using System.Globalization;

namespace Fleet.Hosting;

/// <summary>
/// 每天固定时刻有序退出一次，由 systemd、容器引擎或 procd 的「总是重启」拉起。写法 <c>HH:mm</c>（按 UTC）、
/// <c>HH:mm±hh:mm</c> 或 <c>HH:mm</c> 加 <c>Z</c>：容器里没有时区数据库，所以偏移要写明，例如 <c>04:00+08:00</c>。
/// 进程自己退出，不需要任何执行外部命令的能力。
/// </summary>
public sealed class RestartSchedule
{
    /// <summary>环境变量，设置了就覆盖配置文件里的值。</summary>
    public const string EnvironmentVariable = "MOSHIRO_RESTART_AT";

    /// <summary>刚启动的一段时间内不会触发：重启之后不会因为时刻还没过去而再重启一次，也避免重启风暴。</summary>
    public static readonly TimeSpan MinimumUptime = TimeSpan.FromMinutes(10);

    private readonly TimeSpan _time;
    private readonly TimeSpan _offset;

    private RestartSchedule(TimeSpan time, TimeSpan offset)
    {
        _time = time;
        _offset = offset;
    }

    public static bool TryParse(string? text, out RestartSchedule? schedule)
    {
        schedule = null;
        string value = text?.Trim() ?? "";
        // HH:mm，后面可以跟 Z 或 ±hh:mm
        if (value.Length < 5 || value[2] != ':' ||
            !TryTwoDigits(value, 0, out int hour) || !TryTwoDigits(value, 3, out int minute) || hour > 23 || minute > 59)
        {
            return false;
        }

        TimeSpan offset = TimeSpan.Zero;
        string rest = value[5..];
        if (rest.Length > 0 && rest != "Z")
        {
            if (rest.Length != 6 || rest[0] is not ('+' or '-') || rest[3] != ':' ||
                !TryTwoDigits(rest, 1, out int offsetHours) || !TryTwoDigits(rest, 4, out int offsetMinutes) || offsetMinutes > 59)
            {
                return false;
            }

            offset = new TimeSpan(offsetHours, offsetMinutes, 0);
            if (offset > TimeSpan.FromHours(14))
            {
                return false;
            }

            if (rest[0] == '-')
            {
                offset = -offset;
            }
        }

        schedule = new RestartSchedule(new TimeSpan(hour, minute, 0), offset);
        return true;
    }

    private static bool TryTwoDigits(string text, int start, out int value)
    {
        value = 0;
        if (!char.IsAsciiDigit(text[start]) || !char.IsAsciiDigit(text[start + 1]))
        {
            return false;
        }

        value = (text[start] - '0') * 10 + (text[start + 1] - '0');
        return true;
    }

    /// <summary>
    /// 环境变量优先，其次配置文件的值；都没有就不定时重启。写错了抛 <see cref="InvalidOperationException"/>，
    /// 启动时就能看到，而不是悄悄不生效。
    /// </summary>
    public static RestartSchedule? Resolve(string? configured)
    {
        string? text = Environment.GetEnvironmentVariable(EnvironmentVariable);
        if (string.IsNullOrWhiteSpace(text))
        {
            text = configured;
        }

        if (string.IsNullOrWhiteSpace(text))
        {
            return null;
        }

        return TryParse(text, out RestartSchedule? schedule)
            ? schedule
            : throw new InvalidOperationException($"restart_at must look like 04:00, 04:00Z or 04:00+08:00 (got '{text}')");
    }

    /// <summary>严格晚于 <paramref name="now"/> 的下一个触发时刻。</summary>
    public DateTimeOffset NextAfter(DateTimeOffset now)
    {
        DateTimeOffset local = now.ToOffset(_offset);
        DateTimeOffset candidate = new DateTimeOffset(local.Year, local.Month, local.Day, 0, 0, 0, _offset) + _time;
        return candidate <= local ? candidate.AddDays(1) : candidate;
    }
}