/*
    * Licensed to the Apache Software Foundation (ASF) under one
    * or more contributor license agreements.  See the NOTICE file
    * distributed with this work for additional information
    * regarding copyright ownership.  The ASF licenses this file
    * to you under the Apache License, Version 2.0 (the
    * "License"); you may not use this file except in compliance
    * with the License.  You may obtain a copy of the License at
    *
   *   https://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing,
   * software distributed under the License is distributed on an
   * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
   * KIND, either express or implied.  See the License for the
   * specific language governing permissions and limitations
   * under the License.
   */
  package org.apache.bcel.verifier.structurals;
  
  import java.util.ArrayList;
  import java.util.BitSet;
  import java.util.HashMap;
  import java.util.HashSet;
  import java.util.List;
  import java.util.Map;
  import java.util.Set;
  
  import org.apache.bcel.generic.ASTORE;
  import org.apache.bcel.generic.ATHROW;
  import org.apache.bcel.generic.BranchInstruction;
  import org.apache.bcel.generic.CodeExceptionGen;
  import org.apache.bcel.generic.GotoInstruction;
  import org.apache.bcel.generic.IndexedInstruction;
  import org.apache.bcel.generic.Instruction;
  import org.apache.bcel.generic.InstructionHandle;
  import org.apache.bcel.generic.JsrInstruction;
  import org.apache.bcel.generic.LocalVariableInstruction;
  import org.apache.bcel.generic.MethodGen;
  import org.apache.bcel.generic.RET;
  import org.apache.bcel.generic.ReturnInstruction;
  import org.apache.bcel.generic.Select;
  import org.apache.bcel.verifier.exc.AssertionViolatedException;
  import org.apache.bcel.verifier.exc.StructuralCodeConstraintException;
  
  /**
   * Instances of this class contain information about the subroutines found in a code array of a method. This
   * implementation considers the top-level (the instructions reachable without a JSR or JSR_W starting off from the first
   * instruction in a code array of a method) being a special subroutine; see getTopLevel() for that. Please note that the
   * definition of subroutines in the Java Virtual Machine Specification, Second Edition is somewhat incomplete.
   * Therefore, JustIce uses an own, more rigid notion. Basically, a subroutine is a piece of code that starts at the
   * target of a JSR of JSR_W instruction and ends at a corresponding RET instruction. Note also that the control flow of
   * a subroutine may be complex and non-linear; and that subroutines may be nested. JustIce also mandates subroutines not
   * to be protected by exception handling code (for the sake of control flow predictability). To understand JustIce's
   * notion of subroutines, please read
   *
   * TODO: refer to the paper.
   *
   * @see #getTopLevel()
   */
  public class Subroutines {
      // Node coloring constants
      private enum ColourConstants {
          WHITE, GRAY, BLACK
      }
  
      /**
       * This inner class implements the Subroutine interface.
       */
      private final class SubroutineImpl implements Subroutine {
  
          /**
           * UNSET, a symbol for an uninitialized localVariable field. This is used for the "top-level" Subroutine; for example no
           * subroutine.
           */
          private static final int UNSET = -1;
  
          private final SubroutineImpl[] EMPTY_ARRAY = {};
  
          /**
           * The Local Variable slot where the first instruction of this subroutine (an ASTORE) stores the JsrInstruction's
           * ReturnAddress in and the RET of this subroutine operates on.
           */
          private int localVariable = UNSET;
  
          /** The instructions that belong to this subroutine. */
          private final Set<InstructionHandle> instructions = new HashSet<>(); // Elements: InstructionHandle
  
          /**
           * The JSR or JSR_W instructions that define this subroutine by targeting it.
           */
          private final Set<InstructionHandle> theJSRs = new HashSet<>();
  
          /**
           * The RET instruction that leaves this subroutine.
           */
          private InstructionHandle theRET;
  
          /**
          * Constructs a new instance.
          */
         SubroutineImpl() {
             // empty
         }
 
