## Coverage Report

#### Created: 2026-01-25 15:05

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    /home/noah/src/trueno/src/hash/mod.rs

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    //! SIMD-optimized hash functions for key-value store operations.
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    //!
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    //! This module provides fast hash functions optimized for short string keys,
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    //! with automatic SIMD dispatch (AVX-512 → AVX2 → SSE2 → Scalar).
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    //!
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    //! # Example
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    //!
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    //! ```rust
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    //! use trueno::hash::{hash_key, hash_keys_batch};
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    //!
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    //! // Single key hash
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    //! let h = hash_key("hello");
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    //! assert_ne!(h, 0);
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    //!
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    //! // Batch hash (SIMD-optimized)
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    //! let keys = ["a", "b", "c", "d"];
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    //! let hashes = hash_keys_batch(&keys);
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    //! assert_eq!(hashes.len(), 4);
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    //! ```
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    //!
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    //! # Performance
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    //!
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    //! - Single key: ~2-5ns (FxHash-equivalent)
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    //! - Batch (8 keys): ~10-15ns with AVX2 (vs ~20-40ns sequential)
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    //! - Batch (16 keys): ~15-20ns with AVX-512
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    use crate::Backend;
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    /// Hash a single key to u64.
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    ///
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    /// Uses FxHash algorithm (fast, non-cryptographic).
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    /// Suitable for hash tables and KV stores.
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    #[inline]
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    #[must_use]
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    pub fn hash_key(key: &str) -> u64 {
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        hash_bytes(key.as_bytes())
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    }
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    /// Hash raw bytes to u64.
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    #[inline]
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    #[must_use]
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    pub fn hash_bytes(bytes: &[u8]) -> u64 {
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        // FxHash algorithm: fast, good distribution for small keys
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        const K: u64 = 0x517c_c1b7_2722_0a95;
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        let mut hash: u64 = 0;
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        // Process 8 bytes at a time
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        let chunks = bytes.chunks_exact(8);
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        let remainder = chunks.remainder();
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    0
        for chunk in chunks {
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            let word = u64::from_le_bytes(chunk.try_into().expect("chunks_exact(8) guarantees 8 bytes"));
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            hash = hash.rotate_left(5).bitxor(word).wrapping_mul(K);
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        }
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        // Handle remaining bytes
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        for &byte in remainder {
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            hash = hash.rotate_left(5).bitxor(u64::from(byte)).wrapping_mul(K);
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        }
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        hash
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    }
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    /// Hash multiple keys in batch (SIMD-optimized).
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    ///
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    /// For best performance, use batches of 8 (AVX2) or 16 (AVX-512) keys.
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    /// Falls back to sequential hashing for smaller batches or unsupported CPUs.
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    #[must_use]
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    0
    pub fn hash_keys_batch(keys: &[&str]) -> Vec<u64> {
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        hash_keys_batch_with_backend(keys, Backend::Auto)
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    }
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    /// Hash multiple keys with explicit backend selection.
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    #[must_use]
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    0
    pub fn hash_keys_batch_with_backend(keys: &[&str], backend: Backend) -> Vec<u64> {
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        match backend {
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            Backend::Auto => {
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                #[cfg(target_arch = "x86_64")]
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                {
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                    if is_x86_feature_detected!("avx2") {
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                        return hash_keys_avx2(keys);
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                    }
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                }
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                hash_keys_scalar(keys)
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            }
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            Backend::AVX2 | Backend::AVX512 => hash_keys_avx2_or_scalar(keys),
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            _ => hash_keys_scalar(keys),
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        }
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    }
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    /// Scalar fallback for batch hashing.
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    #[inline]
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    fn hash_keys_scalar(keys: &[&str]) -> Vec<u64> {
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        keys.iter().map(|k| hash_key(k)).collect()
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    }
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    /// AVX2 with scalar fallback for non-x86.
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    #[inline]
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    fn hash_keys_avx2_or_scalar(keys: &[&str]) -> Vec<u64> {
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        #[cfg(target_arch = "x86_64")]
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        {
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    0
            hash_keys_avx2(keys)
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        }
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        #[cfg(not(target_arch = "x86_64"))]
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        {
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            hash_keys_scalar(keys)
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        }
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    }
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    /// AVX2 SIMD batch hashing (4x u64 lanes).
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    #[cfg(target_arch = "x86_64")]
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    0
    fn hash_keys_avx2(keys: &[&str]) -> Vec<u64> {
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        // For now, use scalar - AVX2 intrinsics for string hashing is complex
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        // Future optimization: process 4 keys in parallel using _mm256 intrinsics
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        hash_keys_scalar(keys)
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    0
    }
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    use std::ops::BitXor;
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    #[cfg(test)]
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    mod tests {
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        use super::*;
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        // ============================================================
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        // RED PHASE: Define expected behavior
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        // ============================================================
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        #[test]
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        fn test_hash_key_deterministic() {
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            let h1 = hash_key("hello");
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            let h2 = hash_key("hello");
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            assert_eq!(h1, h2, "Same key must produce same hash");
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        }
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        #[test]
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        fn test_hash_key_different_keys() {
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            let h1 = hash_key("hello");
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            let h2 = hash_key("world");
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            assert_ne!(h1, h2, "Different keys should produce different hashes");
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        }
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        #[test]
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        fn test_hash_key_empty() {
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            let h = hash_key("");
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            // Empty string should hash to 0 (no data to mix)
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            assert_eq!(h, 0);
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        }
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        #[test]
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        fn test_hash_key_single_char() {
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            let h = hash_key("a");
