using Discord.Interactions;
using NodaTime;
using System.Collections.ObjectModel;
using System.Globalization;
using System.Text;

namespace WorldTime;

public class ApplicationCommands : InteractionModuleBase<ShardedInteractionContext> {
    const string ErrNotAllowed = ":x: Only server moderators may use this command.";

    const string EmbedHelpField1 = $"`/help` - {HelpHelp}\n"
        + $"`/list` - {HelpList}\n"
        + $"`/set` - {HelpSet}\n"
        + $"`/remove` - {HelpRemove}";
    const string EmbedHelpField2 = $"`/set-for` - {HelpSetFor}\n`/remove-for` - {HelpRemoveFor}";

    #region Help strings
    const string HelpHelp = "Displays a list of available bot commands.";
    const string HelpList = "Shows the current time for all recently active known users.";
    const string HelpSet = "Adds or updates your time zone to the bot.";
    const string HelpSetFor = "Sets/updates time zone for a given user.";
    const string HelpRemove = "Removes your time zone information from this bot.";
    const string HelpRemoveFor = "Removes time zone for a given user.";
    #endregion
    private const string ErrInvalidZone = ":x: Not a valid zone name."
        + " To find your time zone, refer to: <https://kevinnovak.github.io/Time-Zone-Picker/>.";
    private const string ErrNoUserCache = ":warning: Please try the command again.";

    private static readonly ReadOnlyDictionary<string, string> _tzNameMap;

    public DiscordShardedClient ShardedClient { get; set; } = null!;
    public Database Database { get; set; } = null!;

    static ApplicationCommands() {
        Dictionary<string, string> tzNameMap = new(StringComparer.OrdinalIgnoreCase);
        foreach (var name in DateTimeZoneProviders.Tzdb.Ids) tzNameMap.Add(name, name);
        _tzNameMap = new(tzNameMap);
    }

    [SlashCommand("help", HelpHelp)]
    public async Task CmdHelp() {
        var version = System.Reflection.Assembly.GetExecutingAssembly().GetName().Version!.ToString(3);
        var guildct = ShardedClient.Guilds.Count;
        var uniquetz = await Database.GetDistinctZoneCountAsync();
        await RespondAsync(embed: new EmbedBuilder() {
            Title = "Help & About",
            Description = $"World Time v{version} - Serving {guildct} communities across {uniquetz} time zones.\n\n"
                + "This bot is provided for free, without any paywalled 'premium' features. "
                + "If you've found this bot useful, please consider contributing via the "
                + "bot author's page on Ko-fi: https://ko-fi.com/noithecat.",
            Footer = new EmbedFooterBuilder() {
                IconUrl = Context.Client.CurrentUser.GetAvatarUrl(),
                Text = "World Time"
            }
        }.AddField(inline: false, name: "Commands", value: EmbedHelpField1
        ).AddField(inline: false, name: "Admin commands", value: EmbedHelpField2
        ).AddField(inline: false, name: "Zones", value:
            "This bot accepts zone names from the IANA Time Zone Database (a.k.a. Olson Database). " +
            "A useful tool to determine yours can be found at: https://kevinnovak.github.io/Time-Zone-Picker/"
        ).Build());
    }

    [RequireGuildContext]
    [SlashCommand("list", HelpList)]
    public async Task CmdList([Summary(description: "A specific user whose time to look up.")]SocketGuildUser? user = null) {
        if (!await AreUsersDownloadedAsync(Context.Guild)) {
            await RespondAsync(ErrNoUserCache, ephemeral: true);
            return;
        }

        if (user != null) {
            await CmdListUserAsync(user);
            return;
        }

        var userlist = await Database.GetGuildZonesAsync(Context.Guild.Id);
        if (userlist.Count == 0) {
            await RespondAsync(":x: Nothing to show. Register your time zones with the bot using the `/set` command.");
            return;
        }
        // Order times by popularity to limit how many are shown, group by printed name
        var sortedlist = new SortedDictionary<string, List<ulong>>();
        foreach ((string area, List<ulong> users) in userlist.OrderByDescending(o => o.Value.Count).Take(20)) {
            // Filter further to top 20 distinct timezones, even if they are not displayed in the final result
            var areaprint = TzPrint(area);
            if (!sortedlist.ContainsKey(areaprint)) sortedlist.Add(areaprint, new List<ulong>());
            sortedlist[areaprint].AddRange(users);
        }

        const int MaxSingleLineLength = 750;
        const int MaxSingleOutputLength = 900;

