package service

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

	"github.com/Wei-Shaw/sub2api/internal/pkg/apicompat"
	"github.com/Wei-Shaw/sub2api/internal/pkg/logger"
	"github.com/Wei-Shaw/sub2api/internal/util/responseheaders"
	"github.com/gin-gonic/gin"
	"go.uber.org/zap"
)

// ForwardAsChatCompletions accepts a Chat Completions request body, converts it
// to OpenAI Responses API format, forwards to the OpenAI upstream, and converts
// the response back to Chat Completions format. All account types (OAuth and API
// Key) go through the Responses API conversion path since the upstream only
// exposes the /v1/responses endpoint.
func (s *OpenAIGatewayService) ForwardAsChatCompletions(
	ctx context.Context,
	c *gin.Context,
	account *Account,
	body []byte,
	promptCacheKey string,
	defaultMappedModel string,
) (*OpenAIForwardResult, error) {
	startTime := time.Now()

	// 1. Parse Chat Completions request
	var chatReq apicompat.ChatCompletionsRequest
	if err := json.Unmarshal(body, &chatReq); err != nil {
		return nil, fmt.Errorf("parse chat completions request: %w", err)
	}
	originalModel := chatReq.Model
	clientStream := chatReq.Stream
	includeUsage := chatReq.StreamOptions != nil && chatReq.StreamOptions.IncludeUsage

	// 2. Convert to Responses and forward
	// ChatCompletionsToResponses always sets Stream=true (upstream always streams).
	responsesReq, err := apicompat.ChatCompletionsToResponses(&chatReq)
	if err != nil {
		return nil, fmt.Errorf("convert chat completions to responses: %w", err)
	}

	// 3. Model mapping
	mappedModel := resolveOpenAIForwardModel(account, originalModel, defaultMappedModel)
	responsesReq.Model = mappedModel

	logger.L().Debug("openai chat_completions: model mapping applied",
		zap.Int64("account_id", account.ID),
		zap.String("original_model", originalModel),
		zap.String("mapped_model", mappedModel),
		zap.Bool("stream", clientStream),
	)

	// 4. Marshal Responses request body, then apply OAuth codex transform
	responsesBody, err := json.Marshal(responsesReq)
	if err != nil {
		return nil, fmt.Errorf("marshal responses request: %w", err)
	}

	if account.Type == AccountTypeOAuth {
		var reqBody map[string]any
		if err := json.Unmarshal(responsesBody, &reqBody); err != nil {
			return nil, fmt.Errorf("unmarshal for codex transform: %w", err)
		}
		codexResult := applyCodexOAuthTransform(reqBody, false, false)
		if codexResult.PromptCacheKey != "" {
			promptCacheKey = codexResult.PromptCacheKey
		} else if promptCacheKey != "" {
			reqBody["prompt_cache_key"] = promptCacheKey
		}
		responsesBody, err = json.Marshal(reqBody)
		if err != nil {
			return nil, fmt.Errorf("remarshal after codex transform: %w", err)
		}
	}

	// 5. Get access token
	token, _, err := s.GetAccessToken(ctx, account)
	if err != nil {
		return nil, fmt.Errorf("get access token: %w", err)
	}

	// 6. Build upstream request
	upstreamReq, err := s.buildUpstreamRequest(ctx, c, account, responsesBody, token, true, promptCacheKey, false)
	if err != nil {
		return nil, fmt.Errorf("build upstream request: %w", err)
	}

	if promptCacheKey != "" {
		upstreamReq.Header.Set("session_id", generateSessionUUID(promptCacheKey))
	}

	// 7. Send request
	proxyURL := ""
	if account.Proxy != nil {
		proxyURL = account.Proxy.URL()
	}
	resp, err := s.httpUpstream.Do(upstreamReq, proxyURL, account.ID, account.Concurrency)
	if err != nil {
		safeErr := sanitizeUpstreamErrorMessage(err.Error())
		setOpsUpstreamError(c, 0, safeErr, "")
		appendOpsUpstreamError(c, OpsUpstreamErrorEvent{
			Platform:           account.Platform,
			AccountID:          account.ID,
			AccountName:        account.Name,
			UpstreamStatusCode: 0,
			Kind:               "request_error",
			Message:            safeErr,
		})
		writeChatCompletionsError(c, http.StatusBadGateway, "upstream_error", "Upstream request failed")
		return nil, fmt.Errorf("upstream request failed: %s", safeErr)
	}
	defer func() { _ = resp.Body.Close() }()

