DataTransfer/DataTransfer.Client/Shared/ConnectionCode.cs

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;
}