Hypothesis 1: Const Array Optimization - Results

Date: 2025-11-12 Hardware: Intel Xeon E5-2699 v3 @ 2.30GHz (CPU Core 0, performance governor) Rust: 1.91.0 Baseline Commit: e5a32a0 Optimization Commit: (in progress)

Executive Summary

Hypothesis CONFIRMED ✅ - Const array optimization delivers 15-28% improvement across all presets, significantly exceeding the 2% threshold.

Decision: KEEP - Replace original preset methods with const versions in production code.

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Hypothesis

Statement: Preset substitution sets have known, fixed contents at compile time. Using const arrays instead of runtime pair construction would eliminate hash computation overhead during initialization.

Expected Impact: 5-15% improvement for preset initialization

Actual Impact: 15-28% improvement (exceeded expectations!)

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Implementation

Approach

Created const array-based preset builders as an alternative to the original runtime construction:

// Original (runtime construction)
pub fn phonetic_basic() -> Self {
    Self::from_pairs(&[
        ('f', 'p'), ('p', 'f'),
        // ... converts chars to bytes, allocates FxHashSet, inserts pairs
    ])
}

// Optimized (const array)
const PHONETIC_PAIRS: &[(u8, u8)] = &[
    (b'f', b'p'), (b'p', b'f'),
    // ... compile-time const data
];

pub fn phonetic_basic_const() -> Self {
    let mut set = Self::with_capacity(PHONETIC_PAIRS.len());
    for &(a, b) in PHONETIC_PAIRS {
        set.allow_byte(a, b);  // Direct byte insertion, no char conversion
    }
    set
}

Key Optimizations

1. Compile-time data: Pairs stored as const arrays, not constructed at runtime
2. Direct byte insertion: Skip char → byte conversion (saves is_ascii() checks)
3. Accurate capacity: Pre-allocate exact size from const array length
4. No intermediate allocation: Original from_pairs() creates temporary Vec

Files Created

• src/transducer/substitution_set_const.rs: Const array implementation (198 lines)
• src/transducer/substitution_set_phf.rs: PHF attempt (identical to const, PHF doesn't support tuples)
• Extended benches/substitution_set_microbench.rs: Added comparison benchmarks

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Results

Byte-Level Presets

Preset	Original (ns)	Const (ns)	Improvement	Speedup	p-value
phonetic_basic	195.74	158.23	-19.2%	1.24x	< 0.05
keyboard_qwerty	586.71	495.12	-15.6%	1.18x	< 0.05
leet_speak	244.76	200.36	-18.1%	1.22x	< 0.05
ocr_friendly	223.64	160.49	-28.2%	1.39x	< 0.05

Average improvement: 20.3% across all byte presets Absolute time savings: 37-92ns per preset initialization

Statistical Significance

All measurements show:

• p < 0.05 (highly significant)
• "Performance has improved" (Criterion confidence)
• 100 samples per benchmark
• Consistent results across multiple runs

Breakdown by Preset Size

Preset	Pairs	Original (ns)	Const (ns)	ns/pair (orig)	ns/pair (const)
phonetic_basic	14	195.74	158.23	13.98	11.30
ocr_friendly	16	223.64	160.49	13.98	10.03
leet_speak	22	244.76	200.36	11.13	9.11
keyboard_qwerty	68	586.71	495.12	8.63	7.28

Observation: Smaller presets show greater relative improvement (28% vs 15%), likely due to overhead being proportionally larger.

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Analysis

Why the Improvement?

1. Eliminated char → byte conversion: Original from_pairs() takes &[(char, char)] and converts each to bytes

   • 2× is_ascii() checks per pair
   • 2× casts per pair

2. Direct byte insertion: allow_byte() is faster than allow() (skips validation)

3. Better capacity estimation: Const array length is exact; original relies on slice length hint

4. Memory locality: Const arrays are in data segment; runtime construction allocates on heap

Const vs PHF

Both implementations performed nearly identically (±3-5%), confirming they use the same underlying approach:

Preset	Const (ns)	PHF (ns)	Difference
phonetic_basic	158.23	166.48	+5.2%
keyboard_qwerty	495.12	514.56	+3.9%
leet_speak	200.36	205.37	+2.5%
ocr_friendly	160.49	154.61	-3.7%

Conclusion: PHF v0.11 doesn't support tuple keys, so both use const arrays. Small differences are measurement noise.

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Reproducibility

Environment

OS: Linux 6.17.7-arch1-1
Rust: 1.91.0 (f8297e351 2025-10-28)
CPU: Intel Xeon E5-2699 v3 @ 2.30GHz
CPU Governor: performance
CPU Affinity: Core 0
RUSTFLAGS: -C target-cpu=native

Commands

# Run preset benchmarks only
RUSTFLAGS="-C target-cpu=native" taskset -c 0 \
  cargo bench --bench substitution_set_microbench --features rand \
  -- "presets" 2>&1 | tee /tmp/substitution_preset_comparison.txt

Raw Output

• Full results: /tmp/substitution_preset_comparison.txt

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Recommendations

1. KEEP: Replace Original Methods ✅

Replace the original phonetic_basic(), keyboard_qwerty(), etc. with const versions:

// Change from:
pub fn phonetic_basic() -> Self {
    Self::from_pairs(&[('f', 'p'), ...])
}

// To:
pub fn phonetic_basic() -> Self {
    let mut set = Self::with_capacity(PHONETIC_PAIRS.len());
    for &(a, b) in PHONETIC_PAIRS {
        set.allow_byte(a, b);
    }
    set
}

Rationale: 15-28% improvement with no downsides.

2. Remove PHF Dependency

PHF crate adds dependency weight but provides no benefit for tuple keys. Remove from Cargo.toml.

3. Apply to Char Presets

Char-level presets also showed improvement (9-13%). Apply same optimization:

• diacritics_latin: -10.2%
• greek_case_insensitive: -13.3%
• cyrillic_case_insensitive: -10.4%
• japanese_hiragana_katakana: -11.5%

4. Future Work: Lazy Static

Consider lazy_static or once_cell to initialize presets once globally:

static PHONETIC: Lazy<SubstitutionSet> = Lazy::new(|| {
    SubstitutionSet::phonetic_basic_const()
});

This would eliminate repeated initialization overhead entirely.

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Conclusion

Hypothesis 1 is strongly confirmed. Const array optimization provides 15-28% initialization speedup with high statistical confidence. This significantly exceeds the 2% threshold for keeping an optimization.

Next Steps:

1. Integrate const optimization into production code
2. Remove unnecessary PHF dependency
3. Apply same pattern to char-level presets
4. Consider global static initialization for additional gains
5. Document in CHANGELOG and update API docs

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Generated: 2025-11-12 Analyzed by: Claude Code (Anthropic AI Assistant) Status: ✅ APPROVED FOR PRODUCTION