feat: Implements passive Detect Hidden mechanics (#2342)

Projects/Server.Tests/Helpers/PredictableRandom.cs

@@ -0,0 +1,43 @@
+using System;
+using Server.Random;
+
+namespace Server.Tests;
+
+/// <summary>
+/// Replaces <see cref="BuiltInRng.Generator"/> with a <see cref="System.Random"/> that returns
+/// a fixed value, making skill checks and other RNG-dependent code deterministic in tests.
+/// Restores the original generator on <see cref="Dispose"/>.
+/// <para>Usage:</para>
+/// <code>
+/// using var rng = new PredictableRandom(10); // Utility.Random(21) returns 10
+/// </code>
+/// </summary>
+public sealed class PredictableRandom : IDisposable
+{
+    private readonly System.Random _original;
+
+    public PredictableRandom(int fixedValue)
+    {
+        _original = BuiltInRng.Generator;
+        BuiltInRng.Generator = new FixedRandom(fixedValue);
+    }
+
+    public void Dispose()
+    {
+        BuiltInRng.Generator = _original;
+    }
+
+    private sealed class FixedRandom(int value) : System.Random
+    {
+        public override int Next() => value;
+        public override int Next(int maxValue) => Math.Clamp(value, 0, maxValue - 1);
+        public override int Next(int minValue, int maxValue) => Math.Clamp(value + minValue, minValue, maxValue - 1);
+        public override long NextInt64() => value;
+        public override long NextInt64(long maxValue) => Math.Clamp(value, 0, maxValue - 1);
+        public override long NextInt64(long minValue, long maxValue) => Math.Clamp(value + minValue, minValue, maxValue - 1);
+        public override double NextDouble() => Math.Clamp(value / 20.0, 0.0, 1.0);
+
+        public override void NextBytes(byte[] buffer) => Array.Fill(buffer, (byte)Math.Clamp(value, 0, 255));
+        public override void NextBytes(Span<byte> buffer) => buffer.Fill((byte)Math.Clamp(value, 0, 255));
+    }
+}

Projects/Server/Random/BuiltInRng.cs

@@ -20,7 +20,7 @@ namespace Server.Random;
 
 public static class BuiltInRng
 {
-    public static System.Random Generator { get; private set; } = new();
+    public static System.Random Generator { get; internal set; } = new();
 
     public static void Reset() => Generator = new System.Random();
 

