diff --git a/lib/compress/zstd_ldm.c b/lib/compress/zstd_ldm.c index cc338e697..a6e780b9e 100644 --- a/lib/compress/zstd_ldm.c +++ b/lib/compress/zstd_ldm.c @@ -143,10 +143,10 @@ static void ZSTD_ldm_makeEntryAndInsertByTag(ldmState_t* ldmState, * We count only bytes where pMatch >= pBase and pIn >= pAnchor. */ static size_t ZSTD_ldm_countBackwardsMatch( const BYTE* pIn, const BYTE* pAnchor, - const BYTE* pMatch, const BYTE* pBase) + const BYTE* pMatch, const BYTE* pMatchBase) { size_t matchLength = 0; - while (pIn > pAnchor && pMatch > pBase && pIn[-1] == pMatch[-1]) { + while (pIn > pAnchor && pMatch > pMatchBase && pIn[-1] == pMatch[-1]) { pIn--; pMatch--; matchLength++; @@ -154,6 +154,27 @@ static size_t ZSTD_ldm_countBackwardsMatch( return matchLength; } +/** ZSTD_ldm_countBackwardsMatch_2segments() : + * Returns the number of bytes that match backwards from pMatch, + * even with the backwards match spanning 2 different segments. + * + * On reaching `pMatchBase`, start counting from mEnd */ +static size_t ZSTD_ldm_countBackwardsMatch_2segments( + const BYTE* pIn, const BYTE* pAnchor, + const BYTE* pMatch, const BYTE* pMatchBase, + const BYTE* pExtDictStart, const BYTE* pExtDictEnd) +{ + size_t matchLength = ZSTD_ldm_countBackwardsMatch(pIn, pAnchor, pMatch, pMatchBase); + if (pMatch - matchLength != pMatchBase || pMatchBase == pExtDictStart) { + /* If backwards match is entirely in the extDict or prefix, immediately return */ + return matchLength; + } + DEBUGLOG(7, "ZSTD_ldm_countBackwardsMatch_2segments: found 2-parts backwards match (length in prefix==%zu)", matchLength); + matchLength += ZSTD_ldm_countBackwardsMatch(pIn - matchLength, pAnchor, pExtDictEnd, pExtDictStart); + DEBUGLOG(7, "final backwards match length = %zu", matchLength); + return matchLength; +} + /** ZSTD_ldm_fillFastTables() : * * Fills the relevant tables for the ZSTD_fast and ZSTD_dfast strategies. @@ -329,8 +350,9 @@ static size_t ZSTD_ldm_generateSequences_internal( continue; } curBackwardMatchLength = - ZSTD_ldm_countBackwardsMatch(ip, anchor, pMatch, - lowMatchPtr); + ZSTD_ldm_countBackwardsMatch_2segments(ip, anchor, + pMatch, lowMatchPtr, + dictStart, dictEnd); curTotalMatchLength = curForwardMatchLength + curBackwardMatchLength; } else { /* !extDict */