VolcanicAgenticCore/hosts/Volcanic.AgenticCore.Telegram/Chats/Impersonation.cs

using System.Collections.Concurrent;
using System.Globalization;

namespace Volcanic.AgenticCore.Telegram.Chats;

/// <summary>
/// An admin's (<see cref="TelegramOptions.Admins"/>) <c>/impersonate &lt;user_id&gt;</c>. While it is on, that admin's
/// messages and button presses are handled as the target user's: the allow-list check, the chat, its workspace, session,
/// settings and jobs. Everything the target's chat sends (replies, questions, files, scheduled deliveries) goes to that
/// admin's chat instead, until <c>/impersonate 0</c>. The target user's own messages and button presses are blocked
/// meanwhile and answered with <see cref="BlockedNotice"/>. In private chats the chat id is the user id, so one id maps both.
/// Every admin impersonates independently; a user can be impersonated by one admin at a time, and an admin who is
/// impersonating someone can't be impersonated. Kept in memory: a restart ends it.
/// </summary>
public sealed class Impersonation(TelegramOptions options)
{
    /// <summary>The reply to the impersonated user while an admin works as them.</summary>
    public const string BlockedNotice = "🔒 An admin is working with your account right now. Please try again later.";

    private readonly ConcurrentDictionary<long, long> _targets = new(); // admin → impersonated user
    private readonly Lock _gate = new();

    public IReadOnlyList<long> AdminIds => options.Admins;

    public bool IsAdmin(long? userId) => userId is { } id && options.Admins.Contains(id);

    /// <summary>The user <paramref name="adminId"/> impersonates, or null when they don't.</summary>
    public long? TargetOf(long adminId) => _targets.TryGetValue(adminId, out var target) ? target : null;

    /// <summary>True for an impersonated user themselves: their updates are blocked while an admin works as them.</summary>
    public bool IsBlocked(long? userId) => userId is { } id && AdminOf(id) is not null;

    /// <summary>The user (or private chat) id the bot acts for: the target for an impersonating admin, else <paramref name="id"/>.</summary>
    public long EffectiveId(long id) => TargetOf(id) ?? id;

    /// <summary>Where chat <paramref name="chatId"/>'s output goes: the chat of the admin impersonating it, if any.</summary>
    public long DeliveryChatFor(long chatId) => AdminOf(chatId) ?? chatId;

    private long? AdminOf(long target)
    {
        foreach (var (admin, current) in _targets)
            if (current == target) return admin;
        return null;
    }

    /// <summary>Handle <paramref name="adminId"/>'s <c>/impersonate</c> argument and return the reply for them.</summary>
    public string Apply(long adminId, string arg)
    {
        lock (_gate)
        {
            if (AdminOf(adminId) is { } other)
                return $"Admin {other} is acting as you right now, so you can't impersonate anyone until they stop.";
            if (!long.TryParse(arg.Trim(), NumberStyles.None, CultureInfo.InvariantCulture, out var id))
                return "Usage: /impersonate <user_id> (0 stops). " + (TargetOf(adminId) is { } current ? $"You are acting as user {current}." : "You are not impersonating anyone.");
            if (id == 0 || id == adminId)
                return _targets.TryRemove(adminId, out _) ? "🎭 Impersonation off: you are yourself again." : "You are not impersonating anyone.";
            if (AdminOf(id) is { } taken && taken != adminId)
                return $"Admin {taken} is already acting as user {id}. Only one admin can impersonate a user at a time.";
            if (TargetOf(id) is not null)
                return $"User {id} is an admin who is impersonating someone right now; try again after they stop.";
            _targets[adminId] = id;
            var reply = $"🎭 You are now acting as user {id}: the bot handles your messages as theirs, and their chat's output comes here. /impersonate 0 stops.";
            return options.IsAllowed(id) ? reply : reply + $"\n⚠️ User {id} is not in the allow-list, so the bot ignores your messages until you stop.";
        }
    }
}