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# f2f91ceb0e9f oxygen / vendor / crc-fast / .kiro / specs / crc16-hardware-acceleration / tasks.md
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2cd40ea1 Andy McCall ↶ 1 
# Implementation Plan: CRC-16 Hardware Acceleration
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## Overview
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This implementation adds hardware-accelerated CRC-16 support to the crc-fast library in two phases: first extending the key generator, then extending the algorithm module. The implementation leverages the existing CRC-32 infrastructure by scaling CRC-16 values to 32-bit space.
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## Tasks
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- [x] 1. Extend Key Generator for CRC-16
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  - [x] 1.1 Add CRC-16 exponents constant and update keys() function
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    - Add `CRC16_EXPONENTS` constant (same as CRC32_EXPONENTS)
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    - Update `keys()` function to handle width=16
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    - Update `key()` function to dispatch to `crc16_key()`
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    - _Requirements: 1.3_
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  - [x] 1.2 Implement crc16_key() function
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    - Compute x^exponent mod P(x) for 17-bit CRC-16 polynomial
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    - Handle both forward and reflected modes
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    - Follow the pattern from reference/crc16f/crc16fg.cpp and reference/crc16r/crc16rg.cpp
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    - _Requirements: 1.1, 1.2, 1.7_
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  - [x] 1.3 Implement crc16_mu() function
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    - Compute Barrett reduction constant floor(x^64/P(x))
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    - Handle both forward and reflected modes
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    - _Requirements: 1.4_
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  - [x] 1.4 Implement crc16_polynomial() function
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    - Format polynomial for forward mode: (poly << 16) | (1 << 32)
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    - Format polynomial for reflected mode: (bit_reverse_16(poly) << 1) | 1
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    - _Requirements: 1.5, 1.6_
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  - [x] 1.5 Write property tests for CRC-16 key generation
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    - **Property 1: Forward polynomial formatting**
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    - **Property 2: Reflected polynomial formatting**
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    - **Validates: Requirements 1.5, 1.6**
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  - [x] 1.6 Add unit tests for CRC-16 key generation
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    - Test generated keys match KEYS_8BB7_FORWARD for CRC-16/T10-DIF
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    - Test generated keys match KEYS_1021_REVERSE for CRC-16/IBM-SDLC
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    - Add CRC-16 test configs to TEST_ALL_CONFIGS
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    - _Requirements: 1.1, 1.2, 4.1_
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- [x] 2. Checkpoint - Verify key generation
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  - Ensure all tests pass, ask the user if questions arise.
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- [x] 3. Update CRC-16 Constants and Parameters
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  - [x] 3.1 Update src/crc16/consts.rs with complete constants
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    - Update KEYS_8BB7_FORWARD with 256-byte folding keys (indices 21-22)
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    - Update KEYS_1021_REVERSE with 256-byte folding keys (indices 21-22)
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    - Add SIMD_CONSTANTS if different from CRC-32
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    - _Requirements: 5.1, 5.2, 5.4_
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  - [x] 3.2 Add Width16 struct to src/structs.rs
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    - Implement CrcWidth trait for Width16
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    - _Requirements: 5.3_
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  - [x] 3.3 Update CrcAlgorithm enum and CrcParams::new()
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    - Add Crc16Custom variant to CrcAlgorithm enum
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    - Update CrcParams::new() to handle width=16
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    - Update cache module if needed for CRC-16 key caching
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    - _Requirements: 5.5, 6.1_
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- [x] 4. Implement CRC-16 Algorithm
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  - [x] 4.1 Create src/crc16/algorithm.rs with EnhancedCrcWidth implementation
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    - Implement EnhancedCrcWidth for Width16
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    - Implement create_state() with proper scaling for forward mode
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    - Implement extract_result() with proper scaling
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    - Implement fold_16(), fold_width(), barrett_reduction()
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    - Implement create_coefficient() and perform_final_reduction()
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    - Implement get_last_bytes_table_ptr()
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    - _Requirements: 2.3, 2.4, 2.5, 2.6, 2.7_
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  - [x] 4.2 Implement process_0_to_15 for CRC-16
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    - Handle small inputs (0-15 bytes) for CRC-16
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    - Follow pattern from crc32/algorithm.rs
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    - _Requirements: 2.1, 2.2_
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  - [x] 4.3 Update architecture dispatcher for CRC-16
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    - Update src/arch/mod.rs to handle width=16
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    - Route to Width16 implementation
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    - _Requirements: 2.1, 2.2_
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  - [x] 4.4 Update software fallback for CRC-16
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    - Update src/arch/software.rs to handle CRC-16 algorithms
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    - Add CRC-16 reference implementations from crc crate
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    - _Requirements: 2.1, 2.2_
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- [x] 5. Checkpoint - Verify basic CRC-16 computation
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  - Ensure all tests pass, ask the user if questions arise.
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- [x] 6. Add Public API Support for CRC-16
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  - [x] 6.1 Update src/lib.rs checksum() function
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    - Add cases for Crc16IbmSdlc and Crc16T10Dif
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    - _Requirements: 2.1, 2.2_
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  - [x] 6.2 Update Digest and other public APIs
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    - Ensure Digest works with CRC-16 algorithms
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    - Ensure checksum_combine works with CRC-16
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    - _Requirements: 6.2, 6.3, 6.4_
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- [-] 7. Add Comprehensive Tests
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  - [x] 7.1 Add CRC-16 check value tests
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    - Test "123456789" produces 0x906E for CRC-16/IBM-SDLC
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    - Test "123456789" produces 0xD0DB for CRC-16/T10-DIF
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    - _Requirements: 2.1, 2.2, 4.2_
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  - [x] 7.2 Write property test for CRC-16 computation matches reference
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    - **Property 3: CRC-16 computation matches reference**
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    - **Validates: Requirements 2.1, 2.2, 5.5, 6.1, 6.2, 6.3**
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  - [x] 7.3 Write property test for backwards compatibility
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    - **Property 4: CRC-32 and CRC-64 backwards compatibility**
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    - **Validates: Requirements 3.1, 3.2, 3.4**
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  - [x] 7.4 Write property test for CRC-16 checksum combination
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    - **Property 5: CRC-16 checksum combination round-trip**
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    - **Validates: Requirements 6.4**
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  - [x] 7.5 Add CRC-16 to existing test suites
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    - Add CRC-16 configs to TEST_ALL_CONFIGS in src/test/consts.rs
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    - Ensure all length-based tests include CRC-16
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    - _Requirements: 4.3, 4.4_
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- [x] 8. Final Checkpoint - Ensure all tests pass
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  - Ensure all tests pass, ask the user if questions arise.
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  - Run cargo fmt --check
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  - Run cargo clippy -- -D warnings
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  - Run cargo test
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## Notes
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- Tasks marked with `*` are optional and can be skipped for faster MVP
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- Each task references specific requirements for traceability
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- Checkpoints ensure incremental validation
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- Property tests validate universal correctness properties
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- Unit tests validate specific examples and edge cases
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- The implementation leverages existing CRC-32 infrastructure to minimize code duplication