<!-- START doctoc generated TOC please keep comment here to allow auto update -->
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**Table of Contents**

- [Week 3: Prevention Strategy - Design for Lean Six Sigma (DfLSS)](#week-3-prevention-strategy---design-for-lean-six-sigma-dflss)
  - [1. Compile-Time Guarantees System](#1-compile-time-guarantees-system)
    - [Problem Statement](#problem-statement)
    - [Prevention Solution: PhantomData State Machines](#prevention-solution-phantomdata-state-machines)
      - [Pattern: Registry State Machine](#pattern-registry-state-machine)
  - [2. Architectural Integration Contracts](#2-architectural-integration-contracts)
    - [Problem Statement](#problem-statement-1)
    - [Prevention Solution: Trait-Based Contracts](#prevention-solution-trait-based-contracts)
      - [Contract: TemplateProvider Trait](#contract-templateprovider-trait)
      - [Implementation Contract Tests](#implementation-contract-tests)
  - [3. Error Propagation Strategy](#3-error-propagation-strategy)
    - [Problem Statement](#problem-statement-2)
    - [Prevention Solution: Comprehensive Error Taxonomy](#prevention-solution-comprehensive-error-taxonomy)
      - [Error Hierarchy Design](#error-hierarchy-design)
      - [Error Context Enhancement](#error-context-enhancement)
  - [4. DfLSS Design Review Process](#4-dflss-design-review-process)
    - [Problem Statement](#problem-statement-3)
    - [Prevention Solution: Design Phase Review Checklist](#prevention-solution-design-phase-review-checklist)
      - [Design Review Checklist Template](#design-review-checklist-template)
  - [4. Integration Contracts](#4-integration-contracts)
    - [Is the integration contract clear?](#is-the-integration-contract-clear)
    - [Contract Definition](#contract-definition)
  - [5. Error Visibility](#5-error-visibility)
    - [Are errors visible and propagated?](#are-errors-visible-and-propagated)
    - [Error Taxonomy](#error-taxonomy)
  - [6. Prevention Verification](#6-prevention-verification)
    - [Can we prevent invalid states?](#can-we-prevent-invalid-states)
  - [7. Test Coverage](#7-test-coverage)
    - [Have we tested integration?](#have-we-tested-integration)
  - [8. DfLSS Principles](#8-dflss-principles)
    - [Does this follow DfLSS principles?](#does-this-follow-dflss-principles)
  - [Decision Matrix](#decision-matrix)
  - [Approval](#approval)
  - [Implementation Timeline](#implementation-timeline)
    - [Week 3 Schedule](#week-3-schedule)
  - [Success Criteria](#success-criteria)
  - [Next Steps](#next-steps)

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# Week 3: Prevention Strategy - Design for Lean Six Sigma (DfLSS)

**MISSION**: Design systems to prevent waste and defects before they happen (at design phase).

**PHILOSOPHY**: Prevention is 10x cheaper than detection, 100x cheaper than correction.

---

## 1. Compile-Time Guarantees System

### Problem Statement
- **Current State**: Runtime validation allows invalid states to reach production
- **Risk**: Type errors, invalid sequences, undefined behavior
- **Cost**: Runtime failures, debugging time, customer impact

### Prevention Solution: PhantomData State Machines

**Principle**: Make invalid states unrepresentable through the type system.

#### Pattern: Registry State Machine

```rust
use std::marker::PhantomData;

// Type-level states
pub struct Uninitialized;
pub struct Initialized;
pub struct Validated;

// Registry with type-level state tracking
pub struct Registry<State = Uninitialized> {
    templates: Vec<Template>,
    _state: PhantomData<State>,
}

// Only Uninitialized registry can be created
impl Registry<Uninitialized> {
    pub fn new() -> Self {
        Registry {
            templates: Vec::new(),
            _state: PhantomData,
        }
    }

    // Transition: Uninitialized → Initialized
    pub fn initialize(self, path: &Path) -> Result<Registry<Initialized>, GgenError> {
        let templates = discover_templates(path)?;
        Ok(Registry {
            templates,
            _state: PhantomData,
        })
    }
}

// Only Initialized registry can be validated
impl Registry<Initialized> {
    // Transition: Initialized → Validated
    pub fn validate(self) -> Result<Registry<Validated>, GgenError> {
        for template in &self.templates {
            validate_template(template)?;
        }
        Ok(Registry {
            templates: self.templates,
            _state: PhantomData,
        })
    }
}

// Only Validated registry can be used for rendering
impl Registry<Validated> {
    pub fn search(&self, query: &str) -> Result<Vec<&Template>, GgenError> {
        // Implementation
    }

    pub fn render(&self, template: &Template, context: Context) -> Result<String, GgenError> {
        // Implementation
    }
}
```

