From c86d18be81fd37e7bf3c7e22133d4d85d4254f01 Mon Sep 17 00:00:00 2001
From: ddidderr <ddidderr@paul.network>
Date: Sat, 18 Jul 2026 09:07:42 +0200
Subject: [PATCH] feat(compress): move dedicated dict support predicate to Rust

Dedicated dictionary search support is a pure compression-parameter policy
check. Move its strategy and hash/chain-log comparisons into the Rust
parameter module behind a by-value ZSTD_compressionParameters ABI, while
leaving the existing C fallback path in createCDict_advanced2 unchanged.

Test Plan:
- `cargo test --manifest-path rust/Cargo.toml --no-default-features --features compression dedicated_dict_search_support_matches_strategy_and_log_boundaries` -- passed
- `make -B -C lib -j2 lib` -- passed
- `make -C tests -j2 fuzzer` -- passed
- `./tests/fuzzer -s4560 -t47 -i48 -v` -- passed
- `cargo clippy --manifest-path rust/Cargo.toml --no-default-features --features compression` -- passed
- The same clippy command with `--benches` and `--tests` -- passed
- `cargo +nightly fmt --manifest-path rust/Cargo.toml` -- passed
- `git diff --cached --check` -- passed
---
 lib/compress/zstd_compress.c     | 14 ++-----------
 rust/src/zstd_compress_params.rs | 34 ++++++++++++++++++++++++++++++++
 2 files changed, 36 insertions(+), 12 deletions(-)

diff --git a/lib/compress/zstd_compress.c b/lib/compress/zstd_compress.c
index 198078c00..ce45c5de4 100644
--- a/lib/compress/zstd_compress.c
+++ b/lib/compress/zstd_compress.c
@@ -121,6 +121,7 @@ int ZSTD_rust_params_resolveEnableLdm(
         int mode, ZSTD_compressionParameters cParams);
 int ZSTD_rust_params_resolveExternalRepcodeSearch(int mode, int cLevel);
 int ZSTD_rust_params_cdictIndicesAreTagged(ZSTD_compressionParameters cParams);
+int ZSTD_rust_params_dedicatedDictSearchIsSupported(ZSTD_compressionParameters cParams);
 
 /* CCtx parameter state is mirrored by rust/src/zstd_compress_params_api.rs.
  * Rust owns parameter bounds and clamping; C exposes only the
@@ -912,8 +913,6 @@ size_t ZSTD_CCtx_setPledgedSrcSize(ZSTD_CCtx* cctx, unsigned long long pledgedSr
 static ZSTD_compressionParameters ZSTD_dedicatedDictSearch_getCParams(
         int const compressionLevel,
         size_t const dictSize);
-static int ZSTD_dedicatedDictSearch_isSupported(
-        const ZSTD_compressionParameters* cParams);
 static void ZSTD_dedicatedDictSearch_revertCParams(
         ZSTD_compressionParameters* cParams);
 
@@ -4131,7 +4130,7 @@ ZSTD_CDict* ZSTD_createCDict_advanced2(
             &cctxParams, ZSTD_CONTENTSIZE_UNKNOWN, dictSize, ZSTD_cpm_createCDict);
     }
 
-    if (!ZSTD_dedicatedDictSearch_isSupported(&cParams)) {
+    if (!ZSTD_rust_params_dedicatedDictSearchIsSupported(cParams)) {
         /* Fall back to non-DDSS params */
         cctxParams.enableDedicatedDictSearch = 0;
         cParams = ZSTD_getCParamsFromCCtxParams(
@@ -5675,15 +5674,6 @@ static ZSTD_compressionParameters ZSTD_dedicatedDictSearch_getCParams(int const
     return cParams;
 }
 
-static int ZSTD_dedicatedDictSearch_isSupported(
-        ZSTD_compressionParameters const* cParams)
-{
-    return (cParams->strategy >= ZSTD_greedy)
-        && (cParams->strategy <= ZSTD_lazy2)
-        && (cParams->hashLog > cParams->chainLog)
-        && (cParams->chainLog <= 24);
-}
-
 /**
  * Reverses the adjustment applied to cparams when enabling dedicated dict
  * search. This is used to recover the params set to be used in the working
diff --git a/rust/src/zstd_compress_params.rs b/rust/src/zstd_compress_params.rs
index 422c467cc..7daf98d0a 100644
--- a/rust/src/zstd_compress_params.rs
+++ b/rust/src/zstd_compress_params.rs
@@ -431,6 +431,14 @@ fn cdict_indices_are_tagged(cparams: ZSTD_compressionParameters) -> bool {
     cparams.strategy == ZSTD_FAST || cparams.strategy == ZSTD_DFAST
 }
 
+#[inline]
+fn dedicated_dict_search_is_supported(cparams: ZSTD_compressionParameters) -> bool {
+    cparams.strategy >= ZSTD_GREEDY
+        && cparams.strategy <= ZSTD_LAZY2
+        && cparams.hashLog > cparams.chainLog
+        && cparams.chainLog <= 24
+}
+
 /// C ABI for `ZSTD_CDictIndicesAreTagged()`.
 #[no_mangle]
 pub extern "C" fn ZSTD_rust_params_cdictIndicesAreTagged(
@@ -439,6 +447,14 @@ pub extern "C" fn ZSTD_rust_params_cdictIndicesAreTagged(
     c_int::from(cdict_indices_are_tagged(cparams))
 }
 
+/// C ABI for `ZSTD_dedicatedDictSearch_isSupported()`.
+#[no_mangle]
+pub extern "C" fn ZSTD_rust_params_dedicatedDictSearchIsSupported(
+    cparams: ZSTD_compressionParameters,
+) -> c_int {
+    c_int::from(dedicated_dict_search_is_supported(cparams))
+}
+
 #[inline]
 fn dict_and_window_log(window_log: u32, src_size: u64, dict_size: u64) -> u32 {
     /* 1ULL << ZSTD_WINDOWLOG_MAX, which is smaller for 32-bit builds. */
@@ -1224,6 +1240,24 @@ mod tests {
         );
     }
 
+    #[test]
+    fn dedicated_dict_search_support_matches_strategy_and_log_boundaries() {
+        let supported = |strategy, chain_log, hash_log| {
+            ZSTD_rust_params_dedicatedDictSearchIsSupported(ZSTD_compressionParameters {
+                strategy,
+                chainLog: chain_log,
+                hashLog: hash_log,
+                ..ZSTD_compressionParameters::default()
+            })
+        };
+
+        assert_eq!(supported(ZSTD_GREEDY, 24, 25), 1);
+        assert_eq!(supported(ZSTD_LAZY2, 24, 25), 1);
+        assert_eq!(supported(ZSTD_BTLAZY2, 24, 25), 0);
+        assert_eq!(supported(ZSTD_LAZY2, 24, 24), 0);
+        assert_eq!(supported(ZSTD_LAZY2, 25, 26), 0);
+    }
+
     #[test]
     fn external_sequence_validation_preserves_its_int_mode() {
         for mode in [c_int::MIN, -1, 0, 1, c_int::MAX] {