Support backward decompression

asm/6502/decompress_v1.asm

@@ -7,6 +7,18 @@
 ;
 ; out:
 ; * LZSA_DST_LO and LZSA_DST_HI contain the last decompressed byte address, +1
+;
+; -----------------------------------------------------------------------------
+; Backward decompression is also supported, use lzsa -r -b <original_file> <compressed_file>
+; To use it, also define BACKWARD_DECOMPRESS=1 before including this code!
+;
+; in:
+; * LZSA_SRC_LO/LZSA_SRC_HI must contain the address of the last byte of compressed data
+; * LZSA_DST_LO/LZSA_DST_HI must contain the address of the last byte of the destination buffer
+;
+; out:
+; * LZSA_DST_LO/LZSA_DST_HI contain the last decompressed byte address, -1
+;
 ; -----------------------------------------------------------------------------
 ;
 ;  Copyright (C) 2019 Emmanuel Marty
@@ -96,11 +108,29 @@ PREPARE_COPY_MATCH_Y
 
 COPY_MATCH_LOOP
    LDA $AAAA                            ; get one byte of backreference
+   JSR PUTDST                           ; copy to destination
+
+ifdef BACKWARD_DECOMPRESS
+
+   ; Backward decompression -- put backreference bytes backward
+
+   LDA COPY_MATCH_LOOP+1
+   BNE GETMATCH_DONE
+   DEC COPY_MATCH_LOOP+2
+GETMATCH_DONE
+   DEC COPY_MATCH_LOOP+1
+
+else
+
+   ; Forward decompression -- put backreference bytes forward
+
    INC COPY_MATCH_LOOP+1
    BNE GETMATCH_DONE
    INC COPY_MATCH_LOOP+2
 GETMATCH_DONE
-   JSR PUTDST                           ; copy to destination
+
+endif
+
    DEX
    BNE COPY_MATCH_LOOP
    DEY
@@ -111,6 +141,29 @@ GET_LONG_OFFSET                         ; handle 16 bit offset:
    JSR GETLARGESRC                      ; grab low 8 bits in X, high 8 bits in A
 
 GOT_OFFSET
+
+ifdef BACKWARD_DECOMPRESS
+
+   ; Backward decompression - substract match offset
+
+   STA OFFSHI                           ; store high 8 bits of offset
+   STX OFFSLO
+
+   SEC                                  ; substract dest - match offset
+   LDA PUTDST+1
+OFFSLO = *+1
+   SBC #$AA                             ; low 8 bits
+   STA COPY_MATCH_LOOP+1                ; store back reference address
+   LDA PUTDST+2
+OFFSHI = *+1
+   SBC #$AA                             ; high 8 bits
+   STA COPY_MATCH_LOOP+2                ; store high 8 bits of address
+   SEC
+
+else
+
+   ; Forward decompression - add match offset
+
    STA OFFSHI                           ; store high 8 bits of offset
    TXA
 
@@ -123,6 +176,8 @@ OFFSHI = *+1
    ADC PUTDST+2
    STA COPY_MATCH_LOOP+2                ; store high 8 bits of address
    
+endif
+
    PLA                                  ; retrieve token from stack again
    AND #$0F                             ; isolate match len (MMMM)
    ADC #$02                             ; plus carry which is always set by the high ADC
@@ -145,6 +200,45 @@ OFFSHI = *+1
 DECOMPRESSION_DONE
    RTS
 
+ifdef BACKWARD_DECOMPRESS
+
+   ; Backward decompression -- get and put bytes backward
+
+GETPUT
+   JSR GETSRC
+PUTDST
+LZSA_DST_LO = *+1
+LZSA_DST_HI = *+2
+   STA $AAAA
+   LDA PUTDST+1
+   BNE PUTDST_DONE
+   DEC PUTDST+2
+PUTDST_DONE
+   DEC PUTDST+1
+   RTS
+
+GETLARGESRC
+   JSR GETSRC                           ; grab low 8 bits
+   TAX                                  ; move to X
+                                        ; fall through grab high 8 bits
+
+GETSRC
+LZSA_SRC_LO = *+1
+LZSA_SRC_HI = *+2
+   LDA $AAAA
+   PHA
+   LDA GETSRC+1
+   BNE GETSRC_DONE
+   DEC GETSRC+2
+GETSRC_DONE
+   DEC GETSRC+1
+   PLA
+   RTS
+
+else
+
+   ; Forward decompression -- get and put bytes forward
+
 GETPUT
    JSR GETSRC
 PUTDST
@@ -171,3 +265,5 @@ LZSA_SRC_HI = *+2
    INC GETSRC+2
 GETSRC_DONE
    RTS
+
+endif

