Supermarket Pricing

Level: Advanced 60–90 min

Concepts: Business LogicDesign PatternsIncremental DesignEdge Cases

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Build a checkout system for a supermarket that handles multiple pricing strategies.

Requirements

Step 1: Simple Pricing

Items have a unit price. Scanning items adds them to the total.

Item	Unit Price
A	$50
B	$30
C	$20
D	$15

Scanning A, B, A → total $130.

Step 2: Multi-Buy Discounts

Some items have special pricing when bought in bulk:

Item	Unit Price	Special
A	$50	3 for $130
B	$30	2 for $45
C	$20	—
D	$15	—

Examples:

• A, A, A → $130 (not $150)
• A, A, A, A → $180 ($130 + $50)
• B, B, B → $75 ($45 + $30)
• A, B, A, B, A → $175 ($130 + $45) — order doesn’t matter

Step 3: Buy One Get One Free

Add a BOGOF rule: buy one item, get a second of the same item free.

Item	Rule
C	Buy one get one free

Examples:

• C, C → $20 (second one free)
• C, C, C → $40 ($20 + $20 + free)
• C → $20 (single item, no discount)

Step 4: Weighted Items

Some items are priced by weight rather than unit:

Item	Price
Bananas	$1.99/kg
Apples	$3.49/kg

Bananas (0.5kg) → $1.00 (rounded)

Step 5: Combo Deals

Buy a combination of items for a special price:

Combo	Price
D + C together	$25 (instead of $35)

The combo should only apply once per qualifying set.

Test Cases

Items Scanned	Expected Total
(nothing)	$0
A	$50
A, B	$80
A, A, A	$130
A, A, A, A	$180
B, B	$45
C, C	$20
C, C, C	$40
A, B, A, B, A	$175
D, C	$25 (combo)
D, C, D	$40 ($25 combo + $15)

Bonus

• Add percentage discounts (e.g. 10% off item E)
• Add a loyalty card modifier — loyalty members get an additional 5% off the total
• Add receipt printing with itemized lines showing original and discounted prices
• Make pricing rules configurable at runtime (not hardcoded)

Hint

Start with simple unit pricing. Get the checkout working for one item, then multiple items. Only then add the first pricing rule. Each new pricing strategy is a separate concern — don’t try to build a generic rule engine upfront. Let the design emerge from the tests.