// Package mcp implements a minimal stdio JSON-RPC MCP (Model Context
// Protocol) server over the glpi library — stdlib only, no third-party
// modules. Transport is newline-delimited JSON on the reader/writer
// pair. Tools: change_create, change_list, change_transition,
// change_followup, ci_search, ci_show. An optional agent profile id
// (e.g. 5 = Hotliner) is applied on the first tool call.
package mcp

import (
	"bufio"
	"bytes"
	"context"
	"encoding/json"
	"fmt"
	"io"
	"strconv"
	"sync"

	"git.knownelement.com/ukrrs/mopac-glpi-go/glpi"
)

// protocolVersionDefault answers initialize when the client does not
// name one.
const protocolVersionDefault = "2025-06-18"

// Server is the MCP server over one glpi.Client.
type Server struct {
	client  *glpi.Client
	profile int

	once       sync.Once
	profileErr error
}

// New builds a Server. profileID > 0 switches the session's active
// profile (agent mode) on first use.
func New(c *glpi.Client, profileID int) *Server {
	return &Server{client: c, profile: profileID}
}

// Serve reads newline-delimited JSON-RPC requests until EOF, writing
// one response line per request that carries an id (notifications are
// acknowledged by silence).
func (s *Server) Serve(in io.Reader, out io.Writer) error {
	sc := bufio.NewScanner(in)
	sc.Buffer(make([]byte, 0, 64*1024), 4<<20)
	for sc.Scan() {
		line := bytes.TrimSpace(sc.Bytes())
		if len(line) == 0 {
			continue
		}
		if err := s.handle(line, out); err != nil {
			return err
		}
	}
	return sc.Err()
}

// request is one inbound JSON-RPC message.
type request struct {
	JSONRPC string          `json:"jsonrpc"`
	ID      json.RawMessage `json:"id"`
	Method  string          `json:"method"`
	Params  struct {
		ProtocolVersion string          `json:"protocolVersion"`
		Name            string          `json:"name"`
		Arguments       json.RawMessage `json:"arguments"`
	} `json:"params"`
}

// response is one outbound JSON-RPC message (result XOR error).
type response struct {
	JSONRPC string          `json:"jsonrpc"`
	ID      json.RawMessage `json:"id"`
	Result  any             `json:"result,omitempty"`
	Error   *rpcError       `json:"error,omitempty"`
}

type rpcError struct {
	Code    int    `json:"code"`
	Message string `json:"message"`
}

func (s *Server) handle(line []byte, out io.Writer) error {
	var req request
	if err := json.Unmarshal(line, &req); err != nil || req.Method == "" {
		return nil // cannot route a malformed line; never guess an id
	}
	if len(req.ID) == 0 || string(req.ID) == "null" {
		return nil // notification: no response per JSON-RPC
	}

	resp := response{JSONRPC: "2.0", ID: req.ID}
	switch req.Method {
	case "initialize":
		pv := req.Params.ProtocolVersion
		if pv == "" {
			pv = protocolVersionDefault
		}
		resp.Result = map[string]any{
			"protocolVersion": pv,
			"capabilities":    map[string]any{"tools": map[string]any{}},
			"serverInfo":      map[string]any{"name": "mglpi-mcp", "version": "0.1.0"},
		}
	case "ping":
		resp.Result = map[string]any{}
	case "tools/list":
		resp.Result = map[string]any{"tools": toolDefs()}
	case "tools/call":
		resp.Result = s.callTool(req.Params.Name, req.Params.Arguments)
	default:
		resp.Result = nil
		resp.Error = &rpcError{Code: -32601, Message: fmt.Sprintf("method not found: %s", req.Method)}
	}
	return writeLine(out, resp)
}

// ensureProfile applies the agent profile once per server lifetime (an
// API op; it also warms the session).
func (s *Server) ensureProfile() error {
	s.once.Do(func() {
		if s.profile > 0 {
			s.profileErr = s.client.ChangeActiveProfile(context.Background(), s.profile)
		}
	})
	return s.profileErr
}