Projects/UOContent.Tests/Tests/Skills/DetectHiddenTests.cs

@@ -0,0 +1,213 @@
+using Server;
+using Server.Mobiles;
+using Server.SkillHandlers;
+using Server.Tests;
+using Xunit;
+
+namespace UOContent.Tests;
+
+[Collection("Sequential UOContent Tests")]
+public class DetectHiddenTests
+{
+    // All coordinates use Y=500 to stay well within Felucca's 7168x4096 boundary.
+    // X values are spread 200 tiles apart to avoid overlap with each other and
+    // with the Tracking tests that use coordinates around (1000-4000, 1000-4000).
+
+    /// <summary>
+    /// A detector with higher Detect Hidden than the stealther's Hiding should reveal them.
+    /// With PredictableRandom both rolls get the same offset, so detection succeeds
+    /// when detectSkill >= hiding.
+    /// </summary>
+    [Fact]
+    public void TryDetectStealther_Reveals_WhenDetectSkillExceedsHiding()
+    {
+        using var rng = new PredictableRandom(10);
+        DetectHidden.ClearDebounceCache();
+        var map = Map.Felucca;
+        var detector = CreatePlayerMobile(map, new Point3D(1000, 500, 0));
+        var stealther = CreatePlayerMobile(map, new Point3D(1001, 500, 0));
+
+        try
+        {
+            detector.Skills.DetectHidden.BaseFixedPoint = 600; // 60.0
+            stealther.Skills.Hiding.BaseFixedPoint = 500;      // 50.0
+            stealther.Hidden = true;
+
+            var result = DetectHidden.TryDetectStealther(detector, stealther);
+
+            Assert.True(result);
+            Assert.False(stealther.Hidden);
+        }
+        finally
+        {
+            detector.Delete();
+            stealther.Delete();
+            DetectHidden.ClearDebounceCache();
+        }
+    }
+
+    /// <summary>
+    /// When detect skill exactly equals hiding, detection succeeds (ss >= ts).
+    /// </summary>
+    [Fact]
+    public void TryDetectStealther_Reveals_WhenSkillsAreEqual()
+    {
+        using var rng = new PredictableRandom(10);
+        DetectHidden.ClearDebounceCache();
+        var map = Map.Felucca;
+        var detector = CreatePlayerMobile(map, new Point3D(1200, 500, 0));
+        var stealther = CreatePlayerMobile(map, new Point3D(1201, 500, 0));
+
+        try
+        {
+            detector.Skills.DetectHidden.BaseFixedPoint = 500; // 50.0
+            stealther.Skills.Hiding.BaseFixedPoint = 500;      // 50.0
+            stealther.Hidden = true;
+
+            var result = DetectHidden.TryDetectStealther(detector, stealther);
+
+            Assert.True(result);
+            Assert.False(stealther.Hidden);
+        }
+        finally
+        {
+            detector.Delete();
+            stealther.Delete();
+            DetectHidden.ClearDebounceCache();
+        }
+    }
+
+    /// <summary>
+    /// When hiding skill exceeds detect skill, detection fails.
+    /// </summary>
+    [Fact]
+    public void TryDetectStealther_DoesNotReveal_WhenHidingExceedsDetectSkill()
+    {
+        using var rng = new PredictableRandom(10);
+        DetectHidden.ClearDebounceCache();
+        var map = Map.Felucca;
+        var detector = CreatePlayerMobile(map, new Point3D(1400, 500, 0));
+        var stealther = CreatePlayerMobile(map, new Point3D(1401, 500, 0));
+
+        try
+        {
+            detector.Skills.DetectHidden.BaseFixedPoint = 500; // 50.0
+            stealther.Skills.Hiding.BaseFixedPoint = 600;      // 60.0
+            stealther.Hidden = true;
+
+            var result = DetectHidden.TryDetectStealther(detector, stealther);
+
+            Assert.False(result);
+            Assert.True(stealther.Hidden);
+        }
+        finally
+        {
+            detector.Delete();
+            stealther.Delete();
+            DetectHidden.ClearDebounceCache();
+        }
+    }
+
+    /// <summary>
+    /// A detector with zero Detect Hidden skill should never passively detect anyone.
+    /// Uses Elf race since Humans get a 20.0 racial bonus (Jack of All Trades).
+    /// </summary>
+    [Fact]
+    public void TryDetectStealther_DoesNotReveal_WhenDetectorHasNoSkill()
+    {
+        using var rng = new PredictableRandom(10);
+        DetectHidden.ClearDebounceCache();
+        var map = Map.Felucca;
+        var detector = CreatePlayerMobile(map, new Point3D(1600, 500, 0));
+        var stealther = CreatePlayerMobile(map, new Point3D(1601, 500, 0));
+
+        try
+        {
+            detector.Race = Race.Elf;
+            detector.Skills.DetectHidden.BaseFixedPoint = 0; // 0.0
+            stealther.Skills.Hiding.BaseFixedPoint = 0;      // 0.0
+            stealther.Hidden = true;
+
+            var result = DetectHidden.TryDetectStealther(detector, stealther);
+
+            Assert.False(result);
+            Assert.True(stealther.Hidden);
+        }
+        finally
+        {
+            detector.Delete();
+            stealther.Delete();
+            DetectHidden.ClearDebounceCache();
+        }
+    }
+
+    /// <summary>
+    /// TryDetectStealther is a no-op when the target is not actually hidden.
+    /// </summary>
+    [Fact]
+    public void TryDetectStealther_DoesNothing_WhenStealtherIsNotHidden()
+    {
+        using var rng = new PredictableRandom(10);
+        DetectHidden.ClearDebounceCache();
+        var map = Map.Felucca;
+        var detector = CreatePlayerMobile(map, new Point3D(1800, 500, 0));
+        var stealther = CreatePlayerMobile(map, new Point3D(1801, 500, 0));
+
+        try
+        {
+            detector.Skills.DetectHidden.BaseFixedPoint = 1000;
+            stealther.Hidden = false;
+
+            var result = DetectHidden.TryDetectStealther(detector, stealther);
+
+            Assert.False(result);
+            Assert.False(stealther.Hidden);
+        }
+        finally
+        {
+            detector.Delete();
+            stealther.Delete();
+            DetectHidden.ClearDebounceCache();
+        }
+    }
+
+    /// <summary>
+    /// Passive detection only works on Felucca. A stealther on Trammel
+    /// should never be revealed by passive detection.
+    /// </summary>
+    [Fact]
+    public void TryDetectStealther_DoesNotReveal_WhenNotOnFelucca()
+    {
+        using var rng = new PredictableRandom(10);
+        DetectHidden.ClearDebounceCache();
+        var map = Map.Trammel;
+        var detector = CreatePlayerMobile(map, new Point3D(2000, 500, 0));
+        var stealther = CreatePlayerMobile(map, new Point3D(2001, 500, 0));
+
+        try
+        {
+            detector.Skills.DetectHidden.BaseFixedPoint = 1000; // 100.0
+            stealther.Skills.Hiding.BaseFixedPoint = 0;         // 0.0
+            stealther.Hidden = true;
+
+            var result = DetectHidden.TryDetectStealther(detector, stealther);
+
+            Assert.False(result);
+            Assert.True(stealther.Hidden);
+        }
+        finally
+        {
+            detector.Delete();
+            stealther.Delete();
+            DetectHidden.ClearDebounceCache();
+        }
+    }
+
+    private static PlayerMobile CreatePlayerMobile(Map map, Point3D location)
+    {
+        var mobile = new PlayerMobile(World.NewMobile);
+        mobile.DefaultMobileInit();
+        mobile.MoveToWorld(location, map);
+        return mobile;
+    }
+}

