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SWT is an open source widget toolkit for Java designed to provide efficient, portable access to the user-interface facilities of the operating systems on which it is implemented.
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/*******************************************************************************
 * Copyright (c) 2000, 2008 IBM Corporation and others.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 *     IBM Corporation - initial API and implementation
 *******************************************************************************/
package org.eclipse.swt.layout;

import org.eclipse.swt.*;
import org.eclipse.swt.widgets.*;

/**
 * Instances of this class are used to define the edges of a control
 * within a FormLayout. 
 * 
 * FormAttachments are set into the top, bottom, left,
 * and right fields of the FormData for a control.
 * For example:
 * 
 * 		FormData data = new FormData();
 * 		data.top = new FormAttachment(0,5);
 * 		data.bottom = new FormAttachment(100,-5);
 * 		data.left = new FormAttachment(0,5);
 * 		data.right = new FormAttachment(100,-5);
 * 		button.setLayoutData(data);
 * 
 * 
 * 
 * A FormAttachment defines where to attach the side of
 * a control by using the equation, y = ax + b. The "a" term represents 
 * a fraction of the parent composite's width (from the left) or height
 * (from the top). It can be defined using a numerator and denominator,
 * or just a percentage value. If a percentage is used, the denominator 
 * is set to 100. The "b" term in the equation represents an offset, in
 * pixels, from the attachment position. For example:
 * 
 * 		FormAttachment attach = new FormAttachment (20, -5);
 * 
 * specifies that the side to which the FormAttachment
 * object belongs will lie at 20% of the parent composite, minus 5 pixels.
 * 
 * 
 * Control sides can also be attached to another control.
 * For example:
 * 
 * 		FormAttachment attach = new FormAttachment (button, 10);
 * 
 * specifies that the side to which the FormAttachment
 * object belongs will lie in the same position as the adjacent side of 
 * the button control, plus 10 pixels. The control side can 
 * also be attached to the opposite side of the specified control.
 * For example:
 *  * 		FormData data = new FormData ();
 * 		data.left = new FormAttachment (button, 0, SWT.LEFT);
 * 
 * specifies that the left side of the control will lie in the same position
 * as the left side of the button control. The control can also 
 * be attached in a position that will center the control on the specified 
 * control. For example:
 *  * 		data.left = new FormAttachment (button, 0, SWT.CENTER);
 * 
 * specifies that the left side of the control will be positioned so that it is
 * centered between the left and right sides of the button control.
 * If the alignment is not specified, the default is to attach to the adjacent side.
 * 
 * 
 * @see FormLayout
 * @see FormData
 * @see Sample code and further information
 * 
 * @since 2.0
 */
public final class FormAttachment {
	/**
	 * numerator specifies the numerator of the "a" term in the
	 * equation, y = ax + b, which defines the attachment.
	 */
	public int numerator;
	
	/**
	 * denominator specifies the denominator of the "a" term in the
	 * equation, y = ax + b, which defines the attachment.
	 * 
	 * The default value is 100.
	 */
	public int denominator = 100;
	
	/**
	 * offset specifies the offset, in pixels, of the control side
	 * from the attachment position.
	 * If the offset is positive, then the control side is offset
	 * to the right of or below the attachment position. If it is
	 * negative, then the control side is offset to the left of or
	 * above the attachment position.
	 * 
	 * This is equivalent to the "b" term in the equation y = ax + b.
	 * The default value is 0.
	 */
	public int offset;
	
	/**
	 * control specifies the control to which the control side is
	 * attached.
	 */
	public Control control;
	
	/**
	 * alignment specifies the alignment of the control side that is
	 * attached to a control.
	 * 
	 * For top and bottom attachments, TOP, BOTTOM and CENTER are used. For left 
	 * and right attachments, LEFT, RIGHT and CENTER are used. If any other case
	 * occurs, the default will be used instead.
	 * 
	 * 
	 * 
Possible values are: 
	 *    {@link SWT#TOP}: Attach the side to the top side of the specified control.
	 *    {@link SWT#BOTTOM}: Attach the side to the bottom side of the specified control.
	 *    {@link SWT#LEFT}: Attach the side to the left side of the specified control.
	 *    {@link SWT#RIGHT}: Attach the side to the right side of the specified control.
	 *    {@link SWT#CENTER}: Attach the side at a position which will center the control on the specified control.
	 *    {@link SWT#DEFAULT}: Attach the side to the adjacent side of the specified control.
	 * 
	 */
	public int alignment;

/**
 * Constructs a new instance of this class.
 * Since no numerator, denominator or offset is specified,
 * the attachment is treated as a percentage of the form.
 * The numerator is zero, the denominator is 100 and the
 * offset is zero.
 * 
 * @since 3.2
 */
public FormAttachment () {
}

/**
 * Constructs a new instance of this class given a numerator
 * Since no denominator or offset is specified, the default
 * is to treat the numerator as a percentage of the form, with a 
 * denominator of 100. The offset is zero.
 * 
 * @param numerator the percentage of the position
 * 
 * @since 3.0
 */
public FormAttachment (int numerator) {
	this (numerator, 100, 0);
}

