SIMPLEX: 0.6

DATA: Item

id: string

name: string

price: number

quantity: number

DATA: Cart

items: list of Item

total: number

FUNCTION: add_to_cart(cart, item) → Cart

RULES:

- [add.R1] if item already in cart, increase quantity

- [add.R2] otherwise add item to cart

- [add.R3] recalculate total

DONE_WHEN:

- [add.D1] item is in cart

- [add.D2] total reflects all items

EXAMPLES:

- [add.E1] value: (empty_cart, item_a) → { items: [item_a], total: 10.00 }

- [add.E2] value: (cart_with_a, item_a) → { items: [item_a(qty:2)], total: 20.00 }

- [add.E3] value: (cart_with_a, item_b) → { items: [item_a, item_b], total: 25.00 }

- [add.E4] error: (empty_cart, invalid_item) → "Item must have id, name, and price"

- [add.E5] error: (empty_cart, negative_quantity_item) → "Quantity cannot be negative"

ERRORS:

- [add.X1] invalid item → "Item must have id, name, and price"

- [add.X2] negative quantity → "Quantity cannot be negative"

- any unhandled condition → fail with descriptive message

COVERS:

- add.E1 → add.R2, add.R3, add.D1, add.D2

- add.E2 → add.R1, add.R3, add.D1, add.D2

- add.E3 → add.R2, add.R3, add.D1, add.D2

- add.E4 → add.X1

- add.E5 → add.X2

FUNCTION: remove_from_cart(cart, item_id) → Cart

RULES:

- [remove.R1] find item by id

- [remove.R2] remove it from cart

- [remove.R3] recalculate total

DONE_WHEN:

- [remove.D1] item no longer in cart

- [remove.D2] total is updated

EXAMPLES:

- [remove.E1] value: (cart_with_a_and_b, "a") → { items: [item_b], total: 15.00 }

- [remove.E2] value: (cart_with_a, "a") → { items: [], total: 0.00 }

- [remove.E3] value: (empty_cart, "a") → { items: [], total: 0.00 }

ERRORS:

- [remove.X1] item not found → return cart unchanged (not an error)

- any unhandled condition → fail with descriptive message

COVERS:

- remove.E1 → remove.R1, remove.R2, remove.R3, remove.D1, remove.D2

- remove.E2 → remove.R1, remove.R2, remove.R3, remove.D1, remove.D2

- remove.E3 → remove.R1, remove.R3, remove.D1, remove.D2, remove.X1

FUNCTION: calculate_total(cart) → number

RULES:

- [total.R1] sum price × quantity for all items

DONE_WHEN:

- [total.D1] total is accurate

EXAMPLES:

- [total.E1] value: (empty_cart) → 0.00

- [total.E2] value: (cart_with_a) → 10.00

- [total.E3] value: (cart_with_a_and_b) → 25.00

- [total.E4] value: (invalid_cart) → 0.00

ERRORS:

- [total.X1] invalid cart → 0.00

- any unhandled condition → fail with descriptive message

COVERS:

- total.E1 → total.R1, total.D1

- total.E2 → total.R1, total.D1

- total.E3 → total.R1, total.D1

- total.E4 → total.X1