         /**
          * Adds a new JSR or JSR_W that has this subroutine as its target.
          */
         public void addEnteringJsrInstruction(final InstructionHandle jsrInst) {
             if (jsrInst == null || !(jsrInst.getInstruction() instanceof JsrInstruction)) {
                 throw new AssertionViolatedException("Expecting JsrInstruction InstructionHandle.");
             }
             if (localVariable == UNSET) {
                 throw new AssertionViolatedException("Set the localVariable first.");
             }
             // Something is wrong when an ASTORE is targeted that does not operate on the same local variable than the rest of the
             // JsrInstruction-targets and the RET.
             // (We don't know out leader here so we cannot check if we're really targeted!)
             if (localVariable != ((ASTORE) ((JsrInstruction) jsrInst.getInstruction()).getTarget().getInstruction()).getIndex()) {
                 throw new AssertionViolatedException("Setting a wrong JsrInstruction.");
             }
             theJSRs.add(jsrInst);
         }
 
         /*
          * Adds an instruction to this subroutine. All instructions must have been added before invoking setLeavingRET().
          *
          * @see #setLeavingRET
          */
         void addInstruction(final InstructionHandle ih) {
             if (theRET != null) {
                 throw new AssertionViolatedException("All instructions must have been added before invoking setLeavingRET().");
             }
             instructions.add(ih);
         }
 
         /*
          * Refer to the Subroutine interface for documentation.
          */
         @Override
         public boolean contains(final InstructionHandle inst) {
             return instructions.contains(inst);
         }
 
         /*
          * Satisfies Subroutine.getAccessedLocalIndices().
          */
         @Override
         public int[] getAccessedLocalsIndices() {
             // TODO: Implement caching.
             final Set<Integer> acc = new HashSet<>();
             if (theRET == null && this != getTopLevel()) {
                 throw new AssertionViolatedException("This subroutine object must be built up completely before calculating accessed locals.");
             }
             {
                 for (final InstructionHandle ih : instructions) {
                     // RET is not a LocalVariableInstruction in the current version of BCEL.
                     if (ih.getInstruction() instanceof LocalVariableInstruction || ih.getInstruction() instanceof RET) {
                         final int idx = ((IndexedInstruction) ih.getInstruction()).getIndex();
                         acc.add(Integer.valueOf(idx));
                         // LONG? DOUBLE?.
                         try {
                             // LocalVariableInstruction instances are typed without the need to look into
                             // the constant pool.
                             if (ih.getInstruction() instanceof LocalVariableInstruction) {
                                 final int s = ((LocalVariableInstruction) ih.getInstruction()).getType(null).getSize();
                                 if (s == 2) {
                                     acc.add(Integer.valueOf(idx + 1));
                                 }
                             }
                         } catch (final RuntimeException re) {
                             throw new AssertionViolatedException("BCEL did not like NULL as a ConstantPoolGen object.", re);
                         }
                     }
                 }
             }
 
             {
                 final int[] ret = new int[acc.size()];
                 int j = -1;
                 for (final Integer accessedLocal : acc) {
                     j++;
                     ret[j] = accessedLocal.intValue();
                 }
                 return ret;
             }
         }
 
         /*
          * Refer to the Subroutine interface for documentation.
          */
         @Override
         public InstructionHandle[] getEnteringJsrInstructions() {
             if (this == getTopLevel()) {
                 throw new AssertionViolatedException("getLeavingRET() called on top level pseudo-subroutine.");
             }
             return theJSRs.toArray(InstructionHandle.EMPTY_ARRAY);
         }
 
         /*
          * Refer to the Subroutine interface for documentation.
          */
         @Override
         public InstructionHandle[] getInstructions() {
             return instructions.toArray(InstructionHandle.EMPTY_ARRAY);
         }
 
         /*
          * Refer to the Subroutine interface for documentation.
          */
         @Override
         public InstructionHandle getLeavingRET() {
             if (this == getTopLevel()) {
                 throw new AssertionViolatedException("getLeavingRET() called on top level pseudo-subroutine.");
             }
             return theRET;
         }
 
