package mcp

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

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

// tool is one MCP tool: a name, a description, a JSON-Schema for its arguments,
// and the handler that runs it.
type tool struct {
	Name        string                                  `json:"name"`
	Description string                                  `json:"description"`
	InputSchema json.RawMessage                         `json:"inputSchema"`
	invoke      func(args json.RawMessage) (any, error) `json:"-"`
}

func (s *Server) callTool(req *rpcRequest) *rpcResponse {
	var p struct {
		Name      string          `json:"name"`
		Arguments json.RawMessage `json:"arguments"`
	}
	if err := json.Unmarshal(req.Params, &p); err != nil {
		return errorResponse(req.ID, codeInvalidParams, "invalid params")
	}
	var t *tool
	for i := range s.tools {
		if s.tools[i].Name == p.Name {
			t = &s.tools[i]
			break
		}
	}
	if t == nil {
		return errorResponse(req.ID, codeInvalidParams, "unknown tool: "+p.Name)
	}
	out, err := t.invoke(p.Arguments)
	if err != nil {
		return result(req.ID, toolError(err))
	}
	return result(req.ID, toolText(out))
}

func (s *Server) buildTools() []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: "upsert_promotion",
			Description: "Create a promotion or replace the existing one with the same id. " +
				"`promotion` is a full promotion-rule object (id, name, status, priority, conditions, actions). " +
				"For a tiered volume discount (Volymrabatt) use a cart_total condition plus a tiered_discount action " +
				"whose config.tiers each have minTotal/maxTotal (öre incl. VAT) and percent.",
			InputSchema: object(props{
				"promotion": obj("the full promotion rule object"),
			}, []string{"promotion"}),
			invoke: func(args json.RawMessage) (any, error) {
				var a struct {
					Promotion json.RawMessage `json:"promotion"`
				}
				if err := decode(args, &a); err != nil {
					return nil, err
				}
				var rule promotions.PromotionRule
				if err := json.Unmarshal(a.Promotion, &rule); err != nil {
					return nil, fmt.Errorf("invalid promotion: %w", err)
				}
				if rule.ID == "" {
					return nil, fmt.Errorf("promotion.id is required")
				}
				if rule.Status == "" {
					rule.Status = promotions.StatusActive
				}
				replaced, err := s.store.Upsert(rule)
				if err != nil {
					return nil, err
				}
				return map[string]any{"id": rule.ID, "replaced": replaced}, nil
			},
		},
		{
			Name:        "set_promotion_status",
			Description: "Set a promotion's status (active|inactive|scheduled|expired) to toggle it on or off.",
			InputSchema: object(props{
				"id":     str("the promotion id"),
				"status": str("new status: active|inactive|scheduled|expired"),
			}, []string{"id", "status"}),
			invoke: func(args json.RawMessage) (any, error) {
				var a struct {
					ID     string `json:"id"`
					Status string `json:"status"`
				}
				if err := decode(args, &a); err != nil {
					return nil, err
				}
				if !validStatus(a.Status) {
					return nil, fmt.Errorf("invalid status %q", a.Status)
				}
				found, err := s.store.SetStatus(a.ID, promotions.PromotionStatus(a.Status))
				if err != nil {
					return nil, err
				}
				if !found {
					return nil, fmt.Errorf("promotion %q not found", a.ID)
				}
				return map[string]any{"id": a.ID, "status": a.Status}, nil
			},
		},
		{
			Name:        "delete_promotion",
			Description: "Delete a 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
				}
				found, err := s.store.Delete(a.ID)
				if err != nil {
					return nil, err
				}
				if !found {
					return nil, fmt.Errorf("promotion %q not found", a.ID)
				}
				return map[string]any{"id": a.ID, "deleted": true}, 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)
				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
			},
		},
	}
}

// syntheticCart builds a one-line cart whose gross total is incVat öre (25% VAT
// assumed) so the promotion engine can be evaluated against an arbitrary total.
func syntheticCart(incVat int64, qty int) *cart.CartGrain {
	if qty <= 0 {
		qty = 1
	}
	g := cart.NewCartGrain(0, time.Now())
	g.Items = []*cart.CartItem{
		{Id: 1, Sku: "PREVIEW", Quantity: uint16(qty), Price: *cart.NewPriceFromIncVat(incVat, 25)},
	}
	// Quantity > 1 would multiply the row total; keep the gross equal to incVat by
	// pricing a single unit at the full total.
	g.Items[0].Quantity = 1
	g.UpdateTotals()
	g.Items[0].Quantity = uint16(qty)
	return g
}

func validStatus(s string) bool {
	switch promotions.PromotionStatus(s) {
	case promotions.StatusActive, promotions.StatusInactive, promotions.StatusScheduled, promotions.StatusExpired:
		return true
	}
	return false
}

// ---- result + schema helpers (mirrors the graph-cms MCP edge) -------------

func toolText(v any) map[string]any {
	b, err := json.Marshal(v)
	if err != nil {
		return toolError(err)
	}
	return map[string]any{
		"content": []map[string]any{{"type": "text", "text": string(b)}},
	}
}

func toolError(err error) map[string]any {
	return map[string]any{
		"isError": true,
		"content": []map[string]any{{"type": "text", "text": err.Error()}},
	}
}

// decode unmarshals tool arguments, tolerating empty/absent arguments.
func decode(args json.RawMessage, v any) error {
	if len(args) == 0 || string(args) == "null" {
		return nil
	}
	if err := json.Unmarshal(args, v); err != nil {
		return fmt.Errorf("invalid arguments: %w", err)
	}
	return nil
}

type props map[string]json.RawMessage

func object(p props, required []string) json.RawMessage {
	m := map[string]any{
		"type":       "object",
		"properties": p,
	}
	if len(required) > 0 {
		m["required"] = required
	}
	b, _ := json.Marshal(m)
	return b
}

func str(desc string) json.RawMessage  { return scalar("string", desc) }
func obj(desc string) json.RawMessage  { return scalar("object", desc) }
func integer(d string) json.RawMessage { return scalar("integer", d) }

func scalar(typ, desc string) json.RawMessage {
	b, _ := json.Marshal(map[string]any{"type": typ, "description": desc})
	return b
}