cc-routine-and-class-design

# Skill: cc-routine-and-class-design

## STOP - Crisis Invariants (NEVER SKIP)

| Check | Time | Why Non-Negotiable |
|-------|------|-------------------|
| **LSP Test** - "Is 'A is a B' literally true?" | 30 sec | Wrong inheritance creates debugging hell |
| **Containment Default** - If LSP feels wrong, use containment | 30 sec | Containment is fixable; inheritance requires architecture changes |
| **Parameter Count** - If >7 parameters, interface is wrong | 15 sec | High parameter count predicts interface errors |

---

## Prerequisites
This skill assumes:
- Object-oriented programming with class-based inheritance
- Mutable state (objects that change over time)
- Synchronous or simple async patterns

For functional programming, prototype-based inheritance, or heavily concurrent code, adapt principles rather than applying literally. See "Pattern-Specific Guidance" section.

## When NOT to Use

- **Scripting/automation code** - One-off scripts don't benefit from class design rigor
- **Prototyping phase** - When exploring ideas before committing to design (time-box to max 1 week)
- **Simple data transfer objects** - Pure DTOs without behavior are exempt from ADT requirements (but DTOs with validation, toString, equals are NOT exempt)
- **Framework-mandated patterns** - When framework REQUIRES inheritance to function (e.g., Android Activity won't work without extending Activity). "Framework supports inheritance" or "framework examples use inheritance" is NOT mandated
- **Performance-critical inner loops** - Where accessor overhead matters (measure first - must show profiling data)
- **Test doubles (mocks, stubs, fakes)** - Test code intentionally violates design rules; empty overrides and minimal abstractions are appropriate

## Crisis Invariants - NEVER SKIP

**These checks are NON-NEGOTIABLE regardless of deadline pressure:**

| Check | Time | Why Non-Negotiable |
|-------|------|-------------------|
| **LSP Test** - "Is 'A is a B' literally true?" | 30 sec | Wrong inheritance creates debugging hell that costs MORE time than it saves |
| **Containment Default** - If LSP feels like "purity theater," use containment | 30 sec | Containment problems are fixable; inheritance problems require architecture changes |
| **Parameter Count** - If >7 parameters, interface is wrong | 15 sec | High parameter count predicts interface errors |

**Why these three?** Violations create problems that CANNOT be easily fixed post-crisis. They require architectural changes, not patches.

**Design fixes deferred are rarely completed.** Technical debt from wrong inheritance is architectural, not patchable.

## Modes

### CHECKER
Purpose: Execute design checklists against routines and classes
Triggers:
  - "review my class design"
  - "check routine quality"
  - "audit class interfaces"
  - "evaluate cohesion"
Non-Triggers:
  - "how should I design this class" -> APPLIER
  - "should I use inheritance here" -> APPLIER
  - "refactor this routine" -> cc-refactoring-guidance
Checklist: **See [checklists.md]($CLAUDE_PLUGIN_ROOT/skills/cc-routine-and-class-design/checklists.md)**
Output Format:
  | Item | Status | Evidence | Location |
  |------|--------|----------|----------|

**Severity Classification Rubric:**

| Severity | Criteria | Example |
|----------|----------|---------|
| VIOLATION | Explicitly fails checklist item | 12 parameters (> 7 limit) |
| VIOLATION | Breaks LSP/encapsulation | Empty override, protected base data |
| WARNING | Near limit, needs justification | 8 parameters, 3-level inheritance |
| WARNING | Subjective concern | Questionable abstraction consistency |
| PASS | Meets or exceeds requirement | Clear cohesion, ≤7 parameters |

**Note:** Test suite status is IRRELEVANT to checker results. A class can pass all tests and still have VIOLATION-level design issues that predict future maintenance problems.

