package middleware

import (
	"bytes"
	"crypto/rand"
	"encoding/hex"
	"encoding/json"
	"net/http"
	"strings"

	"github.com/gin-gonic/gin"
)

// Business code constants
const (
	CodeSuccess = 0

	// Common errors (1xxx)
	CodeInvalidParam = 1001
	CodeUnauthorized = 1002
	CodeForbidden    = 1003
	CodeRateLimited  = 1004

	// Client errors (4xxx)
	CodeResourceNotFound = 4001
	CodeResourceConflict = 4002
	CodeInvalidState     = 4003

	// Server errors (5xxx)
	CodeInternalError      = 5001
	CodeServiceUnavailable = 5002
	CodeTimeout            = 5003
)

const (
	businessCodeKey = "response_business_code"
	errorDetailsKey = "response_error_details"
	errorMessageKey = "response_error_message"
)

// Paths that should not be wrapped
var excludedPaths = map[string]bool{
	"/health":           true,
	"/debug/vars":       true,
	"/internal/metrics": true,
}

type responseEnvelope struct {
	Code    int             `json:"code"`
	Message string          `json:"message"`
	Data    json.RawMessage `json:"data"`
	TraceID string          `json:"trace_id"`
	Details any             `json:"details,omitempty"`
}

type envelopeWriter struct {
	gin.ResponseWriter
	body   bytes.Buffer
	status int
	size   int
	wrote  bool
}

func (w *envelopeWriter) WriteHeader(code int) {
	w.status = code
	w.wrote = true
}

func (w *envelopeWriter) WriteHeaderNow() {
	if !w.wrote {
		w.WriteHeader(http.StatusOK)
	}
}

func (w *envelopeWriter) Write(data []byte) (int, error) {
	if !w.wrote {
		w.WriteHeader(http.StatusOK)
	}
	n, err := w.body.Write(data)
	w.size += n
	return n, err
}

func (w *envelopeWriter) WriteString(s string) (int, error) {
	if !w.wrote {
		w.WriteHeader(http.StatusOK)
	}
	n, err := w.body.WriteString(s)
	w.size += n
	return n, err
}

func (w *envelopeWriter) Status() int {
	if w.status == 0 {
		return http.StatusOK
	}
	return w.status
}

func (w *envelopeWriter) Size() int {
	return w.size
}

func (w *envelopeWriter) Written() bool {
	return w.wrote
}

// SetBusinessCode sets an explicit business code for the response envelope.
func SetBusinessCode(c *gin.Context, code int) {
	if c == nil {
		return
	}
	c.Set(businessCodeKey, code)
}

// SetErrorDetails sets additional error details for the response envelope.
func SetErrorDetails(c *gin.Context, details any) {
	if c == nil || details == nil {
		return
	}
	c.Set(errorDetailsKey, details)
}

// SetErrorMessage sets a custom error message for the response envelope.
func SetErrorMessage(c *gin.Context, message string) {
	if c == nil {
		return
	}
	c.Set(errorMessageKey, message)
}

func ResponseEnvelope() gin.HandlerFunc {
	return func(c *gin.Context) {
		// Skip excluded paths
		if isExcludedPath(c.Request.URL.Path) {
			c.Next()
			return
		}

		originalWriter := c.Writer
		writer := &envelopeWriter{ResponseWriter: originalWriter}
		c.Writer = writer

		c.Next()

		status := writer.Status()
		body := writer.body.Bytes()
		if !bodyAllowedForStatus(status) || len(body) == 0 {
			writeStatusOnly(originalWriter, status)
			return
		}

		contentType := originalWriter.Header().Get("Content-Type")
		if !isJSONContentType(contentType) {
			writeThrough(originalWriter, status, body)
			return
		}

		obj, objOK := parseObject(body)
		if objOK && isEnvelopeObject(obj) {
			writeThrough(originalWriter, status, body)
			return
		}

		code := businessCodeFromContext(c)
		if code == 0 {
			code = defaultBusinessCode(status)
		}

		traceID := getTraceID(c)
		isError := status >= http.StatusBadRequest

		var message string
		if customMsg := errorMessageFromContext(c); customMsg != "" {
			message = customMsg
		} else if isError && objOK {
			if raw, ok := obj["error"]; ok {
				var msg string
				if err := json.Unmarshal(raw, &msg); err == nil {
					message = msg
				}
			}
		}
		if message == "" {
			if isError {
				message = http.StatusText(status)
			} else {
				message = "success"
			}
		}

