diff --git a/lib/compress/zstd_compress.c b/lib/compress/zstd_compress.c
index 7721325d78a9d31307e7311034c89c5d689ae010..c6d72f584bcac1ec3c6acd054302c10cfb25e57e 100644
--- a/lib/compress/zstd_compress.c
+++ b/lib/compress/zstd_compress.c
@@ -412,11 +412,12 @@ size_t ZSTD_CCtxParam_setParameter(
CCtxParams->forceWindow = (value > 0);
return CCtxParams->forceWindow;
- case ZSTD_p_forceAttachDict :
- CCtxParams->attachDictPref = value ?
- (value > 0 ? ZSTD_dictForceAttach : ZSTD_dictForceCopy) :
- ZSTD_dictDefaultAttach;
+ case ZSTD_p_forceAttachDict : {
+ const ZSTD_dictAttachPref_e pref = (ZSTD_dictAttachPref_e)value;
+ CLAMPCHECK(pref, ZSTD_dictDefaultAttach, ZSTD_dictForceCopy);
+ CCtxParams->attachDictPref = pref;
return CCtxParams->attachDictPref;
+ }
case ZSTD_p_nbWorkers :
#ifndef ZSTD_MULTITHREAD
diff --git a/lib/compress/zstd_compress_internal.h b/lib/compress/zstd_compress_internal.h
index ec38e5d4b781075a2190355fcb239cfd3f65fb12..ffbb53a78a9743e1632389a24f3349535c9c6d74 100644
--- a/lib/compress/zstd_compress_internal.h
+++ b/lib/compress/zstd_compress_internal.h
@@ -48,12 +48,6 @@ extern "C" {
typedef enum { ZSTDcs_created=0, ZSTDcs_init, ZSTDcs_ongoing, ZSTDcs_ending } ZSTD_compressionStage_e;
typedef enum { zcss_init=0, zcss_load, zcss_flush } ZSTD_cStreamStage;
-typedef enum {
- ZSTD_dictDefaultAttach = 0,
- ZSTD_dictForceAttach = 1,
- ZSTD_dictForceCopy = -1,
-} ZSTD_dictAttachPref_e;
-
typedef struct ZSTD_prefixDict_s {
const void* dict;
size_t dictSize;
diff --git a/lib/zstd.h b/lib/zstd.h
index d7646fdaf6ad63fda7c79ba22b5f83e65a0dc585..30090ca491b781ddc45e5994f21bca53b85b7824 100644
--- a/lib/zstd.h
+++ b/lib/zstd.h
@@ -996,6 +996,38 @@ typedef enum {
* Decoder cannot recognise automatically this format, requiring instructions. */
} ZSTD_format_e;
+typedef enum {
+ /* Note: this enum and the behavior it controls are effectively internal
+ * implementation details of the compressor. They are expected to continue
+ * to evolve and should be considered only in the context of extremely
+ * advanced performance tuning.
+ *
+ * Zstd currently supports the use of a CDict in two ways:
+ *
+ * - The contents of the CDict can be copied into the working context. This
+ * means that the compression can search both the dictionary and input
+ * while operating on a single set of internal tables. This makes
+ * the compression faster per-byte of input. However, the initial copy of
+ * the CDict's tables incurs a fixed cost at the beginning of the
+ * compression. For small compressions (< 8 KB), that copy can dominate
+ * the cost of the compression.
+ *
+ * - The CDict's tables can be used in-place. In this model, compression is
+ * slower per input byte, because the compressor has to search two sets of
+ * tables. However, this model incurs no start-up cost (as long as the
+ * working context's tables can be reused). For small inputs, this can be
+ * faster than copying the CDict's tables.
+ *
+ * Zstd has a simple internal heuristic that selects which strategy to use
+ * at the beginning of a compression. However, if experimentation shows that
+ * Zstd is making poor choices, it is possible to override that choice with
+ * this enum.
+ */
+ ZSTD_dictDefaultAttach = 0, /* Use the default heuristic. */
+ ZSTD_dictForceAttach = 1, /* Never copy the dictionary. */
+ ZSTD_dictForceCopy = 2, /* Always copy the dictionary. */
+} ZSTD_dictAttachPref_e;
+
typedef enum {
/* compression format */
ZSTD_p_format = 10, /* See ZSTD_format_e enum definition.
@@ -1109,29 +1141,14 @@ typedef enum {
ZSTD_p_forceMaxWindow=1100, /* Force back-reference distances to remain < windowSize,
* even when referencing into Dictionary content (default:0) */
- ZSTD_p_forceAttachDict, /* ZSTD supports usage of a CDict in-place
- * (avoiding having to copy the compression tables
- * from the CDict into the working context). Using
- * a CDict in this way saves an initial setup step,
- * but comes at the cost of more work per byte of
- * input. ZSTD has a simple internal heuristic that
- * guesses which strategy will be faster. You can
- * use this flag to override that guess.
+ ZSTD_p_forceAttachDict, /* Controls whether the contents of a CDict are
+ * used in place, or whether they are copied into
+ * the working context.
*
- * Note that the by-reference, in-place strategy is
- * only used when reusing a compression context
- * with compatible compression parameters. (If
- * incompatible / uninitialized, the working
- * context needs to be cleared anyways, which is
- * about as expensive as overwriting it with the
- * dictionary context, so there's no savings in
- * using the CDict by-ref.)
- *
- * Values greater than 0 force attaching the dict.
- * Values less than 0 force copying the dict.
- * 0 selects the default heuristic-guided behavior.
+ * Accepts values from the ZSTD_dictAttachPref_e
+ * enum. See the comments on that enum for an
+ * explanation of the feature.
*/
-
} ZSTD_cParameter;
diff --git a/tests/fuzz/zstd_helpers.c b/tests/fuzz/zstd_helpers.c
index bf5eccff83c7025164bdf915765247ef0c4d6ec8..75d0359a65df545828d3c526842749802a887dcb 100644
--- a/tests/fuzz/zstd_helpers.c
+++ b/tests/fuzz/zstd_helpers.c
@@ -71,7 +71,7 @@ void FUZZ_setRandomParameters(ZSTD_CCtx *cctx, size_t srcSize, uint32_t *state)
setRand(cctx, ZSTD_p_contentSizeFlag, 0, 1, state);
setRand(cctx, ZSTD_p_checksumFlag, 0, 1, state);
setRand(cctx, ZSTD_p_dictIDFlag, 0, 1, state);
- setRand(cctx, ZSTD_p_forceAttachDict, -2, 2, state);
+ setRand(cctx, ZSTD_p_forceAttachDict, 0, 2, state);
/* Select long distance matchig parameters */
setRand(cctx, ZSTD_p_enableLongDistanceMatching, 0, 1, state);
setRand(cctx, ZSTD_p_ldmHashLog, ZSTD_HASHLOG_MIN, 16, state);