         /* Satisfies Subroutine.getRecursivelyAccessedLocalsIndices(). */
         @Override
         public int[] getRecursivelyAccessedLocalsIndices() {
             final Set<Integer> s = new HashSet<>();
             final int[] lvs = getAccessedLocalsIndices();
             for (final int lv : lvs) {
                 s.add(Integer.valueOf(lv));
             }
             getRecursivelyAccessedLocalsIndicesHelper(s, subSubs(), new HashSet<>());
             final int[] ret = new int[s.size()];
             int j = -1;
             for (final Integer index : s) {
                 j++;
                 ret[j] = index.intValue();
             }
             return ret;
         }
 
         /**
          * A recursive helper method for getRecursivelyAccessedLocalsIndices(). Every subroutine is visited at most
          * once: since the computed set is a plain union, re-exploring an already visited subroutine cannot add
          * anything, but doing so once per call path made this helper exponential in the depth of the JSR call graph
          * (and made it recurse forever on a cyclic one).
          *
          * @see #getRecursivelyAccessedLocalsIndices()
          */
         private void getRecursivelyAccessedLocalsIndicesHelper(final Set<Integer> set, final Subroutine[] subs, final Set<Subroutine> visited) {
             for (final Subroutine sub : subs) {
                 if (!visited.add(sub)) {
                     continue;
                 }
                 final int[] lvs = sub.getAccessedLocalsIndices();
                 for (final int lv : lvs) {
                     set.add(Integer.valueOf(lv));
                 }
                 if (sub.subSubs().length != 0) {
                     getRecursivelyAccessedLocalsIndicesHelper(set, sub.subSubs(), visited);
                 }
             }
         }
 
         /**
          * Sets the leaving RET instruction. Must be invoked after all instructions are added. Must not be invoked for top-level
          * 'subroutine'.
          */
         void setLeavingRET() {
             if (localVariable == UNSET) {
                 throw new AssertionViolatedException("setLeavingRET() called for top-level 'subroutine' or forgot to set local variable first.");
             }
             InstructionHandle ret = null;
             for (final InstructionHandle actual : instructions) {
                 if (actual.getInstruction() instanceof RET) {
                     if (ret != null) {
                         throw new StructuralCodeConstraintException("Subroutine with more then one RET detected: '" + ret + "' and '" + actual + "'.");
                     }
                     ret = actual;
                 }
             }
             if (ret == null) {
                 throw new StructuralCodeConstraintException("Subroutine without a RET detected.");
             }
             if (((RET) ret.getInstruction()).getIndex() != localVariable) {
                 throw new StructuralCodeConstraintException(
                     "Subroutine uses '" + ret + "' which does not match the correct local variable '" + localVariable + "'.");
             }
             theRET = ret;
         }
 
         /*
          * Sets the local variable slot the ASTORE that is targeted by the JsrInstructions of this subroutine operates on. This
          * subroutine's RET operates on that same local variable slot, of course.
          */
         void setLocalVariable(final int i) {
             if (localVariable != UNSET) {
                 throw new AssertionViolatedException("localVariable set twice.");
             }
             localVariable = i;
         }
 
         /*
          * Satisfies Subroutine.subSubs().
          */
         @Override
         public Subroutine[] subSubs() {
             final Set<Subroutine> h = new HashSet<>();
 
             for (final InstructionHandle ih : instructions) {
                 final Instruction inst = ih.getInstruction();
                 if (inst instanceof JsrInstruction) {
                     final InstructionHandle targ = ((JsrInstruction) inst).getTarget();
                     h.add(getSubroutine(targ));
                 }
             }
             return h.toArray(EMPTY_ARRAY);
         }
 
         /**
          * Returns a String representation of this object, merely for debugging purposes. (Internal) Warning: Verbosity on a
          * problematic subroutine may cause stack overflow errors due to recursive subSubs() calls. Don't use this, then.
          */
         @Override
         public String toString() {
             final StringBuilder ret = new StringBuilder();
             ret.append("Subroutine: Local variable is '").append(localVariable);
             ret.append("', JSRs are '").append(theJSRs);
             ret.append("', RET is '").append(theRET);
             ret.append("', Instructions: '").append(instructions).append("'.");
 
             ret.append(" Accessed local variable slots: '");
             int[] alv = getAccessedLocalsIndices();
             for (final int element : alv) {
                 ret.append(element);
                 ret.append(" ");
             }
             ret.append("'.");
 
