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Cashier Service Specification

Project Overview

  • Project name: Cashier
  • Type: Elixir library/service
  • Purpose: Manage shopping carts, apply pricing rules, compute totals
  • Target users: Supermarket checkout systems

Requirements

Test Products

Code Name Price
GR1 Green tea £3.11
SR1 Strawberries £5.00
CF1 Coffee £11.23

Pricing Rules

  1. BOGO (Buy One Get One Free): Every 2nd Green Tea free
  2. Bulk Fixed: 3+ strawberries at £4.50 each
  3. Bulk Fraction: 3+ coffees at 2/3 price

Expected Basket Totals

Basket Expected Total
GR1,SR1,GR1,GR1,CF1 £22.45
GR1,GR1 £3.11
SR1,SR1,GR1,SR1 £16.61
GR1,CF1,SR1,CF1,CF1 £30.57

Architecture

  • Hexagonal (Ports & Adapters)
  • Domain: Cart, CartItem, Product
  • Ports: ProductCatalogue, PricingRule
  • Adapters: InMemoryCatalogue, PricingRules
  • Session: GenServer per checkout

API

  • new_checkout() → Session
  • scan(Session, code) → :ok
  • total(Session) → Decimal
  • formatted_total(Session) → "£XX.XX"
  • clear(Session) → :ok
  • stop(Session) → :ok

Validation

  • Product codes: 1-32 bytes
  • Prices: non-negative, max £999,999.99
  • Rules validated at startup
  • Quantity: non-negative

Testing

  • Unit tests for domain
  • Unit tests for rules
  • Property-based tests (StreamData)
  • Integration via Cashier module

Acceptance Criteria

  1. All expected basket totals pass
  2. Empty cart returns £0.00
  3. Zero quantity handled gracefully
  4. Duplicate rules rejected at startup
  5. Session auto-expires after idle