feat(compress): move advanced begin policy into Rust

Move public ZSTD_compressBegin_advanced parameter validation and init-then-begin
sequencing behind a Rust-owned boundary. Keep C responsible for private
ZSTD_CCtx_params initialization and ZSTD_compressBegin_internal through
explicit callbacks.

Test Plan:
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml --release compress_begin_advanced -- --nocapture
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml --release
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --release --all-targets -- -D warnings
- ulimit -v 41943040; make -B -C programs -j1 zstd
- ulimit -v 41943040; make -C tests -j1 test-zstream ZSTREAM_TESTTIME=-T1s

lib/compress/zstd_compress.c

@@ -422,6 +422,31 @@ typedef char ZSTD_rust_compress_using_dict_state_layout[
                 == 3 * sizeof(void*)
          && sizeof(ZSTD_rust_compressUsingDictState) == 4 * sizeof(void*))
         ? 1 : -1];
+typedef void (*ZSTD_rust_compressBeginAdvancedInitParams_f)(
+        void* cctxParams, const ZSTD_parameters* params);
+typedef size_t (*ZSTD_rust_compressBeginAdvancedBegin_f)(
+        void* context, const void* dict, size_t dictSize,
+        const void* cctxParams, U64 pledgedSrcSize);
+typedef struct {
+    void* cctx;
+    void* cctxParams;
+    ZSTD_rust_compressBeginAdvancedInitParams_f initParams;
+    ZSTD_rust_compressBeginAdvancedBegin_f begin;
+} ZSTD_rust_compressBeginAdvancedState;
+size_t ZSTD_rust_compressBeginAdvanced(
+        const ZSTD_rust_compressBeginAdvancedState* state,
+        const void* dict, size_t dictSize,
+        const ZSTD_parameters* params, U64 pledgedSrcSize);
+typedef char ZSTD_rust_compress_begin_advanced_state_layout[
+        (offsetof(ZSTD_rust_compressBeginAdvancedState, cctx) == 0
+         && offsetof(ZSTD_rust_compressBeginAdvancedState, cctxParams)
+                == sizeof(void*)
+         && offsetof(ZSTD_rust_compressBeginAdvancedState, initParams)
+                == 2 * sizeof(void*)
+         && offsetof(ZSTD_rust_compressBeginAdvancedState, begin)
+                == 3 * sizeof(void*)
+         && sizeof(ZSTD_rust_compressBeginAdvancedState) == 4 * sizeof(void*))
+        ? 1 : -1];
 ZSTD_frameProgression ZSTD_rust_frameProgression(U64 consumedSrcSize,
                                                   size_t buffered,
                                                   U64 producedCSize);
@@ -4841,6 +4866,24 @@ static size_t ZSTD_rust_compressBeginUsingDict_begin(
             pledgedSrcSize, ZSTDb_not_buffered);
 }
 
