Add --size-hint=# option

lib/zstd.h

@@ -386,6 +386,7 @@ typedef enum {
      * ZSTD_c_forceAttachDict
      * ZSTD_c_literalCompressionMode
      * ZSTD_c_targetCBlockSize
+     * ZSTD_c_srcSizeHint
      * Because they are not stable, it's necessary to define ZSTD_STATIC_LINKING_ONLY to access them.
      * note : never ever use experimentalParam? names directly;
      *        also, the enums values themselves are unstable and can still change.
@@ -396,6 +397,7 @@ typedef enum {
      ZSTD_c_experimentalParam4=1001,
      ZSTD_c_experimentalParam5=1002,
      ZSTD_c_experimentalParam6=1003,
+     ZSTD_c_experimentalParam7=1004,
 } ZSTD_cParameter;
 
 typedef struct {
@@ -1063,6 +1065,7 @@ ZSTDLIB_API size_t ZSTD_sizeof_DDict(const ZSTD_DDict* ddict);
 /* Advanced parameter bounds */
 #define ZSTD_TARGETCBLOCKSIZE_MIN   64
 #define ZSTD_TARGETCBLOCKSIZE_MAX   ZSTD_BLOCKSIZE_MAX
+#define ZSTD_SRCSIZEHINT_MAX        1e9  /* 1 GB */
 
 /* internal */
 #define ZSTD_HASHLOG3_MAX           17
@@ -1441,6 +1444,11 @@ ZSTDLIB_API size_t ZSTD_CCtx_refPrefix_advanced(ZSTD_CCtx* cctx, const void* pre
  * There is no guarantee on compressed block size (default:0) */
 #define ZSTD_c_targetCBlockSize ZSTD_c_experimentalParam6
 
+/* User's best guess of source size.
+ * Hint is not valid when srcSizeHint == 0.
+ * There is no guarantee that hint is close to actual source size */
+#define ZSTD_c_srcSizeHint ZSTD_c_experimentalParam7
+
 /*! ZSTD_CCtx_getParameter() :
  *  Get the requested compression parameter value, selected by enum ZSTD_cParameter,
  *  and store it into int* value.