fix BactrianDeltaExchangeOperator #1199

Author: walterxie

src/beast/base/inference/operator/kernel/BactrianDeltaExchangeOperator.java

@@ -199,19 +199,27 @@ public final double proposal() {
 
                     // exchange a random delta
                     final double d = getNextDouble(0);
-                    scalar1 -= d;
+//                    scalar1 -= d;
 //                    if (parameterWeights[dim1] != parameterWeights[dim2]) {
 //                        scalar2 += d * parameterWeights[dim1] / parameterWeights[dim2];
 //                    } else {
 //                        scalar2 += d;
 //                    }
-                    
-                    
-                    double w1 = parameterWeights[dim1];
-                    double w2 = parameterWeights[dim2];
-                    double wsum = w1 + w2;
-                    scalar1 = scalar1 + d*w2/wsum;
-                    scalar2 = scalar2 - d*w1/wsum;
+
+                    if (parameterWeights[dim1] != parameterWeights[dim2]) {
+                        final double sumW = parameterWeights[dim1] + parameterWeights[dim2];
+                        scalar1 -= d * parameterWeights[dim2] / sumW;
+                        scalar2 += d * parameterWeights[dim1] / sumW;
+                    } else {
+                        scalar1 -= d / 2; // for equal weights
+                        scalar2 += d / 2;
+                    }
+
+//                    double w1 = parameterWeights[dim1];
+//                    double w2 = parameterWeights[dim2];
+//                    double wsum = w1 + w2;
+//                    scalar1 = scalar1 + d*w2/wsum;
+//                    scalar2 = scalar2 - d*w1/wsum;
 
 
 
@@ -269,12 +277,21 @@ public final double proposal() {
 //                    } else {
 //                        scalar2 += d;
 //                    }
-                    
-                    double w1 = parameterWeights[dim1];
-                    double w2 = parameterWeights[dim2];
-                    double wsum = w1 + w2;
-                    scalar1 = scalar1 + d*w2/wsum;
-                    scalar2 = scalar2 - d*w1/wsum;
+
+                    if (parameterWeights[dim1] != parameterWeights[dim2]) {
+                        final double sumW = parameterWeights[dim1] + parameterWeights[dim2];
+                        scalar1 -= d * parameterWeights[dim2] / sumW;
+                        scalar2 += d * parameterWeights[dim1] / sumW;
+                    } else {
+                        scalar1 -= d / 2; // for equal weights
+                        scalar2 += d / 2;
+                    }
+
+//                    double w1 = parameterWeights[dim1];
+//                    double w2 = parameterWeights[dim2];
+//                    double wsum = w1 + w2;
+//                    scalar1 = scalar1 + d*w2/wsum;
+//                    scalar2 = scalar2 - d*w1/wsum;
 
                 }