// callTool dispatches one tools/call. Tool failures are RESULTS with
// isError=true (protocol errors are the -32601 path only).
func (s *Server) callTool(name string, args json.RawMessage) map[string]any {
	if err := s.ensureProfile(); err != nil {
		return toolError(err)
	}
	var a map[string]any
	if len(args) > 0 {
		if err := json.Unmarshal(args, &a); err != nil {
			return toolError(fmt.Errorf("arguments are not an object"))
		}
	}
	ctx := context.Background()
	var payload any
	switch name {
	case "change_create":
		title, _ := a["title"].(string)
		if title == "" {
			return toolError(fmt.Errorf("change_create requires title"))
		}
		content, _ := a["content"].(string)
		id, err := s.client.CreateChange(ctx, title, content, argInt(a, "urgency", 3), argInt(a, "impact", 3))
		if err != nil {
			return toolError(err)
		}
		payload = map[string]any{"change": map[string]any{"id": id}}
	case "change_list":
		rows, err := s.client.ListChanges(ctx, argInt(a, "status", 0))
		if err != nil {
			return toolError(err)
		}
		payload = map[string]any{"changes": rows}
	case "change_transition":
		id := argInt(a, "id", 0)
		status, ok := glpi.StatusID(argString(a, "status"))
		if !ok {
			status = argInt(a, "status", 0)
		}
		if id == 0 || status == 0 {
			return toolError(fmt.Errorf("change_transition requires id and status"))
		}
		if err := s.client.TransitionChange(ctx, id, status); err != nil {
			return toolError(err)
		}
		payload = map[string]any{"transitioned": map[string]any{"id": id, "status": glpi.StatusName(status)}}
	case "change_followup":
		id := argInt(a, "id", 0)
		content, _ := a["content"].(string)
		if id == 0 || content == "" {
			return toolError(fmt.Errorf("change_followup requires id and content"))
		}
		if err := s.client.AddFollowup(ctx, id, content); err != nil {
			return toolError(err)
		}
		payload = map[string]any{"followup": map[string]any{"items_id": id}}
	case "ci_search":
		rows, err := s.client.SearchCI(ctx, argString(a, "itemtype"), argString(a, "term"))
		if err != nil {
			return toolError(err)
		}
		type result struct {
			ID   int    `json:"id"`
			Name string `json:"name"`
		}
		results := make([]result, 0, len(rows))
		for _, r := range rows {
			results = append(results, result{ID: r.ID, Name: r.Name})
		}
		payload = map[string]any{"results": results}
	case "ci_show":
		id := argInt(a, "id", 0)
		obj, err := s.client.GetItem(ctx, argString(a, "itemtype"), id)
		if err != nil {
			return toolError(err)
		}
		payload = obj
	default:
		return toolError(fmt.Errorf("unknown tool: %s", name))
	}
	b, err := json.Marshal(payload)
	if err != nil {
		return toolError(fmt.Errorf("cannot encode tool result"))
	}
	return map[string]any{
		"content": []map[string]any{{"type": "text", "text": string(b)}},
		"isError": false,
	}
}

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

func argString(a map[string]any, key string) string {
	s, _ := a[key].(string)
	return s
}

func argInt(a map[string]any, key string, def int) int {
	switch v := a[key].(type) {
	case float64:
		return int(v)
	case string:
		if n, err := strconv.Atoi(v); err == nil {
			return n
		}
	}
	return def
}

// toolDefs is the advertised tool surface (stable order).
func toolDefs() []map[string]any {
	schema := func(required ...string) map[string]any {
		props := map[string]any{}
		for _, p := range required {
			props[p] = map[string]any{"type": "string"}
		}
		return map[string]any{
			"type":       "object",
			"properties": props,
			"required":   required,
		}
	}
	full := func(req []string, opt map[string]string) map[string]any {
		props := map[string]any{}
		for _, p := range req {
			props[p] = map[string]any{"type": "string"}
		}
		for p, t := range opt {
			props[p] = map[string]any{"type": t}
		}
		return map[string]any{
			"type":       "object",
			"properties": props,
			"required":   req,
		}
	}
	return []map[string]any{
		{"name": "change_create", "description": "Create a GLPI change (urgency/impact 1-5, 3=medium)", "inputSchema": full([]string{"title"}, map[string]string{"content": "string", "urgency": "integer", "impact": "integer"})},
		{"name": "change_list", "description": "List GLPI changes (optional status filter)", "inputSchema": schema()},
		{"name": "change_transition", "description": "Move a change to a status (name or numeric id)", "inputSchema": schema("id", "status")},
		{"name": "change_followup", "description": "Append a followup note to a change", "inputSchema": schema("id", "content")},
		{"name": "ci_search", "description": "Search CIs of an itemtype by name substring", "inputSchema": schema("itemtype", "term")},
		{"name": "ci_show", "description": "Fetch one CI raw by itemtype and id", "inputSchema": schema("itemtype", "id")},
	}
}

// writeLine emits one compact JSON response line.
func writeLine(out io.Writer, v any) error {
	b, err := json.Marshal(v)
	if err != nil {
		return err
	}
	_, err = out.Write(append(b, '\n'))
	return err
}