zfxaction25/RobotAndDonkey.Game/Utils/SeedString.cs

using System;
using System.Collections.Generic;
using System.Text;

namespace RobotAndDonkey.Game.Utils;

public static class SeedString
{
    /// <summary>
    ///     Encode an int seed to a short, human-friendly string.
    /// </summary>
    /// <param name="seed">Any 32-bit signed integer.</param>
    /// <param name="includeChecksum">Append 1 char to detect typos.</param>
    /// <param name="group">Insert '-' every 4 chars for readability.</param>
    public static string ToString(int seed, bool includeChecksum = true, bool group = false)
    {
        var u = unchecked((uint)seed);

        // Convert to base-32 (most-significant digit first)
        if (u == 0)
            return includeChecksum ? "0" + CheckChar(0) : "0";

        var digits = new List<char>(8);
        while (u > 0)
        {
            var v = (int)(u & 31);
            digits.Add(Alphabet[v]);
            u >>= 5;
        }

        digits.Reverse();

        var sb = new StringBuilder(digits.Count + (includeChecksum ? 1 : 0) + (group ? digits.Count / 4 : 0));
        for (var i = 0; i < digits.Count; i++)
        {
            if (group && i > 0 && i % 4 == 0)
                sb.Append('-');
            sb.Append(digits[i]);
        }

        if (includeChecksum)
            sb.Append(CheckChar(unchecked((uint)seed)));

        return sb.ToString();
    }

    /// <summary>
    ///     Try to parse a seed string back to the original int.
    /// </summary>
    /// <param name="code">Case-insensitive; hyphens/spaces/underscores ignored. O=0, I/L=1 accepted.</param>
    /// <param name="seed">Decoded seed.</param>
    /// <param name="requireChecksum">If true, only accept strings with a valid trailing checksum.</param>
    public static bool TryParse(string code, out int seed, bool requireChecksum = false)
    {
        seed = 0;
        if (string.IsNullOrWhiteSpace(code))
            return false;

        var t = Normalize(code);
        if (t.Length == 0)
            return false;

        // If checksum is required (or present), prefer validating it:
        if (t.Length >= 2)
        {
            if (TryParsePayload(t.AsSpan(0, t.Length - 1), out var u) && TryMap(t[^1], out var chk) && chk == Check5(u))
            {
                seed = unchecked((int)u);
                return true;
            }

            if (requireChecksum)
                return false;
        }

        // Fallback: treat entire string as payload without checksum.
        if (TryParsePayload(t.AsSpan(), out var uv))
        {
            seed = unchecked((int)uv);
            return true;
        }

        return false;
    }

    /// <summary>
    ///     Parse or throw (optionally requiring checksum).
    /// </summary>
    public static int Parse(string code, bool requireChecksum = false)
    {
        return TryParse(code, out var seed, requireChecksum) ? seed : throw new FormatException("Invalid seed string.");
    }

    // ---------- publics ----------

    private static Dictionary<char, int> BuildMap()
    {
        var d = new Dictionary<char, int>(64);
        for (var i = 0; i < Alphabet.Length; i++)
            d[Alphabet[i]] = i;

        // Forgiving mappings
        d['O'] = d['0']; // O -> 0
        d['I'] = d['1']; // I -> 1
        d['L'] = d['1']; // L -> 1
        return d;
    }

    private static string Normalize(string s)
    {
        var sb = new StringBuilder(s.Length);
        foreach (var ch in s)
        {
            if (ch == '-' || ch == '_' || char.IsWhiteSpace(ch))
                continue;
            sb.Append(char.ToUpperInvariant(ch));
        }

        return sb.ToString();
    }

    private static bool TryParsePayload(ReadOnlySpan<char> span, out uint u)
    {
        u = 0;
        if (span.Length == 0)
            return false;
        if (span.Length > 7)
            return false; // 7 base32 digits cover full 32-bit range (5 bits per digit)

        for (var i = 0; i < span.Length; i++)
        {
            if (!TryMap(span[i], out var v))
                return false;
            u = (u << 5) | (uint)v;
        }

        return true;
    }

    private static bool TryMap(char c, out int v)
    {
        return CharToVal.TryGetValue(char.ToUpperInvariant(c), out v);
    }

    // 5-bit checksum: multiplicative hash then take top 5 bits.
    private static int Check5(uint u)
    {
        return (int)((u * 0x9E3779B1u) >> 27);
    }

    private static char CheckChar(uint u)
    {
        return Alphabet[Check5(u)];
    }

    // Crockford Base32 (no I, L, O, U)
    private const string Alphabet = "0123456789ABCDEFGHJKMNPQRSTVWXYZ";
    private static readonly Dictionary<char, int> CharToVal = BuildMap();
}