// Package auth 提供评论区两侧的基础设施：读者会话（cookie one_reader，
// payload 带 reader: 前缀，与后台令牌不可互换）和 GitHub OAuth 客户端。
package auth

import (
	"context"
	"crypto/hmac"
	"crypto/sha256"
	"encoding/base64"
	"encoding/json"
	"errors"
	"fmt"
	"net/http"
	"net/url"
	"strconv"
	"strings"
	"time"
)

// ReaderCookie 是读者会话的 cookie 名（与后台的 one_session 区分开）
const ReaderCookie = "one_reader"

// ReaderSessions 签发 / 校验读者会话令牌：
// base64("reader:<readerID>:<expiryUnix>") + "." + HMAC-SHA256。
type ReaderSessions struct {
	secret []byte
	ttl    time.Duration
}

func NewReaderSessions(secret string, ttl time.Duration) *ReaderSessions {
	if ttl <= 0 {
		ttl = 30 * 24 * time.Hour
	}
	return &ReaderSessions{secret: []byte(secret), ttl: ttl}
}

var ErrBadSession = errors.New("invalid reader session")

func (s *ReaderSessions) Issue(readerID int64) (string, time.Time) {
	exp := time.Now().Add(s.ttl)
	payload := readerPayload(readerID, exp)
	enc := base64.RawURLEncoding.EncodeToString([]byte(payload))
	return enc + "." + s.sign(payload), exp
}

func (s *ReaderSessions) Verify(token string) (int64, error) {
	parts := strings.Split(token, ".")
	if len(parts) != 2 {
		return 0, ErrBadSession
	}
	raw, err := base64.RawURLEncoding.DecodeString(parts[0])
	if err != nil {
		return 0, ErrBadSession
	}
	// 必须以 reader: 开头——后台令牌（base64("admin:<exp>")）复制过来也无效
	if len(raw) < 10 || string(raw)[:7] != "reader:" {
		return 0, ErrBadSession
	}
	// 余下是 "<id>:<exp>"
	rest := string(raw)[7:]
	i := strings.IndexByte(rest, ':')
	if i <= 0 {
		return 0, ErrBadSession
	}
	id, err := strconv.ParseInt(rest[:i], 10, 64)
	if err != nil {
		return 0, ErrBadSession
	}
	expUnix, err := strconv.ParseInt(rest[i+1:], 10, 64)
	if err != nil {
		return 0, ErrBadSession
	}
	if time.Now().After(time.Unix(expUnix, 0)) {
		return 0, ErrBadSession
	}
	// 签名放在最后验证：payload 与 exp 都验过再比对 MAC
	if !hmac.Equal([]byte(s.sign(string(raw))), []byte(parts[1])) {
		return 0, ErrBadSession
	}
	return id, nil
}

func readerPayload(id int64, exp time.Time) string {
	return "reader:" + strconv.FormatInt(id, 10) + ":" + strconv.FormatInt(exp.Unix(), 10)
}

func (s *ReaderSessions) sign(payload string) string {
	mac := hmac.New(sha256.New, s.secret)
	mac.Write([]byte(payload))
	return base64.RawURLEncoding.EncodeToString(mac.Sum(nil))
}

// ---------- GitHub OAuth ----------

// GitHub 持有 OAuth 应用凭据。没配 = GitHub 登录不开放，
// 前台登录卡自动不显示该入口。
type GitHub struct {
	ClientID     string
	ClientSecret string
}

func (g GitHub) Enabled() bool { return g.ClientID != "" && g.ClientSecret != "" }

const (
	githubAuthURL  = "https://github.com/login/oauth/authorize"
	githubTokenURL = "https://github.com/login/oauth/access_token"
	githubUserURL  = "https://api.github.com/user"
)

// LoginURL 生成 GitHub 授权页跳转地址
func (g GitHub) LoginURL(redirectURI, state string) string {
	v := url.Values{}
	v.Set("client_id", g.ClientID)
	v.Set("redirect_uri", redirectURI)
	v.Set("scope", "read:user")
	v.Set("state", state)
	return githubAuthURL + "?" + v.Encode()
}

// GitHubUser 是 GitHub 用户接口里我们关心的字段
type GitHubUser struct {
	Login     string `json:"login"`
	Name      string `json:"name"`
	AvatarURL string `json:"avatar_url"`
	HTMLURL   string `json:"html_url"`
}

// Exchange 用授权码换 access token
func (g GitHub) Exchange(ctx context.Context, code, redirectURI string) (string, error) {
	v := url.Values{}
	v.Set("client_id", g.ClientID)
	v.Set("client_secret", g.ClientSecret)
	v.Set("code", code)
	v.Set("redirect_uri", redirectURI)
	req, err := http.NewRequestWithContext(ctx, http.MethodPost, githubTokenURL, strings.NewReader(v.Encode()))
	if err != nil {
		return "", err
	}
	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
	req.Header.Set("Accept", "application/json")
	res, err := http.DefaultClient.Do(req)
	if err != nil {
		return "", err
	}
	defer res.Body.Close()
	var out struct {
		AccessToken string `json:"access_token"`
	}
	if err := json.NewDecoder(res.Body).Decode(&out); err != nil || out.AccessToken == "" {
		return "", fmt.Errorf("github: token exchange failed")
	}
	return out.AccessToken, nil
}

// FetchUser 拉取 GitHub 用户资料
func (g GitHub) FetchUser(ctx context.Context, accessToken string) (GitHubUser, error) {
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, githubUserURL, nil)
	if err != nil {
		return GitHubUser{}, err
	}
	req.Header.Set("Authorization", "Bearer "+accessToken)
	res, err := http.DefaultClient.Do(req)
	if err != nil {
		return GitHubUser{}, err
	}
	defer res.Body.Close()
	var u GitHubUser
	if err := json.NewDecoder(res.Body).Decode(&u); err != nil || u.Login == "" {
		return GitHubUser{}, fmt.Errorf("github: fetch user failed")
	}
	return u, nil
}