Merge variables if the scope of one is contained within another

This improves the output significantly - by merging in more cases, we
avoid the need to use suffixes (e.g. srcOff2, srcOff3, etc. in the
ArrayUtils class) for what is really the same variable.

Restricting the merging to scopes contained in another prevents variable
declarations for two distinct variables sharing a local variable index
from being lifted outside of an if/else block. This often prevents us
from being able to combine a series of if/else blocks into a longer
change, thus increasing the indentation level.

src/org/jetbrains/java/decompiler/modules/decompiler/vars/VarVersionsProcessor.java

@@ -42,7 +42,7 @@ public class VarVersionsProcessor {
 
     typeProcessor.calculateVarTypes(root, graph);
 
-    simpleMerge(typeProcessor, graph, method);
+    simpleMerge(typeProcessor, graph, method, previousVersionsProcessor);
 
     // FIXME: advanced merging
 
@@ -136,7 +136,7 @@ public class VarVersionsProcessor {
     }
   }
 
-  private static void simpleMerge(VarTypeProcessor typeProcessor, DirectGraph graph, StructMethod mt) {
+  private static void simpleMerge(VarTypeProcessor typeProcessor, DirectGraph graph, StructMethod mt, VarVersionsProcessor prev) {
     Map<VarVersionPair, VarType> mapExprentMaxTypes = typeProcessor.getMapExprentMaxTypes();
     Map<VarVersionPair, VarType> mapExprentMinTypes = typeProcessor.getMapExprentMinTypes();
 
@@ -169,7 +169,7 @@ public class VarVersionsProcessor {
           for (int j = i + 1; j < lstVersions.size(); j++) {
             VarVersionPair secondPair = new VarVersionPair(ent.getKey(), lstVersions.get(j));
 
-            if (!shouldMerge(graph, firstPair, secondPair)) {
+            if (!shouldMerge(graph, mt, firstPair, secondPair, prev)) {
               continue;
             }
 
@@ -211,9 +211,36 @@ public class VarVersionsProcessor {
     }
   }
 
-  private static boolean shouldMerge(DirectGraph graph, VarVersionPair firstPair, VarVersionPair secondPair) {
+  private static boolean shouldMerge(DirectGraph graph, StructMethod mt, VarVersionPair firstPair, VarVersionPair secondPair, VarVersionsProcessor prev) {
+    // the scope of an argument always encompasses the scope of another candidate
+    MethodDescriptor desc = MethodDescriptor.parseDescriptor(mt.getDescriptor());
+
+    int firstLocal = 0;
+    if ((mt.getAccessFlags() & CodeConstants.ACC_STATIC) == 0) {
+      firstLocal++;
+    }
+    for (VarType type : desc.params) {
+      firstLocal += type.stackSize;
+    }
+
+    if (prev != null) {
+      Integer firstIndex = prev.mapOriginalVarIndices.get(firstPair.var);
+      if (firstIndex != null && firstIndex < firstLocal) {
+        return true;
+      }
+
+      Integer secondIndex = prev.mapOriginalVarIndices.get(secondPair.var);
+      if (secondIndex != null && secondIndex < firstLocal) {
+        return true;
+      }
+    } else if (firstPair.var < firstLocal || secondPair.var < firstLocal) {
+      return true;
+    }
+
     // find all scopes the two variable versions are assigned in
-    Set<DirectNode> nodes = new HashSet<>();
+    Set<DirectNode> firstNodes = new HashSet<>();
+    Set<DirectNode> secondNodes = new HashSet<>();
+
     for (DirectNode node : graph.nodes) {
       for (Exprent expr : node.exprents) {
         if (expr.type != Exprent.EXPRENT_ASSIGNMENT) {
@@ -232,15 +259,23 @@ public class VarVersionsProcessor {
         int version = varExpr.getVersion();
 
         if (index == firstPair.var && version == firstPair.version) {
-          nodes.add(node);
+          firstNodes.add(node);
         } else if (index == secondPair.var && version == secondPair.version) {
-          nodes.add(node);
+          secondNodes.add(node);
+        }
+      }
+    }
+
+    // only merge variables if one of the scopes is contained within another
+    for (DirectNode firstNode : firstNodes) {
+      for (DirectNode secondNode : secondNodes) {
+        if (firstNode.statement.containsStatement(secondNode.statement) || secondNode.statement.containsStatement(firstNode.statement)) {
+          return true;
         }
       }
     }
 
-    // only merge variables if they're assigned in the same scope
-    return nodes.size() > 1;
+    return false;
   }
 
   private void setNewVarIndices(VarTypeProcessor typeProcessor, DirectGraph graph, VarVersionsProcessor previousVersionsProcessor) {