package relay

import (
	"bufio"
	"bytes"
	"context"
	"encoding/base64"
	"encoding/json"
	"fmt"
	"io"
	"net/http"
	"strings"
	"time"

	"github.com/rs/zerolog"
)

const (
	// maxProxyBodySize is the maximum request/response body size before truncation.
	// Must fit inside a 64KB relay frame after base64 encoding (~33% expansion) and
	// JSON wrapper overhead (~500 bytes). 47KB * 4/3 ≈ 62.7KB + overhead ≈ 63.2KB < 64KB.
	maxProxyBodySize = 47 * 1024 // 47KB

	// proxyRequestTimeout is the per-request timeout for proxied HTTP calls.
	proxyRequestTimeout = 30 * time.Second
)

// ProxyRequest is the JSON payload inside a DATA frame from the app to the instance.
type ProxyRequest struct {
	ID      string            `json:"id"`
	Method  string            `json:"method"`
	Path    string            `json:"path"`
	Headers map[string]string `json:"headers,omitempty"`
	Body    string            `json:"body,omitempty"` // base64-encoded
}

// ProxyResponse is the JSON payload inside a DATA frame from the instance to the app.
type ProxyResponse struct {
	ID         string            `json:"id"`
	Status     int               `json:"status"`
	Headers    map[string]string `json:"headers,omitempty"`
	Body       string            `json:"body,omitempty"`        // base64-encoded
	Stream     bool              `json:"stream,omitempty"`      // true for all streaming chunks
	StreamDone bool              `json:"stream_done,omitempty"` // true for the final chunk
}

// HTTPProxy proxies DATA frame payloads to the local Pulse API.
type HTTPProxy struct {
	localAddr    string
	client       *http.Client // for normal request/response proxying
	streamClient *http.Client // for SSE streaming (no timeout)
	logger       zerolog.Logger
}

// Close releases idle HTTP connections owned by the proxy clients.
func (p *HTTPProxy) Close() {
	if p == nil {
		return
	}
	if p.client != nil {
		p.client.CloseIdleConnections()
	}
	if p.streamClient != nil {
		p.streamClient.CloseIdleConnections()
	}
}

// NewHTTPProxy creates a proxy that forwards requests to the given local address.
func NewHTTPProxy(localAddr string, logger zerolog.Logger) *HTTPProxy {
	return &HTTPProxy{
		localAddr: localAddr,
		client: &http.Client{
			Timeout: proxyRequestTimeout,
			CheckRedirect: func(req *http.Request, via []*http.Request) error {
				return http.ErrUseLastResponse
			},
		},
		streamClient: &http.Client{
			// No Timeout — streaming responses are long-lived.
			// Cancellation is handled via context.
			CheckRedirect: func(req *http.Request, via []*http.Request) error {
				return http.ErrUseLastResponse
			},
		},
		logger: logger,
	}
}

// HandleRequest processes a DATA frame payload as an HTTP request and returns the response payload.
// The apiToken is the validated token from the channel's CHANNEL_OPEN, injected as X-API-Token.
func (p *HTTPProxy) HandleRequest(payload []byte, apiToken string) ([]byte, error) {
	var req ProxyRequest
	if err := json.Unmarshal(payload, &req); err != nil {
		return p.errorResponse("", http.StatusBadRequest, "invalid request payload"), nil
	}

	req.Method = strings.TrimSpace(req.Method)
	req.Path = strings.TrimSpace(req.Path)

	if req.ID == "" || req.Method == "" || req.Path == "" {
		return p.errorResponse(req.ID, http.StatusBadRequest, "missing required fields (id, method, path)"), nil
	}

	// Ensure path starts with /
	if !strings.HasPrefix(req.Path, "/") {
		req.Path = "/" + req.Path
	}

