5dplomacy/MultiversalDiplomacy/Model/Timelines.cs

using System.Text.Json.Serialization;

namespace MultiversalDiplomacy.Model;

/// <summary>
/// Tracks the relations between seasons.
/// </summary>
public class Timelines(int next, Dictionary<string, Season?> pasts)
{
    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="serial">Integer.</param>
    /// <returns>Timeline identifier.</returns>
    public static string IntToString(int serial) {
        static int downshift(int i ) => (i - (i % 26)) / 26;
        IEnumerable<char> result = [Letters[serial % 26]];
        for (int remainder = downshift(serial); 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());
    }

    /// <summary>
    /// Extract the timeline and turn components of a season designation.
    /// </summary>
    /// <param name="seasonKey">A timeline-turn season designation.</param>
    /// <returns>The timeline and turn components.</returns>
    /// <exception cref="FormatException"></exception>
    public static (string timeline, int turn) SplitKey(string seasonKey)
    {
        int i = 1;
        for (; !char.IsAsciiDigit(seasonKey[i]) && i < seasonKey.Length; i++);
        return int.TryParse(seasonKey.AsSpan(i), out int turn)
            ? (seasonKey[..i], turn)
            : throw new FormatException($"Could not parse turn from {seasonKey}");
    }

    /// <summary>
    /// The next timeline to be created.
    /// </summary>
    public int Next { get; private set; } = next;

    /// <summary>
    /// Map of season designations to their parent seasons. Every season has an entry, so
    /// the set of keys is the set of existing seasons.
    /// </summary>
    public Dictionary<string, Season?> Pasts { get; } = pasts;

    /// <summary>
    /// All seasons in the multiverse.
    /// </summary>
    [JsonIgnore]
    public IEnumerable<Season> Seasons => Pasts.Keys.Select(key => new Season(key));

    /// <summary>
    /// Create a new multiverse with an initial season.
    /// </summary>
    public static Timelines Create()
        => new(StringToInt(Season.First.Timeline) + 1, new() { {Season.First.Key, null} });

    /// <summary>
    /// Create a continuation of a season if it has no futures, otherwise create a fork.
    /// </summary>
    public Timelines WithNewSeason(Season past, out Season future)
    {
        int next;
        (next, future) = GetFutureKeys(past).Any()
            ? (Next + 1, new Season(IntToString(Next), past.Turn + 1))
            : (Next, new Season(past.Timeline, past.Turn + 1));
        return new Timelines(next, new(Pasts.Append(new KeyValuePair<string, Season?>(future.Key, past))));
    }

    /// <summary>
    /// Create a continuation of a season if it has no futures, otherwise create a fork.
    /// </summary>
    public Timelines WithNewSeason(string past, out Season future) => WithNewSeason(new Season(past), out future);

    /// <summary>
    /// Get all seasons that are immediate futures of a season.
    /// </summary>
    /// <param name="season">A season.</param>
    /// <returns>The immediate futures of the season.</returns>
    public IEnumerable<string> GetFutureKeys(Season season)
        => Pasts.Where(kvp => kvp.Value is Season future && future == season).Select(kvp => kvp.Key);

    /// <summary>
    /// Get all seasons that are immediate futures of a season.
    /// </summary>
    /// <param name="season">A season.</param>
    /// <returns>The immediate futures of the season.</returns>
    public IEnumerable<Season> GetFutures(Season season) => GetFutureKeys(season).Select(key => new Season(key));

    /// <summary>
    /// Returns the first season in this season's timeline. The first season is the
    /// root of the first timeline. The earliest season in each alternate timeline is
    /// the root of that timeline.
    /// </summary>
    public Season GetTimelineRoot(Season season)
    {
        return Pasts[season.Key] is Season past && season.Timeline == past.Timeline
        ? GetTimelineRoot(past)
        : season;
    }

    /// <summary>
    /// Returns the first season in this season's timeline. The first season is the
    /// root of the first timeline. The earliest season in each alternate timeline is
    /// the root of that timeline.
    /// </summary>
    public Season GetTimelineRoot(string season) => GetTimelineRoot(new Season(season));

    /// <summary>
    /// Returns whether a season is in an adjacent timeline to another season.
    /// Seasons are considered to be in adjacent timelines if they are in the same timeline,
    /// one is in a timeline that branched from the other's timeline, or both are in timelines
    /// that branched from the same point.
    /// </summary>
    public bool InAdjacentTimeline(Season one, Season two)
    {
        // Timelines are adjacent to themselves. Early out in that case.
        if (one == two) return true;

        // If the timelines aren't identical, one of them isn't the initial trunk.
        // They can still be adjacent if one of them branched off of the other, or
        // if they both branched off of the same point.
        Season rootOne = GetTimelineRoot(one);
        Season rootTwo = GetTimelineRoot(two);
        bool oneForked = Pasts[rootOne.Key] is Season originOne && originOne.Timeline == two.Timeline;
        bool twoForked = Pasts[rootTwo.Key] is Season originTwo && originTwo.Timeline == one.Timeline;
        bool bothForked = Pasts[rootOne.Key] == Pasts[rootTwo.Key];
        return oneForked || twoForked || bothForked;
    }
}