	// 8. Handle error response with failover
	if resp.StatusCode >= 400 {
		respBody, _ := io.ReadAll(io.LimitReader(resp.Body, 2<<20))
		_ = resp.Body.Close()
		resp.Body = io.NopCloser(bytes.NewReader(respBody))

		upstreamMsg := strings.TrimSpace(extractUpstreamErrorMessage(respBody))
		upstreamMsg = sanitizeUpstreamErrorMessage(upstreamMsg)
		if s.shouldFailoverOpenAIUpstreamResponse(resp.StatusCode, upstreamMsg, respBody) {
			upstreamDetail := ""
			if s.cfg != nil && s.cfg.Gateway.LogUpstreamErrorBody {
				maxBytes := s.cfg.Gateway.LogUpstreamErrorBodyMaxBytes
				if maxBytes <= 0 {
					maxBytes = 2048
				}
				upstreamDetail = truncateString(string(respBody), maxBytes)
			}
			appendOpsUpstreamError(c, OpsUpstreamErrorEvent{
				Platform:           account.Platform,
				AccountID:          account.ID,
				AccountName:        account.Name,
				UpstreamStatusCode: resp.StatusCode,
				UpstreamRequestID:  resp.Header.Get("x-request-id"),
				Kind:               "failover",
				Message:            upstreamMsg,
				Detail:             upstreamDetail,
			})
			if s.rateLimitService != nil {
				s.rateLimitService.HandleUpstreamError(ctx, account, resp.StatusCode, resp.Header, respBody)
			}
			return nil, &UpstreamFailoverError{
				StatusCode:             resp.StatusCode,
				ResponseBody:           respBody,
				RetryableOnSameAccount: account.IsPoolMode() && (isPoolModeRetryableStatus(resp.StatusCode) || isOpenAITransientProcessingError(resp.StatusCode, upstreamMsg, respBody)),
			}
		}
		return s.handleChatCompletionsErrorResponse(resp, c, account)
	}

	// 9. Handle normal response
	var result *OpenAIForwardResult
	var handleErr error
	if clientStream {
		result, handleErr = s.handleChatStreamingResponse(resp, c, originalModel, mappedModel, includeUsage, startTime)
	} else {
		result, handleErr = s.handleChatBufferedStreamingResponse(resp, c, originalModel, mappedModel, startTime)
	}

	// Propagate ServiceTier and ReasoningEffort to result for billing
	if handleErr == nil && result != nil {
		if responsesReq.ServiceTier != "" {
			st := responsesReq.ServiceTier
			result.ServiceTier = &st
		}
		if responsesReq.Reasoning != nil && responsesReq.Reasoning.Effort != "" {
			re := responsesReq.Reasoning.Effort
			result.ReasoningEffort = &re
		}
	}

	// Extract and save Codex usage snapshot from response headers (for OAuth accounts)
	if handleErr == nil && account.Type == AccountTypeOAuth {
		if snapshot := ParseCodexRateLimitHeaders(resp.Header); snapshot != nil {
			s.updateCodexUsageSnapshot(ctx, account.ID, snapshot)
		}
	}

	return result, handleErr
}

// handleChatCompletionsErrorResponse reads an upstream error and returns it in
// OpenAI Chat Completions error format.
func (s *OpenAIGatewayService) handleChatCompletionsErrorResponse(
	resp *http.Response,
	c *gin.Context,
	account *Account,
) (*OpenAIForwardResult, error) {
	return s.handleCompatErrorResponse(resp, c, account, writeChatCompletionsError)
}

// handleChatBufferedStreamingResponse reads all Responses SSE events from the
// upstream, finds the terminal event, converts to a Chat Completions JSON
// response, and writes it to the client.
func (s *OpenAIGatewayService) handleChatBufferedStreamingResponse(
	resp *http.Response,
	c *gin.Context,
	originalModel string,
	mappedModel string,
	startTime time.Time,
) (*OpenAIForwardResult, error) {
	requestID := resp.Header.Get("x-request-id")

	scanner := bufio.NewScanner(resp.Body)
	maxLineSize := defaultMaxLineSize
	if s.cfg != nil && s.cfg.Gateway.MaxLineSize > 0 {
		maxLineSize = s.cfg.Gateway.MaxLineSize
	}
	scanner.Buffer(make([]byte, 0, 64*1024), maxLineSize)

	var finalResponse *apicompat.ResponsesResponse
	var usage OpenAIUsage

	for scanner.Scan() {
		line := scanner.Text()
		if !strings.HasPrefix(line, "data: ") || line == "data: [DONE]" {
			continue
		}
		payload := line[6:]

