Dev Log: Game World Time System

using Microsoft.Xna.Framework;
using MonoGame.Extended.ECS.Systems;
using Reoria.Engine.Common;
using Reoria.Engine.Signals.Interfaces;
using GameWorld = MonoGame.Extended.ECS.World;

namespace Reoria.Game.World.Systems;

/// <summary>
/// Simulates in-game time progression, emitting events for minute, hour, and day ticks.
/// Time progresses based on a configurable real-time to game-time ratio.
/// </summary>
public class WorldTimeSystem(ISignalBus signalBus) : Disposable, IUpdateSystem
{
    /// <summary>
    /// Number of in-game minutes in an hour.
    /// </summary>
    public const float MINUTES_PER_HOUR = 60.0f;

    /// <summary>
    /// Number of in-game hours in a day.
    /// </summary>
    public const float HOURS_PER_DAY = 24.0f;

    /// <summary>
    /// Real-world minutes it takes for a full in-game day to pass.
    /// </summary>
    public const float REAL_MINUTES_PER_GAME_DAY = 70.0f;

    /// <summary>
    /// Signal bus for emitting world time-related events.
    /// </summary>
    protected readonly ISignalBus SignalBus = signalBus;

    /// <summary>
    /// The rate at which in-game time progresses relative to real-world time.
    /// </summary>
    public readonly float Timescale = MINUTES_PER_HOUR * HOURS_PER_DAY / REAL_MINUTES_PER_GAME_DAY;

    /// <summary>
    /// The world time recorded at the last update tick.
    /// </summary>
    /// <remarks>Used to detect time unit changes.</remarks>
    protected TimeSpan LastWorldTime { get; set; } = TimeSpan.Zero;

    /// <summary>
    /// The current total elapsed in-game time.
    /// </summary>
    public TimeSpan CurrentWorldTime { get; protected set; } = TimeSpan.Zero;

    /// <summary>
    /// Initializes the system and emits an initialization signal.
    /// </summary>
    /// <param name="world">The ECS world instance.</param>
    public void Initialize(GameWorld world) 
        => this.SignalBus.Emit("WorldTimeSystem.OnInitialize", this);

    /// <summary>
    /// Updates the world time, calculates time progression, and emits signals for time unit ticks.
    /// </summary>
    /// <param name="gameTime">Snapshot of elapsed game time.</param>
    public void Update(GameTime gameTime)
    {
        // Progress world time scaled to the defined timescale
        this.CurrentWorldTime += gameTime.ElapsedGameTime * this.Timescale;
        this.SignalBus.Emit("WorldTimeSystem.OnUpdate", this);

        // Check to see if the number of minutes has changed.
        if (this.LastWorldTime.Minutes != this.CurrentWorldTime.Minutes)
        {
            // It has, please emit a signal to listen for.
            this.SignalBus.Emit("WorldTimeSystem.OnMinuteTick", this);
        }

        // Check to see if the number of hours has changed.
        if (this.LastWorldTime.Hours != this.CurrentWorldTime.Hours)
        {
            // It has, please emit a signal to listen for.
            this.SignalBus.Emit("WorldTimeSystem.OnHourTick", this);
        }

        // Check to see if the number of days has changed.
        if (this.LastWorldTime.Days != this.CurrentWorldTime.Days)
        {
            // It has, please emit a signal to listen for.
            this.SignalBus.Emit("WorldTimeSystem.OnDayTick", this);
        }

        // Update the tracker for the last game tick.
        this.LastWorldTime = this.CurrentWorldTime;
    }
}