        // Build zone listings with users
        var outputlines = new List<string>();
        foreach ((string area, List<ulong> users) in sortedlist) {
            var buffer = new StringBuilder();
            buffer.Append(area[4..] + ": ");
            bool empty = true;
            foreach (var userid in users) {
                var userinstance = Context.Guild.GetUser(userid);
                if (userinstance == null) continue;
                if (empty) empty = !empty;
                else buffer.Append(", ");
                var useradd = FormatName(userinstance);
                if (buffer.Length + useradd.Length > MaxSingleLineLength) {
                    buffer.Append("others...");
                    break;
                } else buffer.Append(useradd);
            }
            if (!empty) outputlines.Add(buffer.ToString());
        }

        // Prepare for output - send buffers out if they become too large
        outputlines.Sort();
        bool hasOutputOneLine = false;
        // First output is shown as an interaction response, followed then as regular channel messages
        async Task doOutput(Embed msg) {
            if (!hasOutputOneLine) {
                await RespondAsync(embed: msg);
                hasOutputOneLine = true;
            } else {
                await ReplyAsync(embed: msg);
            }
        }

        var resultout = new StringBuilder();
        foreach (var line in outputlines) {
            if (resultout.Length + line.Length > MaxSingleOutputLength) {
                await doOutput(new EmbedBuilder().WithDescription(resultout.ToString()).Build());
                resultout.Clear();
            }
            if (resultout.Length > 0) resultout.AppendLine(); // avoids trailing newline by adding to the previous line
            resultout.Append(line);
        }
        if (resultout.Length > 0) {
            await doOutput(new EmbedBuilder().WithDescription(resultout.ToString()).Build());
        }
    }

    private async Task CmdListUserAsync(SocketGuildUser parameter) {
        // Not meant as a command handler - called by CmdList
        var result = await Database.GetUserZoneAsync(parameter);
        if (result == null) {
            bool isself = Context.User.Id == parameter.Id;
            if (isself) await RespondAsync(":x: You do not have a time zone. Set it with `tz.set`.", ephemeral: true);
            else await RespondAsync(":x: The given user does not have a time zone set.", ephemeral: true);
            return;
        }

        var resulttext = TzPrint(result)[4..] + ": " + FormatName(parameter);
        await RespondAsync(embed: new EmbedBuilder().WithDescription(resulttext).Build());
    }

    [SlashCommand("set", HelpSet)]
    public async Task CmdSet([Summary(description: "The new time zone to set.")]string zone) {
        var parsedzone = ParseTimeZone(zone);
        if (parsedzone == null) {
            await RespondAsync(ErrInvalidZone, ephemeral: true);
            return;
        }
        await Database.UpdateUserAsync((SocketGuildUser)Context.User, parsedzone);
        await RespondAsync($":white_check_mark: Your time zone has been set to **{parsedzone}**.");
    }

    [RequireGuildContext]
    [SlashCommand("set-for", HelpSetFor)]
    public async Task CmdSetFor([Summary(description: "The user whose time zone to modify.")] SocketGuildUser user,
                                 [Summary(description: "The new time zone to set.")] string zone) {
        if (!IsUserAdmin((SocketGuildUser)Context.User)) {
            await RespondAsync(ErrNotAllowed, ephemeral: true).ConfigureAwait(false);
            return;
        }

        // Extract parameters
        var newtz = ParseTimeZone(zone);
        if (newtz == null) {
            await RespondAsync(ErrInvalidZone);
            return;
        }

        await Database.UpdateUserAsync(user, newtz).ConfigureAwait(false);
        await RespondAsync($":white_check_mark: Time zone for **{user}** set to **{newtz}**.");
    }

    [RequireGuildContext]
    [SlashCommand("remove", HelpRemove)]
    public async Task CmdRemove() {
        var success = await Database.DeleteUserAsync((SocketGuildUser)Context.User);
        if (success) await RespondAsync(":white_check_mark: Your zone has been removed.");
        else await RespondAsync(":x: You don't have a time zone set.");
    }

