feat(compress): move reduce-index policy into Rust

Move match-table selection and btlazy2 marker policy out of the C overflow
correction wrapper. Rust now decides whether chain and hash3 tables
participate using the strategy, row-matchfinder, dedicated-dictionary, and
log inputs, then delegates cell updates to the existing Rust reducer. C
retains only the private table pointers and scalar projection.

Test Plan:
- cargo fmt --manifest-path rust/Cargo.toml -- --check
- ulimit -v 41943040 && CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
- ulimit -v 41943040 && CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml zstd_compress::tests::reduce_index_for_match_state -- --nocapture
- ulimit -v 41943040 && make -j1
- ulimit -v 41943040 && make -j1 -C tests test-fuzzer FUZZERTEST=-T3s FUZZER_FLAGS=--no-big-tests

lib/compress/zstd_compress.c

@@ -656,6 +656,12 @@ void ZSTD_rust_reduceIndex(U32* hashTable, U32 hashSize,
                            U32* chainTable, U32 chainSize,
                            U32* hashTable3, U32 hashSize3,
                            U32 reducerValue, int preserveChainMark);
+void ZSTD_rust_reduceIndexForMatchState(
+        U32* hashTable, U32 hashLog,
+        U32* chainTable, U32 chainLog,
+        U32* hashTable3, U32 hashLog3,
+        int strategy, int useRowMatchFinder, int dedicatedDictSearch,
+        U32 reducerValue);
 typedef int (*ZSTD_rust_overflowNeedCorrection_f)(
         void* context, const void* src, const void* srcEnd);
 typedef U32 (*ZSTD_rust_overflowCorrect_f)(
@@ -4803,27 +4809,12 @@ size_t ZSTD_copyCCtx(ZSTD_CCtx* dstCCtx, const ZSTD_CCtx* srcCCtx, unsigned long
 *   rescale all indexes to avoid future overflow (indexes are U32) */
 static void ZSTD_reduceIndex (ZSTD_MatchState_t* ms, ZSTD_CCtx_params const* params, const U32 reducerValue)
 {
-    U32 const hSize = (U32)1 << params->cParams.hashLog;
-    U32* chainTable = NULL;
-    U32 chainSize = 0;
-    U32* hashTable3 = NULL;
-    U32 hashSize3 = 0;
-
-    if (ZSTD_allocateChainTable(params->cParams.strategy, params->useRowMatchFinder, (U32)ms->dedicatedDictSearch)) {
-        chainTable = ms->chainTable;
-        chainSize = (U32)1 << params->cParams.chainLog;
-    }
-
-    if (ms->hashLog3) {
-        hashTable3 = ms->hashTable3;
-        hashSize3 = (U32)1 << ms->hashLog3;
-    }
-
-    ZSTD_rust_reduceIndex(ms->hashTable, hSize,
-                          chainTable, chainSize,
-                          hashTable3, hashSize3,
-                          reducerValue,
-                          params->cParams.strategy == ZSTD_btlazy2);
+    ZSTD_rust_reduceIndexForMatchState(
+            ms->hashTable, params->cParams.hashLog,
+            ms->chainTable, params->cParams.chainLog,
+            ms->hashTable3, ms->hashLog3,
+            params->cParams.strategy, params->useRowMatchFinder,
+            ms->dedicatedDictSearch, reducerValue);
 }
 
 

rust/src/zstd_compress.rs

@@ -95,6 +95,7 @@ unsafe extern "C" {
 
 const ZSTD_FAST: c_int = 1;
 const ZSTD_DFAST: c_int = 2;
+const ZSTD_BTLAZY2: c_int = 6;
 const ZSTD_REP_NUM: usize = 3;
 const ZSTD_BM_BUFFERED: c_int = 0;
 const ZSTD_BM_STABLE: c_int = 1;
@@ -6871,6 +6872,53 @@ pub unsafe extern "C" fn ZSTD_rust_reduceIndex(
     }
 }
 
+/// Select the match tables participating in overflow correction and delegate
+/// their cell updates to the Rust reducer.  C supplies private table pointers;
+/// Rust owns the strategy, row-matchfinder, dedicated-dictionary, and marker
+/// policy that determines the participating table set.
+#[no_mangle]
+pub unsafe extern "C" fn ZSTD_rust_reduceIndexForMatchState(
+    hash_table: *mut u32,
+    hash_log: c_uint,
+    chain_table: *mut u32,
+    chain_log: c_uint,
+    hash_table3: *mut u32,
+    hash_log3: c_uint,
+    strategy: c_int,
+    use_row_match_finder: c_int,
+    dedicated_dict_search: c_int,
+    reducer_value: c_uint,
+) {
+    let hash_size = 1u32.wrapping_shl(hash_log);
+    let (chain_table, chain_size) = if ZSTD_rust_params_allocateChainTable(
+        strategy,
+        use_row_match_finder,
+        dedicated_dict_search,
+    ) != 0
+    {
+        (chain_table, 1u32.wrapping_shl(chain_log))
+    } else {
+        (ptr::null_mut(), 0)
+    };
+    let (hash_table3, hash_size3) = if hash_log3 != 0 {
+        (hash_table3, 1u32.wrapping_shl(hash_log3))
+    } else {
+        (ptr::null_mut(), 0)
+    };
+    unsafe {
+        ZSTD_rust_reduceIndex(
+            hash_table,
+            hash_size,
+            chain_table,
+            chain_size,
+            hash_table3,
+            hash_size3,
+            reducer_value,
+            c_int::from(strategy == ZSTD_BTLAZY2),
+        );
+    }
+}
+
 /// Copies a CDict match table into a CCtx, removing short-cache tags when the
 /// C-owned compression parameters say the source table is tagged.
 #[no_mangle]
@@ -11121,6 +11169,35 @@ mod tests {
         assert_eq!(hash_table3[0], 0);
     }
 
+    #[test]
+    fn reduce_index_for_match_state_selects_chain_and_hash3_tables() {
+        let mut hash_table = [0u32; ZSTD_ROWSIZE];
+        let mut chain_table = [0u32; ZSTD_ROWSIZE];
+        let mut hash_table3 = [0u32; ZSTD_ROWSIZE];
+        hash_table[0] = ZSTD_DUBT_UNSORTED_MARK;
+        chain_table[0] = ZSTD_DUBT_UNSORTED_MARK;
+        hash_table3[0] = ZSTD_DUBT_UNSORTED_MARK;
+
+        unsafe {
+            ZSTD_rust_reduceIndexForMatchState(
+                hash_table.as_mut_ptr(),
+                4,
+                chain_table.as_mut_ptr(),
+                4,
+                hash_table3.as_mut_ptr(),
+                4,
+                ZSTD_BTLAZY2,
+                ZSTD_RUST_PS_DISABLE,
+                0,
+                3,
+            );
+        }
+
+        assert_eq!(hash_table[0], 0);
+        assert_eq!(chain_table[0], ZSTD_DUBT_UNSORTED_MARK);
+        assert_eq!(hash_table3[0], 0);
+    }
+
     #[test]
     fn copy_cdict_table_removes_short_cache_tags() {
         let source = [0x1234_56ff, 0xdead_beef, 0x0000_0100, u32::MAX];