package server

import (
	"context"
	"encoding/json"
	"fmt"
	"net/http"
	"strings"
	"time"

	"github.com/edgeai/gateway/internal/adapter"
	"github.com/edgeai/gateway/internal/auth"
	"github.com/edgeai/gateway/internal/config"
	ctxasm "github.com/edgeai/gateway/internal/context"
	"github.com/edgeai/gateway/internal/handler"
	"github.com/edgeai/gateway/internal/middleware"
	"github.com/edgeai/gateway/internal/observability"
	"github.com/edgeai/gateway/internal/router"
	"github.com/edgeai/gateway/internal/session"
	"github.com/edgeai/gateway/internal/task"
	"github.com/edgeai/gateway/pkg/api"
	"github.com/google/uuid"
)

func (s *Server) handleChatCompletions(w http.ResponseWriter, r *http.Request) {
	if r.Method != http.MethodPost {
		handler.WriteError(w, handler.NewGatewayError(handler.ErrInvalidRequest, "method not allowed"))
		return
	}

	requestID := middleware.GetRequestID(r.Context())
	identity := auth.GetAppIdentityFromRequest(r)

	var req api.ChatRequest
	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
		handler.WriteError(w, handler.NewGatewayErrorWithID(handler.ErrInvalidRequest, "invalid JSON body", requestID))
		return
	}

	// Validate required fields
	if req.Model == "" {
		handler.WriteError(w, handler.NewGatewayErrorWithID(handler.ErrInvalidRequest, "model is required", requestID))
		return
	}
	if len(req.Messages) == 0 {
		handler.WriteError(w, handler.NewGatewayErrorWithID(handler.ErrInvalidRequest, "messages is required", requestID))
		return
	}

	// Check model permission
	if identity != nil && !auth.CheckModelPermission(identity, req.Model) {
		handler.WriteError(w, handler.NewGatewayErrorWithID(handler.ErrPermissionDenied, "model not allowed for this application", requestID))
		return
	}

	// Resolve logical model
	target, err := s.modelMap.Resolve(req.Model)
	if err != nil {
		handler.WriteError(w, handler.NewGatewayErrorWithID(handler.ErrModelUnavailable, err.Error(), requestID))
		return
	}

	// Get adapter
	adapterInst, err := s.registry.Get(target.Provider)
	if err != nil {
		handler.WriteError(w, handler.NewGatewayErrorWithID(handler.ErrModelUnavailable, err.Error(), requestID))
		return
	}

	// Parse priority
	priority := config.ParsePriority(req.Priority)
	if priority == 0 && identity != nil && !auth.CheckPriorityPermission(identity, 0) {
		priority = int(task.PriorityNormal) // downgrade to P2 if not allowed P0
	}

	// Backpressure check: reject low-priority requests under high load
	s.backpressure.Update(s.scheduler.RunningCount(), s.scheduler.QueueLength())
	if !s.backpressure.ShouldAccept(priority) {
		reason := s.backpressure.RejectReason(priority)
		s.metrics.IncRequest("backpressure_rejected")
		handler.WriteError(w, handler.NewGatewayErrorWithID(handler.ErrQueueFull, reason, requestID))
		return
	}

	// Circuit breaker check
	if !s.breaker.AllowRequest() {
		s.metrics.IncRequest("circuit_open")
		handler.WriteError(w, handler.NewGatewayErrorWithID(handler.ErrModelUnavailable, "circuit breaker open, please retry later", requestID))
		return
	}

	// Create task
	taskID := uuid.New().String()
	tk := task.NewTask(taskID, requestID, identity.AppID, identity.TenantID, req.Model, task.TaskPriority(priority), req.Stream)
	tk.SessionID = req.SessionID

	// Submit to scheduler
	if err := s.scheduler.Submit(tk); err != nil {
		s.metrics.IncRequest("queue_full")
		handler.WriteError(w, handler.NewGatewayErrorWithID(handler.ErrQueueFull, "queue is full, please retry later", requestID))
		return
	}

	s.metrics.SetQueueLength(s.scheduler.QueueLength())

