Feature/20 rayon parallelism

[xoredtwice]

Summary

This MR introduces an optional parallel feature using Rayon to enable parallel computation of WOTS+ chain segments while preserving deterministic ordering and RFC-compatible behavior.

Changes

• Optional parallel feature (Rayon-based)
• Refactored chain segment computation to use:
  + par_iter().map().collect()
  • Sequential fallback when parallel is disabled
• Minor cleanup, removed duplicate code in verify() and verify_with_randomization_elements() and safety assertions
• Keccak256 function to compare the performace of parallel and sequential flows (Later we should consider proper benchmarking using criterion)

Security & Correctness Notes

• Parallelization only applied at chain-segment level (outer loop)
• Internal hash-chain loop remains sequential (cryptographically required)
• Output ordering preserved (Rayon collect() guarantees order)
• No shared mutable state in parallel blocks
• Signature size invariants maintained

Testing

Tested with:

cargo test
cargo test --features parallel
cargo test --no-default-features --features parallel

All tests pass in both sequential and parallel modes.

[augmentcode]

🤖 Augment PR Summary

Summary: Adds an optional Rayon-based parallel execution path for WOTS+ chain-segment computation while keeping outputs deterministic and RFC-compatible.

Changes:

• Introduced Cargo features: default solana and optional parallel (Rayon).
• Added a SignatureBuffer helper to build signature/public-key byte buffers with reduced duplication and controlled sizing.
• Refactored signing to compute per-segment chains in parallel via par_iter().map(...).collect(), with a sequential fallback.
• Refactored verification to reuse verify_with_randomization_elements() and optionally parallelize per-segment reconstruction.
• Extracted repeated per-segment logic into compute_chain_segment().
• Updated tests to include a Keccak256-based hash function (via sha3), plus determinism and SignatureBuffer behavior tests.

Technical Notes: Parallelism is limited to independent outer segments; internal per-chain hashing remains sequential, and ordering is preserved by collecting from Rayon’s indexed iterators.

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[augmentcode]

Review completed. 4 suggestions posted.

Comment augment review to trigger a new review at any time.

[augmentcode]

use std::time::Instant; is in the main library module but only used in the test module, which can break cargo build-sbf/no-std-ish Solana builds (and also creates an unused-import warning in non-test builds). Consider gating it with #[cfg(test)] or moving it into the tests module.

Severity: high

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[augmentcode]

In the non-solana SignatureBuffer::push_slice branch, an overflow will panic via slice indexing rather than an explicit invariant check, so the failure mode/message is much less clear than the solana branch. Consider adding an explicit length bound check here as well so callers get a predictable panic reason.

Severity: low

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[augmentcode]

sha3 is listed under [dependencies] but appears to be used only from #[cfg(test)] code, so it will be pulled into all downstream builds (including the Solana program). Consider moving it to [dev-dependencies] (or gating it) to avoid inflating the dependency graph for library consumers.

Severity: medium

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[augmentcode]

With default = ["solana"], it’s possible to enable --features parallel alongside solana, but Rayon/threaded parallelism typically won’t work for Solana SBF/BPF targets. Consider documenting or preventing this incompatible feature combination so consumers don’t accidentally break on-chain builds.

Severity: medium

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[daveth3t3chg33k]

Reiterate the init_var for calling upon instantiation of the var