/**
 * Constructs a new instance of this class given a numerator
 * and an offset. Since no denominator is specified, the default
 * is to treat the numerator as a percentage of the form, with a 
 * denominator of 100.
 * 
 * @param numerator the percentage of the position
 * @param offset the offset of the side from the position
 */
public FormAttachment (int numerator, int offset) {
	this (numerator, 100, offset);
}

/**
 * Constructs a new instance of this class given a numerator 
 * and denominator and an offset. The position of the side is
 * given by the fraction of the form defined by the numerator
 * and denominator.
 *
 * @param numerator the numerator of the position
 * @param denominator the denominator of the position
 * @param offset the offset of the side from the position
 */
public FormAttachment (int numerator, int denominator, int offset) {
	if (denominator == 0) SWT.error (SWT.ERROR_CANNOT_BE_ZERO);
	this.numerator = numerator;
	this.denominator = denominator;
	this.offset = offset;
}

/**
 * Constructs a new instance of this class given a control.
 * Since no alignment is specified, the default alignment is
 * to attach the side to the adjacent side of the specified 
 * control. Since no offset is specified, an offset of 0 is
 * used.
 * 
 * @param control the control the side is attached to
 */
public FormAttachment (Control control) {
	this (control, 0, SWT.DEFAULT);
}

/**
 * Constructs a new instance of this class given a control
 * and an offset. Since no alignment is specified, the default
 * alignment is to attach the side to the adjacent side of the 
 * specified control.
 * 
 * @param control the control the side is attached to
 * @param offset the offset of the side from the control
 */
public FormAttachment (Control control, int offset) {
	this (control, offset, SWT.DEFAULT);
}

/**
 * Constructs a new instance of this class given a control,
 * an offset and an alignment.  The possible alignment values are:
 * 
 * {@link SWT#TOP}
 * the side will be attached to the top side of the specified control
 * {@link SWT#BOTTOM}
 * the side will be attached to the bottom side of the specified control
 * {@link SWT#LEFT}
 * the side will be attached to the left side of the specified control
 * {@link SWT#RIGHT}
 * the side will be attached to the right side of the specified control
 * {@link SWT#CENTER}
 * the side will be centered on the same side of the specified control
 * {@link SWT#DEFAULT}
 * the side will be attached to the adjacent side of the specified control
 * 
 * 
 * @param control the control the side is attached to
 * @param offset the offset of the side from the control
 * @param alignment the alignment of the side to the control it is attached to,
 * 		one of TOP, BOTTOM, LEFT, RIGHT, CENTER, or DEFAULT
 */
public FormAttachment (Control control, int offset, int alignment) {
	this.control = control;
	this.offset = offset;
	this.alignment = alignment;
}

FormAttachment divide (int value) {
	return new FormAttachment (numerator, denominator * value, offset / value);
}

int gcd (int m, int n) {
	int temp;
	m = Math.abs (m);
	n = Math.abs (n);
	if (m < n) {
		temp = m;
		m = n;
		n = temp;
	}
	while (n != 0){
		temp = m;
		m = n;
		n = temp % n;
	}
	return m;
}

FormAttachment minus (FormAttachment attachment) {
	FormAttachment solution = new FormAttachment ();
	solution.numerator = numerator * attachment.denominator - denominator * attachment.numerator;
	solution.denominator = denominator * attachment.denominator;
	int gcd = gcd (solution.denominator, solution.numerator);
	solution.numerator = solution.numerator / gcd;
	solution.denominator = solution.denominator / gcd;
	solution.offset = offset - attachment.offset;
	return solution;
}

FormAttachment minus (int value) {
	return new FormAttachment (numerator, denominator, offset - value);
}

FormAttachment plus (FormAttachment attachment) {
	FormAttachment solution = new FormAttachment ();
	solution.numerator = numerator * attachment.denominator + denominator * attachment.numerator;
	solution.denominator = denominator * attachment.denominator;
	int gcd = gcd (solution.denominator, solution.numerator);
	solution.numerator = solution.numerator / gcd;
	solution.denominator = solution.denominator / gcd;
	solution.offset = offset + attachment.offset;
	return solution;
}

FormAttachment plus (int value) {
	return new FormAttachment (numerator, denominator, offset + value);
}

int solveX (int value) {
	if (denominator == 0) SWT.error (SWT.ERROR_CANNOT_BE_ZERO);
	return ((numerator * value) / denominator) + offset;
}

int solveY (int value) {
	if (numerator == 0) SWT.error (SWT.ERROR_CANNOT_BE_ZERO);
	return (value - offset) * denominator / numerator;
}
	
/**
 * Returns a string containing a concise, human-readable
 * description of the receiver.
 *
 * @return a string representation of the FormAttachment
 */
public String toString () {
 	String string = control != null ? control.toString () : numerator + "/" + denominator;
	return "{y = (" + string + (offset >= 0 ? ")x + " + offset: ")x - " + (-offset))+"}";
}

}