package mcp

import (
	"bufio"
	"bytes"
	"context"
	"encoding/json"
	"fmt"
	"io"
	"net/http"
	"strings"
	"sync/atomic"
)

// HTTPConfig configures a Streamable HTTP client transport.
type HTTPConfig struct {
	Endpoint string
	Headers  map[string]string
	Client   *http.Client
}

// httpTransport talks MCP over Streamable HTTP: each outbound message is an
// HTTP POST whose response is either a single JSON message or an SSE stream.
type httpTransport struct {
	endpoint string
	headers  map[string]string
	client   *http.Client

	counter   int64
	sessionID atomic.Value // string
	notify    atomic.Pointer[func(*Message)]
}

// NewHTTPTransport creates a Streamable HTTP transport to endpoint.
func NewHTTPTransport(cfg HTTPConfig) (Transport, error) {
	if cfg.Endpoint == "" {
		return nil, fmt.Errorf("http transport: empty endpoint")
	}
	c := cfg.Client
	if c == nil {
		c = http.DefaultClient
	}
	return &httpTransport{endpoint: cfg.Endpoint, headers: cfg.Headers, client: c}, nil
}

func (t *httpTransport) SetNotificationHandler(h func(*Message)) {
	if h == nil {
		t.notify.Store(nil)
		return
	}
	t.notify.Store(&h)
}

func (t *httpTransport) post(ctx context.Context, msg *Message) (*http.Response, error) {
	body, err := json.Marshal(msg)
	if err != nil {
		return nil, err
	}
	req, err := http.NewRequestWithContext(ctx, http.MethodPost, t.endpoint, bytes.NewReader(body))
	if err != nil {
		return nil, err
	}
	req.Header.Set("Content-Type", "application/json")
	req.Header.Set("Accept", "application/json, text/event-stream")
	for k, v := range t.headers {
		req.Header.Set(k, v)
	}
	if sid, ok := t.sessionID.Load().(string); ok && sid != "" {
		req.Header.Set("Mcp-Session-Id", sid)
	}
	return t.client.Do(req)
}

func (t *httpTransport) Call(ctx context.Context, method string, params any) (*Message, error) {
	id := atomic.AddInt64(&t.counter, 1)
	idRaw := json.RawMessage(fmt.Sprintf("%d", id))
	req, err := NewRequest(idRaw, method, params)
	if err != nil {
		return nil, err
	}
	resp, err := t.post(ctx, req)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()

	if sid := resp.Header.Get("Mcp-Session-Id"); sid != "" {
		t.sessionID.Store(sid)
	}
	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
		b, _ := io.ReadAll(io.LimitReader(resp.Body, 4096))
		return nil, fmt.Errorf("http %s: status %d: %s", method, resp.StatusCode, strings.TrimSpace(string(b)))
	}

	ct := resp.Header.Get("Content-Type")
	want := normalizeID(idRaw)
	switch {
	case strings.HasPrefix(ct, "text/event-stream"):
		return t.readSSE(resp.Body, want)
	default:
		var m Message
		if err := json.NewDecoder(resp.Body).Decode(&m); err != nil {
			return nil, fmt.Errorf("http %s: decode: %w", method, err)
		}
		return &m, nil
	}
}

// readSSE consumes an SSE stream, dispatching peer notifications to the handler
// and returning the response whose id matches want.
func (t *httpTransport) readSSE(r io.Reader, want string) (*Message, error) {
	scanner := bufio.NewScanner(r)
	scanner.Buffer(make([]byte, 0, 64*1024), 16*1024*1024)
	var data strings.Builder
	flush := func() (*Message, bool, error) {
		if data.Len() == 0 {
			return nil, false, nil
		}
		payload := data.String()
		data.Reset()
		var m Message
		if err := json.Unmarshal([]byte(payload), &m); err != nil {
			return nil, false, nil // ignore non-JSON events
		}
		if m.IsResponse() && normalizeID(m.ID) == want {
			return &m, true, nil
		}
		if m.Method != "" {
			if hp := t.notify.Load(); hp != nil {
				(*hp)(&m)
			}
		}
		return nil, false, nil
	}
	for scanner.Scan() {
		line := scanner.Text()
		if line == "" { // event delimiter
			if m, ok, _ := flush(); ok {
				return m, nil
			}
			continue
		}
		if strings.HasPrefix(line, "data:") {
			data.WriteString(strings.TrimSpace(line[len("data:"):]))
		}
	}
	if err := scanner.Err(); err != nil {
		return nil, err
	}
	if m, ok, _ := flush(); ok {
		return m, nil
	}
	return nil, fmt.Errorf("sse stream ended without response for id %s", want)
}

func (t *httpTransport) Notify(ctx context.Context, method string, params any) error {
	note, err := NewNotification(method, params)
	if err != nil {
		return err
	}
	resp, err := t.post(ctx, note)
	if err != nil {
		return err
	}
	defer resp.Body.Close()
	io.Copy(io.Discard, io.LimitReader(resp.Body, 1<<20))
	if resp.StatusCode >= 400 {
		return fmt.Errorf("http notify %s: status %d", method, resp.StatusCode)
	}
	return nil
}

func (t *httpTransport) Close() error { return nil }