             ret.append(" Recursively (via subsub...routines) accessed local variable slots: '");
             alv = getRecursivelyAccessedLocalsIndices();
             for (final int element : alv) {
                 ret.append(element);
                 ret.append(" ");
             }
             ret.append("'.");
 
             return ret.toString();
         }
 
     } // end Inner Class SubrouteImpl
 
     /**
      * A utility method that calculates the successors of a given InstructionHandle <strong>in the same subroutine</strong>. That
      * means, a RET does not have any successors as defined here. A JsrInstruction has its physical successor as its
      * successor (opposed to its target) as defined here.
      */
     private static InstructionHandle[] getSuccessors(final InstructionHandle instruction) {
         final InstructionHandle[] single = new InstructionHandle[1];
 
         final Instruction inst = instruction.getInstruction();
 
         // Terminates method normally.
         // Terminates method abnormally, because JustIce mandates
         // subroutines not to be protected by exception handlers.
         if (inst instanceof RET || inst instanceof ReturnInstruction || inst instanceof ATHROW) {
             return InstructionHandle.EMPTY_ARRAY;
         }
 
         // See method comment.
         if (inst instanceof JsrInstruction) {
             single[0] = instruction.getNext();
             return single;
         }
 
         if (inst instanceof GotoInstruction) {
             single[0] = ((GotoInstruction) inst).getTarget();
             return single;
         }
 
         if (inst instanceof BranchInstruction) {
             if (inst instanceof Select) {
                 // BCEL's getTargets() returns only the non-default targets,
                 // thanks to Eli Tilevich for reporting.
                 final InstructionHandle[] matchTargets = ((Select) inst).getTargets();
                 final InstructionHandle[] ret = new InstructionHandle[matchTargets.length + 1];
                 ret[0] = ((Select) inst).getTarget();
                 System.arraycopy(matchTargets, 0, ret, 1, matchTargets.length);
                 return ret;
             }
             final InstructionHandle[] pair = new InstructionHandle[2];
             pair[0] = instruction.getNext();
             pair[1] = ((BranchInstruction) inst).getTarget();
             return pair;
         }
 
         // default case: Fall through.
         single[0] = instruction.getNext();
         return single;
     }
 
     private static StructuralCodeConstraintException recursiveCallException(final Subroutine sub2) {
         // Don't use toString() here because of possibly infinite recursive subSubs() calls then.
         final SubroutineImpl si = (SubroutineImpl) sub2;
         return new StructuralCodeConstraintException("Subroutine with local variable '" + si.localVariable + "', JSRs '" + si.theJSRs + "', RET '"
             + si.theRET + "' is called by a subroutine which uses the same local variable index as itself; maybe even a recursive call?"
             + " JustIce's clean definition of a subroutine forbids both.");
     }
 
     /**
      * The map containing the subroutines found. Key: InstructionHandle of the leader of the subroutine. Elements:
      * SubroutineImpl objects.
      */
     private final Map<InstructionHandle, Subroutine> subroutines = new HashMap<>();
 
     /**
      * This is referring to a special subroutine, namely the top level. This is not really a subroutine but we use it to
      * distinguish between top level instructions and unreachable instructions.
      */
     // CHECKSTYLE:OFF
     public final Subroutine TOPLEVEL; // TODO can this be made private?
     // CHECKSTYLE:ON
 
     /**
      * Constructs a new instance.
      *
      * @param mg A MethodGen object representing method to create the Subroutine objects of. Assumes that JustIce strict
      *        checks are needed.
      */
     public Subroutines(final MethodGen mg) {
         this(mg, true);
     }
 
     /**
      * Constructs a new instance.
      *
      * @param mg A MethodGen object representing method to create the Subroutine objects of.
      * @param enableJustIceCheck whether to enable additional JustIce checks.
      * @since 6.0
      */
     public Subroutines(final MethodGen mg, final boolean enableJustIceCheck) {
         final InstructionHandle[] all = mg.getInstructionList().getInstructionHandles();
         final CodeExceptionGen[] handlers = mg.getExceptionHandlers();
 
