[sauravGit]

Fixes #266

Repeated entries in discounts.codes[] were each applied, so a fixed-amount code listed N times was subtracted N times and the total could go negative through to a completed order.

Changes:

• Apply each code once per cart and checkout, matched case-insensitively.
• Report repeats with a discount_code_already_applied warning at the repeated entry's path.
• Cap fixed-amount discounts at the remaining total so totals never go below zero.
• Add shared helpers in services/discount_rules.py used by both cart and checkout recalculation.
• Add cart tests for repeated codes and the zero floor, plus a cart-to-checkout test with repeated codes.

[damaz91]

Thank you @sauravGit for reporting #266 and putting together such a clean, well-structured fix!

Extracting the shared helpers into services/discount_rules.py keeps CartService and CheckoutService nicely aligned. In particular:

• Capping fixed_amount discounts against the remaining grand_total before subtracting (while keeping the discount_amount > 0 guard) ensures the receipt ledger always reconciles (sum(non-total entries) == total) and avoids emitting zero-amount discount entries that would violate total.json's exclusiveMaximum: 0 constraint.
• Preserving discounts.codes as submitted while tracking seen_codes case-insensitively ensures $.discounts.codes[i] in messages[] points to the exact repeated entry, and replace_discount_warnings cleanly prevents stale warnings from accumulating across recalculations.

A couple of small, non-blocking suggestions for optional polish if you'd like:

• seen_codes when discount_amount == 0 (services/cart_service.py:385, services/checkout_service.py:1468): If an earlier discount has already brought grand_total to 0 (or a percentage discount truncates to 0 minor units) and a subsequent code in codes[] is also repeated (e.g., ["BIG5000", "EXTRA100", "EXTRA100"]), EXTRA100 evaluates to 0 and is omitted from discounts.applied, but its repeat still emits "Code 'EXTRA100' is already applied". Moving seen_codes.add(code.upper()) inside if discount_amount > 0: would ensure discount_code_already_applied is only emitted for codes that actually applied.
• Direct checkout update test (cart_test.py): Adding a short test exercising PUT /checkout-sessions/{id} directly (applying a repeated code, followed by a PUT that omits discounts to verify warnings don't duplicate, and completing a $0-total checkout) would nicely complement the cart and cart-to-checkout tests.

LGTM!

[sauravGit]

Thank you @sauravGit for reporting #266 and putting together such a clean, well-structured fix!

Extracting the shared helpers into services/discount_rules.py keeps CartService and CheckoutService nicely aligned. In particular:

• Capping fixed_amount discounts against the remaining grand_total before subtracting (while keeping the discount_amount > 0 guard) ensures the receipt ledger always reconciles (sum(non-total entries) == total) and avoids emitting zero-amount discount entries that would violate total.json's exclusiveMaximum: 0 constraint.
• Preserving discounts.codes as submitted while tracking seen_codes case-insensitively ensures $.discounts.codes[i] in messages[] points to the exact repeated entry, and replace_discount_warnings cleanly prevents stale warnings from accumulating across recalculations.

A couple of small, non-blocking suggestions for optional polish if you'd like:

• seen_codes when discount_amount == 0 (services/cart_service.py:385, services/checkout_service.py:1468): If an earlier discount has already brought grand_total to 0 (or a percentage discount truncates to 0 minor units) and a subsequent code in codes[] is also repeated (e.g., ["BIG5000", "EXTRA100", "EXTRA100"]), EXTRA100 evaluates to 0 and is omitted from discounts.applied, but its repeat still emits "Code 'EXTRA100' is already applied". Moving seen_codes.add(code.upper()) inside if discount_amount > 0: would ensure discount_code_already_applied is only emitted for codes that actually applied.
• Direct checkout update test (cart_test.py): Adding a short test exercising PUT /checkout-sessions/{id} directly (applying a repeated code, followed by a PUT that omits discounts to verify warnings don't duplicate, and completing a $0-total checkout) would nicely complement the cart and cart-to-checkout tests.

LGTM!

Thanks for the review, @damaz91! Both suggestions are in this commit 43950fd:

• seen_codes.add now runs only inside if discount_amount > 0: in both services, so discount_code_already_applied is reported only for codes that actually took effect.
• Added test_checkout_update_repeated_codes_zero_total_completes, covering a direct checkout create, an update that omits discounts, and completing a $0-total checkout.
• Also added test_repeated_code_after_zero_effect_is_not_reported for the ["BIG", "EXTRA100", "EXTRA100"] case.