/* Copyright (c) 2010, Carl Burch. License information is located in the
 * com.cburch.logisim.Main source code and at www.cburch.com/logisim/. */

package com.cburch.gray;

import com.cburch.logisim.data.Attribute;
import com.cburch.logisim.data.BitWidth;
import com.cburch.logisim.data.Bounds;
import com.cburch.logisim.data.Value;
import com.cburch.logisim.instance.InstanceFactory;
import com.cburch.logisim.instance.InstancePainter;
import com.cburch.logisim.instance.InstanceState;
import com.cburch.logisim.instance.Port;
import com.cburch.logisim.instance.StdAttr;

/** This component takes a multibit input and outputs the value that follows it
 * in Gray Code. For instance, given input 0100 the output is 1100. */
class GrayIncrementer extends InstanceFactory {
	/* Note that there are no instance variables. There is only one instance of
	 * this class created, which manages all instances of the component. Any
	 * information associated with individual instances should be handled
	 * through attributes. For GrayIncrementer, each instance has a "bit width"
	 * that it works with, and so we'll have an attribute. */

	/** The constructor configures the factory. */
	GrayIncrementer() {
		super("Gray Code Incrementer");
		
		/* This is how we can set up the attributes for GrayIncrementers. In
		 * this case, there is just one attribute - the width - whose default
		 * is 4. The StdAttr class defines several commonly occurring
		 * attributes, including one for "bit width." It's best to use those
		 * StdAttr attributes when appropriate: A user can then select several
		 * components (even from differing factories) with the same attribute
		 * and modify them all at once. */
		setAttributes(new Attribute[] { StdAttr.WIDTH },
				new Object[] { BitWidth.create(4) });
		
		/* The "offset bounds" is the location of the bounding rectangle
		 * relative to the mouse location. Here, we're choosing the component to
		 * be 30x30, and we're anchoring it relative to its primary output
		 * (as is typical for Logisim), which happens to be in the center of the
		 * east edge. Thus, the top left corner of the bounding box is 30 pixels
		 * west and 15 pixels north of the mouse location. */
		setOffsetBounds(Bounds.create(-30, -15, 30, 30));
		
		/* The ports are locations where wires can be connected to this
		 * component. Each port object says where to find the port relative to
		 * the component's anchor location, then whether the port is an
		 * input/output/both, and finally the expected bit width for the port.
		 * The bit width can be a constant (like 1) or an attribute (as here).
		 */
		setPorts(new Port[] {
				new Port(-30, 0, Port.INPUT, StdAttr.WIDTH),
				new Port(0, 0, Port.OUTPUT, StdAttr.WIDTH),
			});
	}

	/** Computes the current output for this component. This method is invoked
	 * any time any of the inputs change their values; it may also be invoked in
	 * other circumstances, even if there is no reason to expect it to change
	 * anything. */
	@Override
	public void propagate(InstanceState state) {
		// First we retrieve the value being fed into the input. Note that in
		// the setPorts invocation above, the component's input was included at
		// index 0 in the parameter array, so we use 0 as the parameter below.
		Value in = state.getPort(0);
		
		// Now compute the output. We've farmed this out to a helper method,
		// since the same logic is needed for the library's other components.
		Value out = nextGray(in);
		
		// Finally we propagate the output into the circuit. The first parameter
		// is 1 because in our list of ports (configured by invocation of
		// setPorts above) the output is at index 1. The second parameter is the
		// value we want to send on that port. And the last parameter is its
		// "delay" - the number of steps it will take for the output to update
		// after its input.
		state.setPort(1, out, out.getWidth() + 1);
	}

	/** Says how an individual instance should appear on the canvas. */
	@Override
	public void paintInstance(InstancePainter painter) {
		// As it happens, InstancePainter contains several convenience methods
		// for drawing, and we'll use those here. Frequently, you'd want to
		// retrieve its Graphics object (painter.getGraphics) so you can draw
		// directly onto the canvas.
		painter.drawRectangle(painter.getBounds(), "G+1");
		painter.drawPorts();
	}
	
	/** Computes the next gray value in the sequence after prev. This static
	 * method just does some bit twiddling; it doesn't have much to do with
	 * Logisim except that it manipulates Value and BitWidth objects. */
	static Value nextGray(Value prev) {
		BitWidth bits = prev.getBitWidth();
		if (!prev.isFullyDefined()) return Value.createError(bits);
		int x = prev.toIntValue();
		int ct = (x >> 16) ^ x; // compute parity of x
		ct = (ct >> 8) ^ ct;
		ct = (ct >> 4) ^ ct;
		ct = (ct >> 2) ^ ct;
		ct = (ct >> 1) ^ ct;
		if ((ct & 1) == 0) { // if parity is even, flip 1's bit
			x = x ^ 1;
		} else { // else flip bit just above last 1
			int y = x ^ (x & (x - 1)); // first compute the last 1
			y = (y << 1) & bits.getMask();
			x = (y == 0 ? 0 : x ^ y);
		}
		return Value.createKnown(bits, x);
	}
}

