even more refactoring

mutation_registry.go

@@ -0,0 +1,301 @@
+package main
+
+import (
+	"fmt"
+	"reflect"
+	"sync"
+)
+
+// mutation_registry.go
+//
+// Mutation Registry Infrastructure
+// --------------------------------
+// This file introduces a generic registry for cart mutations that:
+//
+// 1. Decouples mutation logic from the large type-switch inside CartGrain.Apply.
+// 2. Enforces (at registration time) that every mutation handler has the correct
+//    signature: func(*CartGrain, *T) error
+// 3. Optionally auto-updates cart totals after a mutation if flagged.
+// 4. Provides a single authoritative list of registered mutations for
+//    introspection / coverage testing.
+// 5. Allows incremental migration: you can first register new mutations here,
+//    and later prune the legacy switch cases.
+//
+// Usage Pattern
+// -------------
+//   // Define your mutation proto message (e.g. messages.ApplyCoupon in messages.proto)
+//   // Regenerate protobufs.
+//
+//   // In an init() (ideally in a small file like mutations_apply_coupon.go)
+//   func init() {
+//       RegisterMutation[*messages.ApplyCoupon](
+//           "ApplyCoupon",
+//           func(g *CartGrain, m *messages.ApplyCoupon) error {
+//               // domain logic ...
+//               discount := int64(5000)
+//               if g.TotalPrice < discount {
+//                   discount = g.TotalPrice
+//               }
+//               g.TotalDiscount += discount
+//               g.TotalPrice -= discount
+//               return nil
+//           },
+//           WithTotals(), // we changed price-related fields; recalc totals
+//       )
+//   }
+//
+//   // To invoke dynamically (alternative to the current switch):
+//   if updated, err := ApplyRegistered(grain, incomingMessage); err == nil {
+//       grain = updated
+//   } else if errors.Is(err, ErrMutationNotRegistered) {
+//       // fallback to legacy switch logic
+//   }
+//
+// Migration Strategy
+// ------------------
+// 1. For each existing mutation handled in CartGrain.Apply, add a registry
+//    registration with equivalent logic.
+// 2. Add a test that enumerates all *expected* mutation proto types and asserts
+//    they are present in RegisteredMutationTypes().
+// 3. Once coverage is 100%, replace the switch in CartGrain.Apply with a call
+//    to ApplyRegistered (and optionally keep a minimal default to produce an
+//    "unsupported mutation" error).
+//
+// Thread Safety
+// -------------
+// Registration is typically done at init() time; a RWMutex provides safety
+// should late dynamic registration ever be introduced.
+//
+// Auto Totals
+// -----------
+// Many mutations require recomputing totals. To avoid forgetting this, pass
+// WithTotals() when registering. This will invoke grain.UpdateTotals() after
+// the handler returns successfully.
+//
+// Error Semantics
+// ---------------
+// - If a handler returns an error, totals are NOT recalculated (even if
+//   WithTotals() was specified).
+// - ApplyRegistered returns (nil, ErrMutationNotRegistered) if the message type
+//   is absent.
+//
+// Extensibility
+// -------------
+// It is straightforward to add options like audit hooks, metrics wrappers,
+// or optimistic concurrency guards by extending MutationOption.
+//
+// NOTE: Generics require Go 1.18+. If constrained to earlier Go versions,
+//       replace the generic registration with a non-generic RegisterMutationType
+//       that accepts reflect.Type and an adapter function.
+//
+// ---------------------------------------------------------------------------
+
+var (
+	mutationRegistryMu sync.RWMutex
+	mutationRegistry   = make(map[reflect.Type]*registeredMutation)
+
+	// ErrMutationNotRegistered is returned when no handler exists for a given mutation type.
+	ErrMutationNotRegistered = fmt.Errorf("mutation not registered")
+)
+
+// MutationOption configures additional behavior for a registered mutation.
+type MutationOption func(*mutationOptions)
+
+// mutationOptions holds flags adjustable per registration.
+type mutationOptions struct {
+	updateTotals bool
+}
+
+// WithTotals ensures CartGrain.UpdateTotals() is called after a successful handler.
+func WithTotals() MutationOption {
+	return func(o *mutationOptions) {
+		o.updateTotals = true
+	}
+}
+
+// registeredMutation stores metadata + the execution closure.
+type registeredMutation struct {
+	name         string
+	handler      func(*CartGrain, interface{}) error
+	updateTotals bool
+	msgType      reflect.Type
+}
+
+// RegisterMutation registers a mutation handler for a specific message type T.
+//
+// Parameters:
+//
+//	name    - a human-readable identifier (used for diagnostics / coverage tests).
+//	handler - business logic operating on the cart grain & strongly typed message.
+//	options - optional behavior flags (e.g., WithTotals()).
+//
+// Panics if:
+//   - name is empty
+//   - handler is nil
+//   - duplicate registration for the same message type T
+//
+// Typical call is placed in an init() function.
+func RegisterMutation[T any](name string, handler func(*CartGrain, *T) error, options ...MutationOption) {
+	if name == "" {
+		panic("RegisterMutation: name is required")
+	}
+	if handler == nil {
+		panic("RegisterMutation: handler is nil")
+	}
+
+	// Derive the reflect.Type for *T then its Elem (T) for mapping.