+static void ZSTD_rust_compressBeginAdvanced_initParams(
+        void* cctxParams, const ZSTD_parameters* params)
+{
+    ZSTD_CCtxParams_init_internal(
+            (ZSTD_CCtx_params*)cctxParams, params, ZSTD_NO_CLEVEL);
+}
+
+static size_t ZSTD_rust_compressBeginAdvanced_begin(
+        void* context, const void* dict, size_t dictSize,
+        const void* cctxParams, U64 pledgedSrcSize)
+{
+    return ZSTD_compressBegin_internal(
+            (ZSTD_CCtx*)context, dict, dictSize,
+            ZSTD_dct_auto, ZSTD_dtlm_fast, NULL,
+            (const ZSTD_CCtx_params*)cctxParams,
+            pledgedSrcSize, ZSTDb_not_buffered);
+}
+
 size_t ZSTD_compressBegin_advanced_internal(ZSTD_CCtx* cctx,
                                     const void* dict, size_t dictSize,
                                     ZSTD_dictContentType_e dictContentType,
@@ -4865,12 +4908,14 @@ size_t ZSTD_compressBegin_advanced(ZSTD_CCtx* cctx,
                              const void* dict, size_t dictSize,
                                    ZSTD_parameters params, unsigned long long pledgedSrcSize)
 {
+    ZSTD_rust_compressBeginAdvancedState state;
     ZSTD_CCtx_params cctxParams;
-    ZSTD_CCtxParams_init_internal(&cctxParams, &params, ZSTD_NO_CLEVEL);
-    return ZSTD_compressBegin_advanced_internal(cctx,
-                                            dict, dictSize, ZSTD_dct_auto, ZSTD_dtlm_fast,
-                                            NULL /*cdict*/,
-                                            &cctxParams, pledgedSrcSize);
+    state.cctx = cctx;
+    state.cctxParams = &cctxParams;
+    state.initParams = ZSTD_rust_compressBeginAdvanced_initParams;
+    state.begin = ZSTD_rust_compressBeginAdvanced_begin;
+    return ZSTD_rust_compressBeginAdvanced(
+            &state, dict, dictSize, &params, pledgedSrcSize);
 }
 
 static size_t

rust/src/zstd_compress.rs

@@ -1547,6 +1547,66 @@ pub unsafe extern "C" fn ZSTD_rust_compressAdvanced(
     }
 }
 
+type CompressBeginAdvancedInitParamsFn = unsafe extern "C" fn(*mut c_void, *const ZSTD_parameters);
+type CompressBeginAdvancedBeginFn =
+    unsafe extern "C" fn(*mut c_void, *const c_void, usize, *const c_void, u64) -> usize;
+
+/// Explicit projection for the public `ZSTD_compressBegin_advanced` wrapper.
+///
+/// Rust owns public parameter validation and init-then-begin ordering. C
+/// retains private `ZSTD_CCtx_params` initialization and the final begin
+/// operation behind callbacks.
+#[repr(C)]
+pub struct ZSTD_rust_compressBeginAdvancedState {
+    cctx: *mut c_void,
+    cctx_params: *mut c_void,
+    init_params: CompressBeginAdvancedInitParamsFn,
+    begin: CompressBeginAdvancedBeginFn,
+}
+
+const _: () = {
+    assert!(offset_of!(ZSTD_rust_compressBeginAdvancedState, cctx) == 0);
+    assert!(offset_of!(ZSTD_rust_compressBeginAdvancedState, cctx_params) == size_of::<usize>());
+    assert!(
+        offset_of!(ZSTD_rust_compressBeginAdvancedState, init_params) == 2 * size_of::<usize>()
+    );
+    assert!(offset_of!(ZSTD_rust_compressBeginAdvancedState, begin) == 3 * size_of::<usize>());
+    assert!(size_of::<ZSTD_rust_compressBeginAdvancedState>() == size_of::<[usize; 4]>());
+};
+
+/// Validate parameters and begin a frame through the C-owned operation.
+#[no_mangle]
+pub unsafe extern "C" fn ZSTD_rust_compressBeginAdvanced(
+    state: *const ZSTD_rust_compressBeginAdvancedState,
+    dict: *const c_void,
+    dict_size: usize,
+    params: *const ZSTD_parameters,
+    pledged_src_size: u64,
+) -> usize {
+    if state.is_null() || params.is_null() {
+        return ERROR(ZstdErrorCode::Generic);
+    }
+    let state = unsafe { &*state };
+    if state.cctx.is_null() || state.cctx_params.is_null() {
+        return ERROR(ZstdErrorCode::Generic);
+    }
+    let params = unsafe { &*params };
+    let check_result = ZSTD_rust_params_checkCParams(params.cParams);
+    if ERR_isError(check_result) {
+        return check_result;
+    }
+    unsafe {
+        (state.init_params)(state.cctx_params, params);
+        (state.begin)(
+            state.cctx,
+            dict,
+            dict_size,
+            state.cctx_params,
+            pledged_src_size,
+        )
+    }
+}
+
 type CompressUsingDictInitParamsFn =
     unsafe extern "C" fn(*mut c_void, *const ZSTD_parameters, c_int);
 
