XPathHelper.java

/*--

 Copyright (C) 2000-2012 Jason Hunter & Brett McLaughlin.
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    derived from this software without prior written permission.  For
    written permission, please contact <request_AT_jdom_DOT_org>.

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    may "JDOM" appear in their name, without prior written permission
    from the JDOM Project Management <request_AT_jdom_DOT_org>.

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 created by Jason Hunter <jhunter_AT_jdom_DOT_org> and
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package org.jdom2.xpath;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;

import org.jdom2.Attribute;
import org.jdom2.Comment;
import org.jdom2.Content;
import org.jdom2.Element;
import org.jdom2.Namespace;
import org.jdom2.NamespaceAware;
import org.jdom2.Parent;
import org.jdom2.ProcessingInstruction;
import org.jdom2.Text;
import org.jdom2.filter.Filters;

/**
 * Provides a set of utility methods to generate XPath expressions to select a
 * given node in a document. You can generate absolute XPath expressions to the
 * target node, or relative expressions from a specified start point. If you
 * request a relative expression, the start and target nodes must share some
 * common ancestry.
 * <p>
 * XPaths are required to be namespace-aware. Typically this is done by using
 * a namespace-prefixed query, with the actual namespace URI being set in the
 * context of the XPath expression. This is not possible to express using a
 * simple String return value. As a work-around, this method uses a potentially
 * slower, but more reliable, mechanism for ensuring the correct namespace
 * context is selected. The mechanism will appear like (for Elements):
 * <br>
 * <code>   .../*[local-name() = 'tag' and namespace-uri() = 'uri']</code>
 * <br>
 * Similarly, Attributes will have a syntax similar to:
 * <br>
 * <code>   .../@*[local-name() = 'attname' and namespace-uri() = 'uri'] </code>
 * <br>
 * This mechanism makes it possible to have a simple namespace context, and a
 * simple String value returned from the methods on this class.
 * <p>
 * This class does not provide ways to access document-level content. Nor does
 * it provide ways to access data relative to the Document level. Use absolute
 * methods to access data from the Document level.
 * <p>
 * The methods on this class assume that the Document is above the top-most
 * Element in the XML tree. The top-most Element is the one that does not have
 * a parent Element (although it may have a parent Document). As a result, you
 * can use Element data that is not attached to a JDOM Document.
 * <p>
 * Detatched Attributes, and detached non-Element content are not treated the
 * same. If you try to get an Absolute path to a detached Attribute or
 * non-Element Content you will get an IllegalArgumentException. On the other
 * hand it is legal to get the relative XPath for a detached node to itself (
 * but to some other node will cause an IllegalArgumentException because the
 * nodes do not share a common ancestor).  
 * <p>
 * <strong>Note</strong>: As this class has no knowledge of the document
 * content, the generated XPath expression rely on the document structure. Hence
 * any modification of the structure of the document may invalidate the
 * generated XPaths.
 * </p>
 * 
 * @author Laurent Bihanic
 * @author Rolf Lear
 */
public final class XPathHelper {
	
	/**
	 * Private constructor.
	 */
	private XPathHelper () {
		// make constructor inaccessible.
	}

	/**
	 * Appends the specified path token to the provided buffer followed by the
	 * position specification of the target node in its siblings list (if
	 * needed).
	 * 
	 * @param node
	 *        the target node for the XPath expression.
	 * @param siblings
	 *        the siblings of the target node.
	 * @param pathToken
	 *        the path token identifying the target node.
	 * @param buffer
	 *        the buffer to which appending the XPath sub-expression or
	 *        <code>null</code> if the method shall allocate a new buffer.
	 * @return the XPath sub-expression to select the target node among its
	 *         siblings.
	 */
	private static StringBuilder getPositionPath(Object node, List<?> siblings,
			String pathToken, StringBuilder buffer) {

		buffer.append(pathToken);

		if (siblings != null) {
			int position = 0;
			final Iterator<?> i = siblings.iterator();
			while (i.hasNext()) {
				position++;
				if (i.next() == node)
					break;
			}
			if (position > 1 || i.hasNext()) {
				// the item is not at the first location, ot there are more
				// locations. in other words, indexing is required.
				buffer.append('[').append(position).append(']');
			}
		}
		return buffer;
	}

	/**
	 * Calculate a single stage of an XPath query.
	 * 
	 * @param nsa
	 *        The token to get the relative-to-parent XPath for
	 * @param buffer
	 *        The buffer to append the relative stage to
	 * @return The same buffer as was input.
	 */
	private static final StringBuilder getSingleStep(final NamespaceAware nsa,
			final StringBuilder buffer) {
		if (nsa instanceof Content) {
			
			final Content content = (Content) nsa;
			
			final Parent pnt = content.getParent();

			if (content instanceof Text) { // OR CDATA!
				
