feat(compress): move overflow policy arithmetic to Rust

The window overflow gate mixed two scalar decisions with C-owned pointer and
window state. The early correction test uses the block-start index and must
wait for dictionary invalidation, while the normal fallback uses the block-end
index and the platform-selected ZSTD_CURRENT_MAX threshold. Keeping those
subtractions and the ZSTD_window_t wrapper in C preserves the existing call
contract without making Rust depend on pointer width or private state.

Move the U32 cycle/MAX arithmetic, correction-count scaling, dictionary gate,
and frequent-policy decision into Rust. C passes the two pointer-derived
indices only through scalar results, and supplies the current-max threshold
and active build policy explicitly so 32/64-bit and fuzzing configurations
remain authoritative.

Test Plan:
- `cargo test --manifest-path rust/Cargo.toml --no-default-features
  --features compression` -- 348 passed.
- Compression clippy for the library, benches, and tests, followed by
  nightly fmt and the repeated three clippy checks -- passed.
- `make -C lib -j2 lib-mt` and `make -C lib -j2 lib-nomt` -- passed.
- `make -C tests -j2 fuzzer` and `./tests/fuzzer -s4142 -t63 -i64 -v` --
  passed all 64 cases.
- `make -C tests -j2 test-zstream` -- passed 84 deterministic, 5,477 first
  randomized, and 6,902 new-API randomized cases; the existing unterminated
  string initializer warning remains.

lib/compress/zstd_compress_internal.h

@@ -655,6 +655,13 @@ size_t ZSTD_rust_noCompressBlock(void* dst, size_t dstCapacity,
 size_t ZSTD_rust_rleCompressBlock(void* dst, size_t dstCapacity, BYTE src,
                                   size_t srcSize, U32 lastBlock);
 U32 ZSTD_rust_windowCorrectOverflow(U32 curr, U32 cycleLog, U32 maxDist);
+U32 ZSTD_rust_windowCanOverflowCorrect(U32 curr, U32 cycleLog, U32 maxDist,
+                                       U32 loadedDictEnd,
+                                       U32 nbOverflowCorrections);
+U32 ZSTD_rust_windowNeedOverflowCorrection(U32 curr,
+                                           U32 canOverflowCorrect,
+                                           U32 currentMax,
+                                           int overflowCorrectFrequently);
 
 
 /* ZSTD_minGain() :
@@ -1084,28 +1091,10 @@ MEM_STATIC U32 ZSTD_window_canOverflowCorrect(ZSTD_window_t const window,
                                               U32 loadedDictEnd,
                                               void const* src)
 {
-    U32 const cycleSize = 1u << cycleLog;
     U32 const curr = (U32)((BYTE const*)src - window.base);
-    U32 const minIndexToOverflowCorrect = cycleSize
-                                        + MAX(maxDist, cycleSize)
-                                        + ZSTD_WINDOW_START_INDEX;
-
-    /* Adjust the min index to backoff the overflow correction frequency,
-     * so we don't waste too much CPU in overflow correction. If this
-     * computation overflows we don't really care, we just need to make
-     * sure it is at least minIndexToOverflowCorrect.
-     */
-    U32 const adjustment = window.nbOverflowCorrections + 1;
-    U32 const adjustedIndex = MAX(minIndexToOverflowCorrect * adjustment,
-                                  minIndexToOverflowCorrect);
-    U32 const indexLargeEnough = curr > adjustedIndex;
-
-    /* Only overflow correct early if the dictionary is invalidated already,
-     * so we don't hurt compression ratio.
-     */
-    U32 const dictionaryInvalidated = curr > maxDist + loadedDictEnd;
-
-    return indexLargeEnough && dictionaryInvalidated;
+    return ZSTD_rust_windowCanOverflowCorrect(
+            curr, cycleLog, maxDist, loadedDictEnd,
+            window.nbOverflowCorrections);
 }
 
 /**
@@ -1121,12 +1110,12 @@ MEM_STATIC U32 ZSTD_window_needOverflowCorrection(ZSTD_window_t const window,
                                                   void const* srcEnd)
 {
     U32 const curr = (U32)((BYTE const*)srcEnd - window.base);
-    if (ZSTD_WINDOW_OVERFLOW_CORRECT_FREQUENTLY) {
-        if (ZSTD_window_canOverflowCorrect(window, cycleLog, maxDist, loadedDictEnd, src)) {
-            return 1;
-        }
-    }
-    return curr > ZSTD_CURRENT_MAX;
+    U32 const canOverflowCorrect =
+        ZSTD_window_canOverflowCorrect(window, cycleLog, maxDist,
+                                       loadedDictEnd, src);
+    return ZSTD_rust_windowNeedOverflowCorrection(
+            curr, canOverflowCorrect, ZSTD_CURRENT_MAX,
+            ZSTD_WINDOW_OVERFLOW_CORRECT_FREQUENTLY);
 }
 
 /**

rust/src/zstd_compress.rs

@@ -410,6 +410,76 @@ pub extern "C" fn ZSTD_rust_windowCorrectOverflow(curr: u32, cycle_log: u32, max
     window_correct_overflow(curr, cycle_log, max_dist)
 }
 
