package llm

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

	"gitea.maximumdirect.net/eric/scriptorium/internal/domain"
)

var (
	ErrInvalidConfig     = errors.New("invalid llm client configuration")
	ErrInvalidRequest    = errors.New("invalid generate request")
	ErrRequestFailed     = errors.New("llm request failed")
	ErrUnexpectedStatus  = errors.New("llm returned non-success status")
	ErrMalformedResponse = errors.New("malformed llm response")
)

type OpenAICompatibleConfig struct {
	BaseURL    string
	APIKey     string
	Model      string
	Timeout    time.Duration
	HTTPClient *http.Client
}

type OpenAICompatibleClient struct {
	baseURL      string
	apiKey       string
	defaultModel string
	timeout      time.Duration
	httpClient   *http.Client
}

func NewOpenAICompatibleClient(cfg OpenAICompatibleConfig) (*OpenAICompatibleClient, error) {
	if strings.TrimSpace(cfg.BaseURL) == "" {
		return nil, fmt.Errorf("%w: base URL is required", ErrInvalidConfig)
	}
	if _, err := url.ParseRequestURI(cfg.BaseURL); err != nil {
		return nil, fmt.Errorf("%w: invalid base URL: %v", ErrInvalidConfig, err)
	}

	timeout := cfg.Timeout
	if timeout <= 0 {
		timeout = 10 * time.Minute
	}

	var client *http.Client
	if cfg.HTTPClient != nil {
		client = cfg.HTTPClient
		if client.Timeout == 0 {
			client.Timeout = timeout
		}
	} else {
		client = &http.Client{Timeout: timeout}
	}

	return &OpenAICompatibleClient{
		baseURL:      strings.TrimRight(cfg.BaseURL, "/"),
		apiKey:       cfg.APIKey,
		defaultModel: cfg.Model,
		timeout:      timeout,
		httpClient:   client,
	}, nil
}

func (c *OpenAICompatibleClient) Generate(ctx context.Context, req domain.GenerateRequest) (*domain.GenerateResponse, error) {
	if req.Target.TimeoutSeconds < 0 {
		return nil, fmt.Errorf("%w: timeout_seconds must be greater than or equal to 0", ErrInvalidRequest)
	}

	model := strings.TrimSpace(req.Target.Model)
	if model == "" {
		model = strings.TrimSpace(c.defaultModel)
	}
	if model == "" {
		return nil, fmt.Errorf("%w: model is required", ErrInvalidRequest)
	}

	endpoint := strings.TrimSpace(req.Target.Endpoint)
	if endpoint == "" {
		endpoint = c.baseURL
	}
	endpoint = strings.TrimRight(endpoint, "/") + "/chat/completions"

	wireReq := openAIChatRequest{
		Model: model,
	}

	wireReq.Messages = make([]openAIChatMessage, 0, len(req.Prompt.Messages))
	for _, msg := range req.Prompt.Messages {
		wireReq.Messages = append(wireReq.Messages, openAIChatMessage{
			Role:    msg.Role,
			Content: msg.Content,
		})
	}

	if req.Target.Temperature != 0 {
		wireReq.Temperature = &req.Target.Temperature
	}
	if req.Target.MaxTokens != 0 {
		wireReq.MaxTokens = &req.Target.MaxTokens
	}
	if req.Target.TopP != 0 {
		wireReq.TopP = &req.Target.TopP
	}

	payload, err := json.Marshal(wireReq)
	if err != nil {
		return nil, fmt.Errorf("%w: failed to encode request: %v", ErrRequestFailed, err)
	}

	httpReq, err := http.NewRequestWithContext(ctx, http.MethodPost, endpoint, bytes.NewReader(payload))
	if err != nil {
		return nil, fmt.Errorf("%w: failed to create request: %v", ErrRequestFailed, err)
	}
	httpReq.Header.Set("Content-Type", "application/json")
	if strings.TrimSpace(c.apiKey) != "" {
		httpReq.Header.Set("Authorization", "Bearer "+c.apiKey)
	}

	effectiveTimeout := c.timeout
	if req.Target.TimeoutSeconds > 0 {
		effectiveTimeout = time.Duration(req.Target.TimeoutSeconds) * time.Second
	}

	httpClient := c.httpClient
	if httpClient == nil {
		httpClient = &http.Client{Timeout: effectiveTimeout}
	} else if httpClient.Timeout != effectiveTimeout {
		cloned := *httpClient
		cloned.Timeout = effectiveTimeout
		httpClient = &cloned
	}

	httpResp, err := httpClient.Do(httpReq)
	if err != nil {
		return nil, fmt.Errorf("%w: %v", ErrRequestFailed, err)
	}
	defer httpResp.Body.Close()

	if httpResp.StatusCode < 200 || httpResp.StatusCode >= 300 {
		body, _ := io.ReadAll(io.LimitReader(httpResp.Body, 4096))
		return nil, fmt.Errorf("%w: status=%d body=%q", ErrUnexpectedStatus, httpResp.StatusCode, strings.TrimSpace(string(body)))
	}

	var wireResp openAIChatResponse
	if err := json.NewDecoder(httpResp.Body).Decode(&wireResp); err != nil {
		return nil, fmt.Errorf("%w: failed to decode response: %v", ErrMalformedResponse, err)
	}

	if len(wireResp.Choices) == 0 {
		return nil, fmt.Errorf("%w: no choices returned", ErrMalformedResponse)
	}
	content := wireResp.Choices[0].Message.Content
	if content == "" {
		return nil, fmt.Errorf("%w: first choice has empty message content", ErrMalformedResponse)
	}

	return &domain.GenerateResponse{
		Content: content,
		Usage: domain.TokenUsage{
			PromptTokens:     wireResp.Usage.PromptTokens,
			CompletionTokens: wireResp.Usage.CompletionTokens,
			TotalTokens:      wireResp.Usage.TotalTokens,
		},
	}, nil
}

type openAIChatRequest struct {
	Model       string              `json:"model"`
	Messages    []openAIChatMessage `json:"messages"`
	Temperature *float64            `json:"temperature,omitempty"`
	MaxTokens   *int                `json:"max_tokens,omitempty"`
	TopP        *float64            `json:"top_p,omitempty"`
}

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

type openAIChatResponse struct {
	Choices []struct {
		Message openAIChatMessage `json:"message"`
	} `json:"choices"`
	Usage struct {
		PromptTokens     int `json:"prompt_tokens"`
		CompletionTokens int `json:"completion_tokens"`
		TotalTokens      int `json:"total_tokens"`
	} `json:"usage"`
}