package auth

import (
	"context"
	"encoding/json"
	"fmt"
	"io"
	"net/http"

	"golang.org/x/oauth2"
)

// DiscordEndpoint is the Discord OAuth2 endpoint.
var DiscordEndpoint = oauth2.Endpoint{
	AuthURL:  "https://discord.com/api/oauth2/authorize",
	TokenURL: "https://discord.com/api/oauth2/token",
}

// DiscordUser holds the data returned by /users/@me.
type DiscordUser struct {
	ID       string `json:"id"`
	Username string `json:"username"`
	Avatar   string `json:"avatar"`
}

// AvatarURL returns the full Discord CDN URL for the user's avatar.
func (u *DiscordUser) AvatarURL() string {
	if u.Avatar == "" {
		return ""
	}
	return fmt.Sprintf("https://cdn.discordapp.com/avatars/%s/%s.png?size=128", u.ID, u.Avatar)
}

// NewOAuthConfig builds an oauth2.Config for Discord.
func NewOAuthConfig(clientID, clientSecret, callbackURL string) *oauth2.Config {
	return &oauth2.Config{
		ClientID:     clientID,
		ClientSecret: clientSecret,
		RedirectURL:  callbackURL,
		Scopes:       []string{"identify"},
		Endpoint:     DiscordEndpoint,
	}
}

// AuthURL returns the Discord authorization URL with the given state.
func AuthURL(cfg *oauth2.Config, state string) string {
	return cfg.AuthCodeURL(state, oauth2.AccessTypeOnline)
}

// FetchDiscordUser exchanges the authorization code for a token, then calls /users/@me.
func FetchDiscordUser(ctx context.Context, cfg *oauth2.Config, code string) (*DiscordUser, error) {
	token, err := cfg.Exchange(ctx, code)
	if err != nil {
		return nil, fmt.Errorf("exchange oauth code: %w", err)
	}

	client := cfg.Client(ctx, token)
	resp, err := client.Get("https://discord.com/api/v10/users/@me")
	if err != nil {
		return nil, fmt.Errorf("fetch discord user: %w", err)
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		body, _ := io.ReadAll(resp.Body)
		return nil, fmt.Errorf("discord api %d: %s", resp.StatusCode, body)
	}

	var user DiscordUser
	if err := json.NewDecoder(resp.Body).Decode(&user); err != nil {
		return nil, fmt.Errorf("decode discord user: %w", err)
	}
	return &user, nil
}