/***
 * ASM: a very small and fast Java bytecode manipulation framework
 * Copyright (c) 2000-2007 INRIA, France Telecom
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the copyright holders nor the names of its
 *    contributors may be used to endorse or promote products derived from
 *    this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
 * THE POSSIBILITY OF SUCH DAMAGE.
 */
package alterrs.asm;

/**
 * A label represents a position in the bytecode of a method. Labels are used
 * for jump, goto, and switch instructions, and for try catch blocks. A label
 * designates the <i>instruction</i> that is just after. Note however that there
 * can be other elements between a label and the instruction it designates (such
 * as other labels, stack map frames, line numbers, etc.).
 * 
 * @author Eric Bruneton
 */
public class Label {

	/**
	 * Indicates if this label is only used for debug attributes. Such a label
	 * is not the start of a basic block, the target of a jump instruction, or
	 * an exception handler. It can be safely ignored in control flow graph
	 * analysis algorithms (for optimization purposes).
	 */
	static final int DEBUG = 1;

	/**
	 * Indicates if the position of this label is known.
	 */
	static final int RESOLVED = 2;

	/**
	 * Indicates if this label has been updated, after instruction resizing.
	 */
	static final int RESIZED = 4;

	/**
	 * Indicates if this basic block has been pushed in the basic block stack.
	 * See {@link MethodWriter#visitMaxs visitMaxs}.
	 */
	static final int PUSHED = 8;

	/**
	 * Indicates if this label is the target of a jump instruction, or the start
	 * of an exception handler.
	 */
	static final int TARGET = 16;

	/**
	 * Indicates if a stack map frame must be stored for this label.
	 */
	static final int STORE = 32;

	/**
	 * Indicates if this label corresponds to a reachable basic block.
	 */
	static final int REACHABLE = 64;

	/**
	 * Indicates if this basic block ends with a JSR instruction.
	 */
	static final int JSR = 128;

	/**
	 * Indicates if this basic block ends with a RET instruction.
	 */
	static final int RET = 256;

	/**
	 * Indicates if this basic block is the start of a subroutine.
	 */
	static final int SUBROUTINE = 512;

	/**
	 * Indicates if this subroutine basic block has been visited by a
	 * visitSubroutine(null, ...) call.
	 */
	static final int VISITED = 1024;

	/**
	 * Indicates if this subroutine basic block has been visited by a
	 * visitSubroutine(!null, ...) call.
	 */
	static final int VISITED2 = 2048;

	/**
	 * Field used to associate user information to a label. Warning: this field
	 * is used by the ASM tree package. In order to use it with the ASM tree
	 * package you must override the
	 * {@link alterrs.asm.tree.MethodNode#getLabelNode} method.
	 */
	public Object info;

	/**
	 * Flags that indicate the status of this label.
	 * 
	 * @see #DEBUG
	 * @see #RESOLVED
	 * @see #RESIZED
	 * @see #PUSHED
	 * @see #TARGET
	 * @see #STORE
	 * @see #REACHABLE
	 * @see #JSR
	 * @see #RET
	 */
	int status;

	/**
	 * The line number corresponding to this label, if known.
	 */
	int line;

	/**
	 * The position of this label in the code, if known.
	 */
	int position;

	/**
	 * Number of forward references to this label, times two.
	 */
	private int referenceCount;

	/**
	 * Informations about forward references. Each forward reference is
	 * described by two consecutive integers in this array: the first one is the
	 * position of the first byte of the bytecode instruction that contains the
	 * forward reference, while the second is the position of the first byte of
	 * the forward reference itself. In fact the sign of the first integer
	 * indicates if this reference uses 2 or 4 bytes, and its absolute value
	 * gives the position of the bytecode instruction. This array is also used
	 * as a bitset to store the subroutines to which a basic block belongs. This
	 * information is needed in {@linked MethodWriter#visitMaxs}, after all
	 * forward references have been resolved. Hence the same array can be used
	 * for both purposes without problems.
	 */
	private int[] srcAndRefPositions;