		var event apicompat.ResponsesStreamEvent
		if err := json.Unmarshal([]byte(payload), &event); err != nil {
			logger.L().Warn("openai chat_completions buffered: failed to parse event",
				zap.Error(err),
				zap.String("request_id", requestID),
			)
			continue
		}

		if (event.Type == "response.completed" || event.Type == "response.incomplete" || event.Type == "response.failed") &&
			event.Response != nil {
			finalResponse = event.Response
			if event.Response.Usage != nil {
				usage = OpenAIUsage{
					InputTokens:  event.Response.Usage.InputTokens,
					OutputTokens: event.Response.Usage.OutputTokens,
				}
				if event.Response.Usage.InputTokensDetails != nil {
					usage.CacheReadInputTokens = event.Response.Usage.InputTokensDetails.CachedTokens
				}
			}
		}
	}

	if err := scanner.Err(); err != nil {
		if !errors.Is(err, context.Canceled) && !errors.Is(err, context.DeadlineExceeded) {
			logger.L().Warn("openai chat_completions buffered: read error",
				zap.Error(err),
				zap.String("request_id", requestID),
			)
		}
	}

	if finalResponse == nil {
		writeChatCompletionsError(c, http.StatusBadGateway, "api_error", "Upstream stream ended without a terminal response event")
		return nil, fmt.Errorf("upstream stream ended without terminal event")
	}

	chatResp := apicompat.ResponsesToChatCompletions(finalResponse, originalModel)

	if s.responseHeaderFilter != nil {
		responseheaders.WriteFilteredHeaders(c.Writer.Header(), resp.Header, s.responseHeaderFilter)
	}
	c.JSON(http.StatusOK, chatResp)

	return &OpenAIForwardResult{
		RequestID:    requestID,
		Usage:        usage,
		Model:        originalModel,
		BillingModel: mappedModel,
		Stream:       false,
		Duration:     time.Since(startTime),
	}, nil
}

// handleChatStreamingResponse reads Responses SSE events from upstream,
// converts each to Chat Completions SSE chunks, and writes them to the client.
func (s *OpenAIGatewayService) handleChatStreamingResponse(
	resp *http.Response,
	c *gin.Context,
	originalModel string,
	mappedModel string,
	includeUsage bool,
	startTime time.Time,
) (*OpenAIForwardResult, error) {
	requestID := resp.Header.Get("x-request-id")

	if s.responseHeaderFilter != nil {
		responseheaders.WriteFilteredHeaders(c.Writer.Header(), resp.Header, s.responseHeaderFilter)
	}
	c.Writer.Header().Set("Content-Type", "text/event-stream")
	c.Writer.Header().Set("Cache-Control", "no-cache")
	c.Writer.Header().Set("Connection", "keep-alive")
	c.Writer.Header().Set("X-Accel-Buffering", "no")
	c.Writer.WriteHeader(http.StatusOK)

	state := apicompat.NewResponsesEventToChatState()
	state.Model = originalModel
	state.IncludeUsage = includeUsage

	var usage OpenAIUsage
	var firstTokenMs *int
	firstChunk := true

	scanner := bufio.NewScanner(resp.Body)
	maxLineSize := defaultMaxLineSize
	if s.cfg != nil && s.cfg.Gateway.MaxLineSize > 0 {
		maxLineSize = s.cfg.Gateway.MaxLineSize
	}
	scanner.Buffer(make([]byte, 0, 64*1024), maxLineSize)

	resultWithUsage := func() *OpenAIForwardResult {
		return &OpenAIForwardResult{
			RequestID:    requestID,
			Usage:        usage,
			Model:        originalModel,
			BillingModel: mappedModel,
			Stream:       true,
			Duration:     time.Since(startTime),
			FirstTokenMs: firstTokenMs,
		}
	}

	processDataLine := func(payload string) bool {
		if firstChunk {
			firstChunk = false
			ms := int(time.Since(startTime).Milliseconds())
			firstTokenMs = &ms
		}

		var event apicompat.ResponsesStreamEvent
		if err := json.Unmarshal([]byte(payload), &event); err != nil {
			logger.L().Warn("openai chat_completions stream: failed to parse event",
				zap.Error(err),
				zap.String("request_id", requestID),
			)
			return false
		}