+	var zero *T
+	rtPtr := reflect.TypeOf(zero)
+	if rtPtr.Kind() != reflect.Ptr {
+		panic("RegisterMutation: expected pointer type for generic parameter")
+	}
+	rt := rtPtr.Elem()
+
+	opts := mutationOptions{}
+	for _, opt := range options {
+		opt(&opts)
+	}
+
+	wrapped := func(g *CartGrain, m interface{}) error {
+		typed, ok := m.(*T)
+		if !ok {
+			return fmt.Errorf("mutation type mismatch: have %T want *%s", m, rt.Name())
+		}
+		return handler(g, typed)
+	}
+
+	mutationRegistryMu.Lock()
+	defer mutationRegistryMu.Unlock()
+
+	if _, exists := mutationRegistry[rt]; exists {
+		panic(fmt.Sprintf("RegisterMutation: duplicate registration for type %s", rt.String()))
+	}
+
+	mutationRegistry[rt] = &registeredMutation{
+		name:         name,
+		handler:      wrapped,
+		updateTotals: opts.updateTotals,
+		msgType:      rt,
+	}
+}
+
+// ApplyRegistered attempts to apply a registered mutation.
+// Returns updated grain if successful.
+//
+// If the mutation is not registered, returns (nil, ErrMutationNotRegistered).
+func ApplyRegistered(grain *CartGrain, msg interface{}) (*CartGrain, error) {
+	if grain == nil {
+		return nil, fmt.Errorf("nil grain")
+	}
+	if msg == nil {
+		return nil, fmt.Errorf("nil mutation message")
+	}
+
+	rt := indirectType(reflect.TypeOf(msg))
+	mutationRegistryMu.RLock()
+	entry, ok := mutationRegistry[rt]
+	mutationRegistryMu.RUnlock()
+
+	if !ok {
+		return nil, ErrMutationNotRegistered
+	}
+
+	if err := entry.handler(grain, msg); err != nil {
+		return nil, err
+	}
+
+	if entry.updateTotals {
+		grain.UpdateTotals()
+	}
+
+	return grain, nil
+}
+
+// RegisteredMutations returns metadata for all registered mutations (snapshot).
+func RegisteredMutations() []string {
+	mutationRegistryMu.RLock()
+	defer mutationRegistryMu.RUnlock()
+	out := make([]string, 0, len(mutationRegistry))
+	for _, entry := range mutationRegistry {
+		out = append(out, entry.name)
+	}
+	return out
+}
+
+// RegisteredMutationTypes returns the reflect.Type list of all registered messages.
+// Useful for coverage tests ensuring expected set matches actual set.
+func RegisteredMutationTypes() []reflect.Type {
+	mutationRegistryMu.RLock()
+	defer mutationRegistryMu.RUnlock()
+	out := make([]reflect.Type, 0, len(mutationRegistry))
+	for t := range mutationRegistry {
+		out = append(out, t)
+	}
+	return out
+}
+
+// MustAssertMutationCoverage can be called at startup to ensure every expected
+// mutation type has been registered. It panics with a descriptive message if any
+// are missing. Provide a slice of prototype pointers (e.g. []*messages.AddItem{nil} ...)
+func MustAssertMutationCoverage(expected []interface{}) {
+	mutationRegistryMu.RLock()
+	defer mutationRegistryMu.RUnlock()
+
+	missing := make([]string, 0)
+	for _, ex := range expected {
+		if ex == nil {
+			continue
+		}
+		t := indirectType(reflect.TypeOf(ex))
+		if _, ok := mutationRegistry[t]; !ok {
+			missing = append(missing, t.String())
+		}
+	}
+	if len(missing) > 0 {
+		panic(fmt.Sprintf("mutation registry missing handlers for: %v", missing))
+	}
+}
+
+// indirectType returns the element type if given a pointer; otherwise the type itself.
+func indirectType(t reflect.Type) reflect.Type {
+	for t.Kind() == reflect.Ptr {
+		t = t.Elem()
+	}
+	return t
+}
+
+/*
+Integration Guide
+-----------------
+
+1. Register all existing mutations:
+
+   func init() {
+       RegisterMutation[*messages.AddItem]("AddItem",
+           func(g *CartGrain, m *messages.AddItem) error {
+               // (port logic from existing switch branch)
+               // ...
+               return nil
+           },
+           WithTotals(),
+       )
+       // ... repeat for others
+   }
+
+2. In CartGrain.Apply (early in the method) add:
+
+   if updated, err := ApplyRegistered(c, content); err == nil {
+       return updated, nil
+   } else if err != ErrMutationNotRegistered {
+       return nil, err
+   }
+
+   // existing switch fallback below
+
+3. Once all mutations are registered, remove the legacy switch cases
+   and leave a single ErrMutationNotRegistered path for unknown types.
+
+4. Add a coverage test (see docs for example; removed from source for clarity).
+5. (Optional) Add metrics / tracing wrappers for handlers.
+
+*/