Add API for Attaching Dictionaries

3 files changed, 31 insertions, 2 deletions

diff --git a/lib/lz4frame.c b/lib/lz4frame.c
index a43f595..d4d9397 100644
--- a/lib/lz4frame.c
+++ b/lib/lz4frame.c
@@ -549,9 +549,8 @@ static void LZ4F_applyCDict(void* ctx,
         LZ4_resetStream_fast((LZ4_stream_t *)ctx);
         LZ4_attach_dictionary((LZ4_stream_t *)ctx, cdict ? cdict->fastCtx : NULL);
     } else {
-        LZ4HC_CCtx_internal *internal_ctx = &((LZ4_streamHC_t *)ctx)->internal_donotuse;
         LZ4_resetStreamHC_fast((LZ4_streamHC_t*)ctx, level);
-        internal_ctx->dictCtx = cdict ? &(cdict->HCCtx->internal_donotuse) : NULL;
+        LZ4_attach_HC_dictionary((LZ4_streamHC_t *)ctx, cdict ? cdict->HCCtx : NULL);
     }
 }
 
diff --git a/lib/lz4hc.c b/lib/lz4hc.c
index 124da86..690015f 100644
--- a/lib/lz4hc.c
+++ b/lib/lz4hc.c
@@ -887,6 +887,9 @@ int LZ4_loadDictHC (LZ4_streamHC_t* LZ4_streamHCPtr, const char* dictionary, int
     return dictSize;
 }
 
+void LZ4_attach_HC_dictionary(LZ4_streamHC_t *working_stream, const LZ4_streamHC_t *dictionary_stream) {
+    working_stream->internal_donotuse.dictCtx = dictionary_stream != NULL ? &(dictionary_stream->internal_donotuse) : NULL;
+}
 
 /* compression */
 
diff --git a/lib/lz4hc.h b/lib/lz4hc.h
index cfc5d9e..fb8397e 100644
--- a/lib/lz4hc.h
+++ b/lib/lz4hc.h
@@ -279,5 +279,32 @@ void LZ4_resetStreamHC_fast(LZ4_streamHC_t* LZ4_streamHCPtr, int compressionLeve
 
 int LZ4_compress_HC_extStateHC_fastReset (void* state, const char* src, char* dst, int srcSize, int dstCapacity, int compressionLevel);
 
+/*! LZ4_attach_HC_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_streamHC_t into a
+ *  working LZ4_streamHC_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_loadDictHC() should
+ *  be expected to work.
+ *
+ *  Alternatively, the provided dictionary stream pointer may be NULL, in which
+ *  case any existing dictionary stream is unset.
+ *
+ *  A dictionary should only be attached to a stream without any history (i.e.,
+ *  a stream that has just been reset).
+ *
+ *  The dictionary will remain attached to the working stream only for the
+ *  current stream session. Calls to LZ4_resetStreamHC(_fast) will remove the
+ *  dictionary context association from the working stream. The dictionary
+ *  stream (and source buffer) must remain in-place / accessible / unchanged
+ *  through the lifetime of the stream session.
+ */
+LZ4LIB_API void LZ4_attach_HC_dictionary(LZ4_streamHC_t *working_stream, const LZ4_streamHC_t *dictionary_stream);
+
 #endif   /* LZ4_HC_SLO_098092834 */
 #endif   /* LZ4_HC_STATIC_LINKING_ONLY */