         // Define our "Toplevel" fake subroutine.
         TOPLEVEL = new SubroutineImpl();
 
         // Calculate "real" subroutines.
         final Set<InstructionHandle> subLeaders = new HashSet<>(); // Elements: InstructionHandle
         for (final InstructionHandle element : all) {
             final Instruction inst = element.getInstruction();
             if (inst instanceof JsrInstruction) {
                 subLeaders.add(((JsrInstruction) inst).getTarget());
             }
         }
 
         // Build up the database.
         for (final InstructionHandle astore : subLeaders) {
             final SubroutineImpl sr = new SubroutineImpl();
             sr.setLocalVariable(((ASTORE) astore.getInstruction()).getIndex());
             subroutines.put(astore, sr);
         }
 
         // Fake it a bit. We want a virtual "TopLevel" subroutine.
         subroutines.put(all[0], TOPLEVEL);
         subLeaders.add(all[0]);
 
         // Tell the subroutines about their JsrInstructions.
         // Note that there cannot be a JSR targeting the top-level
         // since "Jsr 0" is disallowed in Pass 3a.
         // Instructions shared by a subroutine and the toplevel are
         // disallowed and checked below, after the BFS.
         for (final InstructionHandle element : all) {
             final Instruction inst = element.getInstruction();
             if (inst instanceof JsrInstruction) {
                 final InstructionHandle leader = ((JsrInstruction) inst).getTarget();
                 ((SubroutineImpl) getSubroutine(leader)).addEnteringJsrInstruction(element);
             }
         }
 
         // Now do a BFS from every subroutine leader to find all the
         // instructions that belong to a subroutine.
         // we don't want to assign an instruction to two or more Subroutine objects.
         final Set<InstructionHandle> instructionsAssigned = new HashSet<>();
 
         // Graph coloring. Key: InstructionHandle, Value: ColourConstants enum.
         final Map<InstructionHandle, ColourConstants> colors = new HashMap<>();
 
         final List<InstructionHandle> qList = new ArrayList<>();
         for (final InstructionHandle actual : subLeaders) {
             // Do some BFS with "actual" as the root of the graph.
             // Init colors: an instruction absent from the map is WHITE. Explicitly coloring every
             // instruction WHITE on every round would make this initialization quadratic in the
             // method size.
             colors.clear();
             colors.put(actual, ColourConstants.GRAY);
             // Init Queue
 
             qList.clear();
             qList.add(actual); // add(Obj) adds to the end, remove(0) removes from the start.
 
             /*
              * BFS ALGORITHM MODIFICATION: Start out with multiple "root" nodes, as exception handlers are starting points of
              * top-level code, too. [why top-level? TODO: Refer to the special JustIce notion of subroutines.]
              */
             if (actual == all[0]) {
                 for (final CodeExceptionGen handler : handlers) {
                     colors.put(handler.getHandlerPC(), ColourConstants.GRAY);
                     qList.add(handler.getHandlerPC());
                 }
             }
             /* CONTINUE NORMAL BFS ALGORITHM */
 
             // Loop until Queue is empty
             while (!qList.isEmpty()) {
                 final InstructionHandle u = qList.remove(0);
                 final InstructionHandle[] successors = getSuccessors(u);
                 for (final InstructionHandle successor : successors) {
                     if (successor != null && colors.get(successor) == null) { // absent from the map means WHITE
                         colors.put(successor, ColourConstants.GRAY);
                         qList.add(successor);
                     }
                 }
                 colors.put(u, ColourConstants.BLACK);
             }
             // BFS ended above. Only instructions visited by this BFS round are in the color map,
             // so this scan is proportional to the round, not to the whole method.
             for (final Map.Entry<InstructionHandle, ColourConstants> entry : colors.entrySet()) {
                 if (entry.getValue() == ColourConstants.BLACK) {
                     final InstructionHandle element = entry.getKey();
                     ((SubroutineImpl) (actual == all[0] ? getTopLevel() : getSubroutine(actual))).addInstruction(element);
                     if (instructionsAssigned.contains(element)) {
                         throw new StructuralCodeConstraintException(
                             "Instruction '" + element + "' is part of more than one subroutine (or of the top level and a subroutine).");
                     }
                     instructionsAssigned.add(element);
                 }
             }
             if (actual != all[0]) { // If we don't deal with the top-level 'subroutine'
                 ((SubroutineImpl) getSubroutine(actual)).setLeavingRET();
             }
         }
 
