dancing-links/PERFORMANCE.md

Fastest JS solver for exact cover problems using Dancing Links

# Performance Optimization Process

This document summarizes the systematic performance optimization work performed on the Dancing Links implementation.

## Optimization Testing Results

After systematic testing of **19 individual optimizations**, we identified 3 successful approaches out of 19 attempts.

## Successful Optimizations (3 out of 19)

### 1. Early Termination for Impossible Constraints

When a column has zero remaining options, the current search path cannot lead to a valid solution. Immediately selecting such columns triggers backtracking sooner, avoiding deeper recursion into impossible branches.

**Result:** Consistent positive performance impact across all benchmarks.

### 2. Pre-calculated Pointers

Storing the next loop iteration target before modifying the current node's links reduces data dependencies in cover/uncover operations where linked list traversal is frequent.

**Result:** Consistent positive performance impact across all benchmarks.

### 3. Unit Propagation for Forced Moves

When a column has exactly one remaining option, that option must be selected in any valid solution. Prioritizing these unit constraints reduces the search space by making forced moves immediately rather than exploring them through normal branching.

**Result:** Significant improvement on constraint problems with cascading effects (like Sudoku), minimal impact on problems without such characteristics (like Pentomino tiling).

## Failed Optimization Patterns

### Complex Micro-optimizations (12 failures)

- **Loop unrolling & fast paths**: Degraded performance
- **Function inlining**: No benefit over existing JavaScript engine optimizations
- **Local variable caching**: No measurable improvement
- **Memory layout optimizations**: Overhead outweighed benefits

### Advanced Algorithmic Improvements (4 failures)

- **Enhanced column selection heuristics**: Computation overhead outweighed algorithmic benefits
- **Complex constraint propagation**: Expensive analysis dominated any search space reduction
- **Symmetry breaking**: Post-processing approach provided no search-time benefits

## Key Insights

### ✅ What Works

1. **Simple algorithmic improvements** that reduce search space
2. **Early termination** for impossible/forced cases
3. **Problem-specific optimizations** that match problem characteristics
4. **Working with language runtime** rather than against it

### ❌ What Doesn't Work

1. **Manual micro-optimizations** - modern JavaScript engines handle these automatically
2. **Complex memory layouts** - property access overhead often dominates
3. **Manual loop optimizations** - can interfere with engine optimizations
4. **Premature caching** - provides no benefit over built-in optimizations

## Optimization Methodology

### Testing Protocol

Each optimization was implemented in isolation and benchmarked against a stable baseline:

1. **Implement**: Single, focused change to current implementation
2. **Benchmark**: Run comprehensive benchmarks multiple times, record averages
3. **Compare**: Calculate performance change vs baseline for each test case
4. **Document**: Record results with analysis
5. **Decide**: Keep if net positive across all benchmarks, revert if negative
6. **Clean**: Ensure clean implementation before next test

### Success Rate

- **Total optimizations tested**: 19
- **Successful optimizations**: 3
- **Pattern**: Simple algorithmic improvements succeed, complex micro-optimizations mostly fail

## Lessons Learned

### Current Understanding

The optimization work demonstrated that:

- Algorithmic improvements can provide substantial gains when they match problem characteristics
- Unit propagation is effective for constraint problems with cascading effects
- Simple optimizations often outperform complex ones in managed runtime environments

### Future Approach

Based on systematic testing:

1. **Focus on algorithmic improvements** over micro-optimizations
2. **Test systematically** - measure everything, assume nothing about performance
3. **Work with the runtime** - simple patterns often optimize better than complex manual optimizations
4. **Consider problem-specific approaches** for targeted use cases

## Recommendations

The systematic optimization effort successfully identified which approaches work and which don't, providing a foundation for future performance work. The current implementation maintains clean, maintainable code while incorporating the optimizations that proved effective through testing.