package adapter

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

	"github.com/edgeai/gateway/pkg/api"
)

// VLLMAdapter implements ModelAdapter for vLLM inference engine.
// vLLM exposes an OpenAI-compatible API at /v1/chat/completions and /v1/models.
type VLLMAdapter struct {
	endpoint   string
	httpClient *http.Client
}

// NewVLLMAdapter creates a new vLLM adapter.
func NewVLLMAdapter(endpoint string) *VLLMAdapter {
	return &VLLMAdapter{
		endpoint:   strings.TrimRight(endpoint, "/"),
		httpClient: newHTTPClient(120 * time.Second),
	}
}

func (a *VLLMAdapter) Name() string {
	return "vllm"
}

// vllmChatRequest is the OpenAI-compatible chat request for vLLM.
type vllmChatRequest struct {
	Model       string    `json:"model"`
	Messages    []vllmMsg `json:"messages"`
	Stream      bool      `json:"stream"`
	MaxTokens   int       `json:"max_tokens,omitempty"`
	Temperature *float64  `json:"temperature,omitempty"`
	TopP        *float64  `json:"top_p,omitempty"`
}

type vllmMsg struct {
	Role    string `json:"role"`
	Content string `json:"content"`
}

// vllmChatResponse is the OpenAI-compatible non-streaming response.
type vllmChatResponse struct {
	ID      string `json:"id"`
	Model   string `json:"model"`
	Choices []struct {
		Index        int     `json:"index"`
		Message      vllmMsg `json:"message"`
		FinishReason string  `json:"finish_reason"`
	} `json:"choices"`
	Usage struct {
		PromptTokens     int `json:"prompt_tokens"`
		CompletionTokens int `json:"completion_tokens"`
		TotalTokens      int `json:"total_tokens"`
	} `json:"usage"`
}

// vllmStreamChunk is a single SSE chunk in vLLM streaming response.
type vllmStreamChunk struct {
	ID      string `json:"id"`
	Model   string `json:"model"`
	Choices []struct {
		Index        int     `json:"index"`
		Delta        vllmMsg `json:"delta"`
		FinishReason *string `json:"finish_reason"`
	} `json:"choices"`
	Usage *struct {
		PromptTokens     int `json:"prompt_tokens"`
		CompletionTokens int `json:"completion_tokens"`
		TotalTokens      int `json:"total_tokens"`
	} `json:"usage,omitempty"`
}

func (a *VLLMAdapter) ChatCompletion(ctx context.Context, req *ChatRequest) (*ChatResponse, error) {
	vllmReq := a.buildRequest(req, false)

	body, err := json.Marshal(vllmReq)
	if err != nil {
		return nil, fmt.Errorf("marshal vllm request: %w", err)
	}

	httpReq, err := http.NewRequestWithContext(ctx, "POST", a.endpoint+"/v1/chat/completions", bytes.NewReader(body))
	if err != nil {
		return nil, fmt.Errorf("create vllm request: %w", err)
	}
	httpReq.Header.Set("Content-Type", "application/json")
	if req.RequestID != "" {
		httpReq.Header.Set("X-Request-ID", req.RequestID)
	}

	resp, err := a.httpClient.Do(httpReq)
	if err != nil {
		return nil, fmt.Errorf("vllm request failed: %w", err)
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		bodyBytes, _ := io.ReadAll(resp.Body)
		return nil, fmt.Errorf("vllm returned status %d: %s", resp.StatusCode, string(bodyBytes))
	}

	var vllmResp vllmChatResponse
	if err := json.NewDecoder(resp.Body).Decode(&vllmResp); err != nil {
		return nil, fmt.Errorf("decode vllm response: %w", err)
	}

	content := ""
	finishReason := "stop"
	if len(vllmResp.Choices) > 0 {
		content = vllmResp.Choices[0].Message.Content
		finishReason = vllmResp.Choices[0].FinishReason
		if finishReason == "" {
			finishReason = "stop"
		}
	}

	return &ChatResponse{
		Content:      content,
		FinishReason: finishReason,
		InputTokens:  vllmResp.Usage.PromptTokens,
		OutputTokens: vllmResp.Usage.CompletionTokens,
		ActualModel:  vllmResp.Model,
	}, nil
}

func (a *VLLMAdapter) ChatCompletionStream(ctx context.Context, req *ChatRequest) (<-chan StreamChunk, error) {
	vllmReq := a.buildRequest(req, true)

	body, err := json.Marshal(vllmReq)
	if err != nil {
		return nil, fmt.Errorf("marshal vllm stream request: %w", err)
	}

	httpReq, err := http.NewRequestWithContext(ctx, "POST", a.endpoint+"/v1/chat/completions", bytes.NewReader(body))
	if err != nil {
		return nil, fmt.Errorf("create vllm stream request: %w", err)
	}
	httpReq.Header.Set("Content-Type", "application/json")
	httpReq.Header.Set("Accept", "text/event-stream")
	if req.RequestID != "" {
		httpReq.Header.Set("X-Request-ID", req.RequestID)
	}