	// Decode body if present
	var bodyReader io.Reader
	if req.Body != "" {
		bodyBytes, err := base64.StdEncoding.DecodeString(req.Body)
		if err != nil {
			return p.errorResponse(req.ID, http.StatusBadRequest, "invalid base64 body"), nil
		}
		if len(bodyBytes) > maxProxyBodySize {
			return p.errorResponse(req.ID, http.StatusRequestEntityTooLarge, "request body exceeds 47KB limit"), nil
		}
		bodyReader = bytes.NewReader(bodyBytes)
	}

	url := fmt.Sprintf("http://%s%s", p.localAddr, req.Path)
	httpReq, err := http.NewRequest(req.Method, url, bodyReader)
	if err != nil {
		return p.errorResponse(req.ID, http.StatusInternalServerError, "failed to create request"), nil
	}

	// Allowlist: only forward safe, content-describing headers.
	// Everything else is stripped to prevent auth-context leakage
	// (X-Proxy-Secret, X-Forwarded-*, Forwarded, Cookie, Authorization, etc.)
	for k, v := range req.Headers {
		headerName := strings.TrimSpace(k)
		if allowedProxyHeader(headerName) {
			httpReq.Header.Set(headerName, v)
		}
	}

	// Inject the API token for Pulse auth middleware
	httpReq.Header.Set("X-API-Token", apiToken)

	p.logger.Debug().
		Str("request_id", req.ID).
		Str("method", req.Method).
		Str("path", req.Path).
		Msg("proxying relay request to local API")

	resp, err := p.client.Do(httpReq)
	if err != nil {
		p.logger.Warn().Err(err).Str("request_id", req.ID).Msg("local API request failed")
		return p.errorResponse(req.ID, http.StatusBadGateway, "local API request failed"), nil
	}
	defer func() {
		if err := resp.Body.Close(); err != nil {
			p.logger.Warn().Err(err).Str("request_id", req.ID).Msg("Failed to close local API response body")
		}
	}()

	// Read response body with size limit
	limitedReader := io.LimitReader(resp.Body, maxProxyBodySize+1)
	respBody, err := io.ReadAll(limitedReader)
	if err != nil {
		return p.errorResponse(req.ID, http.StatusBadGateway, "failed to read response body"), nil
	}

	if len(respBody) > maxProxyBodySize {
		return p.errorResponse(req.ID, http.StatusRequestEntityTooLarge, "response body exceeds 47KB limit"), nil
	}

	// Build response headers (pick relevant ones)
	respHeaders := make(map[string]string)
	for _, key := range []string{"Content-Type", "X-Request-Id", "Cache-Control"} {
		if v := resp.Header.Get(key); v != "" {
			respHeaders[key] = v
		}
	}

	proxyResp := ProxyResponse{
		ID:      req.ID,
		Status:  resp.StatusCode,
		Headers: respHeaders,
	}
	if len(respBody) > 0 {
		proxyResp.Body = base64.StdEncoding.EncodeToString(respBody)
	}

	data, err := json.Marshal(proxyResp)
	if err != nil {
		return p.errorResponse(req.ID, http.StatusInternalServerError, "failed to marshal response"), nil
	}
	return data, nil
}

// HandleStreamRequest processes a DATA frame payload as an HTTP request and streams
// the response as multiple ProxyResponse frames via sendFrame. For non-SSE responses,
// it falls back to single-response behavior identical to HandleRequest.
func (p *HTTPProxy) HandleStreamRequest(ctx context.Context, payload []byte, apiToken string, sendFrame func([]byte)) error {
	var req ProxyRequest
	if err := json.Unmarshal(payload, &req); err != nil {
		sendFrame(p.errorResponse("", http.StatusBadRequest, "invalid request payload"))
		return nil
	}

	req.Method = strings.TrimSpace(req.Method)
	req.Path = strings.TrimSpace(req.Path)

	if req.ID == "" || req.Method == "" || req.Path == "" {
		sendFrame(p.errorResponse(req.ID, http.StatusBadRequest, "missing required fields (id, method, path)"))
		return nil
	}

