feat(compress): move MT overlap window policy to Rust

Move construction of the external-dictionary and active-prefix ranges into a narrow Rust ABI while retaining the C window and logging surface. Preserve byte-range half-open overlap semantics and leave the input-range overlap wrapper available to its other C caller.

Test Plan: cargo test --manifest-path rust/Cargo.toml --no-default-features --features compression (180 passed); root and CLI clippy; make -B -C lib -j2 lib; make -C tests -j2 test-cli-tests (41 passed); make -B -C tests -j2 test-zstream (84 named tests plus 6,845 and 9,628 fuzz cases passed).

lib/compress/zstdmt_compress.c

@@ -141,6 +141,9 @@ size_t ZSTDMT_rust_computeOverlapSize(unsigned windowLog, unsigned chainLog,
                                       int strategy, int overlapLog, int enableLdm);
 int ZSTDMT_rust_isOverlapped(const void* bufferStart, size_t bufferCapacity,
                              const void* rangeStart, size_t rangeSize);
+int ZSTDMT_rust_doesOverlapWindow(const void* bufferStart, size_t bufferCapacity,
+                                  const void* nextSrc, const void* base,
+                                  const void* dictBase, U32 dictLimit, U32 lowLimit);
 
 typedef struct ZSTDMT_bufferPool_s {
     ZSTDMT_RustBufferPool* rustPool;
@@ -1539,24 +1542,18 @@ static int ZSTDMT_isOverlapped(Buffer buffer, Range range)
 
 static int ZSTDMT_doesOverlapWindow(Buffer buffer, ZSTD_window_t window)
 {
-    Range extDict;
-    Range prefix;
-
     DEBUGLOG(5, "ZSTDMT_doesOverlapWindow");
-    extDict.start = window.dictBase + window.lowLimit;
-    extDict.size = window.dictLimit - window.lowLimit;
-
-    prefix.start = window.base + window.dictLimit;
-    prefix.size = window.nextSrc - (window.base + window.dictLimit);
     DEBUGLOG(5, "extDict [0x%zx, 0x%zx)",
-                (size_t)extDict.start,
-                (size_t)extDict.start + extDict.size);
+                (size_t)window.dictBase + window.lowLimit,
+                (size_t)window.dictBase + window.dictLimit);
     DEBUGLOG(5, "prefix  [0x%zx, 0x%zx)",
-                (size_t)prefix.start,
-                (size_t)prefix.start + prefix.size);
+                (size_t)window.base + window.dictLimit,
+                (size_t)window.nextSrc);
 
-    return ZSTDMT_isOverlapped(buffer, extDict)
-        || ZSTDMT_isOverlapped(buffer, prefix);
+    return ZSTDMT_rust_doesOverlapWindow(buffer.start, buffer.capacity,
+                                         window.nextSrc, window.base,
+                                         window.dictBase, window.dictLimit,
+                                         window.lowLimit);
 }
 
 static void ZSTDMT_waitForLdmComplete(ZSTDMT_CCtx* mtctx, Buffer buffer)

rust/src/zstdmt_compress.rs

@@ -250,6 +250,43 @@ pub extern "C" fn ZSTDMT_rust_isOverlapped(
     is_overlapped(bufferStart, bufferCapacity, rangeStart, rangeSize)
 }
 
+/// Return non-zero when a buffer overlaps either the external dictionary or
+/// the active prefix represented by a C `ZSTD_window_t`.
+///
+/// The C window stores the two ranges as pointer/index pairs.  Passing those
+/// scalar fields separately keeps the private C struct out of the Rust ABI;
+/// the byte-distance arithmetic mirrors the original pointer subtraction.
+#[no_mangle]
+pub extern "C" fn ZSTDMT_rust_doesOverlapWindow(
+    bufferStart: *const c_void,
+    bufferCapacity: usize,
+    nextSrc: *const c_void,
+    base: *const c_void,
+    dictBase: *const c_void,
+    dictLimit: u32,
+    lowLimit: u32,
+) -> c_int {
+    let ext_dict_start = dictBase.cast::<u8>().wrapping_add(lowLimit as usize);
+    let ext_dict_size = dictLimit.wrapping_sub(lowLimit) as usize;
+    let prefix_start = base.cast::<u8>().wrapping_add(dictLimit as usize);
+    let prefix_size = (nextSrc as usize)
+        .wrapping_sub(base as usize)
+        .wrapping_sub(dictLimit as usize);
+
+    (is_overlapped(
+        bufferStart,
+        bufferCapacity,
+        ext_dict_start.cast(),
+        ext_dict_size,
+    ) != 0
+        || is_overlapped(
+            bufferStart,
+            bufferCapacity,
+            prefix_start.cast(),
+            prefix_size,
+        ) != 0) as c_int
+}
+
 #[derive(Default)]
 struct BufferPoolState {
     buffer_size: usize,
@@ -926,6 +963,51 @@ mod tests {
         );
     }
 
+    #[test]
+    fn overlap_window_checks_external_dictionary_and_prefix() {
+        let bytes = [0u8; 32];
+        let base = bytes.as_ptr();
+        let next_src = base.wrapping_add(16);
+        let dict_base = base.wrapping_add(16);
+
+        assert_eq!(
+            ZSTDMT_rust_doesOverlapWindow(
+                base.wrapping_add(20).cast(),
+                4,
+                next_src.cast(),
+                base.cast(),
+                dict_base.cast(),
+                8,
+                4,
+            ),
+            1
+        );
+        assert_eq!(
+            ZSTDMT_rust_doesOverlapWindow(
+                base.wrapping_add(12).cast(),
+                4,
+                next_src.cast(),
+                base.cast(),
+                dict_base.cast(),
+                8,
+                4,
+            ),
+            1
+        );
+        assert_eq!(
+            ZSTDMT_rust_doesOverlapWindow(
+                base.cast(),
+                4,
+                next_src.cast(),
+                base.cast(),
+                dict_base.cast(),
+                8,
+                4,
+            ),
+            0
+        );
+    }
+
     #[test]
     fn buffer_pool_reuses_and_resizes_buffers() {
         let pool = unsafe { ZSTDMT_rust_buffer_pool_create(2, DEFAULT_MEM) };