Spacelube slide (#24801)

* Space lube now makes you slide

* review

* oh lord he slippin

---------

Co-authored-by: metalgearsloth <comedian_vs_clown@hotmail.com>

Content.Shared/Slippery/SlidingSystem.cs

@@ -0,0 +1,63 @@
+using Content.Shared.Movement.Events;
+using Content.Shared.Standing;
+using Content.Shared.Stunnable;
+using Robust.Shared.Physics.Events;
+
+namespace Content.Shared.Slippery;
+
+public sealed class SlidingSystem : EntitySystem
+{
+    public override void Initialize()
+    {
+        base.Initialize();
+
+        SubscribeLocalEvent<SlidingComponent, TileFrictionEvent>(OnSlideAttempt);
+        SubscribeLocalEvent<SlidingComponent, StoodEvent>(OnStand);
+        SubscribeLocalEvent<SlidingComponent, StartCollideEvent>(OnStartCollide);
+        SubscribeLocalEvent<SlidingComponent, EndCollideEvent>(OnEndCollide);
+    }
+
+    /// <summary>
+    ///     Modify the friction by the frictionModifier stored on the component.
+    /// </summary>
+    private void OnSlideAttempt(EntityUid uid, SlidingComponent component, ref TileFrictionEvent args)
+    {
+        args.Modifier = component.FrictionModifier;
+    }
+
+    /// <summary>
+    ///     Remove the component when the entity stands up again.
+    /// </summary>
+    private void OnStand(EntityUid uid, SlidingComponent component, ref StoodEvent args)
+    {
+        RemComp<SlidingComponent>(uid);
+    }
+
+    /// <summary>
+    ///     Sets friction to 0 if colliding with a SuperSlippery Entity.
+    /// </summary>
+    private void OnStartCollide(EntityUid uid, SlidingComponent component, ref StartCollideEvent args)
+    {
+        if (!TryComp<SlipperyComponent>(args.OtherEntity, out var slippery) || !slippery.SuperSlippery)
+            return;
+
+        component.CollidingEntities.Add(args.OtherEntity);
+        component.FrictionModifier = 0;
+        Dirty(uid, component);
+    }
+
+    /// <summary>
+    ///     Set friction to normal when ending collision with a SuperSlippery entity.
+    /// </summary>
+    private void OnEndCollide(EntityUid uid, SlidingComponent component, ref EndCollideEvent args)
+    {
+        if (!component.CollidingEntities.Remove(args.OtherEntity))
+            return;
+
+        if (component.CollidingEntities.Count == 0)
+            component.FrictionModifier = SharedStunSystem.KnockDownModifier;
+
+        Dirty(uid, component);
+    }
+
+}