SocialPub/PrivaPub/Infrastructure/Ids/PrivacyIds.cs

using System.Buffers.Binary;
using System.Security.Cryptography;

using MongoDB.Bson;

namespace PrivaPub.Infrastructure.Ids
{
	public static class PrivacyIds
	{
		public static string ForDay(DateTime when) => At(DateTime.SpecifyKind(when, DateTimeKind.Utc).Date);

		public static DateTime PublishedDay(DateTime created) =>
			DateTime.SpecifyKind(created, DateTimeKind.Utc).Date.AddDays(-RandomNumberGenerator.GetInt32(0, PublishedSpreadDays));

		const int PublishedSpreadDays = 14;

		public static string Arrived(DateTime published) =>
			DateTime.SpecifyKind(published, DateTimeKind.Utc) > DateTime.UtcNow - LiveWindow ? ObjectId.GenerateNewId().ToString() : At(published);

		static readonly TimeSpan LiveWindow = TimeSpan.FromHours(1);

		public static string At(DateTime when)
		{
			var seconds = Math.Clamp(new DateTimeOffset(DateTime.SpecifyKind(when, DateTimeKind.Utc)).ToUnixTimeSeconds(), 0,
				DateTimeOffset.UtcNow.AddDays(1).ToUnixTimeSeconds());
			Span<byte> bytes = stackalloc byte[12];
			BinaryPrimitives.WriteInt32BigEndian(bytes, (int)seconds);
			RandomNumberGenerator.Fill(bytes[4..]);
			return Convert.ToHexStringLower(bytes);
		}
	}
}