         if (enableJustIceCheck && handlers.length > 0) {
             // Now make sure no instruction of a Subroutine is protected by exception handling code
             // as is mandated by JustIces notion of subroutines.
             // The handler coverage of every instruction is computed once, with a difference array
             // over instruction list indices. Walking every handler's protected range and, per
             // protected instruction, every subroutine would let a crafted method (thousands of
             // handlers over large ranges) keep this constructor busy nearly forever.
             final Map<InstructionHandle, Integer> instructionIndexes = new HashMap<>();
             for (int i = 0; i < all.length; i++) {
                 instructionIndexes.put(all[i], Integer.valueOf(i));
             }
             final int[] coverageDelta = new int[all.length + 1];
             for (final CodeExceptionGen handler : handlers) {
                 // Note the inclusive/inclusive notation of "generic API" exception handlers!
                 final Integer startIndex = instructionIndexes.get(handler.getStartPC());
                 final Integer endIndex = instructionIndexes.get(handler.getEndPC());
                 if (startIndex == null || endIndex == null || startIndex.intValue() > endIndex.intValue()) {
                     throw new StructuralCodeConstraintException("Exception handler '" + handler + "' does not protect a valid instruction range.");
                 }
                 coverageDelta[startIndex.intValue()]++;
                 coverageDelta[endIndex.intValue() + 1]--;
             }
             final boolean[] isProtected = new boolean[all.length];
             int covered = 0;
             for (int i = 0; i < all.length; i++) {
                 covered += coverageDelta[i];
                 isProtected[i] = covered > 0;
             }
             for (final Subroutine sub : subroutines.values()) {
                 if (sub == subroutines.get(all[0])) {
                     continue;
                 }
                 for (final InstructionHandle protectedIh : sub.getInstructions()) {
                     final Integer index = instructionIndexes.get(protectedIh);
                     if (index != null && isProtected[index.intValue()]) {
                         // Only the error message needs the offending handler; this scan runs at most once.
                         for (final CodeExceptionGen handler : handlers) {
                             final int startIndex = instructionIndexes.get(handler.getStartPC()).intValue();
                             final int endIndex = instructionIndexes.get(handler.getEndPC()).intValue();
                             if (startIndex <= index.intValue() && index.intValue() <= endIndex) {
                                 throw new StructuralCodeConstraintException("Subroutine instruction '" + protectedIh
                                     + "' is protected by an exception handler, '" + handler
                                     + "'. This is forbidden by the JustIce verifier due to its clear definition of subroutines.");
                             }
                         }
                     }
                 }
             }
         }
 
         // Now make sure no subroutine is calling a subroutine
         // that uses the same local variable for the RET as themselves
         // (recursively).
         // This includes that subroutines may not call themselves
         // recursively, even not through intermediate calls to other
         // subroutines.
         noRecursiveCalls(getTopLevel());
 
     }
 
     /**
      * Returns the Subroutine object associated with the given leader (that is, the first instruction of the subroutine).
      * You must not use this to get the top-level instructions modeled as a Subroutine object.
      *
      * @param leader The leader instruction handle.
      * @return The Subroutine object.
      * @see #getTopLevel()
      */
     public Subroutine getSubroutine(final InstructionHandle leader) {
         final Subroutine ret = subroutines.get(leader);
 
         if (ret == null) {
             throw new AssertionViolatedException("Subroutine requested for an InstructionHandle that is not a leader of a subroutine.");
         }
 
         if (ret == TOPLEVEL) {
             throw new AssertionViolatedException("TOPLEVEL special subroutine requested; use getTopLevel().");
         }
 
         return ret;
     }
 
     /**
      * For easy handling, the piece of code that is <strong>not</strong> a subroutine, the top-level, is also modeled as a Subroutine
      * object. It is a special Subroutine object where <B>you must not invoke getEnteringJsrInstructions() or
      * getLeavingRET()</B>.
      *
      * @return The top-level Subroutine.
      * @see Subroutine#getEnteringJsrInstructions()
      * @see Subroutine#getLeavingRET()
      */
     public Subroutine getTopLevel() {
         return TOPLEVEL;
     }
 
