feat(cli): move cycle log policy to Rust

Move the CLI's strategy-dependent cycle-log calculation behind a scalar Rust ABI shim while retaining the C assertion and call-site behavior.

Test Plan:

- cargo test --manifest-path rust/cli/Cargo.toml --no-default-features --features cli,compression,decompression,benchmark (116 tests)

- cargo clippy --manifest-path rust/cli/Cargo.toml --all-targets with the full CLI feature set

- make -B -C programs -j2 zstd zstd-small zstd-frugal

- make -C tests -j2 test-cli-tests (41 scenarios)

programs/fileio.c

@@ -310,6 +310,7 @@ static int FIO_shouldDisplayMultipleFileSummary(FIO_ctx_t const* fCtx)
 int FIO_rust_checkFilenameCollisions(const char** filenameTable, unsigned nbFiles);
 unsigned FIO_rust_highbit64(unsigned long long v);
 unsigned long long FIO_rust_getLargestFileSize(const char** inFileNames, unsigned nbFiles);
+unsigned FIO_rust_cycleLog(unsigned hashLog, int strategy);
 void FIO_rust_setInBuffer(ZSTD_inBuffer* output, const void* buf, size_t s, size_t pos);
 void FIO_rust_setOutBuffer(ZSTD_outBuffer* output, void* buf, size_t s, size_t pos);
 const char* FIO_rust_determineCompressedName(const char* srcFileName, const char* outDirName, const char* suffix);
@@ -824,9 +825,8 @@ typedef struct {
  *  condition for correct operation : hashLog > 1 */
 static U32 ZSTD_cycleLog(U32 hashLog, ZSTD_strategy strat)
 {
-    U32 const btScale = ((U32)strat >= (U32)ZSTD_btlazy2);
     assert(hashLog > 1);
-    return hashLog - btScale;
+    return FIO_rust_cycleLog(hashLog, (int)strat);
 }
 
 static void FIO_adjustParamsForPatchFromMode(FIO_prefs_t* const prefs,

rust/src/fileio_prefs.rs

@@ -24,6 +24,7 @@ const FIO_PATCH_MEM_LIMIT_UNKNOWN_SIZE: c_int = 1;
 const FIO_PATCH_MEM_LIMIT_TOO_LARGE: c_int = 2;
 const UTIL_FILESIZE_UNKNOWN: u64 = u64::MAX;
 const ZSTD_WINDOWLOG_MAX: u32 = if size_of::<usize>() == 4 { 30 } else { 31 };
+const ZSTD_BTLAZY2: c_int = 6;
 static STDOUT_MARK: &[u8] = b"/*stdout*\\\0";
 
 static mut COMPRESSED_NAME_CAPACITY: usize = 0;
@@ -591,6 +592,17 @@ pub extern "C" fn FIO_rust_highbit64(value: u64) -> c_uint {
     (u64::BITS - 1 - value.leading_zeros()) as c_uint
 }
 
+#[inline]
+fn cycle_log(hash_log: c_uint, strategy: c_int) -> c_uint {
+    hash_log.wrapping_sub(c_uint::from(strategy >= ZSTD_BTLAZY2))
+}
+
+/// Rust scalar ABI for the `ZSTD_cycleLog()` policy in `programs/fileio.c`.
+#[no_mangle]
+pub extern "C" fn FIO_rust_cycleLog(hash_log: c_uint, strategy: c_int) -> c_uint {
+    cycle_log(hash_log, strategy)
+}
+
 fn largest_file_size<I>(sizes: I) -> u64
 where
     I: IntoIterator<Item = u64>,
@@ -1085,6 +1097,21 @@ mod tests {
         );
     }
 
+    #[test]
+    fn cycle_log_keeps_lower_strategies_unchanged() {
+        assert_eq!(FIO_rust_cycleLog(20, ZSTD_BTLAZY2 - 1), 20);
+    }
+
+    #[test]
+    fn cycle_log_subtracts_at_btlazy2_threshold() {
+        assert_eq!(FIO_rust_cycleLog(20, ZSTD_BTLAZY2), 19);
+    }
+
+    #[test]
+    fn cycle_log_subtracts_for_upper_strategies() {
+        assert_eq!(FIO_rust_cycleLog(20, ZSTD_BTLAZY2 + 3), 19);
+    }
+
     #[test]
     fn buffer_shims_preserve_fields_and_c_layout() {
         let input_word = size_of::<*const c_void>();