asm/6502/decompress_v2.asm

@@ -8,6 +8,18 @@
 ;
 ; out:
 ; * LZSA_DST_LO and LZSA_DST_HI contain the last decompressed byte address, +1
+;
+; -----------------------------------------------------------------------------
+; Backward decompression is also supported, use lzsa -r -b -f2 <original_file> <compressed_file>
+; To use it, also define BACKWARD_DECOMPRESS=1 before including this code!
+;
+; in:
+; * LZSA_SRC_LO/LZSA_SRC_HI must contain the address of the last byte of compressed data
+; * LZSA_DST_LO/LZSA_DST_HI must contain the address of the last byte of the destination buffer
+;
+; out:
+; * LZSA_DST_LO/LZSA_DST_HI contain the last decompressed byte address, -1
+;
 ; -----------------------------------------------------------------------------
 ;
 ;  Copyright (C) 2019 Emmanuel Marty
@@ -130,6 +142,25 @@ GOT_OFFSET_LO
    STX OFFSHI                           ; store high byte of match offset
 
 REP_MATCH
+ifdef BACKWARD_DECOMPRESS
+
+   ; Backward decompression - substract match offset
+
+   SEC                                  ; add dest + match offset
+   LDA PUTDST+1                         ; low 8 bits
+OFFSLO = *+1
+   SBC #$AA
+   STA COPY_MATCH_LOOP+1                ; store back reference address
+   LDA PUTDST+2
+OFFSHI = *+1
+   SBC #$AA                             ; high 8 bits
+   STA COPY_MATCH_LOOP+2                ; store high 8 bits of address
+   SEC
+
+else
+
+   ; Forward decompression - add match offset
+
    CLC                                  ; add dest + match offset
    LDA PUTDST+1                         ; low 8 bits
 OFFSLO = *+1
@@ -140,6 +171,8 @@ OFFSHI = *+1
    ADC PUTDST+2
    STA COPY_MATCH_LOOP+2                ; store high 8 bits of address
    
+endif
+   
    PLA                                  ; retrieve token from stack again
    AND #$07                             ; isolate match len (MMM)
    ADC #$01                             ; add MIN_MATCH_SIZE_V2 and carry
@@ -173,11 +206,29 @@ PREPARE_COPY_MATCH_Y
 
 COPY_MATCH_LOOP
    LDA $AAAA                            ; get one byte of backreference
+   JSR PUTDST                           ; copy to destination
+
+ifdef BACKWARD_DECOMPRESS
+
+   ; Backward decompression -- put backreference bytes backward
+
+   LDA COPY_MATCH_LOOP+1
+   BNE GETMATCH_DONE
+   DEC COPY_MATCH_LOOP+2
+GETMATCH_DONE
+   DEC COPY_MATCH_LOOP+1
+
+else
+
+   ; Forward decompression -- put backreference bytes forward
+
    INC COPY_MATCH_LOOP+1
    BNE GETMATCH_DONE
    INC COPY_MATCH_LOOP+2
 GETMATCH_DONE
-   JSR PUTDST                           ; copy to destination
+
+endif
+
    DEX
    BNE COPY_MATCH_LOOP
    DEY
@@ -215,6 +266,45 @@ HAS_NIBBLES
    AND #$0F                             ; isolate low 4 bits of nibble
    RTS
 
