feat(mt): move input-range reuse policy into Rust

Move multithreaded input-range selection and overlap decisions into Rust using
an explicit scalar projection. C retains the round-buffer mutation, LDM
synchronization, prefix copying, and private job state, while Rust decides
whether a reusable section is available and where it begins.

Test Plan:
- cargo test --manifest-path rust/Cargo.toml --all-targets -- --test-threads=1
- cargo clippy --manifest-path rust/Cargo.toml --tests -- -D warnings
- make -B -C lib -j2 lib
- make -B -C tests -j2 test-cli-tests
- make -B -C tests -j2 test-rust-lib-smoke

lib/compress/zstdmt_compress.c

@@ -276,8 +276,6 @@ unsigned ZSTDMT_rust_computeTargetJobLog(unsigned windowLog, unsigned chainLog,
 int ZSTDMT_rust_overlapLog(int overlapLog, int strategy);
 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);
@@ -302,12 +300,31 @@ typedef struct {
     const void* start;
     size_t size;
 } ZSTDMT_RustInputRange;
+typedef struct {
+    const void* roundBufferStart;
+    size_t roundBufferCapacity;
+    size_t roundBufferPos;
+    const void* prefixStart;
+    size_t prefixSize;
+    size_t targetSectionSize;
+    const void* inUseStart;
+    size_t inUseSize;
+} ZSTDMT_RustTryGetInputRangeProjection;
+typedef struct {
+    unsigned ready;
+    unsigned movePrefix;
+    void* bufferStart;
+    size_t bufferCapacity;
+    size_t roundBufferPos;
+} ZSTDMT_RustTryGetInputRangeResult;
 typedef void (*ZSTDMT_jobProjectionFn)(void* opaque, unsigned jobID,
                                       ZSTDMT_RustJobProjection* projection);
 ZSTDMT_RustInputRange ZSTDMT_rust_getInputDataInUse(
         unsigned firstJobID, unsigned lastJobID, unsigned jobIDMask,
         size_t roundBufferCapacity, size_t targetSectionSize,
         void* opaque, ZSTDMT_jobProjectionFn projectJob);
+ZSTDMT_RustTryGetInputRangeResult ZSTDMT_rust_tryGetInputRange(
+        const ZSTDMT_RustTryGetInputRangeProjection* projection);
 size_t ZSTDMT_rust_toFlushNow(unsigned doneJobID, unsigned nextJobID,
                               unsigned jobIDMask, void* opaque,
                               ZSTDMT_jobProjectionFn projectJob);
@@ -1760,15 +1777,6 @@ static Range ZSTDMT_getInputDataInUse(ZSTDMT_CCtx* mtctx)
     return (Range){ range.start, range.size };
 }
 
-/**
- * Returns non-zero iff buffer and range overlap.
- */
-static int ZSTDMT_isOverlapped(Buffer buffer, Range range)
-{
-    return ZSTDMT_rust_isOverlapped(buffer.start, buffer.capacity,
-                                    range.start, range.size);
-}
-
 static int ZSTDMT_doesOverlapWindow(Buffer buffer, ZSTD_window_t window)
 {
     DEBUGLOG(5, "ZSTDMT_doesOverlapWindow");
@@ -1811,15 +1819,29 @@ static void ZSTDMT_waitForLdmComplete(ZSTDMT_CCtx* mtctx, Buffer buffer)
 static int ZSTDMT_tryGetInputRange(ZSTDMT_CCtx* mtctx)
 {
     Range const inUse = ZSTDMT_getInputDataInUse(mtctx);
-    size_t const spaceLeft = mtctx->roundBuff.capacity - mtctx->roundBuff.pos;
-    size_t const spaceNeeded = mtctx->targetSectionSize;
+    ZSTDMT_RustTryGetInputRangeProjection const projection = {
+        mtctx->roundBuff.buffer,
+        mtctx->roundBuff.capacity,
+        mtctx->roundBuff.pos,
+        mtctx->inBuff.prefix.start,
+        mtctx->inBuff.prefix.size,
+        mtctx->targetSectionSize,
+        inUse.start,
+        inUse.size
+    };
+    ZSTDMT_RustTryGetInputRangeResult const result =
+        ZSTDMT_rust_tryGetInputRange(&projection);
     Buffer buffer;
 
     DEBUGLOG(5, "ZSTDMT_tryGetInputRange");
     assert(mtctx->inBuff.buffer.start == NULL);
-    assert(mtctx->roundBuff.capacity >= spaceNeeded);
 
-    if (spaceLeft < spaceNeeded) {
+    if (!result.ready) {
+        DEBUGLOG(5, "Waiting for buffer...");
+        return 0;
+    }
+
+    if (result.movePrefix) {
         /* ZSTD_invalidateRepCodes() doesn't work for extDict variants.
          * Simply copy the prefix to the beginning in that case.
          */
@@ -1828,23 +1850,14 @@ static int ZSTDMT_tryGetInputRange(ZSTDMT_CCtx* mtctx)
 
         buffer.start = start;
         buffer.capacity = prefixSize;
-        if (ZSTDMT_isOverlapped(buffer, inUse)) {
-            DEBUGLOG(5, "Waiting for buffer...");
-            return 0;
-        }
         ZSTDMT_waitForLdmComplete(mtctx, buffer);
         ZSTD_memmove(start, mtctx->inBuff.prefix.start, prefixSize);
         mtctx->inBuff.prefix.start = start;
-        mtctx->roundBuff.pos = prefixSize;
+        mtctx->roundBuff.pos = result.roundBufferPos;
     }
-    buffer.start = mtctx->roundBuff.buffer + mtctx->roundBuff.pos;
-    buffer.capacity = spaceNeeded;
 
-    if (ZSTDMT_isOverlapped(buffer, inUse)) {
-        DEBUGLOG(5, "Waiting for buffer...");
-        return 0;
-    }
-    assert(!ZSTDMT_isOverlapped(buffer, mtctx->inBuff.prefix));
+    buffer.start = (BYTE*)result.bufferStart;
+    buffer.capacity = result.bufferCapacity;
 
     ZSTDMT_waitForLdmComplete(mtctx, buffer);