    [RequireGuildContext]
    [SlashCommand("remove-for", HelpRemoveFor)]
    public async Task CmdRemoveFor([Summary(description: "The user whose time zone to remove.")] SocketGuildUser user) {
        if (!IsUserAdmin((SocketGuildUser)Context.User)) {
            await RespondAsync(ErrNotAllowed, ephemeral: true).ConfigureAwait(false);
            return;
        }

        if (await Database.DeleteUserAsync(user))
            await RespondAsync($":white_check_mark: Removed zone information for {user}.");
        else
            await RespondAsync($":white_check_mark: No time zone is set for {user}.");
    }

    #region Helper methods
    /// <summary>
    /// Returns a string displaying the current time in the given time zone.
    /// The result begins with four numbers for sorting purposes. Must be trimmed before output.
    /// </summary>
    private static string TzPrint(string zone) {
        var tzdb = DateTimeZoneProviders.Tzdb;
        DateTimeZone tz = tzdb.GetZoneOrNull(zone)!;
        if (tz == null) throw new Exception("Encountered unknown time zone: " + zone);

        var now = SystemClock.Instance.GetCurrentInstant().InZone(tz);
        var sortpfx = now.ToString("MMdd", DateTimeFormatInfo.InvariantInfo);
        var fullstr = now.ToString("dd'-'MMM' 'HH':'mm' 'x' (UTC'o<g>')'", DateTimeFormatInfo.InvariantInfo);
        return $"{sortpfx}● `{fullstr}`";
    }

    /// <summary>
    /// Checks given time zone input. Returns a valid string for use with NodaTime, or null.
    /// </summary>
    private static string? ParseTimeZone(string tzinput) {
        if (tzinput.Equals("Asia/Calcutta", StringComparison.OrdinalIgnoreCase)) tzinput = "Asia/Kolkata";
        if (_tzNameMap.TryGetValue(tzinput, out var name)) return name;
        return null;
    }

    /// <summary>
    /// Formats a user's name to a consistent, readable format which makes use of their nickname.
    /// </summary>
    private static string FormatName(SocketGuildUser user) {
        static string escapeFormattingCharacters(string input) {
            var result = new StringBuilder();
            foreach (var c in input) {
                if (c is '\\' or '_' or '~' or '*' or '@') {
                    result.Append('\\');
                }
                result.Append(c);
            }
            return result.ToString();
        }

        var username = escapeFormattingCharacters(user.Username);
        if (user.Nickname != null) {
            return $"**{escapeFormattingCharacters(user.Nickname)}** ({username}#{user.Discriminator})";
        }
        return $"**{username}**#{user.Discriminator}";
    }

    /// <summary>
    /// Checks if the given user can be considered a guild admin ('Manage Server' is set).
    /// </summary>
    // TODO replace this with a precondition, or there's also a new permission scheme going around?
    private static bool IsUserAdmin(SocketGuildUser user)
        => user.GuildPermissions.Administrator || user.GuildPermissions.ManageGuild;

    /// <summary>
    /// Checks if the member cache for the specified guild needs to be filled, and sends a request if needed.
    /// </summary>
    /// <remarks>
    /// Due to a quirk in Discord.Net, the user cache cannot be filled until the command handler is no longer executing
    /// regardless of if the request runs on its own thread, thus requiring the user to run the command again.
    /// </remarks>
    /// <returns>
    /// True if the guild's members are already downloaded. If false, the command handler must notify the user.
    /// </returns>
    private static async Task<bool> AreUsersDownloadedAsync(SocketGuild guild) {
        static bool HasMostMembersDownloaded(SocketGuild guild) {
            if (guild.HasAllMembers) return true;
            if (guild.MemberCount > 30) {
                // For guilds of size over 30, require 85% or more of the members to be known
                // (26/30, 42/50, 255/300, etc)
                return guild.DownloadedMemberCount >= (int)(guild.MemberCount * 0.85);
            } else {
                // For smaller guilds, fail if two or more members are missing
                return guild.MemberCount - guild.DownloadedMemberCount <= 2;
            }
        }
        if (HasMostMembersDownloaded(guild)) return true;
        else {
            // Event handler hangs if awaited normally or used with Task.Run
            await Task.Factory.StartNew(guild.DownloadUsersAsync).ConfigureAwait(false);
            return false;
        }
    }
    #endregion
}