Core_Be/Helpers/SandboxTokenHelper.cs

using System.Security.Cryptography;
using System.Text;

namespace Fitness_core_BE.Helpers
{
    // Short, shareable /sandbox/{token} code — replaces the old raw Guid. Made of a
    // time-derived prefix (minutes since a fixed epoch, base62) so codes drift forward
    // over time, plus a random suffix for uniqueness among requests created in the same
    // minute. Not a security boundary: the sandbox is a temporary trial state, not a
    // login credential.
    public static class SandboxTokenHelper
    {
        private const string Alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
        private const int TimePartLength = 5;
        private const int RandomPartLength = 5;
        private static readonly DateTime Epoch = new(2024, 1, 1, 0, 0, 0, DateTimeKind.Utc);

        public static string Generate(DateTime createdAtUtc)
        {
            long minutesSinceEpoch = Math.Max(0, (long)(createdAtUtc - Epoch).TotalMinutes);
            string timePart = ToBaseAlphabet(minutesSinceEpoch).PadLeft(TimePartLength, '0');
            if (timePart.Length > TimePartLength)
                timePart = timePart[^TimePartLength..];

            return timePart + RandomSuffix(RandomPartLength);
        }

        private static string RandomSuffix(int length)
        {
            var bytes = new byte[length];
            RandomNumberGenerator.Fill(bytes);
            var sb = new StringBuilder(length);
            foreach (var b in bytes)
                sb.Append(Alphabet[b % Alphabet.Length]);
            return sb.ToString();
        }

        private static string ToBaseAlphabet(long value)
        {
            if (value <= 0) return "0";
            var sb = new StringBuilder();
            while (value > 0)
            {
                sb.Insert(0, Alphabet[(int)(value % Alphabet.Length)]);
                value /= Alphabet.Length;
            }
            return sb.ToString();
        }
    }
}