feat(compress): move C parameter overrides to Rust

The compression-parameter override helper previously lived in C and selectively
copied each nonzero field from a caller-provided override structure. Move that
pure field-wise operation into the Rust compression-parameter module, while
keeping the existing repr(C) structure and pointer-based ABI at both C call
sites. Zero-valued overrides continue to leave the current parameter untouched.

Test Plan:
- `cargo test --manifest-path rust/Cargo.toml --no-default-features --features compression` -- passed (252 tests)
- `make -B -C lib -j2 lib` -- passed
- `make -C tests -j2 fuzzer` and `tests/fuzzer -s4560 -t47 -i48 -v` -- passed
- required clippy passes, nightly fmt, and `git diff --check` -- passed

lib/compress/zstd_compress.c

@@ -104,6 +104,8 @@ size_t ZSTD_rust_params_maxNbSeq(size_t blockSize, U32 minMatch,
                                  int useSequenceProducer);
 size_t ZSTD_rust_params_resolveMaxBlockSize(size_t maxBlockSize);
 size_t ZSTD_rust_params_getBlockSize(size_t maxBlockSize, U32 windowLog);
+void ZSTD_rust_params_overrideCParams(ZSTD_compressionParameters* cParams,
+                                      const ZSTD_compressionParameters* overrides);
 int ZSTD_rust_params_rowMatchFinderSupported(int strategy);
 int ZSTD_rust_params_rowMatchFinderUsed(int strategy, int mode);
 int ZSTD_rust_params_resolveRowMatchFinderMode(
@@ -1158,19 +1160,6 @@ ZSTD_adjustCParams(ZSTD_compressionParameters cPar,
 static ZSTD_compressionParameters ZSTD_getCParams_internal(int compressionLevel, unsigned long long srcSizeHint, size_t dictSize, ZSTD_CParamMode_e mode);
 static ZSTD_parameters ZSTD_getParams_internal(int compressionLevel, unsigned long long srcSizeHint, size_t dictSize, ZSTD_CParamMode_e mode);
 
-static void ZSTD_overrideCParams(
-              ZSTD_compressionParameters* cParams,
-        const ZSTD_compressionParameters* overrides)
-{
-    if (overrides->windowLog)    cParams->windowLog    = overrides->windowLog;
-    if (overrides->hashLog)      cParams->hashLog      = overrides->hashLog;
-    if (overrides->chainLog)     cParams->chainLog     = overrides->chainLog;
-    if (overrides->searchLog)    cParams->searchLog    = overrides->searchLog;
-    if (overrides->minMatch)     cParams->minMatch     = overrides->minMatch;
-    if (overrides->targetLength) cParams->targetLength = overrides->targetLength;
-    if (overrides->strategy)     cParams->strategy     = overrides->strategy;
-}
-
 ZSTD_compressionParameters ZSTD_getCParamsFromCCtxParams(
         const ZSTD_CCtx_params* CCtxParams, U64 srcSizeHint, size_t dictSize, ZSTD_CParamMode_e mode)
 {
@@ -1181,7 +1170,7 @@ ZSTD_compressionParameters ZSTD_getCParamsFromCCtxParams(
     }
     cParams = ZSTD_getCParams_internal(CCtxParams->compressionLevel, srcSizeHint, dictSize, mode);
     if (CCtxParams->ldmParams.enableLdm == ZSTD_ps_enable) cParams.windowLog = ZSTD_LDM_DEFAULT_WINDOW_LOG;
-    ZSTD_overrideCParams(&cParams, &CCtxParams->cParams);
+    ZSTD_rust_params_overrideCParams(&cParams, &CCtxParams->cParams);
     assert(!ZSTD_checkCParams(cParams));
     /* srcSizeHint == 0 means 0 */
     return ZSTD_adjustCParams_internal(cParams, srcSizeHint, dictSize, mode, CCtxParams->useRowMatchFinder);
@@ -4131,7 +4120,7 @@ ZSTD_CDict* ZSTD_createCDict_advanced2(
     if (cctxParams.enableDedicatedDictSearch) {
         cParams = ZSTD_dedicatedDictSearch_getCParams(
             cctxParams.compressionLevel, dictSize);
-        ZSTD_overrideCParams(&cParams, &cctxParams.cParams);
+        ZSTD_rust_params_overrideCParams(&cParams, &cctxParams.cParams);
     } else {
         cParams = ZSTD_getCParamsFromCCtxParams(
             &cctxParams, ZSTD_CONTENTSIZE_UNKNOWN, dictSize, ZSTD_cpm_createCDict);

rust/src/zstd_compress_params.rs

@@ -905,6 +905,44 @@ fn get_block_size(max_block_size: usize, window_log: u32) -> usize {
     max_block_size.min(1usize << window_log)
 }
 
+#[inline]
+fn override_cparams(
+    cparams: &mut ZSTD_compressionParameters,
+    overrides: &ZSTD_compressionParameters,
+) {
+    if overrides.windowLog != 0 {
+        cparams.windowLog = overrides.windowLog;
+    }
+    if overrides.hashLog != 0 {
+        cparams.hashLog = overrides.hashLog;
+    }
+    if overrides.chainLog != 0 {
+        cparams.chainLog = overrides.chainLog;
+    }
+    if overrides.searchLog != 0 {
+        cparams.searchLog = overrides.searchLog;
+    }
+    if overrides.minMatch != 0 {
+        cparams.minMatch = overrides.minMatch;
+    }
+    if overrides.targetLength != 0 {
+        cparams.targetLength = overrides.targetLength;
+    }
+    if overrides.strategy != 0 {
+        cparams.strategy = overrides.strategy;
+    }
+}
+
+/// Pure leaf for the private `ZSTD_overrideCParams()` helper.
+#[no_mangle]
+pub unsafe extern "C" fn ZSTD_rust_params_overrideCParams(
+    cparams: *mut ZSTD_compressionParameters,
+    overrides: *const ZSTD_compressionParameters,
+) {
+    let (cparams, overrides) = unsafe { (&mut *cparams, &*overrides) };
+    override_cparams(cparams, overrides);
+}
+
 /// Pure leaf for the deprecated `ZSTD_getBlockSize()` path.
 #[no_mangle]
 pub extern "C" fn ZSTD_rust_params_getBlockSize(maxBlockSize: usize, windowLog: u32) -> usize {
@@ -1337,4 +1375,41 @@ mod tests {
         assert_eq!(ZSTD_rust_params_getBlockSize(128 * 1024, 16), 64 * 1024);
         assert_eq!(ZSTD_rust_params_getBlockSize(128 * 1024, 17), 128 * 1024);
     }
+
+    #[test]
+    fn cparam_overrides_replace_only_nonzero_fields() {
+        let mut cparams = ZSTD_compressionParameters {
+            windowLog: 10,
+            chainLog: 11,
+            hashLog: 12,
+            searchLog: 13,
+            minMatch: 14,
+            targetLength: 15,
+            strategy: 16,
+        };
+        let overrides = ZSTD_compressionParameters {
+            windowLog: 20,
+            chainLog: 0,
+            hashLog: 22,
+            searchLog: 0,
+            minMatch: 24,
+            targetLength: 0,
+            strategy: 26,
+        };
+
+        unsafe { ZSTD_rust_params_overrideCParams(&mut cparams, &overrides) };
+
+        assert_eq!(
+            cparams,
+            ZSTD_compressionParameters {
+                windowLog: 20,
+                chainLog: 11,
+                hashLog: 22,
+                searchLog: 13,
+                minMatch: 24,
+                targetLength: 15,
+                strategy: 26,
+            }
+        );
+    }
 }