Initial version

DataTransfer.Client/Shared/ConnectionCode.cs

@@ -0,0 +1,54 @@
+using System.Diagnostics.CodeAnalysis;
+using System.Security.Cryptography;
+
+namespace DataTransfer.Client.Shared;
+
+/// <summary>Generates and parses the one-time codes used to pair two participants.</summary>
+public static class ConnectionCode
+{
+    public const int Length = 10;
+
+    // Ambiguous characters (0/O, 1/I) are excluded. 32 symbols ^ 10 = 50 bits of entropy.
+    private const string Alphabet = "ABCDEFGHJKLMNPQRSTUVWXYZ23456789";
+
+    public static string Generate() => RandomNumberGenerator.GetString(Alphabet, Length);
+
+    /// <summary>Accepts user input case-insensitively, ignoring whitespace and dashes.</summary>
+    public static bool TryNormalize(string? input, [NotNullWhen(true)] out string? code)
+    {
+        code = null;
+        if (string.IsNullOrWhiteSpace(input))
+        {
+            return false;
+        }
+
+        Span<char> buffer = stackalloc char[Length];
+        var count = 0;
+        foreach (var c in input)
+        {
+            if (char.IsWhiteSpace(c) || c == '-')
+            {
+                continue;
+            }
+
+            var upper = char.ToUpperInvariant(c);
+            if (count == Length || !Alphabet.Contains(upper))
+            {
+                return false;
+            }
+
+            buffer[count++] = upper;
+        }
+
+        if (count != Length)
+        {
+            return false;
+        }
+
+        code = new string(buffer);
+        return true;
+    }
+
+    public static string Format(string code) =>
+        code.Length == Length ? $"{code[..5]}-{code[5..]}" : code;
+}