**Compiler Enforcement**:
```rust
// ✅ VALID: Proper state transitions
let registry = Registry::new()
    .initialize(&path)?
    .validate()?;
registry.search("pattern")?;

// ❌ INVALID: Compiler prevents this
let registry = Registry::new();
registry.search("pattern")?;  // ERROR: No method `search` for Registry<Uninitialized>

// ❌ INVALID: Cannot skip validation
let registry = Registry::new().initialize(&path)?;
registry.render(&template, context)?;  // ERROR: No method `render` for Registry<Initialized>
```

**Benefits**:
- **Zero Runtime Cost**: PhantomData is zero-sized
- **Compile-Time Enforcement**: Invalid sequences cannot compile
- **Self-Documenting**: Type signature shows required state
- **Refactoring Safety**: Breaking changes cause compile errors

---

## 2. Architectural Integration Contracts

### Problem Statement
- **Current State**: Template system and CLI system have implicit coupling
- **Risk**: Breaking changes, version skew, integration failures
- **Cost**: Runtime errors, maintenance burden, testing complexity

### Prevention Solution: Trait-Based Contracts

**Principle**: Explicit contracts enforced by the compiler.

#### Contract: TemplateProvider Trait

```rust
use std::path::Path;

/// Contract for any system that provides templates to the CLI
pub trait TemplateProvider: Send + Sync {
    /// Discover all templates in the given path
    fn discover(&self, path: &Path) -> Result<Vec<Template>, GgenError>;

    /// Validate a template's structure and syntax
    fn validate(&self, template: &Template) -> Result<(), GgenError>;

    /// Render a template with the given context
    fn render(&self, template: &Template, context: Context) -> Result<String, GgenError>;

    /// Get template metadata
    fn metadata(&self, template: &Template) -> Result<TemplateMetadata, GgenError>;
}

/// Contract for CLI bridge implementations
pub trait CliBridge: Send + Sync {
    /// Parse CLI arguments into structured command
    fn parse(&self, args: Vec<String>) -> Result<Command, GgenError>;

    /// Execute command with template provider
    fn execute<P: TemplateProvider>(&self, cmd: Command, provider: &P) -> Result<Output, GgenError>;

    /// Format output for display
    fn format(&self, output: Output) -> String;
}
```

#### Implementation Contract Tests

```rust
#[cfg(test)]
mod contract_tests {
    use super::*;

    /// Test that ALL TemplateProvider implementations satisfy the contract
    fn test_template_provider_contract<T: TemplateProvider>(provider: T) {
        // Contract requirement: discover must return valid templates
        let templates = provider.discover(Path::new("templates"))
            .expect("discover must succeed for valid path");

        // Contract requirement: all discovered templates must be valid
        for template in &templates {
            provider.validate(template)
                .expect("all discovered templates must be valid");
        }

        // Contract requirement: render must succeed for valid template
        if let Some(template) = templates.first() {
            let context = Context::default();
            provider.render(template, context)
                .expect("render must succeed for valid template and context");
        }
    }

    #[test]
    fn filesystem_provider_satisfies_contract() {
        test_template_provider_contract(FilesystemTemplateProvider::new());
    }

    #[test]
    fn embedded_provider_satisfies_contract() {
        test_template_provider_contract(EmbeddedTemplateProvider::new());
    }
}
```

**Benefits**:
- **Compile-Time Contract Enforcement**: Breaking changes cause compile errors
- **Multiple Implementations**: Easy to swap providers
- **Testable Contracts**: Generic tests ensure all implementations behave correctly
- **Version Safety**: Trait version changes are explicit

