add faster block splitting heuristic, suitable for dfast strategy

lib/compress/zstd_preSplit.h

@@ -17,11 +17,10 @@
 extern "C" {
 #endif
 
-typedef enum { split_lvl1, split_lvl2, split_lvl3 } ZSTD_SplitBlock_strategy_e;
-
 #define ZSTD_SLIPBLOCK_WORKSPACESIZE 8208
 
-/* note:
+/* @level must be a value between 0 and 3.
+ *        higher levels spend more energy to find block boundaries
  * @workspace must be aligned on 8-bytes boundaries
  * @wkspSize must be at least >= ZSTD_SLIPBLOCK_WORKSPACESIZE
  * note2:
@@ -30,7 +29,7 @@ typedef enum { split_lvl1, split_lvl2, split_lvl3 } ZSTD_SplitBlock_strategy_e;
  * This could be extended to smaller sizes in the future.
  */
 size_t ZSTD_splitBlock(const void* blockStart, size_t blockSize,
-                    ZSTD_SplitBlock_strategy_e splitStrat,
+                    int level,
                     void* workspace, size_t wkspSize);
 
 #if defined (__cplusplus)