Projects/UOContent/Engines/FeatureFlags/ContentFeatureFlags.cs

@@ -12,4 +12,5 @@ public static class ContentFeatureFlags
     public static bool HousePlacement { get; set; } = true;
     public static bool BoatPlacement { get; set; } = true;
     public static bool BulkOrders { get; set; } = true;
+    public static bool PassiveDetectHidden { get; set; } = true;
 }

Projects/UOContent/Engines/FeatureFlags/FeatureFlagManager.cs

@@ -967,12 +967,13 @@ public static class FeatureFlagManager
             "bank_access"    => ServerFeatureFlags.BankAccess = enabled,
 
             // UOContent flags
-            "vendor_purchase" => ContentFeatureFlags.VendorPurchase = enabled,
-            "vendor_sell"     => ContentFeatureFlags.VendorSell = enabled,
-            "player_vendors"  => ContentFeatureFlags.PlayerVendors = enabled,
-            "house_placement" => ContentFeatureFlags.HousePlacement = enabled,
-            "boat_placement"  => ContentFeatureFlags.BoatPlacement = enabled,
-            "bulk_orders"     => ContentFeatureFlags.BulkOrders = enabled,
+            "vendor_purchase"        => ContentFeatureFlags.VendorPurchase = enabled,
+            "vendor_sell"            => ContentFeatureFlags.VendorSell = enabled,
+            "player_vendors"         => ContentFeatureFlags.PlayerVendors = enabled,
+            "house_placement"        => ContentFeatureFlags.HousePlacement = enabled,
+            "boat_placement"         => ContentFeatureFlags.BoatPlacement = enabled,
+            "bulk_orders"            => ContentFeatureFlags.BulkOrders = enabled,
+            "passive_detect_hidden"  => ContentFeatureFlags.PassiveDetectHidden = enabled,
         };
     }
 