		// Extract usage from completion events
		if (event.Type == "response.completed" || event.Type == "response.incomplete" || event.Type == "response.failed") &&
			event.Response != nil && event.Response.Usage != nil {
			usage = OpenAIUsage{
				InputTokens:  event.Response.Usage.InputTokens,
				OutputTokens: event.Response.Usage.OutputTokens,
			}
			if event.Response.Usage.InputTokensDetails != nil {
				usage.CacheReadInputTokens = event.Response.Usage.InputTokensDetails.CachedTokens
			}
		}

		chunks := apicompat.ResponsesEventToChatChunks(&event, state)
		for _, chunk := range chunks {
			sse, err := apicompat.ChatChunkToSSE(chunk)
			if err != nil {
				logger.L().Warn("openai chat_completions stream: failed to marshal chunk",
					zap.Error(err),
					zap.String("request_id", requestID),
				)
				continue
			}
			if _, err := fmt.Fprint(c.Writer, sse); err != nil {
				logger.L().Info("openai chat_completions stream: client disconnected",
					zap.String("request_id", requestID),
				)
				return true
			}
		}
		if len(chunks) > 0 {
			c.Writer.Flush()
		}
		return false
	}

	finalizeStream := func() (*OpenAIForwardResult, error) {
		if finalChunks := apicompat.FinalizeResponsesChatStream(state); len(finalChunks) > 0 {
			for _, chunk := range finalChunks {
				sse, err := apicompat.ChatChunkToSSE(chunk)
				if err != nil {
					continue
				}
				fmt.Fprint(c.Writer, sse) //nolint:errcheck
			}
		}
		// Send [DONE] sentinel
		fmt.Fprint(c.Writer, "data: [DONE]\n\n") //nolint:errcheck
		c.Writer.Flush()
		return resultWithUsage(), nil
	}

	handleScanErr := func(err error) {
		if err != nil && !errors.Is(err, context.Canceled) && !errors.Is(err, context.DeadlineExceeded) {
			logger.L().Warn("openai chat_completions stream: read error",
				zap.Error(err),
				zap.String("request_id", requestID),
			)
		}
	}

	// Determine keepalive interval
	keepaliveInterval := time.Duration(0)
	if s.cfg != nil && s.cfg.Gateway.StreamKeepaliveInterval > 0 {
		keepaliveInterval = time.Duration(s.cfg.Gateway.StreamKeepaliveInterval) * time.Second
	}

	// No keepalive: fast synchronous path
	if keepaliveInterval <= 0 {
		for scanner.Scan() {
			line := scanner.Text()
			if !strings.HasPrefix(line, "data: ") || line == "data: [DONE]" {
				continue
			}
			if processDataLine(line[6:]) {
				return resultWithUsage(), nil
			}
		}
		handleScanErr(scanner.Err())
		return finalizeStream()
	}

	// With keepalive: goroutine + channel + select
	type scanEvent struct {
		line string
		err  error
	}
	events := make(chan scanEvent, 16)
	done := make(chan struct{})
	sendEvent := func(ev scanEvent) bool {
		select {
		case events <- ev:
			return true
		case <-done:
			return false
		}
	}
	go func() {
		defer close(events)
		for scanner.Scan() {
			if !sendEvent(scanEvent{line: scanner.Text()}) {
				return
			}
		}
		if err := scanner.Err(); err != nil {
			_ = sendEvent(scanEvent{err: err})
		}
	}()
	defer close(done)

	keepaliveTicker := time.NewTicker(keepaliveInterval)
	defer keepaliveTicker.Stop()
	lastDataAt := time.Now()

	for {
		select {
		case ev, ok := <-events:
			if !ok {
				return finalizeStream()
			}
			if ev.err != nil {
				handleScanErr(ev.err)
				return finalizeStream()
			}
			lastDataAt = time.Now()
			line := ev.line
			if !strings.HasPrefix(line, "data: ") || line == "data: [DONE]" {
				continue
			}
			if processDataLine(line[6:]) {
				return resultWithUsage(), nil
			}

		case <-keepaliveTicker.C:
			if time.Since(lastDataAt) < keepaliveInterval {
				continue
			}
			// Send SSE comment as keepalive
			if _, err := fmt.Fprint(c.Writer, ":\n\n"); err != nil {
				logger.L().Info("openai chat_completions stream: client disconnected during keepalive",
					zap.String("request_id", requestID),
				)
				return resultWithUsage(), nil
			}
			c.Writer.Flush()
		}
	}
}

// writeChatCompletionsError writes an error response in OpenAI Chat Completions format.
func writeChatCompletionsError(c *gin.Context, statusCode int, errType, message string) {
	c.JSON(statusCode, gin.H{
		"error": gin.H{
			"type":    errType,
			"message": message,
		},
	})
}