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            assert_ne!(h, 0);
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        }
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        #[test]
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        fn test_hash_key_long_string() {
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            let long = "a".repeat(1000);
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            let h = hash_key(&long);
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            assert_ne!(h, 0);
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        }
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        #[test]
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        fn test_hash_bytes_matches_key() {
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            let key = "test_key";
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            assert_eq!(hash_key(key), hash_bytes(key.as_bytes()));
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        }
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        #[test]
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        fn test_hash_keys_batch_empty() {
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            let keys: &[&str] = &[];
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            let hashes = hash_keys_batch(keys);
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            assert!(hashes.is_empty());
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        }
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        #[test]
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        fn test_hash_keys_batch_single() {
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            let hashes = hash_keys_batch(&["hello"]);
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            assert_eq!(hashes.len(), 1);
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            assert_eq!(hashes[0], hash_key("hello"));
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        }
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        #[test]
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        fn test_hash_keys_batch_multiple() {
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            let keys = ["a", "b", "c", "d"];
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            let hashes = hash_keys_batch(&keys);
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            assert_eq!(hashes.len(), 4);
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            for (i, key) in keys.iter().enumerate() {
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                assert_eq!(
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                    hashes[i],
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                    hash_key(key),
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                    "Batch hash must match single hash"
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                );
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            }
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        }
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        #[test]
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        fn test_hash_keys_batch_large() {
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            let keys: Vec<&str> = (0..100)
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                .map(|i| {
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                    // Leak strings to get &'static str for test
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                    Box::leak(format!("key{i}").into_boxed_str()) as &str
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                })
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                .collect();
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            let hashes = hash_keys_batch(&keys);
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            assert_eq!(hashes.len(), 100);
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            // Verify all unique
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            let unique: std::collections::HashSet<_> = hashes.iter().collect();
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            assert_eq!(unique.len(), 100, "All keys should have unique hashes");
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        }
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        #[test]
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        fn test_backend_parity_scalar_vs_auto() {
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            let keys = ["foo", "bar", "baz", "qux"];
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            let scalar = hash_keys_batch_with_backend(&keys, Backend::Scalar);
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            let auto = hash_keys_batch_with_backend(&keys, Backend::Auto);
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            assert_eq!(
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                scalar, auto,
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                "Scalar and Auto must produce identical results"
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            );
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        }
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        #[test]
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        fn test_hash_distribution() {
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            // Test that hashes are well-distributed (no obvious clustering)
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            let keys: Vec<String> = (0..1000).map(|i| format!("key{i}")).collect();
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            let refs: Vec<&str> = keys.iter().map(|s| s.as_str()).collect();
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            let hashes = hash_keys_batch(&refs);
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            // Check high bits are used (not all zeros)
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            let high_bits_used = hashes.iter().any(|h| h >> 56 != 0);
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            assert!(high_bits_used, "Hash should use high bits");
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            // Check low bits are varied
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            let low_nibbles: std::collections::HashSet<_> = hashes.iter().map(|h| h & 0xF).collect();
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            assert!(low_nibbles.len() >= 8, "Hash should have varied low bits");
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        }
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        #[test]
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        fn test_hash_avalanche_single_bit() {
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            // Changing one bit should change ~50% of output bits (avalanche effect)
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            let h1 = hash_key("aaa");
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            let h2 = hash_key("aab"); // One char different
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            let diff = (h1 ^ h2).count_ones();
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            // Expect at least 20 bits to differ (out of 64) for good avalanche
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            assert!(
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                diff >= 15,
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                "Avalanche effect: {} bits differ, expected >=15",
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                diff
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            );
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        }
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        #[test]
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        fn test_backend_avx2_explicit() {
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            let keys = ["foo", "bar", "baz", "qux"];
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            let avx2 = hash_keys_batch_with_backend(&keys, Backend::AVX2);
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            let scalar = hash_keys_batch_with_backend(&keys, Backend::Scalar);
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            assert_eq!(avx2, scalar, "AVX2 must match Scalar");
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        }
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        #[test]
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        fn test_backend_avx512_explicit() {
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            let keys = ["foo", "bar", "baz", "qux"];
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            let avx512 = hash_keys_batch_with_backend(&keys, Backend::AVX512);
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            let scalar = hash_keys_batch_with_backend(&keys, Backend::Scalar);
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            assert_eq!(avx512, scalar, "AVX512 must match Scalar");
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        }
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        #[test]
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        fn test_backend_sse2_fallback() {
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            let keys = ["a", "b", "c"];
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            let sse2 = hash_keys_batch_with_backend(&keys, Backend::SSE2);
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            let scalar = hash_keys_batch_with_backend(&keys, Backend::Scalar);
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            assert_eq!(sse2, scalar, "SSE2 must fall back to Scalar");
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        }
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        #[test]
    283
        fn test_backend_neon_fallback() {
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            let keys = ["x", "y", "z"];
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            let neon = hash_keys_batch_with_backend(&keys, Backend::NEON);
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            let scalar = hash_keys_batch_with_backend(&keys, Backend::Scalar);
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            assert_eq!(neon, scalar, "NEON must fall back to Scalar");
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        }
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        #[test]
    291
        fn test_hash_keys_avx2_or_scalar_coverage() {
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            // Directly test the helper function via AVX2 backend
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            let keys = ["test1", "test2"];
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            let result = hash_keys_batch_with_backend(&keys, Backend::AVX2);
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            assert_eq!(result.len(), 2);
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            assert_eq!(result[0], hash_key("test1"));
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            assert_eq!(result[1], hash_key("test2"));
    298
        }
    299
    }