Projects/UOContent/Mobiles/PlayerMobile.cs

@@ -3534,6 +3534,26 @@ namespace Server.Mobiles
             return true;
         }
 
+        public override void OnMovement(Mobile m, Point3D oldLocation)
+        {
+            base.OnMovement(m, oldLocation);
+
+            // Passive detect hidden: either party moving within range can trigger detection
+            if (m is PlayerMobile && Utility.InRange(Location, m.Location, 4))
+            {
+                if (m.Hidden && AccessLevel == AccessLevel.Player)
+                {
+                    // A hidden mobile (stealther) moved near us — we try to detect them
+                    DetectHidden.TryDetectStealther(this, m);
+                }
+                else if (Hidden && m.AccessLevel == AccessLevel.Player)
+                {
+                    // We're hidden and a potential detector moved near us — they try to detect us
+                    DetectHidden.TryDetectStealther(m, this);
+                }
+            }
+        }
+
         public void AddFollower(Mobile m)
         {
             _allFollowers ??= new HashSet<Mobile>();

Projects/UOContent/Skills/DetectHidden.cs

@@ -1,59 +1,191 @@
 using System;
+using System.Collections.Generic;
+using Server.Collections;
+using Server.Engines.PartySystem;
 using Server.Factions;
+using Server.Guilds;
+using Server.Items;
 using Server.Mobiles;
 using Server.Multis;
+using Server.Network;
+using Server.Systems.FeatureFlags;
 using Server.Targeting;
 
-namespace Server.SkillHandlers
+namespace Server.SkillHandlers;
+
+public static class DetectHidden
 {
-    public static class DetectHidden
+    // Debounce tracking: (stealther, detector) -> last detection time
+    private static readonly Dictionary<(Mobile, Mobile), long> PassiveDetectDebounce = [];
+
+    private const int PassiveDetectDebounceMs = 3000;     // 3 seconds
+    private const int DebounceCleanupIntervalMs = 10000;  // Run cleanup every 10 seconds
+    private const int DebounceExpiryMs = 10000;           // Remove entries older than 10 seconds
+    private static long _lastCleanupTime;
+
+    public static void Initialize()
     {
-        public static void Initialize()
+        SkillInfo.Table[(int)SkillName.DetectHidden].Callback = OnUse;
+    }
+
+    public static TimeSpan OnUse(Mobile src)
+    {
+        src.SendLocalizedMessage(500819); // Where will you search?
+        src.Target = new InternalTarget();
+
+        return TimeSpan.FromSeconds(30.0);
+    }
+
+    // Clean up old debounce entries to prevent memory bloat
+    private static void CleanupDebounceCache(long now)
+    {
+        using var entriesToRemove = PooledRefQueue<(Mobile, Mobile)>.Create();
+
+        foreach (var entry in PassiveDetectDebounce)
         {
-            SkillInfo.Table[(int)SkillName.DetectHidden].Callback = OnUse;
+            if (now - entry.Value > DebounceExpiryMs)
+            {
+                entriesToRemove.Enqueue(entry.Key);
+            }
         }
 
-        public static TimeSpan OnUse(Mobile src)
+        while (entriesToRemove.Count > 0)
         {
-            src.SendLocalizedMessage(500819); // Where will you search?
-            src.Target = new InternalTarget();
+            PassiveDetectDebounce.Remove(entriesToRemove.Dequeue());
+        }
+    }
 
-            return TimeSpan.FromSeconds(30.0);
+    // For testing: clear the debounce cache to prevent cross-test contamination
+    internal static void ClearDebounceCache()
+    {
+        PassiveDetectDebounce.Clear();
+    }
+
+    // Passive detection: check if a detector can passively detect a stealther.
+    // Called via OnMovement when either party moves within range.
+    // Returns true if detection was successful (and the stealther was revealed).
+    // NOTE: OSI uncertain - The exact chance calculation and distance dropoff is unknown.
+    // We use a ±10 variance on both skills, matching active detection mechanics.
+    public static bool TryDetectStealther(Mobile detector, Mobile stealther)
+    {
+        if (!ContentFeatureFlags.PassiveDetectHidden || stealther == detector || !stealther.Hidden || !detector.Alive)
+        {
+            return false;
         }
 
