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Diffstat (limited to 'doc/zstd_compression_format.md')
| -rw-r--r-- | doc/zstd_compression_format.md | 37 |
1 files changed, 32 insertions, 5 deletions
diff --git a/doc/zstd_compression_format.md b/doc/zstd_compression_format.md index e562e628bc9ce..ed758cf598ace 100644 --- a/doc/zstd_compression_format.md +++ b/doc/zstd_compression_format.md @@ -16,7 +16,7 @@ Distribution of this document is unlimited. ### Version -0.3.0 (25/09/18) +0.3.1 (25/10/18) Introduction @@ -913,11 +913,37 @@ Note that blocks which are not `Compressed_Block` are skipped, they do not contr ###### Offset updates rules The newest offset takes the lead in offset history, -shifting others back (up to its previous place if it was already present). +shifting others back by one rank, +up to the previous rank of the new offset _if it was present in history_. + +__Examples__ : + +In the common case, when new offset is not part of history : +`Repeated_Offset3` = `Repeated_Offset2` +`Repeated_Offset2` = `Repeated_Offset1` +`Repeated_Offset1` = `NewOffset` + +When the new offset _is_ part of history, there may be specific adjustments. + +When `NewOffset` == `Repeated_Offset1`, offset history remains actually unmodified. + +When `NewOffset` == `Repeated_Offset2`, +`Repeated_Offset1` and `Repeated_Offset2` ranks are swapped. +`Repeated_Offset3` is unmodified. + +When `NewOffset` == `Repeated_Offset3`, +there is actually no difference with the common case : +all offsets are shifted by one rank, +`NewOffset` (== `Repeated_Offset3`) becomes the new `Repeated_Offset1`. + +Also worth mentioning, the specific corner case when `offset_value` == 3, +and the literal length of the current sequence is zero. +In which case , `NewOffset` = `Repeated_Offset1` - 1_byte. +Here also, from an offset history update perspective, it's just a common case : +`Repeated_Offset3` = `Repeated_Offset2` +`Repeated_Offset2` = `Repeated_Offset1` +`Repeated_Offset1` = `NewOffset` ( == `Repeated_Offset1` - 1_byte ) -This means that when `Repeated_Offset1` (most recent) is used, history is unmodified. -When `Repeated_Offset2` is used, it's swapped with `Repeated_Offset1`. -If any other offset is used, it becomes `Repeated_Offset1` and the rest are shift back by one. Skippable Frames @@ -1629,6 +1655,7 @@ or at least provide a meaningful error code explaining for which reason it canno Version changes --------------- +- 0.3.1 : minor clarification regarding offset history update rules - 0.3.0 : minor edits to match RFC8478 - 0.2.9 : clarifications for huffman weights direct representation, by Ulrich Kunitz - 0.2.8 : clarifications for IETF RFC discuss |