# Requirements: TODO CLI Application

## Functional Requirements

### FR-1: Add Tasks
- **ID**: FR-1
- **Priority**: MUST
- **Description**: User can add a new task with a description
- **Acceptance Criteria**:
  - Command: `todo add "Task description"`
  - Task receives unique ID
  - Task is saved to storage
  - Confirmation message displayed

### FR-2: List Tasks
- **ID**: FR-2
- **Priority**: MUST
- **Description**: User can view all tasks
- **Acceptance Criteria**:
  - Command: `todo list`
  - Shows all tasks with ID, status, description
  - Completed tasks visually differentiated
  - Empty state message if no tasks

### FR-3: Complete Tasks
- **ID**: FR-3
- **Priority**: MUST
- **Description**: User can mark a task as complete
- **Acceptance Criteria**:
  - Command: `todo complete <id>`
  - Task marked with completion timestamp
  - Confirmation message displayed
  - Error if task ID not found

### FR-4: Delete Tasks
- **ID**: FR-4
- **Priority**: MUST
- **Description**: User can delete a task permanently
- **Acceptance Criteria**:
  - Command: `todo delete <id>`
  - Task removed from storage
  - Confirmation message displayed
  - Error if task ID not found

### FR-5: Help Text
- **ID**: FR-5
- **Priority**: MUST
- **Description**: User can view command help
- **Acceptance Criteria**:
  - Command: `todo --help` or `todo -h`
  - Shows all available commands
  - Shows usage examples
  - Shows version number

## Technical Requirements

### TR-1: Node.js Compatibility
- **Requirement**: Support Node.js 16+
- **Rationale**: Modern Node.js features for clean code

### TR-2: Storage Format
- **Requirement**: Store tasks in JSON format
- **Location**: `~/.todo/tasks.json`
- **Format**:
  ```json
  {
    "tasks": [
      {
        "id": "uuid-v4",
        "description": "Task description",
        "completed": false,
        "createdAt": "ISO-8601 timestamp",
        "completedAt": null
      }
    ]
  }
  ```

### TR-3: Error Handling
- **Requirement**: Graceful error handling for all operations
- **Scenarios**:
  - File system errors (permissions, disk full)
  - Invalid input (empty descriptions, invalid IDs)
  - Corrupted data file

### TR-4: CLI Framework
- **Requirement**: Use Commander.js for CLI parsing
- **Rationale**: Industry standard, well-tested, excellent docs

### TR-5: Data Integrity
- **Requirement**: Atomic file writes to prevent corruption
- **Implementation**: Write to temp file, then rename

## Non-Functional Requirements

### NFR-1: Performance
- Command execution < 100ms for typical operations
- Supports up to 10,000 tasks without degradation

### NFR-2: Usability
- Commands follow Unix conventions
- Error messages are clear and actionable
- Help text is comprehensive

### NFR-3: Reliability
- Zero data loss on crashes or interruptions
- Automatic backup before destructive operations
- Corrupted data file recovery

### NFR-4: Maintainability
- Code coverage > 80%
- ESLint compliant
- Clear function documentation
- Modular architecture for easy extension

## Out of Scope

The following are explicitly **not** requirements for v1.0:

- ❌ Multi-user support
- ❌ Cloud synchronization
- ❌ Web interface
- ❌ Task priorities or due dates
- ❌ Recurring tasks
- ❌ Task dependencies
- ❌ Time tracking
- ❌ Notifications or reminders

## Dependencies

### Runtime Dependencies
- `commander`: ^11.0.0 (CLI framework)
- `uuid`: ^9.0.0 (Unique ID generation)

### Development Dependencies
- `jest`: ^29.0.0 (Testing framework)
- `eslint`: ^8.0.0 (Linting)

## Constraints

- Must work offline (no network required)
- Must not require database installation
- Must be installable via npm globally
- Configuration file optional (sensible defaults)