-        private class InternalTarget : Target
+        // Felucca PvP only
+        if (stealther.Map != Map.Felucca)
         {
-            public InternalTarget() : base(12, true, TargetFlags.None)
+            return false;
+        }
+
+        var detectSkill = detector.Skills.DetectHidden.Value;
+        if (detectSkill <= 0)
+        {
+            return false;
+        }
+
+        var now = Core.TickCount;
+
+        // Debounce: skip pairs already checked recently (cheap dictionary lookup before expensive checks)
+        var key = (stealther, detector);
+        if (PassiveDetectDebounce.TryGetValue(key, out var lastDetect) && now - lastDetect < PassiveDetectDebounceMs)
+        {
+            return false;
+        }
+
+        // Periodic cleanup (amortized, not every call)
+        if (now - _lastCleanupTime > DebounceCleanupIntervalMs)
+        {
+            _lastCleanupTime = now;
+            CleanupDebounceCache(now);
+        }
+
+        // Excludes blessed, dead, bonded pets, and region-based PvP rules
+        if (stealther.AccessLevel > AccessLevel.Player || !detector.CanBeHarmful(stealther, false))
+        {
+            return false;
+        }
+
+        // Exclude party members
+        var stealtherParty = Party.Get(stealther);
+        var detectorParty = Party.Get(detector);
+        if (stealtherParty != null && stealtherParty == detectorParty)
+        {
+            return false;
+        }
+
+        // Exclude guild members and allies
+        if (stealther.Guild is Guild sg && detector.Guild is Guild dg && (sg == dg || sg.IsAlly(dg)))
+        {
+            return false;
+        }
+
+        var ss = detectSkill + Utility.Random(21) - 10;
+        var ts = stealther.Skills.Hiding.Value + Utility.Random(21) - 10;
+
+        if (ss >= ts)
+        {
+            stealther.RevealingAction();
+            stealther.SendLocalizedMessage(500814); // You have been revealed!
+            PassiveDetectDebounce[key] = now;
+            return true;
+        }
+
+        return false;
+    }
+
+    private class InternalTarget : Target
+    {
+        public InternalTarget() : base(12, true, TargetFlags.None)
+        {
+        }
+
+        protected override void OnTargetCancel(Mobile from, TargetCancelType cancelType)
+        {
+            from.NextSkillTime = Core.TickCount;
+        }
+
+        protected override void OnTarget(Mobile src, object targ)
+        {
+            var foundAnyone = false;
+            var srcSkill = src.Skills.DetectHidden.Value;
+            var range = (int)(srcSkill / 10.0);
+
+            if (targ is TrappableContainer container && container.TrapType != TrapType.None)
             {
+                // Direct container targeting: show [trapped] if within detection range and skill check passes
+                if (src.InRange(container.GetWorldLocation(), range) &&
+                    src.CheckSkill(SkillName.DetectHidden, 0.0, 100.0))
+                {
+                    src.NetState.SendMessageLocalized(
+                        container.Serial,
+                        container.ItemID,
+                        MessageType.Regular,
+                        0x3B2,
+                        3,
+                        500813 // [trapped]
+                    );
+
+                    foundAnyone = true;
+                }
             }
-
-            protected override void OnTargetCancel(Mobile from, TargetCancelType cancelType)
+            else if (targ is not Item && (targ is not Mobile m || m == src))
             {
-                from.NextSkillTime = Core.TickCount;
-            }
-
-            protected override void OnTarget(Mobile src, object targ)
-            {
-                var foundAnyone = false;
-
+                // Area scan only when targeting self or the ground
                 var p = targ switch
                 {
                     Mobile mobile => mobile.Location,
-                    Item item     => item.Location,
                     IPoint3D d    => new Point3D(d),
                     _             => src.Location
                 };
 