				final List<?> sibs = pnt == null ? null :
					pnt.getContent(Filters.text()); // CDATA
				return getPositionPath(content, sibs, "text()", buffer);
				
			} else if (content instanceof Comment) {
				
				final List<?> sibs = pnt == null ? null :
					pnt.getContent(Filters.comment());
				return getPositionPath(content, sibs, "comment()", buffer);
				
			} else if (content instanceof ProcessingInstruction) {
				
				final List<?> sibs = pnt == null ? null : 
					pnt.getContent(Filters.processinginstruction());
				return getPositionPath(content, sibs,
						"processing-instruction()", buffer);
				
			} else if (content instanceof Element &&
					((Element) content).getNamespace() == Namespace.NO_NAMESPACE) {
				
				// simple XPath to a no-namespace Element.
				
				final String ename = ((Element) content).getName();
				final List<?> sibs = (pnt instanceof Element) ? ((Element)pnt)
						.getChildren(ename) : null;
				return getPositionPath(content, sibs, ename, buffer);
				
			} else if (content instanceof Element) {
				
				// complex XPath to an Element with Namespace...
				// we do not want to have to prefix namespaces because that is
				// essentially impossible to get right with the new JDOM2 API.
				final Element emt = (Element)content;
				
				// Note, the getChildren compares only the URI (not the prefix)
				// so the results are the same as an XPath would be.
				final List<?> sibs = (pnt instanceof Element) ? 
						((Element)pnt).getChildren(emt.getName(), emt.getNamespace()) : null;
				String xps = "*[local-name() = '" + emt.getName() + 
						"' and namespace-uri() = '" + 
						emt.getNamespaceURI() + "']";
				return getPositionPath(content, sibs, xps, buffer);
				
			} else {
				final List<?> sibs = pnt == null ? Collections
						.singletonList(nsa) : pnt.getContent();
				return getPositionPath(content, sibs, "node()", buffer);
				
			}
		} else if (nsa instanceof Attribute) {
			Attribute att = (Attribute) nsa;
			if (att.getNamespace() == Namespace.NO_NAMESPACE) {
				buffer.append("@").append(att.getName());
			} else {
				buffer.append("@*[local-name() = '").append(att.getName());
				buffer.append("' and namespace-uri() = '");
				buffer.append(att.getNamespaceURI()).append("']");
			}
		}

		// do nothing...
		return buffer;
	}

	/**
	 * Returns the path to the specified <code>to</code>Element from the
	 * specified <code>from</code> Element. The from Element must have a common
	 * ancestor Element with the to Element.
	 * <p>
	 * 
	 * @param from
	 *        the Element the generated path shall select relative to.
	 * @param to
	 *        the Content the generated path shall select.
	 * @param sb
	 *        the StringBuilder to append the path to.
	 * @return an XPath expression to select the specified node.
	 * @throws IllegalArgumentException
	 *         if the from and to Elements have no common ancestor.
	 */
	private static StringBuilder getRelativeElementPath(final Element from,
			final Parent to, final StringBuilder sb) {
		if (from == to) {
			sb.append(".");
			return sb;
		}

		// ToStack will be a chain of Elements from the to element to the
		// root element, but will be 'short' if it encounters the from Element
		// itself.
		final ArrayList<Parent> tostack = new ArrayList<Parent>();
		Parent p = to;
		while (p != null && p != from) {
			tostack.add(p);
			p = p.getParent();
		}
		
		// the number of steps we will have in the resulting path (potentially)
		int pos = tostack.size();

		if (p != from) {
			// the from is not a direct ancestor of the to.
			// we need to find where the common ancestor is between from and to
			// we use the 'pos' variable to locate the common ancestor.
			// pos is a pointer in to the tostack.
			Parent f = from;
			int fcnt = 0;
			// note that we search for 'pos' here... it will be set.
			while (f != null && (pos = locate(f, tostack)) < 0) {
				// go up the from ELement's ancestry until we intersect with the
				// to Element's Ancestry.
				fcnt++;
				f = f.getParent();
			}
			if (f == null) {
				throw new IllegalArgumentException(
						"The 'from' and 'to' Element have no common ancestor.");
			}
			// OK, we have counted how far up the ancestry we need to go, so
			// add the steps to the XPath.
			while (--fcnt >= 0) {
				sb.append("../");
			}
		}
		// we have the common point in the ancestry, indicated by 'pos'.
		// we walk down the 'to' side of the tree until we get to the target.
		while (--pos >= 0) {
			getSingleStep(tostack.get(pos), sb);
			sb.append("/");
		}
		// we automatically append '/' in the loop, so we remove the last '/'
		sb.setLength(sb.length() - 1);
		return sb;
	}

