# 🚀 CSP Solver Production Readiness & Advanced Features Plan

## Overview
This plan focuses on transforming our hybrid CSP solver from a research prototype into a production-ready system with advanced capabilities. The plan is structured in four main phases, starting with stability and production readiness.

---

## 📋 **PHASE 1: Production Readiness & Stability**

### **Step 8.1: Error Handling & Recovery** ✅ COMPLETE
**Goal**: Implement comprehensive error handling throughout the solver

**8.1.1: Error Type System**
- [✅] Create simple error enum (`SolverError`) with basic failure cases
- [✅] Replace `Option<Solution>` returns with `Result<Solution, SolverError>`
- [✅] Add basic error context (constraint names, variable info)
- [✅] Handle timeout and memory limit errors gracefully

**8.1.2: Input Validation & Sanitization** ✅ COMPLETE
- [✅] Validate model consistency before solving
- [✅] Check for conflicting constraints early
- [✅] Validate variable domains and bounds
- [✅] Add constraint compatibility checks

**8.1.3: Memory Management & Resource Limits**
- [✅] Implement memory usage monitoring
- [✅] Add configurable memory limits
- [✅] Implement graceful degradation for large problems
- [✅] Add resource cleanup for interrupted solving
  

**Estimated Time**: 2-3 weeks
**Priority**: HIGH

### **Step 8.3: API Stabilization & Configuration**
**Goal**: Create stable, configurable public API

**8.3.1: Configuration System** ✅ COMPLETE
- [✅] **Create `SolverConfig` struct with all tunable parameters** - **COMPLETED**
- [❌] **Add optimization strategy selection** - **REMOVED** (not needed - single hybrid strategy)  
- [✅] **Implement precision and tolerance settings** - **COMPLETED** (via `float_precision_digits`)
- [✅] **Add timeout and memory limit configuration** - **COMPLETED** (foundation with placeholders)

**8.3.2: API Consistency & Documentation** ✅ COMPLETE
- [✅] Review and stabilize public API surface
- [✅] Add comprehensive API documentation
- [✅] Create migration guides for API changes
- [✅] Add API stability guarantees

**Estimated Time**: ~~1-2 weeks~~ **0.5-1 week** (reduced due to SolverConfig completion)
**Priority**: MEDIUM

---

## 🔧 **PHASE 2: Missing Core Constraints**

### **Step 9.1: Essential Missing Constraints** ✅ PARTIALLY COMPLETE  
**Goal**: Implement fundamental constraints that are currently missing

**9.1.1: Basic Arithmetic Constraints**
- [✅] **Division constraint** (`div`): `x / y = z` with domain handling - **COMPLETED**
- [✅] **Modulo constraint** (`mod`): `x mod y = z` - **COMPLETED**
- [✅] **Absolute value** (`abs`): `|x| = y` - **COMPLETED**
- [✅] **Min/Max constraints**: `min(vars...) = z`, `max(vars...) = z` - **COMPLETED**

**9.1.2: Comparison Constraints**  
- [✅] **Short constraint names** (`le`, `ge`, `eq`, `ne`, `lt`, `gt`) - **COMPLETED**
- [✅] **Greater than** (`gt`): `x > y` (strict inequality) - **COMPLETED**
- [✅] **Less than** (`lt`): `x < y` (strict inequality) - **COMPLETED**
- [✅] **Between constraint**: `x ≤ y ≤ z`
- [✅] **Element constraint**: `array[index] = value`

**9.1.3: Logical Constraints**
- [✅] **Boolean AND/OR/NOT** for constraint combinations - **COMPLETED**
- [✅] **If-then-else** constraints: `if condition then constraint1 else constraint2`
- [ ] **Reification**: convert constraints to boolean variables
- [✅] **Cardinality constraints**: exactly/at-most/at-least N variables are true

**Estimated Time**: 3-4 weeks
**Priority**: HIGH

### **Step 9.2: Advanced Global Constraints** ❌ SKIPPED
**Goal**: Implement sophisticated global constraints for complex problems

**9.2.1: Scheduling Constraints**
- [❌] **Cumulative constraint**: resource usage over time
- [❌] **No-overlap constraint**: tasks don't overlap in time
- [❌] **Sequence constraint**: ordering with setup times
- [❌] **Calendar constraints**: working days, shifts, holidays

