Expose dictCtx Functionality in LZ4

3 files changed, 33 insertions, 2 deletions

diff --git a/lib/lz4.c b/lib/lz4.c
index c2d1c32..cee85ec 100644
--- a/lib/lz4.c
+++ b/lib/lz4.c
@@ -1191,6 +1191,10 @@ int LZ4_loadDict (LZ4_stream_t* LZ4_dict, const char* dictionary, int dictSize)
     return dict->dictSize;
 }
 
+void LZ4_attach_dictionary(LZ4_stream_t *working_stream, const LZ4_stream_t *dictionary_stream) {
+    working_stream->internal_donotuse.dictCtx = dictionary_stream != NULL ? &(dictionary_stream->internal_donotuse) : NULL;
+}
+
 
 static void LZ4_renormDictT(LZ4_stream_t_internal* LZ4_dict, const BYTE* src)
 {
diff --git a/lib/lz4.h b/lib/lz4.h
index 427beaf..f90120e 100644
--- a/lib/lz4.h
+++ b/lib/lz4.h
@@ -386,6 +386,34 @@ LZ4LIB_API void LZ4_resetStream_fast (LZ4_stream_t* streamPtr);
  */
 LZ4LIB_API int LZ4_compress_fast_extState_fastReset (void* state, const char* src, char* dst, int srcSize, int dstCapacity, int acceleration);
 
+/*! LZ4_attach_dictionary() :
+ *  This is an experimental API that allows for the efficient use of a
+ *  static dictionary many times.
+ *
+ *  Rather than re-loading the dictionary buffer into a working context before
+ *  each compression, or copying a pre-loaded dictionary's LZ4_stream_t into a
+ *  working LZ4_stream_t, this function introduces a no-copy setup mechanism,
+ *  in which the working stream references the dictionary stream in-place.
+ *
+ *  Several assumptions are made about the state of the dictionary stream.
+ *  Currently, only streams which have been prepared by LZ4_loadDict() should
+ *  be expected to work.
+ *
+ *  Alternatively, the provided dictionary stream pointer may be NULL, in which
+ *  case any existing dictionary stream is unset.
+ *
+ *  If a dictionary is provided, it replaces any pre-existing stream history.
+ *  The dictionary contents are the only history that can be referenced and
+ *  logically immediately precede the data compressed in the first subsequent
+ *  compression call.
+ *
+ *  The dictionary will only remain attached to the working stream through the
+ *  first compression call, at the end of which it is cleared. The dictionary
+ *  stream (and source buffer) must remain in-place / accessible / unchanged
+ *  through the completion of the first compression call on the stream.
+ */
+LZ4LIB_API void LZ4_attach_dictionary(LZ4_stream_t *working_stream, const LZ4_stream_t *dictionary_stream);
+
 
 /*-************************************
  *  Private definitions
diff --git a/lib/lz4frame.c b/lib/lz4frame.c
index 5e11ba2..d973b5f 100644
--- a/lib/lz4frame.c
+++ b/lib/lz4frame.c
@@ -533,8 +533,7 @@ static void LZ4F_applyCDict(void* ctx,
         LZ4_stream_t_internal* internal_ctx = &((LZ4_stream_t *)ctx)->internal_donotuse;
         assert(!internal_ctx->initCheck);
         LZ4_resetStream_fast((LZ4_stream_t *)ctx);
-        /* Point to the dictionary context */
-        internal_ctx->dictCtx = cdict ? &(cdict->fastCtx->internal_donotuse) : NULL;
+        LZ4_attach_dictionary((LZ4_stream_t *)ctx, cdict ? cdict->fastCtx : NULL);
     } else {
         if (cdict) {
             memcpy(ctx, cdict->HCCtx, sizeof(*cdict->HCCtx));