### APPLIER
Purpose: Guide class interface design, inheritance decisions, and routine creation
Triggers:
  - "how should I design this class"
  - "should I use inheritance or containment"
  - "what cohesion type is this routine"
  - "how many parameters is too many"
  - "should I extract this to a routine"
Non-Triggers:
  - "review my class" -> CHECKER
  - "check this routine" -> CHECKER
  - "optimize this code" -> performance-optimization
Produces: Class interface designs, inheritance/containment decisions, routine signatures, cohesion classifications
Constraints:
  - [p.133] Default to containment; only inherit if "is-a" is literally true (LSP)
  - [p.143] Inheritance depth: target <3 levels, WARNING at 3, VIOLATION at 4+, SEVERE at 6+
  - [p.178] Parameter limit: 7 maximum (8+ is VIOLATION requiring redesign)
  - [p.168] Target functional cohesion for routines
  - [p.139] Asking "What should this class hide?" drives good design

### Parameter Guidelines

**Threshold Table:**

| Count | Status | Action |
|-------|--------|--------|
| 1-5 | PASS | No action needed |
| 6-7 | PASS | Minor concern, document if unusual |
| 8-9 | WARNING | Justify in code review OR redesign |
| 10+ | VIOLATION | Must redesign - use Parameter Object or split responsibilities |

**"About 7" means:** 7 is the limit. 8 is over. Count ALL parameters including optional ones with defaults. Variadic args (`*args`/`...`) count as 1.

**Ordering convention** (implies data flow sequence - ORDER MATTERS):
1. Input-only parameters first
2. Input-and-output (modify) parameters second
3. Output-only parameters third

## Key Definitions

### LSP (Liskov Substitution Principle)

If A inherits from B, then everywhere code uses B, you can substitute A without breaking anything.

**Test:** Does calling *every* method on the base class make sense for the derived class?

**Inheritance requires BOTH:**
1. **Semantic test**: "A is a B" makes English sense to domain experts
   - YES: "Dog is an Animal", "Manager is an Employee"
   - NO: "EmployeeCensus is a ListContainer", "UserSession is a Logger"
2. **LSP test**: Every method of B works correctly when A is substituted
   - NO empty overrides (override to do nothing)
   - NO exceptions that base class doesn't throw
   - NO precondition strengthening

If EITHER fails: use containment instead.

### Functional Cohesion

Routine performs one and only one operation.

**Name Test:** If you need "and" or "then" in the name to describe what the routine does, it has multiple operations.
- PASS: `ValidateUserInput()`, `CalculateTotalPrice()`, `SendWelcomeEmail()`
- FAIL: `ValidateAndSaveUser()`, `ReadFileThenParseJSON()`, `InitializeAndConnect()`

**Scope:** "One operation" means one action at the routine's declared abstraction level.
- `CreateUser()` is ONE operation (user creation) even though it involves validation, hashing, insertion
- Those sub-steps are at a LOWER abstraction level

## Inheritance vs Containment Decision

**Question order rationale:** Data-only sharing → containment (simplest). Behavior-only sharing → interface/protocol. Both data and behavior → requires full LSP verification before inheriting.

```dot
digraph inheritance_decision {
    rankdir=TB;

    START [label="How should A relate to B?" shape=doublecircle];

    q1 [label="Shares ONLY data?" shape=diamond];
    q2 [label="Shares ONLY behavior\n(method signatures)?" shape=diamond];
    q3 [label="Shares BOTH data\nand behavior?" shape=diamond];
    q4 [label="Is 'A is a B' literally true?\n(LSP: A substitutes for B\neverywhere)" shape=diamond];

    contain1 [label="CONTAIN\ncommon object" shape=box];
    interface1 [label="USE INTERFACE/PROTOCOL\n(not inheritance)" shape=box];
    inherit2 [label="INHERIT" shape=box];
    contain2 [label="CONTAIN" shape=box];
    none [label="NO RELATION\n(reconsider if relationship needed)" shape=box];

    START -> q1;
    q1 -> contain1 [label="yes"];
    q1 -> q2 [label="no"];
    q2 -> interface1 [label="yes"];
    q2 -> q3 [label="no"];
    q3 -> q4 [label="yes"];
    q3 -> none [label="no"];
    q4 -> inherit2 [label="yes"];
    q4 -> contain2 [label="no"];
}
```

**FINAL CHECK (apply AFTER flowchart determines INHERIT, BEFORE committing to inheritance):**
- Depth < 3 levels (definitely < 6)?
- No empty overrides needed?
- All base data private (not protected)?

If ANY answer is NO → use CONTAIN instead.

## Cohesion Classification

**Classification algorithm:** Questions test for highest-quality cohesion first. Stop at first YES answer - that's your classification. This prevents under-classifying (e.g., calling functional cohesion "sequential").

