using System;
using System.Collections.Generic;
using System.Linq;
using Jellyfin.Data;
using Jellyfin.Data.Enums;
using Jellyfin.Database.Implementations.Entities;
using Jellyfin.Database.Implementations.Enums;

namespace Jellyfin.Plugin.WatchedTogether.Services;

/// <summary>
/// The content restrictions on one user, in a form that can be compared and combined.
/// </summary>
/// <remarks>
/// <para>
/// Every field is read and written the way Jellyfin's <c>BaseItem.IsParentalAllowed</c> reads it,
/// so "stricter" here means "hides at least everything the other hides" there:
/// </para>
/// <list type="bullet">
/// <item>The rating cap allows an item whose score is below the cap, or equal to it with a
/// sub-score no higher than the sub-cap (a null sub-cap allows any). A null cap allows everything.</item>
/// <item>Each blocked unrated kind hides the unrated items of that kind.</item>
/// <item>A blocked tag hides any item carrying it.</item>
/// <item>A non-empty allowed-tag list hides any item carrying none of them. An <em>empty</em> list
/// is not a whitelist at all: it allows everything through this route.</item>
/// </list>
/// </remarks>
public sealed record ContentRestrictions
{
    /// <summary>
    /// The tag written as the whole allowed-tag list when the members' whitelists have nothing in
    /// common. No item carries it, so it hides everything - which an empty list would not, since
    /// Jellyfin reads an empty list as "no whitelist".
    /// </summary>
    public const string NothingAllowedTag = "watched-together:nothing";

    /// <summary>
    /// Gets restrictions that hide everything. Used for a member that cannot be resolved, on the
    /// same principle as library access: an unknown member must not widen the group.
    /// </summary>
    public static ContentRestrictions FullyRestricted { get; } = new()
    {
        MaxParentalRatingScore = 0,
        MaxParentalRatingSubScore = 0,
        BlockUnratedItems = Enum.GetValues<UnratedItem>().ToHashSet(),
        BlockedTags = new HashSet<string>(StringComparer.OrdinalIgnoreCase),
        AllowedTags = new HashSet<string>(StringComparer.OrdinalIgnoreCase),
    };

    /// <summary>
    /// Gets the parental rating cap, or <c>null</c> for no cap.
    /// </summary>
    public int? MaxParentalRatingScore { get; init; }

    /// <summary>
    /// Gets the sub-score cap that applies at exactly <see cref="MaxParentalRatingScore"/>, or
    /// <c>null</c> for any sub-score.
    /// </summary>
    public int? MaxParentalRatingSubScore { get; init; }

    /// <summary>
    /// Gets the kinds of unrated item that are hidden.
    /// </summary>
    public IReadOnlySet<UnratedItem> BlockUnratedItems { get; init; } = new HashSet<UnratedItem>();

    /// <summary>
    /// Gets the tags that hide an item.
    /// </summary>
    public IReadOnlySet<string> BlockedTags { get; init; } = new HashSet<string>(StringComparer.OrdinalIgnoreCase);

    /// <summary>
    /// Gets the whitelist an item must match, <c>null</c> when there is no whitelist, or an empty
    /// set when the whitelist is in force but lets nothing through.
    /// </summary>
    public IReadOnlySet<string>? AllowedTags { get; init; }

    /// <summary>
    /// Gets a value indicating whether a whitelist is in force.
    /// </summary>
    public bool HasAllowedTags => AllowedTags is not null;

    /// <summary>
    /// Reads a user's live restrictions.
    /// </summary>
    /// <param name="user">The user to read.</param>
    /// <returns>The user's restrictions.</returns>
    public static ContentRestrictions FromUser(User user)
    {
        ArgumentNullException.ThrowIfNull(user);

        var allowed = CleanTags(user.GetPreference(PreferenceKind.AllowedTags));

        return new ContentRestrictions
        {
            MaxParentalRatingScore = user.MaxParentalRatingScore,
            MaxParentalRatingSubScore = user.MaxParentalRatingSubScore,
            BlockUnratedItems = user.GetPreferenceValues<UnratedItem>(PreferenceKind.BlockUnratedItems).ToHashSet(),
            BlockedTags = CleanTags(user.GetPreference(PreferenceKind.BlockedTags)),
            AllowedTags = allowed.Count switch
            {
                0 => null,
                // A whitelist consisting only of the sentinel is the stored form of "nothing".
                1 when allowed.Contains(NothingAllowedTag) => new HashSet<string>(StringComparer.OrdinalIgnoreCase),
                _ => allowed,
            },
        };
    }

