package mcp

import (
	"bufio"
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"strings"
	"sync"
)

// Poster is the HTTP seam: internal/webfetch.Fetcher satisfies it. The
// transport takes it as an interface so a test can serve a fake without a
// listener, and so that the ONLY implementation wired in production is the
// guarded fetcher — an MCP endpoint cannot get a bare http.Client this way.
type Poster interface {
	Post(ctx context.Context, rawURL, contentType string, body []byte, hdr map[string]string) (*PostResponse, error)
}

// PostResponse is the shape webfetch returns, restated here so this package
// does not depend on it structurally.
type PostResponse struct {
	Status      int
	ContentType string
	Body        []byte
	Header      map[string]string
}

// httpTransport speaks streamable HTTP: every request is a POST to one
// endpoint, and the reply is either a JSON object or a text/event-stream frame
// carrying one. Both are accepted — servers pick per response, and the two the
// LAN runs disagree about which.
type httpTransport struct {
	poster Poster
	url    string
	extra  map[string]string // static headers, e.g. an Authorization bearer

	mu      sync.Mutex
	session string // Mcp-Session-Id, echoed back when the server issues one
}

func newHTTPTransport(post Poster, endpoint string, extra map[string]string) *httpTransport {
	return &httpTransport{poster: post, url: endpoint, extra: extra}
}

func (t *httpTransport) Call(ctx context.Context, req *rpcRequest) (*rpcResponse, error) {
	body, err := t.send(ctx, req)
	if err != nil {
		return nil, err
	}
	frame, err := decodeFrame(body, req.ID)
	if err != nil {
		return nil, err
	}
	var resp rpcResponse
	if err := json.Unmarshal(frame, &resp); err != nil {
		return nil, fmt.Errorf("mcp: decode response: %w", err)
	}
	// The id check the stdio transport already did. Without it a server that
	// sends a request of its own (sampling/createMessage, roots/list) mid-stream
	// has that request accepted as the answer: it unmarshals into an rpcResponse
	// with neither result nor error, and the call reports success with nothing
	// in it. An empty string and no error is the one answer that lies — the act
	// is logged as done and the tool never ran.
	if resp.ID == nil || *resp.ID != req.ID {
		return nil, fmt.Errorf("mcp: response id mismatch (wanted %d)", req.ID)
	}
	if resp.Error == nil && len(resp.Result) == 0 {
		return nil, errors.New("mcp: response carries neither result nor error")
	}
	return &resp, nil
}

func (t *httpTransport) Notify(ctx context.Context, method string, params any) error {
	_, err := t.send(ctx, &rpcRequest{JSONRPC: "2.0", Method: method, Params: params})
	return err
}

func (t *httpTransport) send(ctx context.Context, req *rpcRequest) ([]byte, error) {
	req.JSONRPC = "2.0"
	raw, err := json.Marshal(req)
	if err != nil {
		return nil, err
	}
	hdr := map[string]string{}
	// Configured headers first, so nothing here can be overwritten by them:
	// a real remote server needs a bearer token, and the Vikunja one on
	// loopback is only reachable without one because it is unauthenticated.
	for k, v := range t.extra {
		hdr[k] = v
	}
	hdr["Accept"] = "application/json, text/event-stream"
	t.mu.Lock()
	if t.session != "" {
		hdr["Mcp-Session-Id"] = t.session
	}
	t.mu.Unlock()

	resp, err := t.poster.Post(ctx, t.url, "application/json", raw, hdr)
	if err != nil {
		return nil, err
	}
	if sid := headerGet(resp.Header, "Mcp-Session-Id"); sid != "" {
		t.mu.Lock()
		t.session = sid
		t.mu.Unlock()
	}
	return resp.Body, nil
}

func (t *httpTransport) Close() error {
	t.mu.Lock()
	t.session = ""
	t.mu.Unlock()
	return nil
}

func headerGet(h map[string]string, key string) string {
	if h == nil {
		return ""
	}
	if v, ok := h[key]; ok {
		return v
	}
	lower := strings.ToLower(key)
	for k, v := range h {
		if strings.ToLower(k) == lower {
			return v
		}
	}
	return ""
}

// decodeFrame pulls the JSON object out of a body that is either raw JSON or
// SSE. For SSE we take the last data: payload that parses AND carries our own
// id with a result or an error in it. Matching on the presence of an "id" key
// alone is not enough: a JSON-RPC request from the server has one too.
func decodeFrame(body []byte, id int64) ([]byte, error) {
	trimmed := bytes.TrimSpace(body)
	if len(trimmed) == 0 {
		return nil, errors.New("mcp: empty response body")
	}
	if trimmed[0] == '{' || trimmed[0] == '[' {
		return trimmed, nil
	}
	var last []byte
	sc := bufio.NewScanner(bytes.NewReader(trimmed))
	sc.Buffer(make([]byte, 0, 64<<10), maxLine)
	for sc.Scan() {
		line := strings.TrimSpace(sc.Text())
		if !strings.HasPrefix(line, "data:") {
			continue
		}
		payload := strings.TrimSpace(strings.TrimPrefix(line, "data:"))
		if payload == "" {
			continue
		}
		var probe struct {
			ID     *int64          `json:"id"`
			Result json.RawMessage `json:"result"`
			Error  json.RawMessage `json:"error"`
			Method string          `json:"method"`
		}
		if json.Unmarshal([]byte(payload), &probe) != nil {
			continue
		}
		if probe.Method != "" || probe.ID == nil || *probe.ID != id {
			continue
		}
		if len(probe.Result) == 0 && len(probe.Error) == 0 {
			continue
		}
		last = []byte(payload)
	}
	if err := sc.Err(); err != nil {
		return nil, fmt.Errorf("mcp: read event stream: %w", err)
	}
	if last == nil {
		return nil, fmt.Errorf("mcp: no JSON-RPC response for id %d in event stream", id)
	}
	return last, nil
}