h-school/src/HSchool.People/Seed.cs

namespace HSchool.People;

/// <summary>
/// Per-family streams derived from the school seed. Family N never consumes family N-1's rolls,
/// so appending a thirteenth family leaves the first twelve unchanged.
/// </summary>
public static class Seed
{
    public const int ChildCountSalt = 1;
    public const int AppearanceSalt = 2;
    public const int IntakeSalt = 3;
    public const int SeatShuffleSalt = 4;
    public const int HouseholdSalt = 5;
    public const int ApplicantSalt = 6;
    public const int CommuteSalt = 7;
    public const int SkillGrantSalt = 8;
    public const int NativeLanguageSalt = 9;
    public const int ClimatePresetSalt = 10;
    public const int ApparelSalt = 11;
    public const int WhisperSalt = 12;

    /// <summary>A stream that belongs to the school rather than to one family.</summary>
    public static int ForSchool(int schoolSeed, int salt) => Mix(schoolSeed, familyIndex: -1, salt);

    public static int Mix(int schoolSeed, int familyIndex, int salt)
    {
        var z = Mix64((uint)schoolSeed);
        z = Mix64(z ^ (uint)(familyIndex + 1));
        z = Mix64(z ^ (uint)(salt + 1));
        return (int)z;
    }

    /// <summary>A stream that belongs to one person on one calendar day — commute slack, not looks.</summary>
    public static int Mix(int schoolSeed, string personId, int dayNumber, int salt)
    {
        ArgumentNullException.ThrowIfNull(personId);
        var z = Mix64((uint)schoolSeed);
        z = Mix64(z ^ Stable(personId));
        z = Mix64(z ^ (uint)(dayNumber + 1));
        z = Mix64(z ^ (uint)(salt + 1));
        return (int)z;
    }

    private static uint Stable(string value)
    {
        ulong z = 0;
        foreach (var character in value)
        {
            z = Mix64(z ^ character);
        }

        return (uint)z;
    }

    private static ulong Mix64(ulong z)
    {
        z += 0x9E3779B97F4A7C15UL;
        z = (z ^ (z >> 30)) * 0xBF58476D1CE4E5B9UL;
        z = (z ^ (z >> 27)) * 0x94D049BB133111EBUL;
        return z ^ (z >> 31);
    }
}