---

## 3. Error Propagation Strategy

### Problem Statement
- **Current State**: Errors can be silently ignored or lost
- **Risk**: Silent failures, missing diagnostics, debugging difficulty
- **Cost**: Production incidents, customer impact, debugging time

### Prevention Solution: Comprehensive Error Taxonomy

**Principle**: Every error must be visible, propagated, and actionable.

#### Error Hierarchy Design

```rust
use thiserror::Error;

/// Top-level error type for ggen
#[derive(Error, Debug)]
pub enum GgenError {
    // === Template Errors ===
    #[error("Template not found: {path}")]
    TemplateNotFound {
        path: String,
        context: String,
    },

    #[error("Invalid template syntax in {file}:{line}: {reason}")]
    InvalidTemplateSyntax {
        file: String,
        line: usize,
        reason: String,
        snippet: String,
    },

    #[error("Template validation failed: {0}")]
    TemplateValidationFailed(#[from] ValidationError),

    // === CLI Errors ===
    #[error("Invalid command: {command}")]
    InvalidCommand {
        command: String,
        suggestion: Option<String>,
    },

    #[error("Missing required argument: {arg}")]
    MissingArgument {
        arg: String,
        usage: String,
    },

    // === Integration Errors ===
    #[error("API incompatibility: {message}")]
    ApiIncompatible {
        message: String,
        expected_version: String,
        actual_version: String,
    },

    #[error("Contract violation: {contract} failed: {reason}")]
    ContractViolation {
        contract: String,
        reason: String,
        fix_suggestion: String,
    },

    // === System Errors ===
    #[error("IO error: {0}")]
    Io(#[from] std::io::Error),

    #[error("Configuration error: {0}")]
    Config(#[from] ConfigError),

    // === Root Cause Tracking ===
    #[error("Multiple errors occurred")]
    Multiple {
        errors: Vec<GgenError>,
        primary: Box<GgenError>,
    },
}

/// Validation-specific errors with detailed diagnostics
#[derive(Error, Debug)]
pub enum ValidationError {
    #[error("Missing required field: {field}")]
    MissingField {
        field: String,
        location: String,
    },

    #[error("Type mismatch: expected {expected}, got {actual}")]
    TypeMismatch {
        expected: String,
        actual: String,
        location: String,
    },

    #[error("Constraint violation: {constraint}")]
    ConstraintViolation {
        constraint: String,
        value: String,
        allowed_values: Vec<String>,
    },
}

/// Result type alias for ggen operations
pub type Result<T> = std::result::Result<T, GgenError>;
```

#### Error Context Enhancement

```rust
use std::backtrace::Backtrace;

/// Extension trait for adding context to errors
pub trait ErrorContext<T> {
    fn context(self, msg: impl Into<String>) -> Result<T>;
    fn with_context<F>(self, f: F) -> Result<T>
    where
        F: FnOnce() -> String;
}

impl<T, E: Into<GgenError>> ErrorContext<T> for std::result::Result<T, E> {
    fn context(self, msg: impl Into<String>) -> Result<T> {
        self.map_err(|e| {
            let error: GgenError = e.into();
            // Enhance error with context
            error
        })
    }

    fn with_context<F>(self, f: F) -> Result<T>
    where
        F: FnOnce() -> String,
    {
        self.map_err(|e| {
            let error: GgenError = e.into();
            // Enhance error with lazy context
            error
        })
    }
}

// Usage example
fn load_template(path: &Path) -> Result<Template> {
    std::fs::read_to_string(path)
        .context(format!("Failed to read template at {}", path.display()))?
        .parse()
        .with_context(|| format!("Failed to parse template at {}", path.display()))
}
```

**Benefits**:
- **No Silent Failures**: Result<T, E> forces error handling
- **Rich Diagnostics**: Every error has context and suggestions
- **Compile-Time Enforcement**: Compiler prevents ignored errors
- **Actionable Messages**: Errors guide users to solutions

---

## 4. DfLSS Design Review Process

### Problem Statement
- **Current State**: Defects discovered during testing or production
- **Risk**: Late detection, expensive fixes, customer impact
- **Cost**: Rework, debugging, hotfixes, reputation damage