+ifdef BACKWARD_DECOMPRESS
+
+   ; Backward decompression -- get and put bytes backward
+
+GETPUT
+   JSR GETSRC
+PUTDST
+LZSA_DST_LO = *+1
+LZSA_DST_HI = *+2
+   STA $AAAA
+   LDA PUTDST+1
+   BNE PUTDST_DONE
+   DEC PUTDST+2
+PUTDST_DONE
+   DEC PUTDST+1
+   RTS
+
+GETLARGESRC
+   JSR GETSRC                           ; grab low 8 bits
+   TAX                                  ; move to X
+                                        ; fall through grab high 8 bits
+
+GETSRC
+LZSA_SRC_LO = *+1
+LZSA_SRC_HI = *+2
+   LDA $AAAA
+   PHA
+   LDA GETSRC+1
+   BNE GETSRC_DONE
+   DEC GETSRC+2
+GETSRC_DONE
+   DEC GETSRC+1
+   PLA
+   RTS
+
+else
+
+   ; Forward decompression -- get and put bytes forward
+
 GETPUT
    JSR GETSRC
 PUTDST
@@ -241,3 +331,5 @@ LZSA_SRC_HI = *+2
    INC GETSRC+2
 GETSRC_DONE
    RTS
+
+endif

src/expand_context.c

@@ -35,6 +35,7 @@
 #include "expand_context.h"
 #include "expand_block_v1.h"
 #include "expand_block_v2.h"
+#include "lib.h"
 
 /**
  * Decompress one data block
@@ -45,14 +46,31 @@
  * @param nOutDataOffset starting index of where to store decompressed bytes in output buffer (and size of previously decompressed bytes)
  * @param nBlockMaxSize total size of output decompression buffer, in bytes
  * @param nFormatVersion version of format to use (1-2)
+ * @param nFlags compression flags (LZSA_FLAG_xxx)
  *
  * @return size of decompressed data in bytes, or -1 for error
  */
-int lzsa_decompressor_expand_block(const unsigned char *pInBlock, int nBlockSize, unsigned char *pOutData, int nOutDataOffset, int nBlockMaxSize, const int nFormatVersion) {
+int lzsa_decompressor_expand_block(unsigned char *pInBlock, int nBlockSize, unsigned char *pOutData, int nOutDataOffset, int nBlockMaxSize, const int nFormatVersion, const int nFlags) {
-   if (nFormatVersion == 1)
+   int nDecompressedSize;
-      return lzsa_decompressor_expand_block_v1(pInBlock, nBlockSize, pOutData, nOutDataOffset, nBlockMaxSize);
+
-   else if (nFormatVersion == 2)
+   if (nFlags & LZSA_FLAG_RAW_BACKWARD) {
-      return lzsa_decompressor_expand_block_v2(pInBlock, nBlockSize, pOutData, nOutDataOffset, nBlockMaxSize);
+      lzsa_reverse_buffer(pInBlock, nBlockSize);
-   else
+   }
-      return -1;
+
+   if (nFormatVersion == 1)
+      nDecompressedSize = lzsa_decompressor_expand_block_v1(pInBlock, nBlockSize, pOutData, nOutDataOffset, nBlockMaxSize);
+   else if (nFormatVersion == 2)
+      nDecompressedSize = lzsa_decompressor_expand_block_v2(pInBlock, nBlockSize, pOutData, nOutDataOffset, nBlockMaxSize);
+   else
+      nDecompressedSize = -1;
+
+   if (nDecompressedSize != -1 && (nFlags & LZSA_FLAG_RAW_BACKWARD)) {
+      lzsa_reverse_buffer(pOutData + nOutDataOffset, nDecompressedSize);
+   }
+
+   if (nFlags & LZSA_FLAG_RAW_BACKWARD) {
+      lzsa_reverse_buffer(pInBlock, nBlockSize);
+   }
+
+   return nDecompressedSize;
 }

src/expand_context.h

@@ -48,10 +48,11 @@ extern "C" {
  * @param nOutDataOffset starting index of where to store decompressed bytes in output buffer (and size of previously decompressed bytes)
  * @param nBlockMaxSize total size of output decompression buffer, in bytes
  * @param nFormatVersion version of format to use (1-2)
+ * @param nFlags compression flags (LZSA_FLAG_xxx)
  *
  * @return size of decompressed data in bytes, or -1 for error
  */
-int lzsa_decompressor_expand_block(const unsigned char *pInBlock, int nBlockSize, unsigned char *pOutData, int nOutDataOffset, int nBlockMaxSize, const int nFormatVersion);
+int lzsa_decompressor_expand_block(unsigned char *pInBlock, int nBlockSize, unsigned char *pOutData, int nOutDataOffset, int nBlockMaxSize, const int nFormatVersion, const int nFlags);
 