	if !strings.HasPrefix(req.Path, "/") {
		req.Path = "/" + req.Path
	}

	var bodyReader io.Reader
	if req.Body != "" {
		bodyBytes, err := base64.StdEncoding.DecodeString(req.Body)
		if err != nil {
			sendFrame(p.errorResponse(req.ID, http.StatusBadRequest, "invalid base64 body"))
			return nil
		}
		if len(bodyBytes) > maxProxyBodySize {
			sendFrame(p.errorResponse(req.ID, http.StatusRequestEntityTooLarge, "request body exceeds 47KB limit"))
			return nil
		}
		bodyReader = bytes.NewReader(bodyBytes)
	}

	url := fmt.Sprintf("http://%s%s", p.localAddr, req.Path)
	httpReq, err := http.NewRequestWithContext(ctx, req.Method, url, bodyReader)
	if err != nil {
		sendFrame(p.errorResponse(req.ID, http.StatusInternalServerError, "failed to create request"))
		return nil
	}

	for k, v := range req.Headers {
		headerName := strings.TrimSpace(k)
		if allowedProxyHeader(headerName) {
			httpReq.Header.Set(headerName, v)
		}
	}
	httpReq.Header.Set("X-API-Token", apiToken)

	p.logger.Debug().
		Str("request_id", req.ID).
		Str("method", req.Method).
		Str("path", req.Path).
		Msg("proxying relay request (stream-capable)")

	resp, err := p.streamClient.Do(httpReq)
	if err != nil {
		p.logger.Warn().Err(err).Str("request_id", req.ID).Msg("local API request failed")
		sendFrame(p.errorResponse(req.ID, http.StatusBadGateway, "local API request failed"))
		return nil
	}
	defer func() {
		if err := resp.Body.Close(); err != nil {
			p.logger.Warn().Err(err).Str("request_id", req.ID).Msg("Failed to close streamed local API response body")
		}
	}()

	// Check if this is an SSE response
	ct := resp.Header.Get("Content-Type")
	if !strings.HasPrefix(ct, "text/event-stream") {
		// Non-streaming: read full body and send a single response (same as HandleRequest)
		limitedReader := io.LimitReader(resp.Body, maxProxyBodySize+1)
		respBody, err := io.ReadAll(limitedReader)
		if err != nil {
			sendFrame(p.errorResponse(req.ID, http.StatusBadGateway, "failed to read response body"))
			return nil
		}
		if len(respBody) > maxProxyBodySize {
			sendFrame(p.errorResponse(req.ID, http.StatusRequestEntityTooLarge, "response body exceeds 47KB limit"))
			return nil
		}

		respHeaders := make(map[string]string)
		for _, key := range []string{"Content-Type", "X-Request-Id", "Cache-Control"} {
			if v := resp.Header.Get(key); v != "" {
				respHeaders[key] = v
			}
		}

		proxyResp := ProxyResponse{
			ID:      req.ID,
			Status:  resp.StatusCode,
			Headers: respHeaders,
		}
		if len(respBody) > 0 {
			proxyResp.Body = base64.StdEncoding.EncodeToString(respBody)
		}
		data, err := json.Marshal(proxyResp)
		if err != nil {
			sendFrame(p.errorResponse(req.ID, http.StatusInternalServerError, "failed to marshal response"))
			return nil
		}
		sendFrame(data)
		return nil
	}

	// SSE streaming mode: send an initial header frame
	respHeaders := make(map[string]string)
	respHeaders["Content-Type"] = "text/event-stream"
	if v := resp.Header.Get("X-Request-Id"); v != "" {
		respHeaders["X-Request-Id"] = v
	}

	initResp := ProxyResponse{
		ID:      req.ID,
		Status:  resp.StatusCode,
		Headers: respHeaders,
		Stream:  true,
	}
	initData, err := json.Marshal(initResp)
	if err != nil {
		sendFrame(p.errorResponse(req.ID, http.StatusInternalServerError, "failed to marshal stream init"))
		return nil
	}
	sendFrame(initData)