	// Wait for task to be dequeued
	queueStart := time.Now()
	ctx, cancel := context.WithTimeout(r.Context(), time.Duration(s.cfg.Timeouts.DefaultQueueMs)*time.Millisecond)
	defer cancel()

	dequeued, err := s.scheduler.GetNext(ctx)
	if err != nil {
		s.scheduler.Complete(tk.ID)
		s.metrics.IncRequest("queue_timeout")
		tk.Transition(task.StateTimedOut)
		handler.WriteError(w, handler.NewGatewayErrorWithID(handler.ErrQueueTimeout, "queue timeout", requestID))
		return
	}

	// Record queue time
	dequeued.QueueMs = int(time.Since(queueStart) / time.Millisecond)

	// Transition to RUNNING
	dequeued.Transition(task.StateRunning)
	s.metrics.SetRunningTasks(s.scheduler.RunningCount())

	// Build adapter request — assemble context if session_id provided
	messages := req.Messages
	if req.SessionID != "" {
		sess, err := s.sessions.Get(req.SessionID)
		if err == nil && sess != nil {
			// Load session history and assemble context
			history := s.loadSessionHistory(sess)
			if len(history) > 0 {
				assembler := ctxasm.NewAssembler(&s.cfg.Context)
				result := assembler.Assemble(history, req.Messages, target.ContextWindow, target.MaxOutputTokens, req.ContextPolicy)
				messages = result.Messages
			}
		}
	}

	adapterReq := &adapter.ChatRequest{
		RequestID:   requestID,
		Model:       target.ActualModel,
		Messages:    messages,
		MaxTokens:   target.MaxOutputTokens,
		Temperature: req.Temperature,
		TopP:        req.TopP,
		Stream:      req.Stream,
		CancelCh:    dequeued.Cancelled(),
	}

	if req.MaxOutputTokens > 0 {
		adapterReq.MaxTokens = req.MaxOutputTokens
	}

	if req.Stream {
		s.handleStreaming(w, r, adapterInst, adapterReq, dequeued, requestID, target, req.Model)
	} else {
		s.handleNonStreaming(w, r, adapterInst, adapterReq, dequeued, requestID, target, req.Model)
	}
}

func (s *Server) handleStreaming(w http.ResponseWriter, r *http.Request, adapterInst adapter.ModelAdapter, req *adapter.ChatRequest, tk *task.Task, requestID string, _ *router.ModelTarget, logicalModel string) {
	sse := handler.NewSSEWriter(w)
	if sse == nil {
		s.scheduler.Complete(tk.ID)
		handler.WriteError(w, handler.NewGatewayErrorWithID(handler.ErrInternalError, "streaming not supported", requestID))
		return
	}

	tk.Transition(task.StateStreaming)
	inferenceStart := time.Now()

	ch, err := adapterInst.ChatCompletionStream(r.Context(), req)
	if err != nil {
		s.scheduler.Complete(tk.ID)
		tk.Transition(task.StateFailed)
		s.metrics.IncTask("failed")
		handler.WriteError(w, handler.NewGatewayErrorWithID(handler.ErrModelUnavailable, err.Error(), requestID))
		return
	}

	inputTokens, outputTokens, err := handler.StreamChatCompletion(sse, ch, requestID, tk.ID, logicalModel)
	if err != nil {
		s.logger.Error("streaming error", observability.F().Event("stream_error").TaskID(tk.ID).Reason(err.Error()))
		tk.Transition(task.StateFailed)
		s.metrics.IncTask("failed")
		s.breaker.RecordError()
	} else {
		tk.Transition(task.StateCompleted)
		s.metrics.IncTask("completed")
		s.breaker.RecordSuccess()
	}

	tk.InferenceMs = int(time.Since(inferenceStart) / time.Millisecond)
	tk.TotalMs = int(time.Since(tk.CreatedAt) / time.Millisecond)