 #ifdef __cplusplus
 }

src/expand_inmem.c

@@ -98,8 +98,8 @@ size_t lzsa_get_max_decompressed_size_inmem(const unsigned char *pFileData, size
  *
  * @return actual decompressed size, or -1 for error
  */
-size_t lzsa_decompress_inmem(const unsigned char *pFileData, unsigned char *pOutBuffer, size_t nFileSize, size_t nMaxOutBufferSize, const unsigned int nFlags, int *pFormatVersion) {
+size_t lzsa_decompress_inmem(unsigned char *pFileData, unsigned char *pOutBuffer, size_t nFileSize, size_t nMaxOutBufferSize, const unsigned int nFlags, int *pFormatVersion) {
-   const unsigned char *pCurFileData = pFileData;
+   unsigned char *pCurFileData = pFileData;
    const unsigned char *pEndFileData = pCurFileData + nFileSize;
    unsigned char *pCurOutBuffer = pOutBuffer;
    const unsigned char *pEndOutBuffer = pCurOutBuffer + nMaxOutBufferSize;
@@ -107,7 +107,7 @@ size_t lzsa_decompress_inmem(const unsigned char *pFileData, unsigned char *pOut
    const int nHeaderSize = lzsa_get_header_size();
 
    if (nFlags & LZSA_FLAG_RAW_BLOCK) {
-      return (size_t)lzsa_decompressor_expand_block(pFileData, (int)nFileSize, pOutBuffer, 0, (int)nMaxOutBufferSize, *pFormatVersion);
+      return (size_t)lzsa_decompressor_expand_block(pFileData, (int)nFileSize, pOutBuffer, 0, (int)nMaxOutBufferSize, *pFormatVersion, nFlags);
    }
 
    /* Check header */
@@ -139,7 +139,7 @@ size_t lzsa_decompress_inmem(const unsigned char *pFileData, unsigned char *pOut
          if ((pCurFileData + nBlockDataSize) > pEndFileData)
             return -1;
 
-         nDecompressedSize = lzsa_decompressor_expand_block(pCurFileData, nBlockDataSize, pCurOutBuffer - nPreviousBlockSize, nPreviousBlockSize, (int)(pEndOutBuffer - pCurOutBuffer + nPreviousBlockSize), *pFormatVersion);
+         nDecompressedSize = lzsa_decompressor_expand_block(pCurFileData, nBlockDataSize, pCurOutBuffer - nPreviousBlockSize, nPreviousBlockSize, (int)(pEndOutBuffer - pCurOutBuffer + nPreviousBlockSize), *pFormatVersion, nFlags);
          if (nDecompressedSize < 0)
             return -1;
 

src/expand_inmem.h

@@ -61,7 +61,7 @@ size_t lzsa_get_max_decompressed_size_inmem(const unsigned char *pFileData, size
  *
  * @return actual decompressed size, or -1 for error
  */
-size_t lzsa_decompress_inmem(const unsigned char *pFileData, unsigned char *pOutBuffer, size_t nFileSize, size_t nMaxOutBufferSize, const unsigned int nFlags, int *pFormatVersion);
+size_t lzsa_decompress_inmem(unsigned char *pFileData, unsigned char *pOutBuffer, size_t nFileSize, size_t nMaxOutBufferSize, const unsigned int nFlags, int *pFormatVersion);
 
