go-cart-actor/pkg/promotions/mcp/public.go

package mcp

import (
	"encoding/json"
	"fmt"
	"time"

	"git.k6n.net/mats/go-cart-actor/pkg/promotions"
)

// NewPublic builds an MCP server exposing only the read-only promotion tools:
// list_promotions, get_promotion, preview_cart_discount, and evaluate_promotion.
// This is mounted on the public-facing endpoint (no auth needed since all tools
// are safe to expose).
func NewPublic(store *promotions.Store, eval *promotions.PromotionService) *Server {
	if eval == nil {
		eval = promotions.NewPromotionService(nil)
	}
	s := &Server{store: store, eval: eval}
	s.tools = s.buildPublicTools()
	return s
}

func (s *Server) buildPublicTools() []tool {
	return []tool{
		{
			Name:        "list_promotions",
			Description: "List all promotion rules the cart engine evaluates, optionally filtered by status (active|inactive|scheduled|expired).",
			InputSchema: object(props{
				"status": str("optional status filter: active|inactive|scheduled|expired"),
			}, nil),
			invoke: func(args json.RawMessage) (any, error) {
				var a struct {
					Status string `json:"status"`
				}
				if err := decode(args, &a); err != nil {
					return nil, err
				}
				rules := s.store.List()
				if a.Status != "" {
					filtered := rules[:0:0]
					for _, r := range rules {
						if string(r.Status) == a.Status {
							filtered = append(filtered, r)
						}
					}
					rules = filtered
				}
				return map[string]any{"promotions": rules, "count": len(rules)}, nil
			},
		},
		{
			Name:        "get_promotion",
			Description: "Fetch a single promotion rule by id.",
			InputSchema: object(props{"id": str("the promotion id")}, []string{"id"}),
			invoke: func(args json.RawMessage) (any, error) {
				var a struct {
					ID string `json:"id"`
				}
				if err := decode(args, &a); err != nil {
					return nil, err
				}
				r, ok := s.store.Get(a.ID)
				if !ok {
					return nil, fmt.Errorf("promotion %q not found", a.ID)
				}
				return r, nil
			},
		},
		{
			Name: "preview_cart_discount",
			Description: "Evaluate the current active promotions against a hypothetical cart total and report the " +
				"matched discount. cartTotalIncVat is in öre incl. VAT (e.g. 5 000 000 = 50 000 kr). Useful for " +
				"verifying Volymrabatt tiers without touching a real cart.",
			InputSchema: object(props{
				"cartTotalIncVat": integer("cart total incl. VAT, in öre"),
				"itemQuantity":    integer("optional total item quantity"),
				"customerSegment": str("optional customer segment"),
			}, []string{"cartTotalIncVat"}),
			invoke: func(args json.RawMessage) (any, error) {
				var a struct {
					CartTotalIncVat int64  `json:"cartTotalIncVat"`
					ItemQuantity    int    `json:"itemQuantity"`
					CustomerSegment string `json:"customerSegment"`
				}
				if err := decode(args, &a); err != nil {
					return nil, err
				}
				g := syntheticCart(a.CartTotalIncVat, a.ItemQuantity, s.defaultVatRate())
				opts := []promotions.ContextOption{promotions.WithNow(time.Now())}
				if a.CustomerSegment != "" {
					opts = append(opts, promotions.WithCustomerSegment(a.CustomerSegment))
				}
				ctx := promotions.NewContextFromCart(g, opts...)
				results, actions := s.eval.EvaluateAll(s.store.Snapshot(), ctx)
				s.eval.ApplyResults(g, results, ctx)
				return map[string]any{
					"subtotalIncVat":    a.CartTotalIncVat,
					"discountIncVat":    g.TotalDiscount.IncVat,
					"netIncVat":         g.TotalPrice.IncVat,
					"matchedActions":    actions,
					"appliedPromotions": g.AppliedPromotions,
				}, nil
			},
		},
		{
			Name: "evaluate_promotion",
			Description: "Evaluate promotions against a hypothetical cart and return the full evaluation result: " +
				"which promotions match, what discount they apply, and any pending next-tier nudges (\"spend X more for Y%\"). " +
				"When promotionId is set, only that specific promotion is evaluated (useful for testing a new rule). " +
				"When omitted, all active promotions are evaluated. cartTotalIncVat is in öre incl. VAT.",
			InputSchema: object(props{
				"cartTotalIncVat": integer("cart total incl. VAT, in öre"),
				"promotionId":     str("optional: evaluate only this promotion (by id)"),
				"itemQuantity":    integer("optional total item quantity"),
				"customerSegment": str("optional customer segment"),
			}, []string{"cartTotalIncVat"}),
			invoke: func(args json.RawMessage) (any, error) {
				var a struct {
					CartTotalIncVat int64  `json:"cartTotalIncVat"`
					PromotionId     string `json:"promotionId"`
					ItemQuantity    int    `json:"itemQuantity"`
					CustomerSegment string `json:"customerSegment"`
				}
				if err := decode(args, &a); err != nil {
					return nil, err
				}

				rules := s.store.Snapshot()
				if a.PromotionId != "" {
					filtered := rules[:0:0]
					for _, r := range rules {
						if r.ID == a.PromotionId {
							filtered = append(filtered, r)
							break
						}
					}
					if len(filtered) == 0 {
						return nil, fmt.Errorf("promotion %q not found", a.PromotionId)
					}
					rules = filtered
				}

				g := syntheticCart(a.CartTotalIncVat, a.ItemQuantity, s.defaultVatRate())
				opts := []promotions.ContextOption{promotions.WithNow(time.Now())}
				if a.CustomerSegment != "" {
					opts = append(opts, promotions.WithCustomerSegment(a.CustomerSegment))
				}
				ctx := promotions.NewContextFromCart(g, opts...)
				results, actions := s.eval.EvaluateAll(rules, ctx)
				evalResults := make([]map[string]any, 0, len(results))
				for _, r := range results {
					evalResults = append(evalResults, map[string]any{
						"ruleId":       r.Rule.ID,
						"name":         r.Rule.Name,
						"applicable":   r.Applicable,
						"failedReason": r.FailedReason,
						"matchedActions": r.MatchedActions,
					})
				}
				s.eval.ApplyResults(g, results, ctx)
				return map[string]any{
					"subtotalIncVat":    a.CartTotalIncVat,
					"discountIncVat":    g.TotalDiscount.IncVat,
					"netIncVat":         g.TotalPrice.IncVat,
					"matchedActions":    actions,
					"appliedPromotions": g.AppliedPromotions,
					"evaluationResults": evalResults,
				}, nil
			},
		},
	}
}