+#[inline]
+fn window_can_overflow_correct(
+    curr: u32,
+    cycle_log: u32,
+    max_dist: u32,
+    loaded_dict_end: u32,
+    nb_overflow_corrections: u32,
+) -> bool {
+    let cycle_size = 1u32.wrapping_shl(cycle_log);
+    let min_index_to_overflow_correct = cycle_size
+        .wrapping_add(max_dist.max(cycle_size))
+        .wrapping_add(ZSTD_WINDOW_START_INDEX);
+    let adjustment = nb_overflow_corrections.wrapping_add(1);
+    let adjusted_index = min_index_to_overflow_correct
+        .wrapping_mul(adjustment)
+        .max(min_index_to_overflow_correct);
+    let index_large_enough = curr > adjusted_index;
+    let dictionary_invalidated = curr > max_dist.wrapping_add(loaded_dict_end);
+    index_large_enough && dictionary_invalidated
+}
+
+/// Return whether the C-owned window state should enter overflow correction.
+///
+/// C retains both pointer subtractions and the `ZSTD_window_t` state. The
+/// early-correction result is supplied as a scalar because it is based on the
+/// block-start index, while `curr` is the block-end index used by the normal
+/// current-limit fallback. The current limit and fuzzing policy are supplied
+/// by C so their 32/64-bit and build-mode choices remain authoritative.
+#[inline]
+fn window_need_overflow_correction(
+    curr: u32,
+    can_overflow_correct: bool,
+    current_max: u32,
+    overflow_correct_frequently: bool,
+) -> bool {
+    (overflow_correct_frequently && can_overflow_correct) || curr > current_max
+}
+
+#[no_mangle]
+pub extern "C" fn ZSTD_rust_windowCanOverflowCorrect(
+    curr: u32,
+    cycle_log: u32,
+    max_dist: u32,
+    loaded_dict_end: u32,
+    nb_overflow_corrections: u32,
+) -> u32 {
+    window_can_overflow_correct(
+        curr,
+        cycle_log,
+        max_dist,
+        loaded_dict_end,
+        nb_overflow_corrections,
+    ) as u32
+}
+
+#[no_mangle]
+pub extern "C" fn ZSTD_rust_windowNeedOverflowCorrection(
+    curr: u32,
+    can_overflow_correct: u32,
+    current_max: u32,
+    overflow_correct_frequently: c_int,
+) -> u32 {
+    window_need_overflow_correction(
+        curr,
+        can_overflow_correct != 0,
+        current_max,
+        overflow_correct_frequently != 0,
+    ) as u32
+}
+
 #[inline]
 fn next_input_size_hint(
     in_buffer_mode: c_int,
@@ -1440,6 +1510,62 @@ mod tests {
         );
     }
 
+    #[test]
+    fn window_overflow_need_uses_the_explicit_current_max_boundary() {
+        assert!(!window_need_overflow_correction(1024, false, 1024, false));
+        assert!(window_need_overflow_correction(1025, false, 1024, false));
+        assert_eq!(ZSTD_rust_windowNeedOverflowCorrection(1024, 1, 1024, 0), 0);
+        assert_eq!(ZSTD_rust_windowNeedOverflowCorrection(1025, 0, 1024, 0), 1);
+    }
+
+    #[test]
+    fn window_overflow_need_honors_frequent_dictionary_policy() {
+        let can_correct = window_can_overflow_correct(27, 3, 16, 0, 0);
+        let dictionary_still_valid = window_can_overflow_correct(27, 3, 16, 20, 0);
+        assert!(can_correct);
+        assert!(!dictionary_still_valid);
+        assert!(window_need_overflow_correction(27, can_correct, 1024, true));
+        assert!(!window_need_overflow_correction(
+            27,
+            dictionary_still_valid,
+            1024,
+            true
+        ));
+        assert_eq!(
+            ZSTD_rust_windowNeedOverflowCorrection(27, can_correct as u32, 1024, 1),
+            1
+        );
+        assert_eq!(
+            ZSTD_rust_windowNeedOverflowCorrection(27, dictionary_still_valid as u32, 1024, 1,),
+            0
+        );
+        assert!(!window_need_overflow_correction(27, true, 1024, false));
+    }
+
+    #[test]
+    fn window_overflow_need_scales_the_frequency_with_correction_count() {
+        assert!(window_can_overflow_correct(27, 3, 16, 0, 0));
+        assert!(!window_can_overflow_correct(27, 3, 16, 0, 1));
+        assert!(window_can_overflow_correct(53, 3, 16, 0, 1));
+        assert_eq!(ZSTD_rust_windowCanOverflowCorrect(53, 3, 16, 0, 1), 1);
+    }
+
+    #[test]
+    fn window_overflow_need_preserves_u32_wrap_boundaries() {
+        assert!(window_can_overflow_correct(
+            3,
+            31,
+            0x8000_0000,
+            0x8000_0000,
+            0,
+        ));
+        assert!(window_can_overflow_correct(27, 3, 16, 0, u32::MAX,));
+        assert_eq!(
+            ZSTD_rust_windowCanOverflowCorrect(3, 31, 0x8000_0000, 0x8000_0000, 0),
+            1
+        );
+    }
+
     #[test]
     fn dict_too_big_uses_a_strict_chunk_size_boundary() {
         assert!(!dict_too_big(0));