 #ifdef __cplusplus
 }

src/expand_streaming.c

@@ -196,7 +196,7 @@ lzsa_status_t lzsa_decompress_stream(lzsa_stream_t *pInStream, lzsa_stream_t *pO
                nDecompressedSize = nBlockSize;
             }
             else {
-               nDecompressedSize = lzsa_decompressor_expand_block(pInBlock, nBlockSize, pOutData, BLOCK_SIZE, BLOCK_SIZE, nFormatVersion);
+               nDecompressedSize = lzsa_decompressor_expand_block(pInBlock, nBlockSize, pOutData, BLOCK_SIZE, BLOCK_SIZE, nFormatVersion, nFlags);
                if (nDecompressedSize < 0) {
                   nDecompressionError = LZSA_ERROR_DECOMPRESSION;
                   break;

src/lib.h

@@ -69,6 +69,24 @@ typedef enum _lzsa_status_t {
 /* Compression flags */
 #define LZSA_FLAG_FAVOR_RATIO    (1<<0)      /**< 1 to compress with the best ratio, 0 to trade some compression ratio for extra decompression speed */
 #define LZSA_FLAG_RAW_BLOCK      (1<<1)      /**< 1 to emit raw block */
+#define LZSA_FLAG_RAW_BACKWARD   (1<<2)      /**< 1 to compress or decompress raw block backward */
+
+/**
+ * Reverse bytes in the specified buffer
+ *
+ * @param pBuffer pointer to buffer whose contents are to be reversed
+ * @param nBufferSize size of buffer in bytes
+ */
+static inline void lzsa_reverse_buffer(unsigned char *pBuffer, const int nBufferSize) {
+   int nMidPoint = nBufferSize / 2;
+   int i, j;
+
+   for (i = 0, j = nBufferSize - 1; i < nMidPoint; i++, j--) {
+      unsigned char c = pBuffer[i];
+      pBuffer[i] = pBuffer[j];
+      pBuffer[j] = c;
+   }
+}
 
 #ifdef __cplusplus
 }

src/lzsa.c

@@ -45,8 +45,9 @@
 #define OPT_VERBOSE        1
 #define OPT_RAW            2
 #define OPT_FAVOR_RATIO    4
+#define OPT_RAW_BACKWARD   8
 
-#define TOOL_VERSION "1.0.4"
+#define TOOL_VERSION "1.0.5"
 
 /*---------------------------------------------------------------------------*/
 
@@ -109,6 +110,8 @@ static int do_compress(const char *pszInFilename, const char *pszOutFilename, co
       nFlags |= LZSA_FLAG_FAVOR_RATIO;
    if (nOptions & OPT_RAW)
       nFlags |= LZSA_FLAG_RAW_BLOCK;
+   if (nOptions & OPT_RAW_BACKWARD)
+      nFlags |= LZSA_FLAG_RAW_BACKWARD;
 
    if (nOptions & OPT_VERBOSE) {
       nStartTime = do_get_time();
@@ -160,6 +163,8 @@ static int do_decompress(const char *pszInFilename, const char *pszOutFilename,
    nFlags = 0;
    if (nOptions & OPT_RAW)
       nFlags |= LZSA_FLAG_RAW_BLOCK;
+   if (nOptions & OPT_RAW_BACKWARD)
+      nFlags |= LZSA_FLAG_RAW_BACKWARD;
 
    if (nOptions & OPT_VERBOSE) {
       nStartTime = do_get_time();
@@ -308,6 +313,8 @@ static int do_compare(const char *pszInFilename, const char *pszOutFilename, con
    nFlags = 0;
    if (nOptions & OPT_RAW)
       nFlags |= LZSA_FLAG_RAW_BLOCK;
+   if (nOptions & OPT_RAW_BACKWARD)
+      nFlags |= LZSA_FLAG_RAW_BACKWARD;
 
    if (nOptions & OPT_VERBOSE) {
       nStartTime = do_get_time();
@@ -419,6 +426,8 @@ static int do_self_test(const unsigned int nOptions, const int nMinMatchSize, in
       nFlags |= LZSA_FLAG_FAVOR_RATIO;
    if (nOptions & OPT_RAW)
       nFlags |= LZSA_FLAG_RAW_BLOCK;
+   if (nOptions & OPT_RAW_BACKWARD)
+      nFlags |= LZSA_FLAG_RAW_BACKWARD;
 