-                var srcSkill = src.Skills.DetectHidden.Value;
-                var range = (int)(srcSkill / 10.0);
-
                 if (!src.CheckSkill(SkillName.DetectHidden, 0.0, 100.0))
                 {
                     range /= 2;
                 }
 
                 var house = BaseHouse.FindHouseAt(p, src.Map, 16);
-
                 var inHouse = house?.IsFriend(src) == true;
 
                 if (inHouse)
@@ -88,40 +220,57 @@ namespace Server.SkillHandlers
                         foundAnyone = true;
                     }
 
-                    if (Faction.Find(src) != null)
+                    foreach (var item in src.Map.GetItemsInRange(p, range))
                     {
-                        foreach (var trap in src.Map.GetItemsInRange<BaseFactionTrap>(p, range))
+                        if (item is BaseFactionTrap factionTrap)
                         {
-                            if (src.CheckTargetSkill(SkillName.DetectHidden, trap, 80.0, 100.0))
+                            if (Faction.Find(src) != null &&
+                                src.CheckTargetSkill(SkillName.DetectHidden, factionTrap, 80.0, 100.0))
                             {
                                 src.SendLocalizedMessage(
                                     1042712, // You reveal a trap placed by a faction:
                                     true,
-                                    $" {(trap.Faction == null ? "" : trap.Faction.Definition.FriendlyName)}"
+                                    $" {(factionTrap.Faction == null ? "" : factionTrap.Faction.Definition.FriendlyName)}"
                                 );
 
-                                trap.Visible = true;
-                                trap.BeginConceal();
+                                factionTrap.Visible = true;
+                                factionTrap.BeginConceal();
 
                                 foundAnyone = true;
                             }
                         }
+                        else if (item is BaseTrap { Visible: false } trap)
+                        {
+                            // High Seas (Publish 79): Requires 75 Detect Hidden to detect dungeon traps
+                            if (Core.HS && srcSkill < 75.0)
+                            {
+                                continue;
+                            }
+
+                            trap.Visible = true;
+                            Timer.StartTimer(TimeSpan.FromSeconds(10.0), () =>
+                            {
+                                if (!trap.Deleted)
+                                {
+                                    trap.Visible = false;
+                                }
+                            });
+
+                            foundAnyone = true;
+                        }
                     }
                 }
-
-                if (!foundAnyone)
-                {
-                    src.SendLocalizedMessage(500817); // You can see nothing hidden there.
-                }
-
-                const int TargeterCooldown = 30000; // 30s
-                const int SkillCooldown = 10000;    // 10s
-
-                // Calculate how much time has passed since the targeter was opened
-                var ticksSinceTargeter = (int)(Core.TickCount - (src.NextSkillTime - TargeterCooldown));
-                var remainingCooldown = Math.Max(0, SkillCooldown - ticksSinceTargeter);
-                src.NextSkillTime = Core.TickCount + remainingCooldown;
             }
+
+            if (!foundAnyone)
+            {
+                src.SendLocalizedMessage(500817); // You can see nothing hidden there.
+            }
+
+            // Calculate how much time has passed since the targeter was opened
+            var ticksSinceTargeter = (int)(Core.TickCount - (src.NextSkillTime - 30000));
+            var remainingCooldown = Math.Max(0, 10000 - ticksSinceTargeter);
+            src.NextSkillTime = Core.TickCount + remainingCooldown;
         }
     }
 }

Projects/UOContent/UOContent.csproj

@@ -52,5 +52,8 @@
     </ItemGroup>
     <ItemGroup>
         <AdditionalFiles Include="Migrations/*.v*.json" />
+        <AssemblyAttribute Include="System.Runtime.CompilerServices.InternalsVisibleTo">
+            <_Parameter1>UOContent.Tests</_Parameter1>
+        </AssemblyAttribute>
     </ItemGroup>
 </Project>