Added : ability to manually select the dictionary ID of a newly created dictionary

lib/dictBuilder/zdict.c

@@ -819,10 +819,10 @@ size_t ZDICT_trainFromBuffer_unsafe(
                             ZDICT_params_t params)
 {
     U32 const dictListSize = MAX( MAX(DICTLISTSIZE, nbSamples), (U32)(maxDictSize/16));
-    dictItem* dictList = (dictItem*)malloc(dictListSize * sizeof(*dictList));
+    dictItem* const dictList = (dictItem*)malloc(dictListSize * sizeof(*dictList));
     unsigned selectivity = params.selectivityLevel;
     unsigned compressionLevel = params.compressionLevel;
-    size_t targetDictSize = maxDictSize;
+    size_t const targetDictSize = maxDictSize;
     size_t sBuffSize;
     size_t dictSize = 0;
 
@@ -865,17 +865,16 @@ size_t ZDICT_trainFromBuffer_unsafe(
     /* create dictionary */
     {   U32 dictContentSize = ZDICT_dictSize(dictList);
         size_t hSize;
-        BYTE* ptr;
-        U32 u;
 
         /* build dict content */
-        ptr = (BYTE*)dictBuffer + maxDictSize;
-        for (u=1; u<dictList->pos; u++) {
-            U32 l = dictList[u].length;
-            ptr -= l;
-            if (ptr<(BYTE*)dictBuffer) return ERROR(GENERIC);   /* should not happen */
-            memcpy(ptr, (const char*)samplesBuffer+dictList[u].pos, l);
-        }
+        {   U32 u;
+            BYTE* ptr = (BYTE*)dictBuffer + maxDictSize;
+            for (u=1; u<dictList->pos; u++) {
+                U32 l = dictList[u].length;
+                ptr -= l;
+                if (ptr<(BYTE*)dictBuffer) return ERROR(GENERIC);   /* should not happen */
+                memcpy(ptr, (const char*)samplesBuffer+dictList[u].pos, l);
+        }   }
 
         /* fast mode dict content */
         if (selectivity==1) {  /* note could also be used to complete a dictionary, but not necessarily better */
@@ -888,7 +887,8 @@ size_t ZDICT_trainFromBuffer_unsafe(
        /* dictionary header */
         MEM_writeLE32(dictBuffer, ZSTD_DICT_MAGIC);
         {   U64 const randomID = XXH64((char*)dictBuffer + maxDictSize - dictContentSize, dictContentSize, 0);
-            MEM_writeLE32((char*)dictBuffer+4, (U32)(randomID>>11));
+            U32 const dictID = params.dictID ? params.dictID : (U32)(randomID>>11);
+            MEM_writeLE32((char*)dictBuffer+4, dictID);
         }
         hSize = 8;
 

lib/dictBuilder/zdict_static.h

@@ -54,7 +54,8 @@ typedef struct {
     unsigned selectivityLevel;   /* 0 means default; larger => bigger selection => larger dictionary */
     unsigned compressionLevel;   /* 0 means default; target a specific zstd compression level */
     unsigned notificationLevel;  /* Write to stderr; 0 = none (default); 1 = errors; 2 = progression; 3 = details; 4 = debug; */
-    unsigned reserved[3];        /* space for future parameters */
+    unsigned dictID;             /* 0 means auto mode (32-bits random value); other : force dictID value */
+    unsigned reserved[2];        /* space for future parameters */
 } ZDICT_params_t;
 
 
@@ -65,7 +66,7 @@ typedef struct {
     Same as ZDICT_trainFromBuffer() with control over more parameters.
     `parameters` is optional and can be provided with values set to 0 to mean "default".
     @return : size of dictionary stored into `dictBuffer` (<= `dictBufferSize`)
-              or an error code, which can be tested by DiB_isError().
+              or an error code, which can be tested by ZDICT_isError().
     note : ZDICT_trainFromBuffer_advanced() will send notifications into stderr if instructed to, using ZDICT_setNotificationLevel()
 */
 size_t ZDICT_trainFromBuffer_advanced(void* dictBuffer, size_t dictBufferCapacity,