) +
// Skip the CRC table.
(indexObjectCRCWidth * total) +
// Skip until the desired index in the small offsets table.
(indexObjectSmallOffsetWidth * at)
}
// v2LargeOffsetOffset returns the offset of an object's large (4-byte) offset,
// given by the index "at".
func v2LargeOffsetOffset(at, total, hashlen int64) int64 {
// Skip the packfile index header and the L1 fanout table.
return indexOffsetV2Start +
// Skip the name table.
(hashlen * total) +
// Skip the CRC table.
(indexObjectCRCWidth * total) +
// Skip the small offsets table.
(indexObjectSmallOffsetWidth * total) +
// Seek to the large offset within the large offset(s) table.
(indexObjectLargeOffsetWidth * at)
}
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