**9.2.2: Assignment & Routing Constraints**
- [❌] **Assignment constraint**: one-to-one mappings
- [❌] **Circuit constraint**: Hamiltonian cycle (TSP)
- [❌] **Path constraint**: simple paths in graphs
- [❌] **Bin packing**: items into containers with capacity

**9.2.3: Counting & Grouping Constraints**
- [❌] **Global cardinality**: count occurrences of values
- [❌] **Among constraint**: count variables in a set
- [❌] **Distribute constraint**: distribute values across variables
- [❌] **Balance constraint**: equal distribution

**Estimated Time**: 4-5 weeks
**Priority**: MEDIUM

---

## 🔗 **PHASE 3: Advanced Constraint Optimization**

### **Step 10.1: Non-linear Constraint Support**
**Goal**: Handle non-linear mathematical constraints

**10.1.1: Mathematical Functions**
- [X] **Power constraints**: `x^n = y`
- [X] **Square root**: `sqrt(x) = y`
- [X] **Trigonometric**: `sin(x) = y`, `cos(x) = y`
- [X] **Exponential/Logarithmic**: `exp(x) = y`, `log(x) = y`

**10.1.2: Non-linear Optimization Integration**
- [X] Integrate with `nlopt` for non-linear problems
- [X] Add interval arithmetic for bounds propagation
- [X] Implement constraint linearization techniques
- [X] Add non-linear constraint satisfaction methods

**Estimated Time**: 5-6 weeks
**Priority**: MEDIUM

### **Step 10.2: Global Constraint Optimization**
**Goal**: Optimize global constraints using hybrid techniques

**10.2.1: Global Constraint Decomposition**
- [ ] Automatically decompose global constraints
- [ ] Identify separable sub-constraints
- [ ] Apply optimization to decomposed parts
- [ ] Recombine optimized solutions

**10.2.2: Specialized Global Solvers** ✅ COMPLETED
- [✅] Custom AllDifferent optimization solver - **COMPLETED**
- [✅] Cumulative constraint optimization - **COMPLETED**
- [✅] Assignment problem solvers (Hungarian algorithm) - **COMPLETED**
- [✅] Network flow constraint optimization - **COMPLETED**

**Estimated Time**: 4 weeks
**Priority**: LOW

### **Step 10.3: Objective Expression Analysis & AST**
**Goal**: Enable optimization of complex mathematical expressions in objectives

**10.3.1: Expression AST Implementation**
- [ ] Design View expression tree structure for mathematical expressions
- [ ] Implement AST parsing for objective expressions (`x * 2.0`, `x + 10.0`, etc.)
- [ ] Add expression type classification (linear, quadratic, polynomial)
- [ ] Create expression simplification and normalization algorithms

**10.3.2: Advanced Objective Analysis**  
- [ ] Detect linear transformations (`minimize x * coeff + const`)
- [ ] Identify separable expressions (`minimize x + y * coeff`)
- [ ] Handle negation and scaling automatically (`minimize -x` → `maximize x`)
- [ ] Support common mathematical patterns and transformations

**10.3.3: Optimization Integration**
- [ ] Extend `extract_simple_variable` with AST analysis capabilities
- [ ] Add transformation-aware optimization (scale results by coefficients)
- [ ] Implement coefficient and offset handling in precision optimizers
- [ ] Add expression complexity scoring for optimization routing decisions

**10.3.4: Performance & Compatibility**
- [ ] Ensure AST parsing overhead is <10% of optimization time
- [ ] Maintain backwards compatibility with direct variable objectives
- [ ] Add comprehensive test suite for expression patterns
- [ ] Document supported expression types and limitations

**Estimated Time**: 3-4 weeks
**Priority**: MEDIUM-HIGH (high impact on user experience - expands optimization coverage)

---

## 🔌 **PHASE 4: External Solver Integration**

### **Step 11.1: Optimization Library Interfaces**
**Goal**: Interface with commercial and open-source optimization libraries