@@ -10657,6 +10717,92 @@ mod tests {
         assert!(context.events.is_empty());
     }
 
+    #[derive(Default)]
+    struct CompressBeginAdvancedTestContext {
+        events: Vec<&'static str>,
+        params: ZSTD_parameters,
+        dict: *const c_void,
+        dict_size: usize,
+        cctx_params: *const c_void,
+        pledged_src_size: u64,
+        result: usize,
+    }
+
+    unsafe extern "C" fn compress_begin_advanced_test_init(
+        context: *mut c_void,
+        params: *const ZSTD_parameters,
+    ) {
+        let context = unsafe { &mut *context.cast::<CompressBeginAdvancedTestContext>() };
+        context.events.push("init");
+        context.params = unsafe { *params };
+    }
+
+    unsafe extern "C" fn compress_begin_advanced_test_begin(
+        context: *mut c_void,
+        dict: *const c_void,
+        dict_size: usize,
+        cctx_params: *const c_void,
+        pledged_src_size: u64,
+    ) -> usize {
+        let context = unsafe { &mut *context.cast::<CompressBeginAdvancedTestContext>() };
+        context.events.push("begin");
+        context.dict = dict;
+        context.dict_size = dict_size;
+        context.cctx_params = cctx_params;
+        context.pledged_src_size = pledged_src_size;
+        context.result
+    }
+
+    fn compress_begin_advanced_test_state(
+        context: &mut CompressBeginAdvancedTestContext,
+    ) -> ZSTD_rust_compressBeginAdvancedState {
+        ZSTD_rust_compressBeginAdvancedState {
+            cctx: (context as *mut CompressBeginAdvancedTestContext).cast(),
+            cctx_params: (context as *mut CompressBeginAdvancedTestContext).cast(),
+            init_params: compress_begin_advanced_test_init,
+            begin: compress_begin_advanced_test_begin,
+        }
+    }
+
+    #[test]
+    fn compress_begin_advanced_validates_and_preserves_init_begin_order() {
+        let mut context = CompressBeginAdvancedTestContext {
+            result: 29,
+            ..CompressBeginAdvancedTestContext::default()
+        };
+        let state = compress_begin_advanced_test_state(&mut context);
+        let params = compress_advanced_test_params();
+        let dict = [1u8, 2, 3];
+
+        let result = unsafe {
+            ZSTD_rust_compressBeginAdvanced(&state, dict.as_ptr().cast(), dict.len(), &params, 123)
+        };
+
+        assert_eq!(result, context.result);
+        assert_eq!(context.events, ["init", "begin"]);
+        assert_eq!(context.params, params);
+        assert_eq!(context.dict, dict.as_ptr().cast());
+        assert_eq!(context.dict_size, dict.len());
+        assert_eq!(
+            context.cctx_params,
+            (&context as *const CompressBeginAdvancedTestContext).cast()
+        );
+        assert_eq!(context.pledged_src_size, 123);
+    }
+
+    #[test]
+    fn compress_begin_advanced_stops_before_callbacks_on_invalid_parameters() {
+        let mut context = CompressBeginAdvancedTestContext::default();
+        let state = compress_begin_advanced_test_state(&mut context);
+        let mut params = compress_advanced_test_params();
+        params.cParams.windowLog = 0;
+
+        let result = unsafe { ZSTD_rust_compressBeginAdvanced(&state, ptr::null(), 0, &params, 0) };
+
+        assert_eq!(result, ERROR(ZstdErrorCode::ParameterOutOfBound));
+        assert!(context.events.is_empty());
+    }
+
     #[derive(Default)]
     struct CompressUsingDictTestContext {
         events: Vec<&'static str>,