Give player species slight passive regen (#20638)

* Implementation

* Reviews

Content.Shared/Damage/Systems/PassiveDamageSystem.cs

@@ -0,0 +1,55 @@
+using Content.Shared.Damage.Components;
+using Content.Shared.Mobs.Systems;
+using Content.Shared.Mobs.Components;
+using Content.Shared.FixedPoint;
+using Robust.Shared.Timing;
+
+namespace Content.Shared.Damage;
+
+public sealed class PassiveDamageSystem : EntitySystem
+{
+    [Dependency] private readonly DamageableSystem _damageable = default!;
+    [Dependency] private readonly IGameTiming _timing = default!;
+    [Dependency] private readonly MobStateSystem _mobState = default!;
+
+    public override void Initialize()
+    {
+        base.Initialize();
+
+        SubscribeLocalEvent<PassiveDamageComponent, MapInitEvent>(OnPendingMapInit);
+    }
+
+    private void OnPendingMapInit(EntityUid uid, PassiveDamageComponent component, MapInitEvent args)
+    {
+        component.NextDamage = _timing.CurTime + TimeSpan.FromSeconds(1f);
+    }
+
+    // Every tick, attempt to damage entities
+    public override void Update(float frameTime)
+    {
+        base.Update(frameTime);
+        var curTime = _timing.CurTime;
+
+        // Go through every entity with the component
+        var query = EntityQueryEnumerator<PassiveDamageComponent, DamageableComponent, MobStateComponent>();
+        while (query.MoveNext(out var uid, out var comp, out var damage, out var mobState))
+        {   
+            // Make sure they're up for a damage tick
+            if (comp.NextDamage > curTime)
+                continue;
+
+            if (comp.DamageCap != 0 && damage.TotalDamage >= comp.DamageCap)
+                continue;
+            
+            // Set the next time they can take damage
+            comp.NextDamage = curTime + TimeSpan.FromSeconds(1f);
+
+            // Damage them
+            foreach (var allowedState in comp.AllowedStates)
+            {
+                if(allowedState == mobState.CurrentState)
+                    _damageable.TryChangeDamage(uid, comp.Damage, true, false, damage);
+            }
+        }
+    }
+}