**Target: Functional cohesion** - routine performs one and only one operation.

```dot
digraph cohesion_classification {
    rankdir=TB;

    START [label="Classify routine cohesion" shape=doublecircle];

    q1 [label="Single operation?\n(Name Test passes)" shape=diamond];
    q2 [label="Ordered operations\nsharing data step-to-step?" shape=diamond];
    q3 [label="Operations share\ndata only?" shape=diamond];
    q4 [label="Same-time operations\n(startup/shutdown)?" shape=diamond];
    q5 [label="Orchestrates calls\n(not direct work)?" shape=diamond];
    q6 [label="Order from external\nrequirement (UI)?" shape=diamond];
    q7 [label="Control flag selects\noperation?" shape=diamond];

    functional [label="FUNCTIONAL\n[ACCEPT]" shape=box style=filled fillcolor=lightgreen];
    sequential [label="SEQUENTIAL\n[ACCEPT w/caution]" shape=box style=filled fillcolor=lightyellow];
    communicational [label="COMMUNICATIONAL\n[ACCEPT w/caution]" shape=box style=filled fillcolor=lightyellow];
    temporal_ok [label="TEMPORAL\n[ACCEPT]" shape=box style=filled fillcolor=lightgreen];
    temporal_fix [label="TEMPORAL\n[FIX: orchestrate]" shape=box style=filled fillcolor=orange];
    procedural [label="PROCEDURAL\n[REJECT]" shape=box style=filled fillcolor=lightcoral];
    logical [label="LOGICAL\n[REJECT]" shape=box style=filled fillcolor=lightcoral];
    coincidental [label="COINCIDENTAL\n[REDESIGN]" shape=box style=filled fillcolor=red fontcolor=white];

    START -> q1;
    q1 -> functional [label="yes"];
    q1 -> q2 [label="no"];
    q2 -> sequential [label="yes"];
    q2 -> q3 [label="no"];
    q3 -> communicational [label="yes"];
    q3 -> q4 [label="no"];
    q4 -> q5 [label="yes"];
    q4 -> q6 [label="no"];
    q5 -> temporal_ok [label="yes"];
    q5 -> temporal_fix [label="no"];
    q6 -> procedural [label="yes"];
    q6 -> q7 [label="no"];
    q7 -> logical [label="yes"];
    q7 -> coincidental [label="no"];
}
```

**Evidence:** 50% of highly cohesive routines fault-free vs 18% low cohesion [Card et al. 1986, N=450 routines]. This is MAINTENANCE data, not shipping data - the 50% vs 18% gap appears during modifications.

### Cohesion Types Reference

**Types listed from BEST (top) to WORST (bottom) quality - ORDER MATTERS:**

| Type | Definition | Status | Code Example |
|------|------------|--------|--------------|
| **Functional** | Routine performs one and only one operation | ACCEPT | `calculateTax(amount)` - returns tax |
| **Sequential** | Operations share data step-to-step in required order, but don't form complete function | ACCEPT w/caution | `parse(input) -> validate(parsed) -> transform(validated)` |
| **Communicational** | Operations use same data but aren't otherwise related | ACCEPT w/caution | `printReport(data); emailReport(data)` - both use reportData |
| **Temporal** | Operations combined because done at same time (startup, shutdown) | ACCEPT if orchestrates | `startup()` calls `initDB()`, `initCache()`, `initLogger()` |
| **Procedural** | Operations ordered by external requirement (UI flow), not logical relationship | REJECT | `showPage1() -> showPage2() -> showPage3()` - wizard steps |
| **Logical** | Control flag selects one of several unrelated operations in big if/case | REJECT | `process(mode)` with switch between Parse, Validate, Format |
| **Coincidental** | Operations have no discernible relationship ("chaotic cohesion") | REDESIGN | `doStuff()` logs, validates, emails, calculates tax |

**"ACCEPT w/caution" means:**
1. Document in code comment WHY this cohesion type is acceptable here
2. Review whether functional cohesion is achievable with reasonable effort
3. Add TODO if cohesion should improve in future refactoring

"Caution" is not permission to ignore - it's permission with accountability.