	s.metrics.AddTokens(inputTokens, outputTokens)
	s.scheduler.Complete(tk.ID)
	s.metrics.SetRunningTasks(s.scheduler.RunningCount())
	s.metrics.SetQueueLength(s.scheduler.QueueLength())
	s.metrics.IncRequest("stream_ok")
}

func (s *Server) handleNonStreaming(w http.ResponseWriter, r *http.Request, adapterInst adapter.ModelAdapter, req *adapter.ChatRequest, tk *task.Task, requestID string, target *router.ModelTarget, logicalModel string) {
	ctx, cancel := context.WithTimeout(r.Context(), time.Duration(s.cfg.Timeouts.DefaultInferenceMs)*time.Millisecond)
	defer cancel()

	inferenceStart := time.Now()

	resp, err := adapterInst.ChatCompletion(ctx, req)
	if err != nil {
		// Try overload fallback strategies
		fallbackResp, fbErr := s.tryOverloadFallback(r.Context(), req, target, logicalModel)
		if fbErr == nil && fallbackResp != nil {
			tk.Degraded = true
			tk.InferenceMs = int(time.Since(inferenceStart) / time.Millisecond)
			tk.TotalMs = int(time.Since(tk.CreatedAt) / time.Millisecond)
			s.scheduler.Complete(tk.ID)
			tk.Transition(task.StateCompleted)
			s.metrics.IncTask("completed")
			s.metrics.IncRequest("ok")
			s.metrics.IncRequest("overload_fallback")
			s.metrics.AddTokens(fallbackResp.InputTokens, fallbackResp.OutputTokens)
			s.breaker.RecordSuccess()
			s.scheduler.Complete(tk.ID)
			s.metrics.SetRunningTasks(s.scheduler.RunningCount())
			s.metrics.SetQueueLength(s.scheduler.QueueLength())

			chatResp := handler.BuildChatResponse(requestID, tk.ID, logicalModel, fallbackResp)
			chatResp.Degraded = true
			chatResp.Timing = s.buildTiming(tk)
			handler.WriteJSON(w, http.StatusOK, chatResp)
			return
		}

		s.scheduler.Complete(tk.ID)
		tk.Transition(task.StateFailed)
		s.metrics.IncTask("failed")
		s.metrics.IncRequest("error")
		s.breaker.RecordError()

		if strings.Contains(err.Error(), "timeout") || ctx.Err() != nil {
			tk.Transition(task.StateTimedOut)
			handler.WriteError(w, handler.NewGatewayErrorWithID(handler.ErrInferenceTimeout, "inference timeout", requestID))
		} else {
			handler.WriteError(w, handler.NewGatewayErrorWithID(handler.ErrModelUnavailable, err.Error(), requestID))
		}
		return
	}

	tk.InferenceMs = int(time.Since(inferenceStart) / time.Millisecond)
	tk.TotalMs = int(time.Since(tk.CreatedAt) / time.Millisecond)

	tk.Transition(task.StateCompleted)
	s.metrics.IncTask("completed")
	s.metrics.IncRequest("ok")
	s.metrics.AddTokens(resp.InputTokens, resp.OutputTokens)
	s.breaker.RecordSuccess()
	s.scheduler.Complete(tk.ID)
	s.metrics.SetRunningTasks(s.scheduler.RunningCount())
	s.metrics.SetQueueLength(s.scheduler.QueueLength())

	chatResp := handler.BuildChatResponse(requestID, tk.ID, logicalModel, resp)
	chatResp.Timing = s.buildTiming(tk)
	handler.WriteJSON(w, http.StatusOK, chatResp)
}