     /**
      * This utility method makes sure that no subroutine is calling a subroutine that uses the same local
      * variable for the RET as themselves (recursively). This includes that subroutines may not call themselves recursively,
      * even not through intermediate calls to other subroutines.
      *
      * Every subroutine is fully validated exactly once, memoizing the RET local variable indices used anywhere in its
      * call subtree. The former implementation re-explored a subroutine once per call path, which is exponential in the
      * number of subroutines for a layered JSR call graph.
      *
      * @throws StructuralCodeConstraintException Thrown if the above constraint is not satisfied.
      */
     private void noRecursiveCalls(final Subroutine sub) {
         noRecursiveCalls(sub, new BitSet(), new HashMap<>(), new HashMap<>());
     }
 
     /**
      * The recursive helper for {@link #noRecursiveCalls(Subroutine)}.
      *
      * @param sub the subroutine whose callees are validated.
      * @param pathLocals compact ids (see {@code localIds}) of the RET local variables used by the subroutines on the current call path.
      * @param validated maps every fully validated subroutine to the compact ids of the RET local variables used by it and its entire call subtree.
      * @param localIds maps a RET local variable index to a compact id so the bit sets stay small.
      * @return the compact ids of the RET local variables used by {@code sub}'s callees and their call subtrees.
      * @throws StructuralCodeConstraintException Thrown if a subroutine calls a subroutine using the same RET local variable.
      */
     private BitSet noRecursiveCalls(final Subroutine sub, final BitSet pathLocals, final Map<Subroutine, BitSet> validated,
         final Map<Integer, Integer> localIds) {
         final BitSet subtreeLocals = new BitSet();
 
         for (final Subroutine sub2 : sub.subSubs()) {
             final Integer index = Integer.valueOf(((RET) sub2.getLeavingRET().getInstruction()).getIndex());
             final int localId = localIds.computeIfAbsent(index, k -> Integer.valueOf(localIds.size())).intValue();
 
             BitSet childLocals = validated.get(sub2);
             if (childLocals == null) {
                 if (pathLocals.get(localId)) {
                     // sub2 uses a RET local variable also used by a subroutine on the current call path;
                     // this also covers (possibly indirect) recursive calls.
                     throw recursiveCallException(sub2);
                 }
                 pathLocals.set(localId);
                 childLocals = noRecursiveCalls(sub2, pathLocals, validated, localIds);
                 pathLocals.clear(localId);
                 childLocals.set(localId);
                 validated.put(sub2, childLocals);
             } else if (childLocals.intersects(pathLocals)) {
                 // A subroutine in sub2's (already validated) call subtree uses a RET local variable also
                 // used by a subroutine on the current call path.
                 throw recursiveCallException(sub2);
             }
             subtreeLocals.or(childLocals);
         }
         return subtreeLocals;
     }
 
     /**
      * Returns the subroutine object associated with the given instruction. This is a costly operation, you should consider
      * using getSubroutine(InstructionHandle). Returns 'null' if the given InstructionHandle lies in so-called 'dead code',
      * for example code that can never be executed.
      *
      * @param any The instruction handle.
      * @return The Subroutine object or null.
      * @see #getSubroutine(InstructionHandle)
      * @see #getTopLevel()
      */
     public Subroutine subroutineOf(final InstructionHandle any) {
         for (final Subroutine s : subroutines.values()) {
             if (s.contains(any)) {
                 return s;
             }
         }
         System.err.println("DEBUG: Please verify '" + any.toString(true) + "' lies in dead code.");
         return null;
         // throw new AssertionViolatedException("No subroutine for InstructionHandle found (DEAD CODE?).");
     }
 
     /**
      * Returns a String representation of this object; merely for debugging puposes.
      */
     @Override
     public String toString() {
         return "---\n" + subroutines + "\n---\n";
     }
 }