Package org.eclipse.jgit.internal.storage.pack

Class DeltaIndex

  • public class DeltaIndex
extends Object
    Index of blocks in a source file.

    The index can be passed a result buffer, and output an instruction sequence that transforms the source buffer used by the index into the result buffer. The instruction sequence can be executed by BinaryDelta to recreate the result buffer.

    An index stores the entire contents of the source buffer, but also a table of block identities mapped to locations where the block appears in the source buffer. The mapping table uses 12 bytes for every 16 bytes of source buffer, and is therefore ~75% of the source buffer size. The overall index is ~1.75x the size of the source buffer. This relationship holds for any JVM, as only a constant number of objects are allocated per index. Callers can use the method getIndexSize() to obtain a reasonably accurate estimate of the complete heap space used by this index.

    A DeltaIndex is thread-safe. Concurrent threads can use the same index to encode delta instructions for different result buffers.

    • Constructor Detail

      • DeltaIndex

        public DeltaIndex​(byte[] sourceBuffer)
        Construct an index from the source file.
        Parameters:
        sourceBuffer - the source file's raw contents. The buffer will be held by the index instance to facilitate matching, and therefore must not be modified by the caller.

    • Method Detail

      • estimateIndexSize

        public static long estimateIndexSize​(int sourceLength)
        Estimate the size of an index for a given source.

        This is roughly a worst-case estimate. The actual index may be smaller.

        Parameters:
        sourceLength - length of the source, in bytes.
        Returns:
        estimated size. Approximately 1.75 * sourceLength.

      • getSourceSize

        public long getSourceSize()
        Get size of the source buffer this index has scanned.
        Returns:
        size of the source buffer this index has scanned.

      • getIndexSize

        public long getIndexSize()
        Get an estimate of the memory required by this index.
        Returns:
        an approximation of the number of bytes used by this index in memory. The size includes the cached source buffer size from getSourceSize(), as well as a rough approximation of JVM object overheads.

      • encode

        public void encode​(OutputStream out,
                   byte[] res)
            throws IOException
        Generate a delta sequence to recreate the result buffer.

        There is no limit on the size of the delta sequence created. This is the same as encode(out, res, 0).

        Parameters:
        out - stream to receive the delta instructions that can transform this index's source buffer into res. This stream should be buffered, as instructions are written directly to it in small bursts.
        res - the desired result buffer. The generated instructions will recreate this buffer when applied to the source buffer stored within this index.
        Throws:
        IOException - the output stream refused to write the instructions.

      • encode

        public boolean encode​(OutputStream out,
                      byte[] res,
                      int deltaSizeLimit)
               throws IOException
        Generate a delta sequence to recreate the result buffer.
        Parameters:
        out - stream to receive the delta instructions that can transform this index's source buffer into res. This stream should be buffered, as instructions are written directly to it in small bursts. If the caller might need to discard the instructions (such as when deltaSizeLimit would be exceeded) the caller is responsible for discarding or rewinding the stream when this method returns false.
        res - the desired result buffer. The generated instructions will recreate this buffer when applied to the source buffer stored within this index.
        deltaSizeLimit - maximum number of bytes that the delta instructions can occupy. If the generated instructions would be longer than this amount, this method returns false. If 0, there is no limit on the length of delta created.
        Returns:
        true if the delta is smaller than deltaSizeLimit; false if the encoder aborted because the encoded delta instructions would be longer than deltaSizeLimit bytes.
        Throws:
        IOException - the output stream refused to write the instructions.