    pGeneratedData = (unsigned char*)malloc(4 * BLOCK_SIZE);
    if (!pGeneratedData) {
@@ -590,6 +599,8 @@ static int do_compr_benchmark(const char *pszInFilename, const char *pszOutFilen
       nFlags |= LZSA_FLAG_FAVOR_RATIO;
    if (nOptions & OPT_RAW)
       nFlags |= LZSA_FLAG_RAW_BLOCK;
+   if (nOptions & OPT_RAW_BACKWARD)
+      nFlags |= LZSA_FLAG_RAW_BACKWARD;
 
    if (pszDictionaryFilename) {
       fprintf(stderr, "in-memory benchmarking does not support dictionaries\n");
@@ -720,6 +731,8 @@ static int do_dec_benchmark(const char *pszInFilename, const char *pszOutFilenam
    nFlags = 0;
    if (nOptions & OPT_RAW)
       nFlags |= LZSA_FLAG_RAW_BLOCK;
+   if (nOptions & OPT_RAW_BACKWARD)
+      nFlags |= LZSA_FLAG_RAW_BACKWARD;
 
    if (pszDictionaryFilename) {
       fprintf(stderr, "in-memory benchmarking does not support dictionaries\n");
@@ -987,6 +1000,13 @@ int main(int argc, char **argv) {
          else
             bArgsError = true;
       }
+      else if (!strcmp(argv[i], "-b")) {
+         if ((nOptions & OPT_RAW_BACKWARD) == 0) {
+            nOptions |= OPT_RAW_BACKWARD;
+         }
+         else
+            bArgsError = true;
+      }
       else {
          if (!pszInFilename)
             pszInFilename = argv[i];
@@ -999,6 +1019,11 @@ int main(int argc, char **argv) {
       }
    }
 
+   if (!bArgsError && (nOptions & OPT_RAW_BACKWARD) && !(nOptions & OPT_RAW)) {
+      fprintf(stderr, "error: -b (compress backwards) requires -r (raw block format)\n");
+      return 100;
+   }
+
    if (!bArgsError && cCommand == 't') {
       return do_self_test(nOptions, nMinMatchSize, nFormatVersion);
    }
@@ -1014,6 +1039,7 @@ int main(int argc, char **argv) {
       fprintf(stderr, "       -v: be verbose\n");
       fprintf(stderr, "       -f <value>: LZSA compression format (1-2)\n");
       fprintf(stderr, "       -r: raw block format (max. 64 Kb files)\n");
+      fprintf(stderr, "       -b: compress backward (requires -r and a backward decompressor)\n");
       fprintf(stderr, "       -D <filename>: use dictionary file\n");
       fprintf(stderr, "       -m <value>: minimum match size (3-5) (default: 3)\n");
       fprintf(stderr, "       --prefer-ratio: favor compression ratio (default)\n");

src/shrink_context.c

@@ -37,6 +37,7 @@
 #include "shrink_block_v2.h"
 #include "format.h"
 #include "matchfinder.h"
+#include "lib.h"
 
 /**
  * Initialize compression context
@@ -188,25 +189,45 @@ void lzsa_compressor_destroy(lzsa_compressor *pCompressor) {
  *
  * @return size of compressed data in output buffer, or -1 if the data is uncompressible
  */
-int lzsa_compressor_shrink_block(lzsa_compressor *pCompressor, const unsigned char *pInWindow, const int nPreviousBlockSize, const int nInDataSize, unsigned char *pOutData, const int nMaxOutDataSize) {
+int lzsa_compressor_shrink_block(lzsa_compressor *pCompressor, unsigned char *pInWindow, const int nPreviousBlockSize, const int nInDataSize, unsigned char *pOutData, const int nMaxOutDataSize) {
+   int nCompressedSize;
+
+   if (pCompressor->flags & LZSA_FLAG_RAW_BACKWARD) {
+      lzsa_reverse_buffer(pInWindow + nPreviousBlockSize, nInDataSize);
+   }
+
    if (lzsa_build_suffix_array(pCompressor, pInWindow, nPreviousBlockSize + nInDataSize))
-      return -1;
+      nCompressedSize = -1;
+   else {
       if (nPreviousBlockSize) {
          lzsa_skip_matches(pCompressor, 0, nPreviousBlockSize);
       }
       lzsa_find_all_matches(pCompressor, nPreviousBlockSize, nPreviousBlockSize + nInDataSize);
 
