// Package ai 提供 OpenAI 兼容接口的调用，用于自动生成 "What's New" 更新说明。
package ai

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

// Config 是 AI 接入配置（管理员在后台填写）。
type Config struct {
	Enabled  bool
	BaseURL  string
	APIKey   string
	Model    string
	Language string // zh / en
}

// Ready 判断是否具备调用条件。
func (c Config) Ready() bool {
	return c.Enabled && strings.TrimSpace(c.BaseURL) != "" && strings.TrimSpace(c.Model) != ""
}

// Endpoint 归一化出 chat/completions 地址。
func (c Config) Endpoint() string {
	base := strings.TrimRight(strings.TrimSpace(c.BaseURL), "/")
	switch {
	case base == "":
		return ""
	case strings.HasSuffix(base, "/chat/completions"):
		return base
	case strings.HasSuffix(base, "/v1"):
		return base + "/chat/completions"
	default:
		return base + "/v1/chat/completions"
	}
}

// CommitLine 是喂给模型的一条提交摘要。
type CommitLine struct {
	SHA     string
	Author  string
	Subject string
}

// SummaryInput 是一次推送的全部素材。
type SummaryInput struct {
	RepoName  string
	Ref       string
	Commits   []CommitLine
	DiffStat  string
	Additions int
	Deletions int
}

type chatRequest struct {
	Model       string        `json:"model"`
	Messages    []chatMessage `json:"messages"`
	Temperature float64       `json:"temperature"`
}

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

type chatResponse struct {
	Choices []struct {
		Message struct {
			Content string `json:"content"`
		} `json:"message"`
	} `json:"choices"`
	Error *struct {
		Message string `json:"message"`
	} `json:"error"`
}

func systemPrompt(lang string) string {
	if lang == "en" {
		return `You are the changelog assistant of "gitcat", a self-hosted Git service for a small studio.
Given the commits and change stats of one git push, write a short visitor-friendly "What's New" summary.
Rules: English; at most 3 bullet points, each starting with "- "; summarize the intent instead of copying commit messages; do not invent features; keep it under 80 words; no headings, no extra commentary.`
	}
	return `你是 "gitcat"（工作室自建 Git 服务）的更新日志助手。
请根据一次代码推送的提交信息与变更统计，写一段面向访问者的「What's New」更新说明。
要求：使用简体中文；最多 3 条要点，每条以「- 」开头并单独成行；归纳改动意图而不是照抄提交信息；不要编造未提及的内容；总字数控制在 120 字以内；不要输出标题或额外说明。`
}

// GenerateWhatsNew 调用模型生成更新说明。
func GenerateWhatsNew(ctx context.Context, cfg Config, in SummaryInput) (string, error) {
	if !cfg.Ready() {
		return "", fmt.Errorf("AI 未启用或配置不完整")
	}
	var sb strings.Builder
	fmt.Fprintf(&sb, "仓库：%s\n分支：%s\n提交数：%d\n", in.RepoName, in.Ref, len(in.Commits))
	if in.Additions > 0 || in.Deletions > 0 {
		fmt.Fprintf(&sb, "代码行变化：+%d / -%d\n", in.Additions, in.Deletions)
	}
	if in.DiffStat != "" {
		sb.WriteString("变更文件统计：\n")
		sb.WriteString(truncate(in.DiffStat, 3000))
		sb.WriteString("\n")
	}
	sb.WriteString("提交列表：\n")
	for i, c := range in.Commits {
		if i >= 40 {
			fmt.Fprintf(&sb, "…… 其余 %d 条提交已省略\n", len(in.Commits)-i)
			break
		}
		fmt.Fprintf(&sb, "- %s %s: %s\n", shortSHA(c.SHA), c.Author, c.Subject)
	}

	body, err := json.Marshal(chatRequest{
		Model: cfg.Model,
		Messages: []chatMessage{
			{Role: "system", Content: systemPrompt(cfg.Language)},
			{Role: "user", Content: sb.String()},
		},
		Temperature: 0.3,
	})
	if err != nil {
		return "", err
	}

	ctx, cancel := context.WithTimeout(ctx, 90*time.Second)
	defer cancel()
	req, err := http.NewRequestWithContext(ctx, http.MethodPost, cfg.Endpoint(), bytes.NewReader(body))
	if err != nil {
		return "", err
	}
	req.Header.Set("Content-Type", "application/json")
	if cfg.APIKey != "" {
		req.Header.Set("Authorization", "Bearer "+cfg.APIKey)
	}
	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		return "", err
	}
	defer resp.Body.Close()
	raw, err := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
	if err != nil {
		return "", err
	}
	var parsed chatResponse
	if err := json.Unmarshal(raw, &parsed); err != nil {
		return "", fmt.Errorf("AI 返回内容无法解析（HTTP %d）", resp.StatusCode)
	}
	if parsed.Error != nil && parsed.Error.Message != "" {
		return "", fmt.Errorf("AI 接口错误：%s", parsed.Error.Message)
	}
	if resp.StatusCode >= 300 {
		return "", fmt.Errorf("AI 接口返回 HTTP %d", resp.StatusCode)
	}
	if len(parsed.Choices) == 0 {
		return "", fmt.Errorf("AI 未返回任何内容")
	}
	out := strings.TrimSpace(parsed.Choices[0].Message.Content)
	if out == "" {
		return "", fmt.Errorf("AI 返回内容为空")
	}
	return truncate(out, 2000), nil
}

// TestConnection 用一条极小的请求验证配置是否可用。
func TestConnection(ctx context.Context, cfg Config) (string, error) {
	cfg.Enabled = true
	return GenerateWhatsNew(ctx, cfg, SummaryInput{
		RepoName: "gitcat-demo",
		Ref:      "main",
		Commits: []CommitLine{
			{SHA: "0123456789abcdef", Author: "gitcat", Subject: "add hello world page"},
		},
		DiffStat: " hello.txt | 1 +\n 1 file changed, 1 insertion(+)",
	})
}

func shortSHA(s string) string {
	if len(s) > 7 {
		return s[:7]
	}
	return s
}

// truncate 按字符（而非字节）截断，避免切断多字节字符。
func truncate(s string, max int) string {
	r := []rune(s)
	if len(r) <= max {
		return s
	}
	cut := string(r[:max])
	if i := strings.LastIndexAny(cut, "\n"); i > len(cut)/2 {
		cut = cut[:i]
	}
	return cut + "\n…"
}