commons-bcel-master/src/main/java/org/apache/bcel/verifier/structurals/OperandStack.java
/*
 * 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
 *
 *      http://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 org.apache.bcel.generic.ObjectType;
import org.apache.bcel.generic.ReferenceType;
import org.apache.bcel.generic.Type;
import org.apache.bcel.verifier.exc.AssertionViolatedException;
import org.apache.bcel.verifier.exc.StructuralCodeConstraintException;

/**
 * This class implements a stack used for symbolic JVM stack simulation. [It's used as an operand stack substitute.]
 * Elements of this stack are {@link Type} objects.
 */
public class OperandStack implements Cloneable {

    /** We hold the stack information here. */
    private ArrayList<Type> stack = new ArrayList<>();

    /** The maximum number of stack slots this OperandStack instance may hold. */
    private final int maxStack;

    /**
     * Creates an empty stack with a maximum of maxStack slots.
     */
    public OperandStack(final int maxStack) {
        this.maxStack = maxStack;
    }

    /**
     * Creates an otherwise empty stack with a maximum of maxStack slots and the ObjectType 'obj' at the top.
     */
    public OperandStack(final int maxStack, final ObjectType obj) {
        this.maxStack = maxStack;
        this.push(obj);
    }

    /**
     * Clears the stack.
     */
    public void clear() {
        stack = new ArrayList<>();
    }

    /**
     * Returns a deep copy of this object; that means, the clone operates on a new stack. However, the Type objects on the
     * stack are shared.
     */
    @Override
    public Object clone() {
        final OperandStack newstack = new OperandStack(this.maxStack);
        @SuppressWarnings("unchecked") // OK because this.stack is the same type
        final ArrayList<Type> clone = (ArrayList<Type>) this.stack.clone();
        newstack.stack = clone;
        return newstack;
    }

    /**
     * Returns true if and only if this OperandStack equals another, meaning equal lengths and equal objects on the stacks.
     */
    @Override
    public boolean equals(final Object o) {
        if (!(o instanceof OperandStack)) {
            return false;
        }
        final OperandStack s = (OperandStack) o;
        return this.stack.equals(s.stack);
    }

    /**
     * Returns a (typed!) clone of this.
     *
     * @see #clone()
     */
    public OperandStack getClone() {
        return (OperandStack) this.clone();
    }

    /**
     * @return a hash code value for the object.
     */
    @Override
    public int hashCode() {
        return stack.hashCode();
    }

    /**
     * Replaces all occurrences of u in this OperandStack instance with an "initialized" ObjectType.
     */
    public void initializeObject(final UninitializedObjectType u) {
        for (int i = 0; i < stack.size(); i++) {
            if (stack.get(i) == u) {
                stack.set(i, u.getInitialized());
            }
        }
    }

    /**
     * Returns true IFF this OperandStack is empty.
     */
    public boolean isEmpty() {
        return stack.isEmpty();
    }

    /**
     * Returns the number of stack slots this stack can hold.
     */
    public int maxStack() {
        return this.maxStack;
    }

    /**
     * Merges another stack state into this instance's stack state. See the Java Virtual Machine Specification, Second
     * Edition, page 146: 4.9.2 for details.
     */
    public void merge(final OperandStack s) {
        try {
            if (slotsUsed() != s.slotsUsed() || size() != s.size()) {
                throw new StructuralCodeConstraintException("Cannot merge stacks of different size:\nOperandStack A:\n" + this + "\nOperandStack B:\n" + s);
            }

