Advance blocked split previews to the last safe Core delta

Assets/MagnetismHeroes/Tests/PlayMode/PlaygroundSafetyPlayTests.cs

@@ -182,11 +182,15 @@ namespace MagnetismHeroes.Tests
             Assert.That(_app.Presenter.CanDropGroup("group-0"), Is.True);
             var validDelta = _app.Presenter.DropDelta; var validPoint = _app.Presenter.Balls["safety-2"].transform.position;
             _app.Input.Move(point + new Vector2(140, 0));
-            Assert.That(_app.Presenter.DropDelta, Is.EqualTo(validDelta));
-            Assert.That(_app.Presenter.Balls["safety-2"].transform.position, Is.EqualTo(validPoint));
+            var stopped = _app.Presenter.Balls["safety-2"].transform.position;
+            Assert.That(stopped.x, Is.GreaterThan(validPoint.x + .2f), "Continue the safe part of the split drag up to the obstacle.");
+            Assert.That(stopped.x, Is.EqualTo(.689f).Within(.003f), "The whole unit sphere must stop just before the thin wall.");
+            Assert.That(_app.Presenter.DropDelta.x, Is.GreaterThan(validDelta.x + .2f));
+            var corePreview = _app.Session.Board.PreviewSplit("group-0", new[] { "safety-2" }, BoardPresenter.ToPosition(_app.Presenter.DropDelta));
+            foreach (var ball in corePreview) Assert.That(_app.Presenter.Balls[ball.id].transform.position, Is.EqualTo(BoardPresenter.ToVector(ball.position)));
             _app.Input.Move(point + new Vector2(128, 0));
             var reverse = _app.Presenter.Balls["safety-2"].transform.position;
-            Assert.That(reverse.x, Is.LessThan(validPoint.x - .01f));
+            Assert.That(reverse.x, Is.LessThan(stopped.x - .01f), "Reverse must act immediately at the last safe split delta.");
             var preview = _app.Presenter.Balls.ToDictionary(pair => pair.Key, pair => pair.Value.transform.position);
             _app.Input.End(point + new Vector2(128, 0)); yield return null;
             Assert.That(_app.Session.Board.Groups.Length, Is.EqualTo(2));

Assets/MagnetismHeroes/UnityRuntime/Presentation/BoardPresenter.cs

@@ -144,20 +144,40 @@ namespace MagnetismHeroes.UnityRuntime.Presentation
                 CancelPreview(); _previewGroup = groupId; _splitPreview = true; _previewValid = false;
                 return false;
             }
+            if (!CanSweepSplit(positions, selected))
+            {
+                if (!hadValidPreview) { _previewValid = false; return false; }
+                // Search Core input deltas, so the last safe arrays and the committed delta stay identical.
+                var previousDelta = DropDelta;
+                positions = _board.PreviewSplit(groupId, selected, ToPosition(previousDelta));
+                float safe = 0, blocked = 1;
+                for (int iteration = 0; iteration < 20; iteration++)
+                {
+                    float fraction = (safe + blocked) * .5f;
+                    var candidateDelta = Vector3.Lerp(previousDelta, delta, fraction);
+                    var candidate = _board.PreviewSplit(groupId, selected, ToPosition(candidateDelta));
+                    if (candidate != null && candidate.All(ball => _environment.IsSafeSphere(ToVector(ball.position), transform))
+                        && CanSweepSplit(candidate, selected))
+                    { safe = fraction; positions = candidate; }
+                    else blocked = fraction;
+                }
+                delta = Vector3.Lerp(previousDelta, delta, safe);
+            }
+            foreach (var ball in positions) _balls[ball.id].transform.position = ToVector(ball.position);
+            DropDelta = delta; _previewValid = true;
+            PositionBadges(); return true;
+        }
+
+        private bool CanSweepSplit(IReadOnlyList<BallState> positions, IReadOnlyList<string> selected)
+        {
             foreach (var ball in positions.Where(ball => selected.Contains(ball.id)))
             {
                 var start = _balls[ball.id].transform.position;
                 var movement = ToVector(ball.position) - start;
                 if ((_environment.ConstrainTranslation(new[] { start }, movement, transform) - movement).sqrMagnitude > .00000001f)
-                {
-                    // Keep exactly the last Core-produced split and its input delta; never commit a clipped shape.
-                    _previewValid = hadValidPreview;
-                    return hadValidPreview;
-                }
+                    return false;
             }
-            foreach (var ball in positions) _balls[ball.id].transform.position = ToVector(ball.position);
-            DropDelta = delta; _previewValid = true;
-            PositionBadges(); return true;
+            return true;
         }
 
         public void ShowSelection(IReadOnlyCollection<string> selected)