### Prevention Solution: Design Phase Review Checklist

**Principle**: Find and prevent defects at design phase (10x cheaper than testing, 100x cheaper than production).

#### Design Review Checklist Template

```markdown
# DfLSS Design Review Checklist

**Feature**: [Feature name]
**Designer**: [Name]
**Reviewer**: [Name]
**Date**: [Date]

## 1. Failure Mode Analysis (FMEA)

### Have we identified all failure modes?
- [ ] Input validation failures
- [ ] State transition errors
- [ ] Resource exhaustion
- [ ] Concurrency issues
- [ ] Integration failures
- [ ] Error propagation gaps

### Failure Mode Table
| Failure Mode | Severity (1-10) | Probability (1-10) | Detection (1-10) | RPN | Mitigation |
|--------------|-----------------|-------------------|------------------|-----|------------|
| [Mode 1]     | [S]             | [P]               | [D]              | [S×P×D] | [Action] |

**Critical Failures (RPN > 100)**: [List and require mitigation]

## 2. Contradiction Resolution (TRIZ)

### Have we resolved contradictions?
- [ ] Performance vs. Safety trade-offs addressed
- [ ] Flexibility vs. Type Safety balanced
- [ ] Ergonomics vs. Explicitness resolved
- [ ] No "we'll fix it later" deferred decisions

### Contradiction Matrix
| Improving Parameter | Worsening Parameter | Resolution Strategy |
|---------------------|---------------------|---------------------|
| [Param 1]           | [Param 2]           | [Strategy]          |

## 3. Compile-Time Guarantees

### Do we have compile-time guarantees?
- [ ] Invalid states made unrepresentable (PhantomData)
- [ ] State transitions enforced by type system
- [ ] No runtime validation that could be compile-time
- [ ] API usage errors caught by compiler

### Type-Level Enforcement
```rust
// Example: Show type-level state machine
```

## 4. Integration Contracts

### Is the integration contract clear?
- [ ] Traits define explicit contracts
- [ ] All implementations tested against contract
- [ ] Version compatibility explicit
- [ ] Breaking changes cause compile errors

### Contract Definition
```rust
// Example: Show trait definition
```

## 5. Error Visibility

### Are errors visible and propagated?
- [ ] All functions return Result<T, E>
- [ ] Error types have rich context
- [ ] No silent failures possible
- [ ] Error messages actionable

### Error Taxonomy
```rust
// Example: Show error enum
```

## 6. Prevention Verification

### Can we prevent invalid states?
- [ ] State machine prevents invalid sequences
- [ ] Types encode invariants
- [ ] Compiler prevents misuse
- [ ] Runtime validation minimized

## 7. Test Coverage

### Have we tested integration?
- [ ] Unit tests for components
- [ ] Integration tests for contracts
- [ ] Property tests for invariants
- [ ] Performance tests for SLOs

## 8. DfLSS Principles

### Does this follow DfLSS principles?
- [ ] Prevention over detection
- [ ] Design for quality
- [ ] Waste elimination built-in
- [ ] Continuous improvement path clear

## Decision Matrix

| Criterion | Weight | Score (1-10) | Weighted |
|-----------|--------|--------------|----------|
| Defect Prevention | 30% | [Score] | [W×S] |
| Compile-Time Safety | 25% | [Score] | [W×S] |
| Integration Clarity | 20% | [Score] | [W×S] |
| Error Visibility | 15% | [Score] | [W×S] |
| Test Coverage | 10% | [Score] | [W×S] |
| **TOTAL** | 100% | - | **[Sum]** |

**Minimum Passing Score**: 7.0/10

## Approval

- [ ] All critical failures mitigated (RPN < 100)
- [ ] All contradictions resolved
- [ ] Compile-time guarantees implemented
- [ ] Integration contracts defined
- [ ] Error propagation complete
- [ ] Tests planned
- [ ] DfLSS principles followed
- [ ] Score ≥ 7.0/10

**Reviewer Signature**: _______________  **Date**: _______________

**Approved for Implementation**: [ ] Yes [ ] No (see comments)

**Comments**:
```

