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# Task 2: Implement MentionParserService for @username extraction

**Dependencies:** Task 1

### Goal
Create a service that parses text content to extract @username mentions, validates them against organization membership, and filters out self-mentions and code blocks.

### Implementation Context
**Files to Create:**
- `src/lib/services/mention-parser-service.ts`
- `src/lib/services/mention-parser-service.test.ts`

**Key Requirements:**
- Extract @username patterns using regex
- Validate usernames exist in the organization
- Filter out mentions in code blocks (``` and `)
- Remove duplicate mentions
- Exclude self-mentions
- Handle edge cases (invalid usernames, deactivated users)

**Technical Notes:**
- Regex pattern: `/@([a-zA-Z0-9_-]+)(?!\w)` for word boundary matching
- Reuse existing Prisma client from `src/lib/db.ts`
- Follow service pattern from `src/lib/services/` directory
- Use existing `UnifiedLogger` from `src/lib/utils/error-logging.ts`

**Simplicity Decisions:**
- Simple regex parsing instead of NLP or AST parsing
- Synchronous validation (no caching for MVP)
- Direct database queries (no Redis cache for username lookups)

### Scope Definition
**Deliverables:**
- `MentionParserService` class with `extractMentions()` method
- Regex-based @username extraction
- Organization membership validation
- Code block filtering logic
- Comprehensive unit tests

**Exclusions:**
- @display-name syntax (username only)
- Autocomplete suggestions
- Mention preview/validation UI
- Username caching layer

### Implementation Steps
1. Create `src/lib/services/mention-parser-service.ts` with class `MentionParserService`
2. Implement `extractMentions(content: string, organizationId: string, currentUserId: string): Promise<User[]>`:
- Strip code blocks using regex: /```[\s\S]*?```/g and /`[^`]+`/g
- Extract @username patterns using `/@([a-zA-Z0-9_-]+)(?!\w)/g`
- Remove duplicates using Set
- Filter out current user's username (self-mention)
- Query database for users by username within organization
- Return array of valid User objects
3. Add error handling for database query failures
4. Add logging for invalid usernames and validation failures
5. **Test: Extract single valid mention**
- **Setup:** Content with "@john_doe", user exists in org
- **Action:** Call extractMentions()
- **Expect:** Returns array with john_doe User object
6. **Test: Filter self-mentions**
- **Setup:** Content with "@current_user", currentUserId matches
- **Action:** Call extractMentions()
- **Expect:** Returns empty array
7. **Test: Ignore mentions in code blocks**
- **Setup:** Content with "```@john_doe```" and "`@jane_smith`"
- **Action:** Call extractMentions()
- **Expect:** Returns empty array
8. **Test: Handle multiple mentions with duplicates**
- **Setup:** Content with "@john_doe and @jane_smith and @john_doe"
- **Action:** Call extractMentions()
- **Expect:** Returns array with john_doe and jane_smith (no duplicates)
9. **Test: Skip invalid usernames**
- **Setup:** Content with "@nonexistent_user"
- **Action:** Call extractMentions()
- **Expect:** Returns empty array, logs warning

### Success Criteria
- Service correctly extracts @username patterns from text
- Validates usernames against organization membership
- Filters out self-mentions automatically
- Ignores mentions within code blocks
- Removes duplicate mentions
- All unit tests pass
- Handles edge cases gracefully (invalid usernames, DB errors)

### Scope Constraint
Implement only mention parsing and validation. Do not implement notification sending or database storage of mentions.