// tryOverloadFallback attempts fallback strategies when the primary model fails.
func (s *Server) tryOverloadFallback(ctx context.Context, req *adapter.ChatRequest, target *router.ModelTarget, logicalModel string) (*adapter.ChatResponse, error) {
	exclude := map[string]bool{target.Endpoint: true}

	result := s.overload.Resolve(logicalModel, exclude)
	if result.Rejected {
		return nil, fmt.Errorf("overload: %s", result.Reason)
	}

	fallbackAdapter, err := s.registry.Get(result.Target.Provider)
	if err != nil {
		return nil, fmt.Errorf("overload: adapter not found: %w", err)
	}

	fallbackReq := &adapter.ChatRequest{
		RequestID:   req.RequestID,
		Model:       result.Target.ActualModel,
		Messages:    req.Messages,
		MaxTokens:   result.Target.MaxOutputTokens,
		Temperature: req.Temperature,
		TopP:        req.TopP,
		Stream:      false,
		CancelCh:    req.CancelCh,
	}

	s.logger.Info("overload fallback",
		observability.F().
			Event("overload_fallback").
			Set("strategy", router.StrategyName(result.Strategy)).
			Set("from", logicalModel).
			Set("to", result.Target.LogicalModel))

	fbCtx, fbCancel := context.WithTimeout(ctx, time.Duration(s.cfg.Timeouts.DefaultInferenceMs)*time.Millisecond)
	defer fbCancel()

	return fallbackAdapter.ChatCompletion(fbCtx, fallbackReq)
}

func (s *Server) handleModels(w http.ResponseWriter, r *http.Request) {
	if r.Method != http.MethodGet {
		handler.WriteError(w, handler.NewGatewayError(handler.ErrInvalidRequest, "method not allowed"))
		return
	}

	models := s.modelMap.List()
	data := make([]api.ModelInfo, len(models))
	for i, m := range models {
		data[i] = api.ModelInfo{
			ID:      m,
			Object:  "model",
			OwnedBy: "edgeai-gateway",
		}
	}

	resp := api.ModelListResponse{
		Object: "list",
		Data:   data,
	}
	handler.WriteJSON(w, http.StatusOK, resp)
}

func (s *Server) handleSessions(w http.ResponseWriter, r *http.Request) {
	requestID := middleware.GetRequestID(r.Context())
	identity := auth.GetAppIdentityFromRequest(r)

	switch r.Method {
	case http.MethodPost:
		var req api.SessionRequest
		if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
			handler.WriteError(w, handler.NewGatewayErrorWithID(handler.ErrInvalidRequest, "invalid JSON body", requestID))
			return
		}
		if req.ApplicationID == "" {
			handler.WriteError(w, handler.NewGatewayErrorWithID(handler.ErrInvalidRequest, "application_id is required", requestID))
			return
		}

		sessionID := uuid.New().String()
		tenantID := ""
		if identity != nil {
			tenantID = identity.TenantID
		}

		sess, err := s.sessions.Create(sessionID, req.ApplicationID, tenantID, req.UserID, req.Config)
		if err != nil {
			handler.WriteError(w, handler.NewGatewayErrorWithID(handler.ErrInternalError, err.Error(), requestID))
			return
		}

		resp := api.SessionResponse{
			SessionID:     sess.ID,
			ApplicationID: sess.ApplicationID,
			UserID:        sess.UserID,
			CreatedAt:     sess.CreatedAt.Format(time.RFC3339),
			LastActive:    sess.LastActive.Format(time.RFC3339),
		}
		handler.WriteJSON(w, http.StatusCreated, resp)

	case http.MethodGet:
		// List sessions (simplified: return empty for now)
		handler.WriteJSON(w, http.StatusOK, map[string]any{"sessions": []any{}})

	default:
		handler.WriteError(w, handler.NewGatewayError(handler.ErrInvalidRequest, "method not allowed"))
	}
}

func (s *Server) handleSessionByID(w http.ResponseWriter, r *http.Request) {
	requestID := middleware.GetRequestID(r.Context())
	sessionID := strings.TrimPrefix(r.URL.Path, "/v1/sessions/")

	switch r.Method {
	case http.MethodGet:
		sess, err := s.sessions.Get(sessionID)
		if err != nil || sess == nil {
			handler.WriteError(w, handler.NewGatewayErrorWithID(handler.ErrInvalidRequest, "session not found", requestID))
			return
		}
		handler.WriteJSON(w, http.StatusOK, sess)