	// ------------------------------------------------------------------------

	/*
	 * Fields for the control flow and data flow graph analysis algorithms (used
	 * to compute the maximum stack size or the stack map frames). A control
	 * flow graph contains one node per "basic block", and one edge per "jump"
	 * from one basic block to another. Each node (i.e., each basic block) is
	 * represented by the Label object that corresponds to the first instruction
	 * of this basic block. Each node also stores the list of its successors in
	 * the graph, as a linked list of Edge objects.
	 * 
	 * The control flow analysis algorithms used to compute the maximum stack
	 * size or the stack map frames are similar and use two steps. The first
	 * step, during the visit of each instruction, builds information about the
	 * state of the local variables and the operand stack at the end of each
	 * basic block, called the "output frame", <i>relatively</i> to the frame
	 * state at the beginning of the basic block, which is called the "input
	 * frame", and which is <i>unknown</i> during this step. The second step, in
	 * {@link MethodWriter#visitMaxs}, is a fix point algorithm that computes
	 * information about the input frame of each basic block, from the input
	 * state of the first basic block (known from the method signature), and by
	 * the using the previously computed relative output frames.
	 * 
	 * The algorithm used to compute the maximum stack size only computes the
	 * relative output and absolute input stack heights, while the algorithm
	 * used to compute stack map frames computes relative output frames and
	 * absolute input frames.
	 */

	/**
	 * Start of the output stack relatively to the input stack. The exact
	 * semantics of this field depends on the algorithm that is used.
	 * <p/>
	 * When only the maximum stack size is computed, this field is the number of
	 * elements in the input stack.
	 * <p/>
	 * When the stack map frames are completely computed, this field is the
	 * offset of the first output stack element relatively to the top of the
	 * input stack. This offset is always negative or null. A null offset means
	 * that the output stack must be appended to the input stack. A -n offset
	 * means that the first n output stack elements must replace the top n input
	 * stack elements, and that the other elements must be appended to the input
	 * stack.
	 */
	int inputStackTop;

	/**
	 * Maximum height reached by the output stack, relatively to the top of the
	 * input stack. This maximum is always positive or null.
	 */
	int outputStackMax;

	/**
	 * Information about the input and output stack map frames of this basic
	 * block. This field is only used when {@link ClassWriter#COMPUTE_FRAMES}
	 * option is used.
	 */
	Frame frame;

	/**
	 * The successor of this label, in the order they are visited. This linked
	 * list does not include labels used for debug info only. If
	 * {@link ClassWriter#COMPUTE_FRAMES} option is used then, in addition, it
	 * does not contain successive labels that denote the same bytecode position
	 * (in this case only the first label appears in this list).
	 */
	Label successor;

	/**
	 * The successors of this node in the control flow graph. These successors
	 * are stored in a linked list of {@link Edge Edge} objects, linked to each
	 * other by their {@link Edge#next} field.
	 */
	Edge successors;

	/**
	 * The next basic block in the basic block stack. This stack is used in the
	 * main loop of the fix point algorithm used in the second step of the
	 * control flow analysis algorithms. It is also used in
	 * {@link #visitSubroutine} to avoid using a recursive method.
	 * 
	 * @see MethodWriter#visitMaxs
	 */
	Label next;

	// ------------------------------------------------------------------------
	// Constructor
	// ------------------------------------------------------------------------

	/**
	 * Constructs a new label.
	 */
	public Label() {
	}

	// ------------------------------------------------------------------------
	// Methods to compute offsets and to manage forward references
	// ------------------------------------------------------------------------

	/**
	 * Returns the offset corresponding to this label. This offset is computed
	 * from the start of the method's bytecode. <i>This method is intended for
	 * {@link Attribute} sub classes, and is normally not needed by class
	 * generators or adapters.</i>
	 * 
	 * @return the offset corresponding to this label.
	 * @throws IllegalStateException
	 *             if this label is not resolved yet.
	 */
	public int getOffset() {
		if ((status & RESOLVED) == 0) {
			throw new IllegalStateException(
					"Label offset position has not been resolved yet");
		}
		return position;
	}

	/**
	 * Puts a reference to this label in the bytecode of a method. If the
	 * position of the label is known, the offset is computed and written
	 * directly. Otherwise, a null offset is written and a new forward reference
	 * is declared for this label.
	 * 
	 * @param owner
	 *            the code writer that calls this method.
	 * @param out
	 *            the bytecode of the method.
	 * @param source
	 *            the position of first byte of the bytecode instruction that
	 *            contains this label.
	 * @param wideOffset
	 *            <tt>true</tt> if the reference must be stored in 4 bytes, or
	 *            <tt>false</tt> if it must be stored with 2 bytes.
	 * @throws IllegalArgumentException
	 *             if this label has not been created by the given code writer.
	 */
	void put(final MethodWriter owner, final ByteVector out, final int source,
			final boolean wideOffset) {
		if ((status & RESOLVED) == 0) {
			if (wideOffset) {
				addReference(-1 - source, out.length);
				out.putInt(-1);
			} else {
				addReference(source, out.length);
				out.putShort(-1);
			}
		} else {
			if (wideOffset) {
				out.putInt(position - source);
			} else {
				out.putShort(position - source);
			}
		}
	}