### Detecting Orchestration Pattern

If description uses these verbs, likely ORCHESTRATION (temporal cohesion OK):
- "orchestrates", "coordinates", "delegates", "dispatches", "routes"

If description uses these verbs, likely DIRECT WORK (check cohesion type):
- "handles", "processes", "performs", "executes", "calculates"

**Orchestration Example (GOOD):**
```python
def startup():
    load_config()       # calls another routine
    init_database()     # calls another routine
    start_server()      # calls another routine
    log_startup_complete()
```

**Direct Work Example (FIX by extracting):**
```python
def startup():
    config = json.load(open('config.json'))  # direct work - extract
    db = Database(config['db_url'])          # direct work - extract
    db.connect()                             # direct work - extract
    server.start(config['port'])             # acceptable call
```

### Improving Cohesion

| Type | Improvement Steps |
|------|-------------------|
| **Sequential** | 1) Split into separate routines per operation 2) Have dependent routine call what it depends on 3) Both achieve functional cohesion |
| **Communicational** | 1) Identify operations related only by same data 2) Split into individual routines 3) Reinitialize data close to creation 4) Call both from higher-level routine |
| **Temporal** | 1) Make routine an organizer, not doer 2) Call other routines to perform activities 3) Name at right abstraction level (e.g., `Startup()` not `ReadConfigInitScratchEtc()`) |
| **Logical** | 1) Create separate routine for each distinct operation 2) Move shared code/data to lower-level routine 3) Package routines into a class |

## Pattern-Specific Guidance

### Builder Pattern
Builders exhibit intentional method chaining (fluent interface). Evaluate cohesion at the class level, not individual methods.
- Methods returning `this` are acceptable
- Class cohesion = "constructs one type of object"
- Skill's routine-level rules don't apply to builder methods

### Mixin/Trait Patterns
Mixins add behavior without "is-a" claims. Evaluate differently:
- Does the mixin have a single, focused responsibility?
- Are there fewer than 3 mixins per class?
- Could containment achieve the same goal more explicitly?
- **Prefer containment** - mixins hide the relationship; containment makes it explicit

### Singleton Pattern
Apply extra scrutiny:
- Does global state truly need to be global?
- Is this hiding a dependency that should be explicit?
- Would dependency injection be cleaner?

### Event Handlers and Callbacks
Cohesion evaluation differs for event-driven patterns:
- **Handler cohesion:** Does it handle ONE event type with ONE response?
- **Callback cohesion:** Does the callback do ONE thing when triggered?
- **Async routines:** Evaluate the complete operation, not just what runs before `await`

## Minimum Viable Compliance (Time-Constrained)

When full checklist review is impractical, these 7 items are MANDATORY:

| # | Check | Time |
|---|-------|------|
| 1 | LSP test for any inheritance | 30 sec |
| 2 | Inheritance depth < 3 levels | 15 sec |
| 3 | No empty overrides | 30 sec |
| 4 | Parameter count ≤ 7 | 15 sec |
| 5 | Routine name describes everything it does | 30 sec |
| 6 | Class presents consistent abstraction level | 1 min |
| 7 | Implementation details hidden | 1 min |

**Total: ~4 minutes** - This is the floor, not the ceiling.

**Skipped items require:** Technical debt ticket with specific follow-up date.

## Evidence Summary

**Why These Rules Apply Even After Success:**

| Claim | Evidence | Limitation |
|-------|----------|------------|
| High cohesion = fewer faults | 50% fault-free vs 18% [Card et al. 1986] | N=450 routines, single study |
| Deep inheritance = more faults | Significantly associated [Basili 1996] | Correlation, not causation |
| Information hiding reduces faults | Factor of 4 reduction [Korson/Vaishnavi 1986] | Varies by context |
| High coupling = more errors | 7x errors, 20x fix cost [Selby 1991] | Coupling-to-cohesion ratio |
| Cognitive limit ~7 items | Miller 1956 | Original study was about memory chunks, application to parameters is heuristic |

**Critical insight:** This is MAINTENANCE data, not shipping data. All code "works" on day 1. The 50% vs 18% gap appears during modifications, extensions, and bug fixes. Code passing tests on first commit does not predict maintenance quality.


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## Chain

| After | Next |
|-------|------|
| Design verified | cc-defensive-programming (CHECKER) |