# Specification: Shopping Cart

**ID**: SPEC-100
**Version**: 1.0
**Date**: 2025-10-24
**Status**: Active
**Author**: Product Team

## Overview

This specification defines the requirements for a shopping cart feature in an e-commerce application. The shopping cart allows users to add, remove, and manage items before checkout.

## Background

Customers need a way to collect items they wish to purchase before completing a transaction. The cart must persist across sessions and provide real-time price calculations.

## Requirements (EARS Format)

### Functional Requirements

**REQ-001**: The system shall allow users to add items to their shopping cart.

**REQ-002**: WHEN a user adds an item to the cart, the system shall update the cart total in real-time.

**REQ-003**: The system shall allow users to remove items from their shopping cart.

**REQ-004**: IF the cart is empty, THEN the system shall display a message "Your cart is empty".

**REQ-005**: The system shall persist cart contents across browser sessions using local storage.

### Non-Functional Requirements

**NFR-001**: The system shall calculate cart totals within 100 milliseconds.

**NFR-002**: The cart interface shall be responsive and work on mobile devices with screens as small as 320px width.

## Examples

### Adding Items
```python
cart = ShoppingCart()
cart.add_item(item_id="SKU-123", quantity=2)
assert cart.item_count == 2
```

### Calculating Totals
```python
cart.add_item(item_id="SKU-123", price=10.00, quantity=2)
cart.add_item(item_id="SKU-456", price=5.00, quantity=1)
assert cart.total == 25.00
```

## Related Specifications

- SPEC-101: Checkout Process
- SPEC-102: Payment Processing

## Revision History

- 2025-10-24: Initial version
