using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;

namespace PKHeX.Core;

/// <summary>
/// Contains many <see cref="ITrainerInfo"/> instances to match against a <see cref="GameVersion"/>.
/// </summary>
public sealed class TrainerDatabase
{
    private readonly Dictionary<GameVersion, List<ITrainerInfo>> Database = [];

    /// <summary>
    /// Gets the number of unique versions in the database.
    /// </summary>
    public int CountVersions => Database.Count;

    /// <summary>
    /// Gets the number of trainers in the database.
    /// </summary>
    public int CountTrainers => Database.Sum(z => z.Value.Count);

    /// <summary>
    /// Checks if the database contains any trainers for the specified <see cref="version"/>.
    /// </summary>
    /// <param name="version"></param>
    public bool HasVersion(GameVersion version) => Database.ContainsKey(version);

    /// <summary>
    /// Gets all trainers from the database for the specified saved <see cref="version"/>.
    /// </summary>
    /// <param name="version">Saved Version to fetch trainers for</param>
    public ReadOnlySpan<ITrainerInfo> GetTrainers(GameVersion version)
    {
        if (Database.TryGetValue(version, out var list))
            return CollectionsMarshal.AsSpan(list);
        return default;
    }

    /// <summary>
    /// Fetches an appropriate trainer based on the requested <see cref="version"/>.
    /// </summary>
    /// <param name="version">Version the trainer should originate from</param>
    /// <param name="language">Language to request for</param>
    /// <returns>Null if no trainer found for this version.</returns>
    public ITrainerInfo? GetTrainer(GameVersion version, LanguageID? language = null)
    {
        if (version <= 0)
            return null;

        if (!version.IsValidSavedVersion())
            return GetTrainerFromGroup(version, language);

        if (Database.TryGetValue(version, out var list))
            return list[GetRandomIndex(list.Count)];

        return null;
    }

    private static int GetRandomIndex(int count) => count == 1 ? 0 : Util.Rand.Next(count);

    /// <summary>
    /// Fetches an appropriate trainer based on the requested <see cref="version"/> group.
    /// </summary>
    /// <param name="version">Version the trainer should originate from</param>
    /// <param name="lang">Language to request for</param>
    /// <returns>Null if no trainer found for this version.</returns>
    private ITrainerInfo? GetTrainerFromGroup(GameVersion version, LanguageID? lang = null)
    {
        var possible = Database.Where(z => version.Contains(z.Key)).ToList();
        if (possible.Count == 0)
            return null;

        if (lang is not null)
        {
            possible = possible.Select(z =>
            {
                var filtered = z.Value.Where(x => x.Language == (int)lang).ToList();
                return new KeyValuePair<GameVersion, List<ITrainerInfo>>(z.Key, filtered);
            }).Where(z => z.Value.Count != 0).ToList();
        }
        var span = CollectionsMarshal.AsSpan(possible);
        return GetRandomTrainer(span);
    }

    /// <summary>
    /// Fetches an appropriate trainer based on the requested <see cref="context"/>.
    /// </summary>
    /// <param name="context">Generation the trainer should inhabit</param>
    /// <param name="lang">Language to request for</param>
    /// <returns>Null if no trainer found for this version.</returns>
    public ITrainerInfo? GetTrainerFromContext(EntityContext context, LanguageID? lang = null)
    {
        var possible = Database.Where(z => z.Key.Context == context).ToList();
        if (possible.Count == 0)
            return null;

        if (lang is not null)
        {
            possible = possible.Select(z =>
            {
                var filtered = z.Value.Where(x => x.Language == (int)lang).ToList();
                return new KeyValuePair<GameVersion, List<ITrainerInfo>>(z.Key, filtered);
            }).Where(z => z.Value.Count != 0).ToList();
        }
        var span = CollectionsMarshal.AsSpan(possible);
        return GetRandomTrainer(span);
    }

    private static ITrainerInfo? GetRandomTrainer(ReadOnlySpan<KeyValuePair<GameVersion, List<ITrainerInfo>>> possible)
    {
        if (possible.Length == 0)
            return null;
        var group = possible[GetRandomIndex(possible.Length)];
        var span = group.Value;
        return span[GetRandomIndex(span.Count)];
    }

    /// <summary>
    /// Adds the <see cref="trainer"/> to the <see cref="Database"/>.
    /// </summary>
    /// <param name="trainer">Trainer details to add.</param>
    public void Register(ITrainerInfo trainer)
    {
        var version = trainer.Version;
        if (!Database.TryGetValue(version, out var list))
        {
            Database.Add(version, [trainer]);
            return;
        }

        if (list.Contains(trainer))
            return;
        list.Add(trainer);
    }

    /// <summary>
    /// Adds the trainer details of the <see cref="pk"/> to the <see cref="Database"/>.
    /// </summary>
    /// <param name="pk">Pokémon with Trainer details to add.</param>
    /// <remarks>A copy of the object will be made to prevent modifications, just in case.</remarks>
    public void RegisterCopy(PKM pk) => Register(GetTrainerReference(pk));

    /// <summary>
    /// Adds the trainer details of the <see cref="info"/> to the <see cref="Database"/>.
    /// </summary>
    /// <param name="info">Pokémon with Trainer details to add.</param>
    /// <remarks>A copy of the object will be made to prevent modifications, just in case.</remarks>
    public void RegisterCopy(ITrainerInfo info) => Register(new SimpleTrainerInfo(info));

    private static SimpleTrainerInfo GetTrainerReference(PKM pk)
    {
        var (cr, c, r) = GetRegion3DS(pk);
        return GetTrainerReference(pk, cr, c, r);
    }

    private static SimpleTrainerInfo GetTrainerReference(PKM pk, byte cr, byte c, byte r) => new(pk.Version)
    {
        TID16 = pk.TID16,
        SID16 = pk.SID16,
        OT = pk.OriginalTrainerName,
        Gender = pk.OriginalTrainerGender,
        Language = pk.Language,
        Generation = pk.Generation,
        ConsoleRegion = cr,
        Country = c,
        Region = r,
    };

    private static (byte ConsoleRegion, byte Country, byte Region) GetRegion3DS(PKM pk)
    {
        if (pk is IRegionOriginReadOnly x)
            return (x.ConsoleRegion, x.Country, x.Region);
        if (pk.Version.IsGen6() || pk.Version.IsGen7())
        {
            if (pk.Language == (int)LanguageID.Japanese)
                return (0, 1, 0);
            return (1, 7, 49);
        }
        return default;
    }

    /// <summary>
    /// Clears all trainer details from the <see cref="Database"/>.
    /// </summary>
    public void Clear() => Database.Clear();
}