#### Design Review Process Flow

```
┌─────────────────────────────────────────────────────────────┐
│                    DESIGN REVIEW PROCESS                     │
└─────────────────────────────────────────────────────────────┘

1. DESIGN PHASE (Before Coding)
   ├─ Complete design review checklist
   ├─ FMEA analysis (identify all failure modes)
   ├─ TRIZ contradiction resolution
   ├─ Type-level design (PhantomData state machines)
   ├─ Contract definition (traits)
   └─ Error taxonomy design

2. REVIEW MEETING
   ├─ Present design to reviewer
   ├─ Walk through FMEA table
   ├─ Discuss type-level guarantees
   ├─ Review integration contracts
   └─ Assess against DfLSS principles

3. DECISION
   ├─ Score ≥ 7.0/10 → APPROVE
   ├─ Score < 7.0/10 → REVISE
   └─ Critical RPN > 100 → BLOCK (must mitigate)

4. IMPLEMENTATION (After Approval)
   ├─ Code following approved design
   ├─ Tests verify design assumptions
   ├─ CI enforces design constraints
   └─ Post-implementation review confirms design

5. LESSONS LEARNED
   ├─ Document design decisions
   ├─ Update checklist with new patterns
   ├─ Train team on new techniques
   └─ Continuous improvement of process
```

**Benefits**:
- **Early Detection**: Find defects at design phase (10x cheaper)
- **Systematic Prevention**: Checklist ensures nothing missed
- **Measurable Quality**: Score provides objective assessment
- **Knowledge Transfer**: Checklist embeds best practices

---

## 5. Kaizen Continuous Improvement Cycle

### Problem Statement
- **Current State**: Same issues recur, no systematic learning
- **Risk**: Repeated failures, process stagnation, team frustration
- **Cost**: Wasted effort, missed improvements, competitive disadvantage

### Prevention Solution: Monthly Kaizen + Quarterly Reviews

**Principle**: Continuous, incremental improvement driven by data and team insights.

#### Monthly Kaizen Cycle (4-Week Cadence)

```
┌─────────────────────────────────────────────────────────────┐
│                    MONTHLY KAIZEN CYCLE                      │
└─────────────────────────────────────────────────────────────┘

WEEK 1: IDENTIFY PROBLEMS (Andon Signals)
├─ Collect Andon signal data
│  ├─ Compiler errors count
│  ├─ Test failures count
│  ├─ Linting warnings count
│  └─ SLO violations count
├─ Team retrospective
│  ├─ What blocked us?
│  ├─ What slowed us down?
│  ├─ What caused rework?
│  └─ What frustrated us?
└─ Prioritize top 3 problems (Pareto 80/20)

WEEK 2: ANALYZE ROOT CAUSES (5 Whys)
├─ For each top problem:
│  ├─ Why did this happen? (Level 1)
│  ├─ Why did that happen? (Level 2)
│  ├─ Why did that happen? (Level 3)
│  ├─ Why did that happen? (Level 4)
│  └─ Why did that happen? (Level 5 - Root Cause)
├─ Validate root cause with data
└─ Design countermeasures

WEEK 3: IMPLEMENT COUNTERMEASURES (Fixes)
├─ Implement prevention systems
├─ Update design review checklist
├─ Add new type-level guarantees
├─ Enhance error messages
└─ Document new patterns

WEEK 4: VERIFY IMPROVEMENTS (Metrics)
├─ Measure impact
│  ├─ Did errors decrease?
│  ├─ Did cycle time improve?
│  ├─ Did rework reduce?
│  └─ Did team satisfaction increase?
├─ Adjust countermeasures if needed
└─ Standardize successful improvements
```

#### Quarterly Strategic Reviews

```
┌─────────────────────────────────────────────────────────────┐
│                  QUARTERLY STRATEGIC REVIEW                  │
└─────────────────────────────────────────────────────────────┘

QUARTER REVIEW (Every 3 Months)

1. METRICS TREND ANALYSIS
   ├─ 3-month trend charts
   │  ├─ Defect density (defects per KLOC)
   │  ├─ Cycle time (idea to production)
   │  ├─ Rework percentage
   │  ├─ Test pass rate
   │  └─ SLO compliance
   ├─ Identify patterns
   └─ Celebrate wins

2. SYSTEMIC PATTERN IDENTIFICATION
   ├─ What problems recur?
   ├─ What root causes appear multiple times?
   ├─ What process gaps exist?
   └─ What training gaps exist?

3. DfLSS PROCESS UPDATE
   ├─ Update design review checklist
   ├─ Add new prevention patterns
   ├─ Enhance error taxonomy
   ├─ Improve type-level guarantees
   └─ Document new best practices

4. TEAM TRAINING
   ├─ Share lessons learned
   ├─ Teach new techniques
   ├─ Practice new patterns
   └─ Update onboarding materials

5. STRATEGIC GOALS
   ├─ Set next quarter objectives
   ├─ Define success metrics
   ├─ Assign champions
   └─ Schedule reviews
```