		// Get details: from context first, then from original response
		details := errorDetailsFromContext(c)
		if details == nil && isError && objOK {
			if raw, ok := obj["details"]; ok {
				var d any
				if err := json.Unmarshal(raw, &d); err == nil {
					details = d
				}
			}
		}

		var data json.RawMessage
		if isError {
			data = json.RawMessage("null")
		} else {
			data = json.RawMessage(body)
		}

		envelope := responseEnvelope{
			Code:    code,
			Message: message,
			Data:    data,
			TraceID: traceID,
			Details: details,
		}
		payload, err := json.Marshal(envelope)
		if err != nil {
			writeThrough(originalWriter, status, body)
			return
		}

		originalWriter.Header().Set("Content-Type", "application/json; charset=utf-8")
		originalWriter.Header().Del("Content-Length")
		writeThrough(originalWriter, status, payload)
	}
}

func isExcludedPath(path string) bool {
	if excludedPaths[path] {
		return true
	}
	// Also exclude swagger paths
	if strings.HasPrefix(path, "/swagger") {
		return true
	}
	return false
}

func businessCodeFromContext(c *gin.Context) int {
	if c == nil {
		return 0
	}
	value, ok := c.Get(businessCodeKey)
	if !ok {
		return 0
	}
	code, ok := value.(int)
	if !ok {
		return 0
	}
	return code
}

func errorDetailsFromContext(c *gin.Context) any {
	if c == nil {
		return nil
	}
	value, ok := c.Get(errorDetailsKey)
	if !ok {
		return nil
	}
	return value
}

func errorMessageFromContext(c *gin.Context) string {
	if c == nil {
		return ""
	}
	value, ok := c.Get(errorMessageKey)
	if !ok {
		return ""
	}
	msg, ok := value.(string)
	if !ok {
		return ""
	}
	return msg
}

func getTraceID(c *gin.Context) string {
	if c == nil {
		return generateTraceID()
	}
	// Try to get from context first (set by RequestID middleware)
	if id, ok := c.Get("request_id"); ok {
		if s, ok := id.(string); ok && s != "" {
			return s
		}
	}
	// Fallback to header
	if id := c.GetHeader("X-Request-ID"); id != "" {
		return id
	}
	// Generate UUID as fallback
	return generateTraceID()
}

func generateTraceID() string {
	b := make([]byte, 16)
	_, _ = rand.Read(b)
	return hex.EncodeToString(b)
}

func defaultBusinessCode(status int) int {
	switch {
	case status >= http.StatusOK && status < http.StatusMultipleChoices:
		return CodeSuccess
	case status == http.StatusBadRequest:
		return CodeInvalidParam
	case status == http.StatusUnauthorized:
		return CodeUnauthorized
	case status == http.StatusForbidden:
		return CodeForbidden
	case status == http.StatusNotFound:
		return CodeResourceNotFound
	case status == http.StatusConflict:
		return CodeResourceConflict
	case status == http.StatusTooManyRequests:
		return CodeRateLimited
	case status >= http.StatusBadRequest && status < http.StatusInternalServerError:
		return CodeInvalidParam
	case status == http.StatusServiceUnavailable:
		return CodeServiceUnavailable
	case status == http.StatusGatewayTimeout:
		return CodeTimeout
	case status >= http.StatusInternalServerError:
		return CodeInternalError
	default:
		return CodeSuccess
	}
}

func bodyAllowedForStatus(status int) bool {
	switch {
	case status >= 100 && status <= 199:
		return false
	case status == http.StatusNoContent:
		return false
	case status == http.StatusNotModified:
		return false
	}
	return true
}

func isJSONContentType(contentType string) bool {
	if contentType == "" {
		return false
	}
	return strings.Contains(strings.ToLower(contentType), "application/json")
}

func parseObject(body []byte) (map[string]json.RawMessage, bool) {
	var obj map[string]json.RawMessage
	if err := json.Unmarshal(body, &obj); err != nil {
		return nil, false
	}
	return obj, true
}

func isEnvelopeObject(obj map[string]json.RawMessage) bool {
	if obj == nil {
		return false
	}
	// Only recognize new envelope format: must have trace_id and code must be number
	codeRaw, hasCode := obj["code"]
	_, hasData := obj["data"]
	_, hasMessage := obj["message"]
	_, hasTraceID := obj["trace_id"]

	if !hasCode || !hasData || !hasMessage || !hasTraceID {
		return false
	}

	// Code must be a number (int)
	var codeNum int
	if json.Unmarshal(codeRaw, &codeNum) != nil {
		return false
	}

	return true
}

func writeStatusOnly(w gin.ResponseWriter, status int) {
	w.WriteHeader(status)
}

func writeThrough(w gin.ResponseWriter, status int, body []byte) {
	w.WriteHeader(status)
	_, _ = w.Write(body)
}