using System.Data;
using MiaoJiZhang.Domain.Entities;
using MiaoJiZhang.Domain.Enums;
using MiaoJiZhang.Infrastructure.Persistence;
using Microsoft.EntityFrameworkCore;

namespace MiaoJiZhang.Api.Services;

public sealed record AiChatQuotaStatus(
    int Limit,
    int Used,
    int Remaining,
    AiQuotaPeriod Period,
    DateTime WindowStartedAt,
    DateTime ResetAt,
    bool Allowed);

public sealed class AiChatQuotaService(AppDbContext db)
{
    public async Task<AiChatQuotaStatus> TryConsumeAsync(
        long userId,
        CancellationToken ct = default)
    {
        await using var transaction = await db.Database.BeginTransactionAsync(
            IsolationLevel.Serializable,
            ct);
        var user = await db.Users.SingleAsync(item => item.Id == userId, ct);
        var status = GetStatus(user);
        if (!status.Allowed)
        {
            await transaction.CommitAsync(ct);
            return status;
        }

        if (user.AiChatWindowStartedAt != status.WindowStartedAt)
        {
            user.AiChatWindowStartedAt = status.WindowStartedAt;
            user.AiChatUsed = 0;
        }
        user.AiChatUsed++;
        await db.SaveChangesAsync(ct);
        await transaction.CommitAsync(ct);
        return GetStatus(user) with { Allowed = true };
    }

    public static AiChatQuotaStatus GetStatus(User user)
    {
        var (start, end) = CurrentWindow(user.AiChatPeriod);
        var used = user.AiChatWindowStartedAt == start
            ? Math.Max(0, user.AiChatUsed)
            : 0;
        var limit = Math.Max(0, user.AiChatLimit);
        var remaining = limit == 0 ? -1 : Math.Max(0, limit - used);
        return new AiChatQuotaStatus(
            limit,
            used,
            remaining,
            user.AiChatPeriod,
            start,
            end,
            limit == 0 || used < limit);
    }

    public static (DateTime Start, DateTime End) CurrentWindow(
        AiQuotaPeriod period)
    {
        var now = ChinaClock.Now;
        var start = period switch
        {
            AiQuotaPeriod.Week => now.Date.AddDays(-(now.DayOfWeek == DayOfWeek.Sunday
                ? 6
                : (int)now.DayOfWeek - 1)),
            AiQuotaPeriod.Month => new DateTime(now.Year, now.Month, 1),
            _ => now.Date,
        };
        var end = period switch
        {
            AiQuotaPeriod.Week => start.AddDays(7),
            AiQuotaPeriod.Month => start.AddMonths(1),
            _ => start.AddDays(1),
        };
        return (ChinaClock.ToUtc(start), ChinaClock.ToUtc(end));
    }

    public static bool TryParsePeriod(string? value, out AiQuotaPeriod period)
    {
        period = value?.Trim().ToLowerInvariant() switch
        {
            "week" => AiQuotaPeriod.Week,
            "month" => AiQuotaPeriod.Month,
            _ => AiQuotaPeriod.Day,
        };
        return value?.Trim().ToLowerInvariant() is "day" or "week" or "month";
    }

    public static string PeriodKey(AiQuotaPeriod period) =>
        period.ToString().ToLowerInvariant();
}