	// Read SSE events line-by-line and forward as individual frames
	scanner := bufio.NewScanner(resp.Body)
	scanner.Buffer(make([]byte, maxProxyBodySize), maxProxyBodySize)

	var eventBuf strings.Builder

	for scanner.Scan() {
		// Check if context was cancelled (relay disconnected)
		if ctx.Err() != nil {
			return ctx.Err()
		}

		line := scanner.Text()

		if line == "" {
			// Empty line = end of SSE event
			if eventBuf.Len() > 0 {
				eventText := eventBuf.String()
				eventBuf.Reset()

				chunk := ProxyResponse{
					ID:     req.ID,
					Status: resp.StatusCode,
					Body:   base64.StdEncoding.EncodeToString([]byte(eventText)),
					Stream: true,
				}
				chunkData, err := json.Marshal(chunk)
				if err != nil {
					p.logger.Warn().Err(err).Msg("failed to marshal SSE chunk")
					continue
				}
				sendFrame(chunkData)
			}
		} else {
			// Bound total buffered event size, not just per-line scanner token size.
			// Without this, many small lines before a blank separator can grow
			// eventBuf unbounded and exhaust memory.
			added := len(line)
			if eventBuf.Len() > 0 {
				added++ // newline separator inserted between lines
			}
			if eventBuf.Len()+added > maxProxyBodySize {
				sendFrame(p.errorResponse(req.ID, http.StatusRequestEntityTooLarge, "stream event exceeds 47KB limit"))
				return nil
			}

			if eventBuf.Len() > 0 {
				eventBuf.WriteByte('\n')
			}
			eventBuf.WriteString(line)
		}
	}

	// Check for scanner error before sending completion.
	// If scanning failed (e.g. token too long, transport read error), send an
	// error response instead of stream_done so the client knows it's incomplete.
	if err := scanner.Err(); err != nil {
		if ctx.Err() != nil {
			return ctx.Err()
		}
		p.logger.Warn().Err(err).Str("request_id", req.ID).Msg("SSE scanner error")
		sendFrame(p.errorResponse(req.ID, http.StatusBadGateway, "stream read error"))
		return nil
	}

	// Flush any remaining buffered event
	if eventBuf.Len() > 0 {
		eventText := eventBuf.String()
		chunk := ProxyResponse{
			ID:     req.ID,
			Status: resp.StatusCode,
			Body:   base64.StdEncoding.EncodeToString([]byte(eventText)),
			Stream: true,
		}
		chunkData, _ := json.Marshal(chunk)
		sendFrame(chunkData)
	}

	// Send stream-done frame (only on clean completion)
	doneResp := ProxyResponse{
		ID:         req.ID,
		Status:     resp.StatusCode,
		StreamDone: true,
	}
	doneData, _ := json.Marshal(doneResp)
	sendFrame(doneData)

	return nil
}

// allowedProxyHeaders is the set of headers that may be forwarded from relay
// requests to the local Pulse API. All other headers are stripped to prevent
// auth-context leakage (X-Proxy-Secret, X-Forwarded-*, etc.).
var allowedProxyHeaders = map[string]bool{
	"accept":            true,
	"accept-encoding":   true,
	"accept-language":   true,
	"content-type":      true,
	"content-length":    true,
	"if-match":          true,
	"if-none-match":     true,
	"if-modified-since": true,
}

func allowedProxyHeader(name string) bool {
	return allowedProxyHeaders[strings.ToLower(strings.TrimSpace(name))]
}

func (p *HTTPProxy) errorResponse(requestID string, status int, message string) []byte {
	resp := ProxyResponse{
		ID:     requestID,
		Status: status,
		Headers: map[string]string{
			"Content-Type": "application/json",
		},
	}
	body, _ := json.Marshal(map[string]string{"error": message})
	resp.Body = base64.StdEncoding.EncodeToString(body)
	data, _ := json.Marshal(resp)
	return data
}