	/**
	 * Adds a forward reference to this label. This method must be called only
	 * for a true forward reference, i.e. only if this label is not resolved
	 * yet. For backward references, the offset of the reference can be, and
	 * must be, computed and stored directly.
	 * 
	 * @param sourcePosition
	 *            the position of the referencing instruction. This position
	 *            will be used to compute the offset of this forward reference.
	 * @param referencePosition
	 *            the position where the offset for this forward reference must
	 *            be stored.
	 */
	private void addReference(final int sourcePosition,
			final int referencePosition) {
		if (srcAndRefPositions == null) {
			srcAndRefPositions = new int[6];
		}
		if (referenceCount >= srcAndRefPositions.length) {
			int[] a = new int[srcAndRefPositions.length + 6];
			System.arraycopy(srcAndRefPositions, 0, a, 0,
					srcAndRefPositions.length);
			srcAndRefPositions = a;
		}
		srcAndRefPositions[referenceCount++] = sourcePosition;
		srcAndRefPositions[referenceCount++] = referencePosition;
	}

	/**
	 * Resolves all forward references to this label. This method must be called
	 * when this label is added to the bytecode of the method, i.e. when its
	 * position becomes known. This method fills in the blanks that where left
	 * in the bytecode by each forward reference previously added to this label.
	 * 
	 * @param owner
	 *            the code writer that calls this method.
	 * @param position
	 *            the position of this label in the bytecode.
	 * @param data
	 *            the bytecode of the method.
	 * @return <tt>true</tt> if a blank that was left for this label was to
	 *         small to store the offset. In such a case the corresponding jump
	 *         instruction is replaced with a pseudo instruction (using unused
	 *         opcodes) using an unsigned two bytes offset. These pseudo
	 *         instructions will need to be replaced with true instructions with
	 *         wider offsets (4 bytes instead of 2). This is done in
	 *         {@link MethodWriter#resizeInstructions}.
	 * @throws IllegalArgumentException
	 *             if this label has already been resolved, or if it has not
	 *             been created by the given code writer.
	 */
	boolean resolve(final MethodWriter owner, final int position,
			final byte[] data) {
		boolean needUpdate = false;
		this.status |= RESOLVED;
		this.position = position;
		int i = 0;
		while (i < referenceCount) {
			int source = srcAndRefPositions[i++];
			int reference = srcAndRefPositions[i++];
			int offset;
			if (source >= 0) {
				offset = position - source;
				if (offset < Short.MIN_VALUE || offset > Short.MAX_VALUE) {
					/*
					 * changes the opcode of the jump instruction, in order to
					 * be able to find it later (see resizeInstructions in
					 * MethodWriter). These temporary opcodes are similar to
					 * jump instruction opcodes, except that the 2 bytes offset
					 * is unsigned (and can therefore represent values from 0 to
					 * 65535, which is sufficient since the size of a method is
					 * limited to 65535 bytes).
					 */
					int opcode = data[reference - 1] & 0xFF;
					if (opcode <= Opcodes.JSR) {
						// changes IFEQ ... JSR to opcodes 202 to 217
						data[reference - 1] = (byte) (opcode + 49);
					} else {
						// changes IFNULL and IFNONNULL to opcodes 218 and 219
						data[reference - 1] = (byte) (opcode + 20);
					}
					needUpdate = true;
				}
				data[reference++] = (byte) (offset >>> 8);
				data[reference] = (byte) offset;
			} else {
				offset = position + source + 1;
				data[reference++] = (byte) (offset >>> 24);
				data[reference++] = (byte) (offset >>> 16);
				data[reference++] = (byte) (offset >>> 8);
				data[reference] = (byte) offset;
			}
		}
		return needUpdate;
	}

