5dplomacy/MultiversalDiplomacy/Model/TimelineFactory.cs

namespace MultiversalDiplomacy.Model;

/// <summary>
/// A shared counter for handing out new timeline designations.
/// </summary>
public class TimelineFactory
{
    private static readonly char[] Letters = [
        'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j',
        'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't',
        'u', 'v', 'w', 'x', 'y', 'z',
    ];

    /// <summary>
    /// Convert a string timeline identifier to its serial number.
    /// </summary>
    /// <param name="timeline">Timeline identifier.</param>
    /// <returns>Integer.</returns>
    public static int StringToInt(string timeline)
    {
        int result = Array.IndexOf(Letters, timeline[0]);
        for (int i = 1; i < timeline.Length; i++) {
            // The result is incremented by one because timeline designations are not a true base26 system.
            // The "ones digit" maps a-z 0-25, but the "tens digit" maps a to 1, so "10" (26) is "aa" and not "a0"
            result = (result + 1) * 26;
            result += Array.IndexOf(Letters, timeline[i]);
        }
        return result;
    }

    /// <summary>
    /// Convert a timeline serial number to its string identifier.
    /// </summary>
    /// <param name="designation">Integer.</param>
    /// <returns>Timeline identifier.</returns>
    public static string IntToString(int designation) {
        static int downshift(int i ) => (i - (i % 26)) / 26;
        IEnumerable<char> result = [Letters[designation % 26]];
        for (int remainder = downshift(designation); remainder > 0; remainder = downshift(remainder) - 1) {
            // We subtract 1 after downshifting for the same reason we add 1 above after upshifting.
            // 
            result = result.Prepend(Letters[(remainder % 26 + 25) % 26]);
        }
        return new string(result.ToArray());
    }

    private int nextTimeline = 0;

    public string NextTimeline() => IntToString(nextTimeline++);
}