#### Kaizen Metrics Dashboard

```rust
/// Metrics tracked for continuous improvement
#[derive(Debug, Clone)]
pub struct KaizenMetrics {
    // Defect Metrics
    pub defect_density: f64,        // defects per 1000 lines of code
    pub defects_found_in_design: usize,
    pub defects_found_in_testing: usize,
    pub defects_found_in_production: usize,

    // Cycle Time Metrics
    pub design_to_code_hours: f64,
    pub code_to_test_hours: f64,
    pub test_to_deploy_hours: f64,
    pub total_cycle_time_hours: f64,

    // Rework Metrics
    pub rework_percentage: f64,
    pub compiler_error_fixes: usize,
    pub test_failure_fixes: usize,
    pub design_changes: usize,

    // Quality Metrics
    pub test_pass_rate: f64,
    pub slo_compliance_rate: f64,
    pub code_review_approval_rate: f64,

    // Team Metrics
    pub team_satisfaction_score: f64,  // 1-10 scale
    pub learning_hours: f64,
    pub kaizen_improvements_implemented: usize,
}

impl KaizenMetrics {
    /// Calculate improvement from baseline
    pub fn improvement_from(&self, baseline: &KaizenMetrics) -> KaizenImprovement {
        KaizenImprovement {
            defect_reduction: (baseline.defect_density - self.defect_density) / baseline.defect_density * 100.0,
            cycle_time_reduction: (baseline.total_cycle_time_hours - self.total_cycle_time_hours) / baseline.total_cycle_time_hours * 100.0,
            rework_reduction: (baseline.rework_percentage - self.rework_percentage),
            quality_improvement: (self.test_pass_rate - baseline.test_pass_rate) * 100.0,
        }
    }
}

#[derive(Debug, Clone)]
pub struct KaizenImprovement {
    pub defect_reduction: f64,      // percentage reduction
    pub cycle_time_reduction: f64,  // percentage reduction
    pub rework_reduction: f64,      // percentage points reduction
    pub quality_improvement: f64,   // percentage points improvement
}
```

**Benefits**:
- **Data-Driven**: Decisions based on metrics, not opinions
- **Systematic**: Regular cadence ensures continuous focus
- **Team-Driven**: Everyone participates in improvement
- **Measurable**: Clear metrics show impact

---

## Implementation Timeline

### Week 3 Schedule

**Monday-Tuesday**: Design Systems
- PhantomData state machines design
- Trait-based contracts design
- Error taxonomy design

**Wednesday-Thursday**: Process Creation
- DfLSS design review checklist
- Kaizen cycle definition
- Metrics dashboard design

**Friday**: Documentation & Training
- Prevention strategy guide
- Team training materials
- Implementation plan

---

## Success Criteria

**Week 3 Goals**:
1. ✅ All 5 prevention systems designed and documented
2. ✅ DfLSS process embedded in workflow
3. ✅ Kaizen cycle operational with first retrospective
4. ✅ Team trained and ready to use prevention systems
5. ✅ Foundation laid for zero-defect future
6. ✅ Metrics tracking in place

**Measurable Outcomes**:
- Design review checklist complete (8+ sections)
- Prevention patterns documented (5+ patterns)
- Kaizen cycle defined (4-week + quarterly)
- Metrics dashboard implemented
- Team training completed (100% participation)

---

## Next Steps

**After Week 3**:
1. **Week 4**: Implementation of prevention systems
2. **Month 2**: First full Kaizen cycle
3. **Quarter 2**: First quarterly review
4. **Year 1**: Achieve Six Sigma quality (3.4 defects per million)

---

**Remember**: Prevention is 10x cheaper than detection, 100x cheaper than correction. Week 3 is investment in prevention that pays dividends forever.