            for (int i = 0; i < size(); i++) {
                // If the object _was_ initialized and we're supposed to merge
                // in some uninitialized object, we reject the code (see vmspec2, 4.9.4, last paragraph).
                if (!(stack.get(i) instanceof UninitializedObjectType) && s.stack.get(i) instanceof UninitializedObjectType) {
                    throw new StructuralCodeConstraintException("Backwards branch with an uninitialized object on the stack detected.");
                }
                // Even harder, we're not initialized but are supposed to broaden
                // the known object type
                if (!stack.get(i).equals(s.stack.get(i)) && stack.get(i) instanceof UninitializedObjectType
                    && !(s.stack.get(i) instanceof UninitializedObjectType)) {
                    throw new StructuralCodeConstraintException("Backwards branch with an uninitialized object on the stack detected.");
                }
                // on the other hand...
                if (stack.get(i) instanceof UninitializedObjectType && !(s.stack.get(i) instanceof UninitializedObjectType)) { // that has been initialized by
                                                                                                                               // now
                    stack.set(i, ((UninitializedObjectType) stack.get(i)).getInitialized()); // note that.
                }
                if (!stack.get(i).equals(s.stack.get(i))) {
                    if (!(stack.get(i) instanceof ReferenceType) || !(s.stack.get(i) instanceof ReferenceType)) {
                        throw new StructuralCodeConstraintException("Cannot merge stacks of different types:\nStack A:\n" + this + "\nStack B:\n" + s);
                    }
                    stack.set(i, ((ReferenceType) stack.get(i)).getFirstCommonSuperclass((ReferenceType) s.stack.get(i)));
                }
            }
        } catch (final ClassNotFoundException e) {
            // FIXME: maybe not the best way to handle this
            throw new AssertionViolatedException("Missing class: " + e, e);
        }
    }

    /**
     * Returns the element on top of the stack. The element is not popped off the stack!
     */
    public Type peek() {
        return peek(0);
    }

    /**
     * Returns the element that's i elements below the top element; that means, iff i==0 the top element is returned. The
     * element is not popped off the stack!
     */
    public Type peek(final int i) {
        return stack.get(size() - i - 1);
    }

    /**
     * Returns the element on top of the stack. The element is popped off the stack.
     */
    public Type pop() {
        return stack.remove(size() - 1);
    }

    /**
     * Pops i elements off the stack. Always returns null.
     *
     * @return Always returns null.
     */
    public Type pop(final int count) {
        for (int j = 0; j < count; j++) {
            pop();
        }
        return null;
    }

    /**
     * Pushes a Type object onto the stack.
     */
    public void push(final Type type) {
        if (type == null) {
            throw new AssertionViolatedException("Cannot push NULL onto OperandStack.");
        }
        if (type == Type.BOOLEAN || type == Type.CHAR || type == Type.BYTE || type == Type.SHORT) {
            throw new AssertionViolatedException("The OperandStack does not know about '" + type + "'; use Type.INT instead.");
        }
        if (slotsUsed() >= maxStack) {
            throw new AssertionViolatedException("OperandStack too small, should have thrown proper Exception elsewhere. Stack: " + this);
        }
        stack.add(type);
    }

    /**
     * Returns the size of this OperandStack; that means, how many Type objects there are.
     */
    public int size() {
        return stack.size();
    }

    /**
     * Returns the number of stack slots used.
     *
     * @see #maxStack()
     */
    public int slotsUsed() {
        /*
         * XXX change this to a better implementation using a variable that keeps track of the actual slotsUsed()-value
         * monitoring all push()es and pop()s.
         */
        int slots = 0;
        for (int i = 0; i < stack.size(); i++) {
            slots += peek(i).getSize();
        }
        return slots;
    }

    /**
     * Returns a String representation of this OperandStack instance.
     */
    @Override
    public String toString() {
        final StringBuilder sb = new StringBuilder();
        sb.append("Slots used: ");
        sb.append(slotsUsed());
        sb.append(" MaxStack: ");
        sb.append(maxStack);
        sb.append(".\n");
        for (int i = 0; i < size(); i++) {
            sb.append(peek(i));
            sb.append(" (Size: ");
            sb.append(String.valueOf(peek(i).getSize()));
            sb.append(")\n");
        }
        return sb.toString();
    }

}