**11.1.1: Linear Programming Integration**
- [X] **HiGHS** integration for LP problems
- [X] **COIN-OR CLP** interface
- [X] **GLPK** integration for educational use
- [X] Automatic LP problem extraction from CSP

**11.1.2: Mixed-Integer Programming**
- [X] **CPLEX** interface (if available)
- [X] **Gurobi** integration
- [X] **SCIP** open-source solver interface
- [X] **CBC** (COIN-OR) integration

**11.1.3: Specialized Solvers**
- [X] **OR-Tools** integration for complex constraints
- [X] **Choco** solver interface
- [X] **Gecode** integration
- [X] **MiniZinc** model translation

**Estimated Time**: 6-8 weeks
**Priority**: LOW

### **Step 11.2: Solver Selection & Coordination**
**Goal**: Intelligently choose and coordinate multiple solvers

**11.2.1: Automatic Solver Selection**
- [X] Problem classification for solver choice
- [X] Performance-based solver ranking
- [X] Fallback chain for solver failures
- [X] Parallel solver racing

**11.2.2: Solution Coordination**
- [X] Merge solutions from multiple solvers
- [X] Validate solutions across solvers
- [X] Handle solver disagreements
- [X] Performance comparison and reporting

**Estimated Time**: 3-4 weeks
**Priority**: LOW

---

## 🎯 **PHASE 5: Domain-Specific Extensions**

### **Step 12.1: Scheduling Solver Extensions**
**Goal**: Specialized support for scheduling problems

**12.1.1: Job Shop Scheduling**
- [X] Resource allocation with precedence
- [X] Machine assignment optimization
- [X] Makespan minimization
- [X] Critical path analysis

**12.1.2: Employee Scheduling**
- [X] Shift assignment with preferences
- [X] Skill-based assignment
- [X] Work regulations compliance
- [X] Fairness and balance constraints

**Estimated Time**: 4-5 weeks
**Priority**: LOW

### **Step 12.2: Packing & Routing Extensions**
**Goal**: Specialized solvers for packing and routing problems

**12.2.1: Bin Packing Variants**
- [ ] 2D/3D bin packing
- [ ] Knapsack problem variants
- [ ] Cutting stock problems
- [ ] Container loading optimization

**12.2.2: Vehicle Routing**
- [ ] Basic VRP solver
- [ ] Time windows constraints
- [ ] Capacity constraints
- [ ] Multiple depot support

**Estimated Time**: 5-6 weeks
**Priority**: LOW

---

## 📊 **Implementation Priority Ranking**

### **IMMEDIATE (Phase 1 - Weeks 1-6)**
1. **Step 8.1**: Error Handling & Recovery
2. **Step 8.3**: API Stabilization

### **SHORT TERM (Phase 2 - Weeks 7-12)**
4. **Step 9.1**: Essential Missing Constraints

### **MEDIUM TERM (Weeks 13-20)**
6. **Step 10.1**: Non-linear Constraints
7. **Step 10.3**: Objective Expression Analysis & AST
8. **Step 11.1**: External Solver Integration (basic)

### **LONG TERM (Weeks 21+)**
9. **Step 10.2**: Global Constraint Optimization
10. **Step 11.2**: Solver Coordination
11. **Step 12.1-12.2**: Domain-Specific Extensions

---

## 🎯 **Success Metrics**

### **Production Readiness**
- [ ] Zero panics in production code
- [ ] 99.9% API stability between minor versions
- [ ] Comprehensive error recovery

### **Constraint Coverage**
- [ ] 25+ constraint types implemented
- [ ] Support for 90% of common CSP problems
- [ ] Optimization support for 70% of constraints

### **Integration Success**
- [ ] 3+ external solver integrations
- [ ] Automatic solver selection accuracy >90%
- [ ] Performance within 10% of specialized solvers

---

## 🔄 **Review & Decision Points**

Please review this plan and let me know:

1. **Priority adjustments**: Which phases/steps should be prioritized?
2. **Scope modifications**: Any features to add/remove/modify?
3. **Timeline considerations**: Any deadline constraints?
4. **Resource allocation**: Focus areas for immediate development?

This plan provides a roadmap for transforming the hybrid CSP solver into a production-ready, feature-complete constraint solving system.