       if (pCompressor->format_version == 1) {
-      return lzsa_optimize_and_write_block_v1(pCompressor, pInWindow, nPreviousBlockSize, nInDataSize, pOutData, nMaxOutDataSize);
+         nCompressedSize = lzsa_optimize_and_write_block_v1(pCompressor, pInWindow, nPreviousBlockSize, nInDataSize, pOutData, nMaxOutDataSize);
+         if (nCompressedSize != -1 && (pCompressor->flags & LZSA_FLAG_RAW_BACKWARD)) {
+            lzsa_reverse_buffer(pOutData, nCompressedSize);
+         }
       }
       else if (pCompressor->format_version == 2) {
-      return lzsa_optimize_and_write_block_v2(pCompressor, pInWindow, nPreviousBlockSize, nInDataSize, pOutData, nMaxOutDataSize);
+         nCompressedSize = lzsa_optimize_and_write_block_v2(pCompressor, pInWindow, nPreviousBlockSize, nInDataSize, pOutData, nMaxOutDataSize);
+         if (nCompressedSize != -1 && (pCompressor->flags & LZSA_FLAG_RAW_BACKWARD)) {
+            lzsa_reverse_buffer(pOutData, nCompressedSize);
+         }
       }
       else {
-      return -1;
+         nCompressedSize = -1;
       }
    }
 
+   if (pCompressor->flags & LZSA_FLAG_RAW_BACKWARD) {
+      lzsa_reverse_buffer(pInWindow + nPreviousBlockSize, nInDataSize);
+   }
+
+   return nCompressedSize;
+}
+
 /**
  * Get the number of compression commands issued in compressed data blocks
  *

src/shrink_context.h

@@ -123,7 +123,7 @@ void lzsa_compressor_destroy(lzsa_compressor *pCompressor);
  *
  * @return size of compressed data in output buffer, or -1 if the data is uncompressible
  */
-int lzsa_compressor_shrink_block(lzsa_compressor *pCompressor, const unsigned char *pInWindow, const int nPreviousBlockSize, const int nInDataSize, unsigned char *pOutData, const int nMaxOutDataSize);
+int lzsa_compressor_shrink_block(lzsa_compressor *pCompressor, unsigned char *pInWindow, const int nPreviousBlockSize, const int nInDataSize, unsigned char *pOutData, const int nMaxOutDataSize);
 
 /**
  * Get the number of compression commands issued in compressed data blocks

src/shrink_inmem.c

@@ -62,7 +62,7 @@ size_t lzsa_get_max_compressed_size_inmem(size_t nInputSize) {
  *
  * @return actual compressed size, or -1 for error
  */
-size_t lzsa_compress_inmem(const unsigned char *pInputData, unsigned char *pOutBuffer, size_t nInputSize, size_t nMaxOutBufferSize,
+size_t lzsa_compress_inmem(unsigned char *pInputData, unsigned char *pOutBuffer, size_t nInputSize, size_t nMaxOutBufferSize,
                              const unsigned int nFlags, const int nMinMatchSize, const int nFormatVersion) {
    lzsa_compressor compressor;
    size_t nOriginalSize = 0;

src/shrink_inmem.h

@@ -61,7 +61,7 @@ size_t lzsa_get_max_compressed_size_inmem(size_t nInputSize);
  *
  * @return actual compressed size, or -1 for error
  */
-size_t lzsa_compress_inmem(const unsigned char *pInputData, unsigned char *pOutBuffer, size_t nInputSize, size_t nMaxOutBufferSize,
+size_t lzsa_compress_inmem(unsigned char *pInputData, unsigned char *pOutBuffer, size_t nInputSize, size_t nMaxOutBufferSize,
    const unsigned int nFlags, const int nMinMatchSize, const int nFormatVersion);
 
 #ifdef __cplusplus