org.eclipse.jgit.storage.file
Class PackIndexWriter

java.lang.Object
  extended by org.eclipse.jgit.storage.file.PackIndexWriter

public abstract class PackIndexWriter
extends Object

Creates a table of contents to support random access by PackFile.

Pack index files (the .idx suffix in a pack file pair) provides random access to any object in the pack by associating an ObjectId to the byte offset within the pack where the object's data can be read.


Field Summary
protected  List<? extends PackedObjectInfo> entries
          The entries this writer must pack.
protected  DigestOutputStream out
          The index data stream we are responsible for creating.
protected  byte[] packChecksum
          SHA-1 checksum for the entire pack data.
protected  byte[] tmp
          A temporary buffer for use during IO to {link #out}.
protected static byte[] TOC
          Magic constant indicating post-version 1 format.
 
Constructor Summary
protected PackIndexWriter(OutputStream dst)
          Create a new writer instance.
 
Method Summary
static PackIndexWriter createOldestPossible(OutputStream dst, List<? extends PackedObjectInfo> objs)
          Create a new writer for the oldest (most widely understood) format.
static PackIndexWriter createVersion(OutputStream dst, int version)
          Create a new writer instance for a specific index format version.
 void write(List<? extends PackedObjectInfo> toStore, byte[] packDataChecksum)
          Write all object entries to the index stream.
protected  void writeChecksumFooter()
          Output the standard two-checksum index footer.
protected  void writeFanOutTable()
          Output the standard 256 entry first-level fan-out table.
protected abstract  void writeImpl()
          Writes the index file to out.
protected  void writeTOC(int version)
          Output the version 2 (and later) TOC header, with version number.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Field Detail

TOC

protected static final byte[] TOC
Magic constant indicating post-version 1 format.


out

protected final DigestOutputStream out
The index data stream we are responsible for creating.


tmp

protected final byte[] tmp
A temporary buffer for use during IO to {link #out}.


entries

protected List<? extends PackedObjectInfo> entries
The entries this writer must pack.


packChecksum

protected byte[] packChecksum
SHA-1 checksum for the entire pack data.

Constructor Detail

PackIndexWriter

protected PackIndexWriter(OutputStream dst)
Create a new writer instance.

Parameters:
dst - the stream this instance outputs to. If not already buffered it will be automatically wrapped in a buffered stream.
Method Detail

createOldestPossible

public static PackIndexWriter createOldestPossible(OutputStream dst,
                                                   List<? extends PackedObjectInfo> objs)
Create a new writer for the oldest (most widely understood) format.

This method selects an index format that can accurate describe the supplied objects and that will be the most compatible format with older Git implementations.

Index version 1 is widely recognized by all Git implementations, but index version 2 (and later) is not as well recognized as it was introduced more than a year later. Index version 1 can only be used if the resulting pack file is under 4 gigabytes in size; packs larger than that limit must use index version 2.

Parameters:
dst - the stream the index data will be written to. If not already buffered it will be automatically wrapped in a buffered stream. Callers are always responsible for closing the stream.
objs - the objects the caller needs to store in the index. Entries will be examined until a format can be conclusively selected.
Returns:
a new writer to output an index file of the requested format to the supplied stream.
Throws:
IllegalArgumentException - no recognized pack index version can support the supplied objects. This is likely a bug in the implementation.

createVersion

public static PackIndexWriter createVersion(OutputStream dst,
                                            int version)
Create a new writer instance for a specific index format version.

Parameters:
dst - the stream the index data will be written to. If not already buffered it will be automatically wrapped in a buffered stream. Callers are always responsible for closing the stream.
version - index format version number required by the caller. Exactly this formatted version will be written.
Returns:
a new writer to output an index file of the requested format to the supplied stream.
Throws:
IllegalArgumentException - the version requested is not supported by this implementation.

write

public void write(List<? extends PackedObjectInfo> toStore,
                  byte[] packDataChecksum)
           throws IOException
Write all object entries to the index stream.

After writing the stream passed to the factory is flushed but remains open. Callers are always responsible for closing the output stream.

Parameters:
toStore - sorted list of objects to store in the index. The caller must have previously sorted the list using PackedObjectInfo's native Comparable implementation.
packDataChecksum - checksum signature of the entire pack data content. This is traditionally the last 20 bytes of the pack file's own stream.
Throws:
IOException - an error occurred while writing to the output stream, or this index format cannot store the object data supplied.

writeImpl

protected abstract void writeImpl()
                           throws IOException
Writes the index file to out.

Implementations should go something like:

 writeFanOutTable();
 for (final PackedObjectInfo po : entries)
        writeOneEntry(po);
 writeChecksumFooter();
 

Where the logic for writeOneEntry is specific to the index format in use. Additional headers/footers may be used if necessary and the entries collection may be iterated over more than once if necessary. Implementors therefore have complete control over the data.

Throws:
IOException - an error occurred while writing to the output stream, or this index format cannot store the object data supplied.

writeTOC

protected void writeTOC(int version)
                 throws IOException
Output the version 2 (and later) TOC header, with version number.

Post version 1 all index files start with a TOC header that makes the file an invalid version 1 file, and then includes the version number. This header is necessary to recognize a version 1 from a version 2 formatted index.

Parameters:
version - version number of this index format being written.
Throws:
IOException - an error occurred while writing to the output stream.

writeFanOutTable

protected void writeFanOutTable()
                         throws IOException
Output the standard 256 entry first-level fan-out table.

The fan-out table is 4 KB in size, holding 256 32-bit unsigned integer counts. Each count represents the number of objects within this index whose AnyObjectId.getFirstByte() matches the count's position in the fan-out table.

Throws:
IOException - an error occurred while writing to the output stream.

writeChecksumFooter

protected void writeChecksumFooter()
                            throws IOException
Output the standard two-checksum index footer.

The standard footer contains two checksums (20 byte SHA-1 values):

  1. Pack data checksum - taken from the last 20 bytes of the pack file.
  2. Index data checksum - checksum of all index bytes written, including the pack data checksum above.

Throws:
IOException - an error occurred while writing to the output stream.


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