protected override void WriteSkipData(int level, IndexOutput skipBuffer)
{
// To efficiently store payloads in the posting lists we do not store the length of
// every payload. Instead we omit the length for a payload if the previous payload had
// the same length.
// However, in order to support skipping the payload length at every skip point must be known.
// So we use the same length encoding that we use for the posting lists for the skip data as well:
// Case 1: current field does not store payloads
// SkipDatum --> DocSkip, FreqSkip, ProxSkip
// DocSkip,FreqSkip,ProxSkip --> VInt
// DocSkip records the document number before every SkipInterval th document in TermFreqs.
// Document numbers are represented as differences from the previous value in the sequence.
// Case 2: current field stores payloads
// SkipDatum --> DocSkip, PayloadLength?, FreqSkip,ProxSkip
// DocSkip,FreqSkip,ProxSkip --> VInt
// PayloadLength --> VInt
// In this case DocSkip/2 is the difference between
// the current and the previous value. If DocSkip
// is odd, then a PayloadLength encoded as VInt follows,
// if DocSkip is even, then it is assumed that the
// current payload length equals the length at the previous
// skip point
if (CurStorePayloads)
{
int delta = CurDoc - LastSkipDoc[level];
if (CurPayloadLength == LastSkipPayloadLength[level])
{
// the current payload length equals the length at the previous skip point,
// so we don't store the length again
skipBuffer.WriteVInt32(delta * 2);
}
else
{
// the payload length is different from the previous one. We shift the DocSkip,
// set the lowest bit and store the current payload length as VInt.
skipBuffer.WriteVInt32(delta * 2 + 1);
skipBuffer.WriteVInt32(CurPayloadLength);
LastSkipPayloadLength[level] = CurPayloadLength;
}
}
else
{
// current field does not store payloads
skipBuffer.WriteVInt32(CurDoc - LastSkipDoc[level]);
}
skipBuffer.WriteVInt32((int)(CurFreqPointer - LastSkipFreqPointer[level]));
skipBuffer.WriteVInt32((int)(CurProxPointer - LastSkipProxPointer[level]));
LastSkipDoc[level] = CurDoc;
LastSkipFreqPointer[level] = CurFreqPointer;
LastSkipProxPointer[level] = CurProxPointer;
}