Code Deduplication

# Code Deduplication Skill


**Purpose:** Prevent semantic duplication and code bloat. Maintain a capability index so Claude always knows what exists before writing something new.

---

## Core Philosophy

```
┌─────────────────────────────────────────────────────────────────┐
│  CHECK BEFORE YOU WRITE                                         │
│  ─────────────────────────────────────────────────────────────  │
│  AI doesn't copy/paste - it reimplements.                       │
│  The problem isn't duplicate code, it's duplicate PURPOSE.      │
│                                                                 │
│  Before writing ANY new function:                               │
│  1. Check CODE_INDEX.md for existing capabilities               │
│  2. Search codebase for similar functionality                   │
│  3. Extend existing code if possible                            │
│  4. Only create new if nothing suitable exists                  │
├─────────────────────────────────────────────────────────────────┤
│  AFTER WRITING: Update the index immediately.                   │
│  PERIODICALLY: Run /audit-duplicates to catch overlap.          │
└─────────────────────────────────────────────────────────────────┘
```

---

## Code Index Structure

Maintain `CODE_INDEX.md` in project root, organized by **capability** not file location:

```markdown
# Code Index

*Last updated: [timestamp]*
*Run `/update-code-index` to regenerate*

## Quick Reference

| Category | Count | Location |
|----------|-------|----------|
| Date/Time | 5 functions | src/utils/dates.ts |
| Validation | 8 functions | src/utils/validate.ts |
| API Clients | 12 functions | src/api/*.ts |
| Auth | 6 functions | src/auth/*.ts |

---

## Date/Time Operations

| Function | Location | Does What | Params |
|----------|----------|-----------|--------|
| `formatDate()` | utils/dates.ts:15 | Formats Date → "Jan 15, 2024" | `(date: Date, format?: string)` |
| `formatRelative()` | utils/dates.ts:32 | Formats Date → "2 days ago" | `(date: Date)` |
| `parseDate()` | utils/dates.ts:48 | Parses string → Date | `(str: string, format?: string)` |
| `isExpired()` | auth/tokens.ts:22 | Checks if timestamp past now | `(timestamp: number)` |
| `addDays()` | utils/dates.ts:61 | Adds days to date | `(date: Date, days: number)` |

---

## Validation

| Function | Location | Does What | Params |
|----------|----------|-----------|--------|
| `isEmail()` | utils/validate.ts:10 | Validates email format | `(email: string)` |
| `isPhone()` | utils/validate.ts:25 | Validates phone with country | `(phone: string, country?: string)` |
| `isURL()` | utils/validate.ts:42 | Validates URL format | `(url: string)` |
| `isUUID()` | utils/validate.ts:55 | Validates UUID v4 | `(id: string)` |
| `sanitizeHTML()` | utils/sanitize.ts:12 | Strips XSS from input | `(html: string)` |
| `sanitizeSQL()` | utils/sanitize.ts:28 | Escapes SQL special chars | `(input: string)` |

---

## String Operations

| Function | Location | Does What | Params |
|----------|----------|-----------|--------|
| `slugify()` | utils/strings.ts:8 | Converts to URL slug | `(str: string)` |
| `truncate()` | utils/strings.ts:20 | Truncates with ellipsis | `(str: string, len: number)` |
| `capitalize()` | utils/strings.ts:32 | Capitalizes first letter | `(str: string)` |
| `pluralize()` | utils/strings.ts:40 | Adds s/es correctly | `(word: string, count: number)` |

---

## API Clients

| Function | Location | Does What | Returns |
|----------|----------|-----------|---------|
| `fetchUser()` | api/users.ts:15 | GET /users/:id | `Promise<User>` |
| `fetchUsers()` | api/users.ts:28 | GET /users with pagination | `Promise<User[]>` |
| `createUser()` | api/users.ts:45 | POST /users | `Promise<User>` |
| `update