DiscordChatExporter/DiscordChatExporter.Core/Discord/Snowflake.cs

using System;
using System.Diagnostics.CodeAnalysis;
using System.Globalization;
using System.Text.RegularExpressions;

namespace DiscordChatExporter.Core.Discord;

public readonly partial record struct Snowflake(ulong Value)
{
    public DateTimeOffset ToDate() => DateTimeOffset.FromUnixTimeMilliseconds(
        (long)((Value >> 22) + 1420070400000UL)
    ).ToLocalTime();

    [ExcludeFromCodeCoverage]
    public override string ToString() => Value.ToString(CultureInfo.InvariantCulture);
}

public partial record struct Snowflake
{
    public static Snowflake Zero { get; } = new(0);

    public static Snowflake FromDate(DateTimeOffset date) => new(
        ((ulong)date.ToUnixTimeMilliseconds() - 1420070400000UL) << 22
    );

    public static Snowflake? TryParse(string? str, IFormatProvider? formatProvider = null)
    {
        if (string.IsNullOrWhiteSpace(str))
            return null;

        // As number
        if (Regex.IsMatch(str, @"^\d+$") && ulong.TryParse(str, NumberStyles.Number, formatProvider, out var value))
        {
            return new Snowflake(value);
        }

        // As date
        if (DateTimeOffset.TryParse(str, formatProvider, DateTimeStyles.None, out var date))
        {
            return FromDate(date);
        }

        return null;
    }

    public static Snowflake Parse(string str, IFormatProvider? formatProvider) =>
        TryParse(str, formatProvider) ?? throw new FormatException($"Invalid snowflake '{str}'.");

    public static Snowflake Parse(string str) => Parse(str, null);
}

public partial record struct Snowflake : IComparable<Snowflake>
{
    public int CompareTo(Snowflake other) => Value.CompareTo(other.Value);
}