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Helpers/SandboxTokenHelper.cs

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+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();
+        }
+    }
+}