Fix flipping of chests and stairs (#526)

* Fix flipping of chests and stairs

* Check if the new property value is valid before updating

* Only for horizontal flips double chests/stairs should be modified

worldedit-core/src/main/java/com/sk89q/worldedit/extent/transform/BlockTransformExtent.java

@@ -25,6 +25,7 @@ import com.sk89q.worldedit.extent.AbstractDelegateExtent;
 import com.sk89q.worldedit.extent.Extent;
 import com.sk89q.worldedit.math.BlockVector3;
 import com.sk89q.worldedit.math.Vector3;
+import com.sk89q.worldedit.math.transform.AffineTransform;
 import com.sk89q.worldedit.math.transform.Transform;
 import com.sk89q.worldedit.registry.state.BooleanProperty;
 import com.sk89q.worldedit.registry.state.DirectionalProperty;
@@ -162,6 +163,38 @@ public class BlockTransformExtent extends AbstractDelegateExtent {
                             }
                         }
                     }
+                } else if (property.getName().equals("type") && transform instanceof AffineTransform) {
+                    // chests
+                    if (((AffineTransform) transform).isHorizontalFlip()) {
+                        String value = (String) block.getState(property);
+                        String newValue = null;
+                        if ("left".equals(value)) {
+                            newValue = "right";
+                        } else if ("right".equals(value)) {
+                            newValue = "left";
+                        }
+                        if (newValue != null && enumProp.getValues().contains(newValue)) {
+                            result = result.with(enumProp, newValue);
+                        }
+                    }
+                } else if (property.getName().equals("shape") && transform instanceof AffineTransform) {
+                    // stairs
+                    if (((AffineTransform) transform).isHorizontalFlip()) {
+                        String value = (String) block.getState(property);
+                        String newValue = null;
+                        if ("outer_left".equals(value)) {
+                            newValue = "outer_right";
+                        } else if ("outer_right".equals(value)) {
+                            newValue = "outer_left";
+                        } else if ("inner_left".equals(value)) {
+                            newValue = "inner_right";
+                        } else if ("inner_right".equals(value)) {
+                            newValue = "inner_left";
+                        }
+                        if (newValue != null && enumProp.getValues().contains(newValue)) {
+                            result = result.with(enumProp, newValue);
+                        }
+                    }
                 }
             } else if (property instanceof IntegerProperty) {
                 IntegerProperty intProp = (IntegerProperty) property;

worldedit-core/src/main/java/com/sk89q/worldedit/math/transform/AffineTransform.java

@@ -312,6 +312,14 @@ public class AffineTransform implements Transform {
         }
     }
 
+    /**
+     * Returns if this affine transform is representing a horizontal flip.
+     */
+    public boolean isHorizontalFlip() {
+        // use the determinant of the x-z submatrix to check if this is a horizontal flip
+        return m00 * m22 - m02 * m20 < 0;
+    }
+
     @Override
     public String toString() {
         return String.format("Affine[%g %g %g %g, %g %g %g %g, %g %g %g %g]}", m00, m01, m02, m03, m10, m11, m12, m13, m20, m21, m22, m23);