Recipe Manager

Prototype: NewRecipeDraft(name string,ingredients,steps []string) *RecipeDraft; Snapshot() Draft copies lists both in and out; no name validation/permissions.

Final: NewRecipeManager(). AddUser(user string,permissions []string) (bool,error) requires nonempty user and only read/write permission strings; duplicate permissions harmless, empty allowed. Validate even for existing user; duplicate valid registration false preserving permissions. Add(user,name string,ingredients,steps []string) (*string,error) validates nonempty name first, then requires write. Unauthorized/name-used returns nil without consuming recipeN ID; success appends v1. Update(user,id,name,...)(*int,error) same name validation; requires write, existing ID, and name unused by another current recipe. Appends even identical content; returns version. Get(user,id string,version *int) *Recipe requires read; nil version latest, positive existing version otherwise; nil denied/missing/invalid. History(user,id) []Recipe read-only, oldest first. Rollback(user,id string,version int) *int requires write and valid target with no current-name conflict; append selected content as latest, preserve ID/history. ByIngredient(user,ingredient) []string read-only; exact current membership once per recipe; order by ingredient slice length, current name, ID. Empty list items, duplicates and order retained. Invalid/rejected mutations preserve names, versions and next ID.

All identifiers and strings compare exactly and case-sensitively. Numeric inputs are typed int; booleans cannot be passed as amounts. Arithmetic and logical timestamps fit int. All supplied collection inputs that are retained must be copied. Every collection or pointer snapshot must be detached; separate service instances own independent state. A nil pointer or nil optional slice denotes absence, while an ordinary empty result collection is nonnil. Invalid calls leave state unchanged unless the contract explicitly accepts time and expires state on a valid unsuccessful call. The exported signatures in exercise.go are the API specification.

Edit exercise.go; retain the public types and signatures. prototype and final are separate stages. The executable tests recover panics as failures, report named checks, and exit nonzero on any failure. A successful run must execute at least one check. Tests use logical time or explicit synchronization where relevant.