	/**
	 * Returns the first label of the series to which this label belongs. For an
	 * isolated label or for the first label in a series of successive labels,
	 * this method returns the label itself. For other labels it returns the
	 * first label of the series.
	 * 
	 * @return the first label of the series to which this label belongs.
	 */
	Label getFirst() {
		return !ClassReader.FRAMES || frame == null ? this : frame.owner;
	}

	// ------------------------------------------------------------------------
	// Methods related to subroutines
	// ------------------------------------------------------------------------

	/**
	 * Returns true is this basic block belongs to the given subroutine.
	 * 
	 * @param id
	 *            a subroutine id.
	 * @return true is this basic block belongs to the given subroutine.
	 */
	boolean inSubroutine(final long id) {
		if ((status & Label.VISITED) != 0) {
			return (srcAndRefPositions[(int) (id >>> 32)] & (int) id) != 0;
		}
		return false;
	}

	/**
	 * Returns true if this basic block and the given one belong to a common
	 * subroutine.
	 * 
	 * @param block
	 *            another basic block.
	 * @return true if this basic block and the given one belong to a common
	 *         subroutine.
	 */
	boolean inSameSubroutine(final Label block) {
		if ((status & VISITED) == 0 || (block.status & VISITED) == 0) {
			return false;
		}
		for (int i = 0; i < srcAndRefPositions.length; ++i) {
			if ((srcAndRefPositions[i] & block.srcAndRefPositions[i]) != 0) {
				return true;
			}
		}
		return false;
	}

	/**
	 * Marks this basic block as belonging to the given subroutine.
	 * 
	 * @param id
	 *            a subroutine id.
	 * @param nbSubroutines
	 *            the total number of subroutines in the method.
	 */
	void addToSubroutine(final long id, final int nbSubroutines) {
		if ((status & VISITED) == 0) {
			status |= VISITED;
			srcAndRefPositions = new int[(nbSubroutines - 1) / 32 + 1];
		}
		srcAndRefPositions[(int) (id >>> 32)] |= (int) id;
	}

	/**
	 * Finds the basic blocks that belong to a given subroutine, and marks these
	 * blocks as belonging to this subroutine. This method follows the control
	 * flow graph to find all the blocks that are reachable from the current
	 * block WITHOUT following any JSR target.
	 * 
	 * @param JSR
	 *            a JSR block that jumps to this subroutine. If this JSR is not
	 *            null it is added to the successor of the RET blocks found in
	 *            the subroutine.
	 * @param id
	 *            the id of this subroutine.
	 * @param nbSubroutines
	 *            the total number of subroutines in the method.
	 */
	void visitSubroutine(final Label JSR, final long id, final int nbSubroutines) {
		// user managed stack of labels, to avoid using a recursive method
		// (recursivity can lead to stack overflow with very large methods)
		Label stack = this;
		while (stack != null) {
			// removes a label l from the stack
			Label l = stack;
			stack = l.next;
			l.next = null;

			if (JSR != null) {
				if ((l.status & VISITED2) != 0) {
					continue;
				}
				l.status |= VISITED2;
				// adds JSR to the successors of l, if it is a RET block
				if ((l.status & RET) != 0) {
					if (!l.inSameSubroutine(JSR)) {
						Edge e = new Edge();
						e.info = l.inputStackTop;
						e.successor = JSR.successors.successor;
						e.next = l.successors;
						l.successors = e;
					}
				}
			} else {
				// if the l block already belongs to subroutine 'id', continue
				if (l.inSubroutine(id)) {
					continue;
				}
				// marks the l block as belonging to subroutine 'id'
				l.addToSubroutine(id, nbSubroutines);
			}
			// pushes each successor of l on the stack, except JSR targets
			Edge e = l.successors;
			while (e != null) {
				// if the l block is a JSR block, then 'l.successors.next' leads
				// to the JSR target (see {@link #visitJumpInsn}) and must
				// therefore not be followed
				if ((l.status & Label.JSR) == 0 || e != l.successors.next) {
					// pushes e.successor on the stack if it not already added
					if (e.successor.next == null) {
						e.successor.next = stack;
						stack = e.successor;
					}
				}
				e = e.next;
			}
		}
	}

	// ------------------------------------------------------------------------
	// Overriden Object methods
	// ------------------------------------------------------------------------

	/**
	 * Returns a string representation of this label.
	 * 
	 * @return a string representation of this label.
	 */
	public String toString() {
		return "L" + System.identityHashCode(this);
	}
}
