The MT rsync initialization previously computed the rolling-hash prime
power through the C-only ZSTD_rollingHash_primePower helper. Move only that
scalar exponentiation behind the existing Rust MT ABI boundary. The C caller
still owns the MT context assignment, RSYNC_LENGTH constant, and surrounding
initialization.
The Rust helper mirrors ZSTD_ipow with wrapping u64 multiplication and applies
u32 wrapping subtraction before widening the exponent, preserving the C
length == 0 behavior. Rolling-hash scanning, rotation, and all other MT state
remain unchanged.
Test Plan:
- Focused rolling_hash_prime_power Rust tests -- passed (2 tests).
- Required clippy/fmt sequence in normal, bench, and test modes -- passed.
- make -B -C lib -j2 lib -- passed.
- make -C tests test-rust-lib-smoke -- passed.
- tests/fuzzer -s4560 -t56 -i57 -v -- passed (57 tests).
- make -C tests -j2 test-zstream -- passed (84 named, 5,305 + 7,351 randomized).
- git diff --check and git diff --cached --check -- passed.
ddidderr
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