	resp, err := a.httpClient.Do(httpReq)
	if err != nil {
		return nil, fmt.Errorf("vllm stream request failed: %w", err)
	}

	if resp.StatusCode != http.StatusOK {
		bodyBytes, _ := io.ReadAll(resp.Body)
		resp.Body.Close()
		return nil, fmt.Errorf("vllm stream returned status %d: %s", resp.StatusCode, string(bodyBytes))
	}

	ch := make(chan StreamChunk, 100)
	go func() {
		defer close(ch)
		defer resp.Body.Close()

		scanner := bufio.NewScanner(resp.Body)
		scanner.Buffer(make([]byte, 0, 64*1024), 1024*1024)

		for scanner.Scan() {
			line := scanner.Text()
			if !strings.HasPrefix(line, "data: ") {
				continue
			}
			data := strings.TrimPrefix(line, "data: ")
			if data == "[DONE]" {
				ch <- StreamChunk{Done: true, FinishReason: "stop"}
				return
			}

			// Check for cancellation
			select {
			case <-req.CancelCh:
				ch <- StreamChunk{Done: true, FinishReason: "cancelled"}
				return
			default:
			}

			var chunk vllmStreamChunk
			if err := json.Unmarshal([]byte(data), &chunk); err != nil {
				ch <- StreamChunk{Error: fmt.Errorf("decode stream chunk: %w", err)}
				return
			}

			if len(chunk.Choices) > 0 {
				choice := chunk.Choices[0]
				if choice.Delta.Content != "" {
					ch <- StreamChunk{Delta: choice.Delta.Content}
				}
				if choice.FinishReason != nil {
					inputTokens, outputTokens := 0, 0
					if chunk.Usage != nil {
						inputTokens = chunk.Usage.PromptTokens
						outputTokens = chunk.Usage.CompletionTokens
					}
					ch <- StreamChunk{
						Done:         true,
						FinishReason: *choice.FinishReason,
						InputTokens:  inputTokens,
						OutputTokens: outputTokens,
					}
					return
				}
			}
		}

		if err := scanner.Err(); err != nil {
			ch <- StreamChunk{Error: fmt.Errorf("stream read error: %w", err)}
			return
		}

		ch <- StreamChunk{Done: true, FinishReason: "stop"}
	}()

	return ch, nil
}

func (a *VLLMAdapter) ListModels(ctx context.Context) ([]ModelInfo, error) {
	httpReq, err := http.NewRequestWithContext(ctx, "GET", a.endpoint+"/v1/models", nil)
	if err != nil {
		return nil, fmt.Errorf("create list models request: %w", err)
	}

	resp, err := a.httpClient.Do(httpReq)
	if err != nil {
		return nil, fmt.Errorf("list models failed: %w", err)
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("list models returned status %d", resp.StatusCode)
	}

	var modelsResp struct {
		Data []struct {
			ID      string `json:"id"`
			Context int    `json:"max_model_len,omitempty"`
		} `json:"data"`
	}
	if err := json.NewDecoder(resp.Body).Decode(&modelsResp); err != nil {
		return nil, fmt.Errorf("decode models response: %w", err)
	}

	models := make([]ModelInfo, len(modelsResp.Data))
	for i, m := range modelsResp.Data {
		models[i] = ModelInfo{Name: m.ID, ContextWindow: m.Context}
	}
	return models, nil
}

func (a *VLLMAdapter) HealthCheck(ctx context.Context) error {
	httpReq, err := http.NewRequestWithContext(ctx, "GET", a.endpoint+"/v1/models", nil)
	if err != nil {
		return fmt.Errorf("create health check request: %w", err)
	}

	resp, err := a.httpClient.Do(httpReq)
	if err != nil {
		return fmt.Errorf("health check failed: %w", err)
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		return fmt.Errorf("health check returned status %d", resp.StatusCode)
	}
	return nil
}

func (a *VLLMAdapter) Cancel(requestID string) error {
	// vLLM supports cancellation via context cancellation (closing HTTP connection).
	return nil
}

func (a *VLLMAdapter) buildRequest(req *ChatRequest, stream bool) vllmChatRequest {
	msgs := make([]vllmMsg, len(req.Messages))
	for i, m := range req.Messages {
		content, _ := m.Content.(string)
		msgs[i] = vllmMsg{Role: m.Role, Content: content}
	}

	vllmReq := vllmChatRequest{
		Model:    req.Model,
		Messages: msgs,
		Stream:   stream,
	}

	if req.MaxTokens > 0 {
		vllmReq.MaxTokens = req.MaxTokens
	}
	if req.Temperature != nil {
		vllmReq.Temperature = req.Temperature
	}
	if req.TopP != nil {
		vllmReq.TopP = req.TopP
	}

	return vllmReq
}

// Ensure vllmMsg satisfies the api.Message content interface when needed.
var _ = api.Message{}