	case http.MethodDelete:
		if err := s.sessions.Delete(sessionID); err != nil {
			handler.WriteError(w, handler.NewGatewayErrorWithID(handler.ErrInternalError, err.Error(), requestID))
			return
		}
		handler.WriteJSON(w, http.StatusOK, map[string]string{"status": "deleted"})

	default:
		handler.WriteError(w, handler.NewGatewayError(handler.ErrInvalidRequest, "method not allowed"))
	}
}

// loadSessionHistory returns the message history from a session.
func (s *Server) loadSessionHistory(sess *session.Session) []api.Message {
	if sess == nil {
		return nil
	}
	return sess.Messages
}

// buildTiming constructs Timing metadata from a task.
func (s *Server) buildTiming(tk *task.Task) *api.Timing {
	return &api.Timing{
		QueueMs:      tk.QueueMs,
		FirstTokenMs: tk.FirstTokenMs,
		InferenceMs:  tk.InferenceMs,
		TotalMs:      tk.TotalMs,
	}
}

func (s *Server) handleTasks(w http.ResponseWriter, r *http.Request) {
	if r.Method != http.MethodGet {
		handler.WriteError(w, handler.NewGatewayError(handler.ErrInvalidRequest, "method not allowed"))
		return
	}
	handler.WriteJSON(w, http.StatusOK, map[string]any{"tasks": []any{}})
}

func (s *Server) handleTaskByID(w http.ResponseWriter, r *http.Request) {
	requestID := middleware.GetRequestID(r.Context())
	taskID := strings.TrimPrefix(r.URL.Path, "/v1/tasks/")

	switch r.Method {
	case http.MethodGet:
		tk, ok := s.scheduler.GetTask(taskID)
		if !ok {
			handler.WriteError(w, handler.NewGatewayErrorWithID(handler.ErrInvalidRequest, "task not found", requestID))
			return
		}
		resp := map[string]any{
			"task_id":       tk.ID,
			"request_id":    tk.RequestID,
			"session_id":    tk.SessionID,
			"state":         string(tk.GetState()),
			"priority":      int(tk.Priority),
			"logical_model": tk.LogicalModel,
			"stream":        tk.Stream,
			"created_at":    tk.CreatedAt.Format(time.RFC3339),
			"degraded":      tk.Degraded,
		}
		if tk.StartedAt != nil {
			resp["started_at"] = tk.StartedAt.Format(time.RFC3339)
		}
		if tk.CompletedAt != nil {
			resp["completed_at"] = tk.CompletedAt.Format(time.RFC3339)
		}
		if tk.CancelReason != "" {
			resp["cancel_reason"] = tk.CancelReason
		}
		if tk.ErrorMessage != "" {
			resp["error_message"] = tk.ErrorMessage
		}
		resp["timing"] = s.buildTiming(tk)
		handler.WriteJSON(w, http.StatusOK, resp)

	case http.MethodDelete:
		tk, ok := s.scheduler.GetTask(taskID)
		if !ok {
			handler.WriteError(w, handler.NewGatewayErrorWithID(handler.ErrInvalidRequest, "task not found", requestID))
			return
		}
		if tk.IsTerminal() {
			handler.WriteError(w, handler.NewGatewayErrorWithID(handler.ErrInvalidRequest, "task already in terminal state", requestID))
			return
		}
		tk.Cancel("client requested cancellation")
		s.scheduler.Complete(tk.ID)
		s.metrics.SetRunningTasks(s.scheduler.RunningCount())
		s.metrics.SetQueueLength(s.scheduler.QueueLength())
		handler.WriteJSON(w, http.StatusOK, map[string]string{"status": "cancelled"})

	default:
		handler.WriteError(w, handler.NewGatewayError(handler.ErrInvalidRequest, "method not allowed"))
	}
}