    /// <summary>
    /// Combines two sets of restrictions, keeping the stricter value of each field.
    /// </summary>
    /// <param name="other">The restrictions to combine with.</param>
    /// <returns>Restrictions at least as strict as both.</returns>
    public ContentRestrictions CombineStrictest(ContentRestrictions other)
    {
        ArgumentNullException.ThrowIfNull(other);

        var (score, subScore) = RatingCapIsAtMost(this, other)
            ? (MaxParentalRatingScore, MaxParentalRatingSubScore)
            : (other.MaxParentalRatingScore, other.MaxParentalRatingSubScore);

        var blockUnrated = new HashSet<UnratedItem>(BlockUnratedItems);
        blockUnrated.UnionWith(other.BlockUnratedItems);

        var blocked = new HashSet<string>(BlockedTags, StringComparer.OrdinalIgnoreCase);
        blocked.UnionWith(other.BlockedTags);

        // Only members with a whitelist constrain; a member without one accepts the other's.
        IReadOnlySet<string>? allowed;
        if (AllowedTags is null)
        {
            allowed = other.AllowedTags;
        }
        else if (other.AllowedTags is null)
        {
            allowed = AllowedTags;
        }
        else
        {
            var both = new HashSet<string>(AllowedTags, StringComparer.OrdinalIgnoreCase);
            both.IntersectWith(other.AllowedTags);
            allowed = both;
        }

        return new ContentRestrictions
        {
            MaxParentalRatingScore = score,
            MaxParentalRatingSubScore = subScore,
            BlockUnratedItems = blockUnrated,
            BlockedTags = blocked,
            AllowedTags = allowed,
        };
    }

    /// <summary>
    /// Determines whether these restrictions hide at least everything <paramref name="other"/>
    /// hides.
    /// </summary>
    /// <param name="other">The restrictions to compare against.</param>
    /// <returns><c>true</c> if every field here is at least as restrictive.</returns>
    public bool IsAtLeastAsStrictAs(ContentRestrictions other)
    {
        ArgumentNullException.ThrowIfNull(other);

        if (!RatingCapIsAtMost(this, other))
        {
            return false;
        }

        if (!BlockUnratedItems.IsSupersetOf(other.BlockUnratedItems))
        {
            return false;
        }

        if (!BlockedTags.IsSupersetOf(other.BlockedTags))
        {
            return false;
        }

        // No whitelist on the other side constrains nothing. Otherwise ours must exist and let
        // through no more than theirs does.
        return other.AllowedTags is null
            || (AllowedTags is not null && AllowedTags.IsSubsetOf(other.AllowedTags));
    }

    /// <summary>
    /// Gets the allowed-tag list in the form Jellyfin stores it: empty for no whitelist, the
    /// sentinel for a whitelist that allows nothing.
    /// </summary>
    /// <returns>The tags to write to the user's policy.</returns>
    public string[] AllowedTagsForPolicy()
    {
        if (AllowedTags is null)
        {
            return [];
        }

        return AllowedTags.Count == 0 ? [NothingAllowedTag] : AllowedTags.ToArray();
    }

    /// <summary>
    /// Compares two rating caps: true when <paramref name="a"/>'s cap allows no more than
    /// <paramref name="b"/>'s. A null cap allows everything; at an equal score, a null sub-cap
    /// allows every sub-score.
    /// </summary>
    private static bool RatingCapIsAtMost(ContentRestrictions a, ContentRestrictions b)
    {
        if (b.MaxParentalRatingScore is null)
        {
            return true;
        }

        if (a.MaxParentalRatingScore is null)
        {
            return false;
        }

        if (a.MaxParentalRatingScore.Value != b.MaxParentalRatingScore.Value)
        {
            return a.MaxParentalRatingScore.Value < b.MaxParentalRatingScore.Value;
        }

        if (b.MaxParentalRatingSubScore is null)
        {
            return true;
        }

        return a.MaxParentalRatingSubScore is not null
            && a.MaxParentalRatingSubScore.Value <= b.MaxParentalRatingSubScore.Value;
    }

    private static HashSet<string> CleanTags(IEnumerable<string> tags)
        => tags.Where(t => !string.IsNullOrWhiteSpace(t))
            .Select(t => t.Trim())
            .ToHashSet(StringComparer.OrdinalIgnoreCase);
}