	/**
	 * Do an identity search in an array for a specific value.
	 * 
	 * @param f
	 *        The Element to search for.
	 * @param tostack
	 *        The list to search in.
	 * @return the position of the f value in the tostack.
	 */
	private static int locate(final Parent f, final List<Parent> tostack) {
		// a somewhat naive search... ArrayList it is fast enough though.
		int ret = tostack.size();
		while (--ret >= 0) {
			if (f == tostack.get(ret)) {
				return ret;
			}
		}
		return -1;
	}

	/**
	 * Returns the relative path from the given from Content to the specified to
	 * Content as an XPath expression.
	 * 
	 * @param from
	 *        the Content from which the the generated path shall be applied.
	 * @param to
	 *        the Content the generated path shall select.
	 * @return an XPath expression to select the specified node.
	 * @throws IllegalArgumentException
	 *         if <code>to</code> and <code>from</code> are not part of the same
	 *         XML tree
	 */
	public static String getRelativePath(final Content from, final Content to) {
		if (from == null) {
			throw new NullPointerException(
					"Cannot create a path from a null target");
		}
		if (to == null) {
			throw new NullPointerException(
					"Cannot create a path to a null target");
		}
		StringBuilder sb = new StringBuilder();
		if (from == to) {
			return ".";
		}
		final Element efrom = (from instanceof Element) ? (Element) from : from
				.getParentElement();
		if (from != efrom) {
			sb.append("../");
		}
		if (to instanceof Element) {
			getRelativeElementPath(efrom, (Element) to, sb);
		} else {
			final Parent telement = to.getParent();
			if (telement == null) {
				throw new IllegalArgumentException(
						"Cannot get a relative XPath to detached content.");
			}
			getRelativeElementPath(efrom, telement, sb);
			sb.append("/");
			getSingleStep(to, sb);
		}
		return sb.toString();
	}

	/**
	 * Returns the relative path from the given from Content to the specified to
	 * Attribute as an XPath expression.
	 * 
	 * @param from
	 *        the Content from which the the generated path shall be applied.
	 * @param to
	 *        the Attribute the generated path shall select.
	 * @return an XPath expression to select the specified node.
	 * @throws IllegalArgumentException
	 *         if <code>to</code> and <code>from</code> are not part of the same
	 *         XML tree
	 */
	public static String getRelativePath(final Content from, final Attribute to) {
		if (from == null) {
			throw new NullPointerException(
					"Cannot create a path from a null Content");
		}
		if (to == null) {
			throw new NullPointerException(
					"Cannot create a path to a null Attribute");
		}
		final Element t = to.getParent();
		if (t == null) {
			throw new IllegalArgumentException(
					"Cannot create a path to detached Attribute");
		}
		StringBuilder sb = new StringBuilder(getRelativePath(from, t));
		sb.append("/");
		getSingleStep(to, sb);
		return sb.toString();
	}

	/**
	 * Returns the relative path from the given from Attribute to the specified
	 * to Attribute as an XPath expression.
	 * 
	 * @param from
	 *        the Attribute from which the the generated path shall be applied.
	 * @param to
	 *        the Attribute the generated path shall select.
	 * @return an XPath expression to select the specified node.
	 * @throws IllegalArgumentException
	 *         if <code>to</code> and <code>from</code> are not part of the same
	 *         XML tree
	 */
	public static String getRelativePath(final Attribute from,
			final Attribute to) {
		if (from == null) {
			throw new NullPointerException(
					"Cannot create a path from a null 'from'");
		}
		if (to == null) {
			throw new NullPointerException(
					"Cannot create a path to a null target");
		}
		if (from == to) {
			return ".";
		}

		final Element f = from.getParent();
		if (f == null) {
			throw new IllegalArgumentException(
					"Cannot create a path from a detached attrbibute");
		}

		return "../" + getRelativePath(f, to);
	}

	/**
	 * Returns the relative path from the given from Attribute to the specified
	 * to Content as an XPath expression.
	 * 
	 * @param from
	 *        the Attribute from which the the generated path shall be applied.
	 * @param to
	 *        the Content the generated path shall select.
	 * @return an XPath expression to select the specified node.
	 * @throws IllegalArgumentException
	 *         if <code>to</code> and <code>from</code> are not part of the same
	 *         XML tree
	 */
	public static String getRelativePath(final Attribute from, final Content to) {
		if (from == null) {
			throw new NullPointerException(
					"Cannot create a path from a null 'from'");
		}
		if (to == null) {
			throw new NullPointerException(
					"Cannot create a path to a null target");
		}
		final Element f = from.getParent();
		if (f == null) {
			throw new IllegalArgumentException(
					"Cannot create a path from a detached attrbibute");
		}
		if (f == to) {
			return "..";
		}
		return "../" + getRelativePath(f, to);
	}

	/**
	 * Returns the absolute path to the specified to Content.
	 * 
	 * @param to
	 *        the Content the generated path shall select.
	 * @return an XPath expression to select the specified node.
	 * @throws IllegalArgumentException
	 *         if <code>to</code> is not an Element and it is detached.
	 */
	public static String getAbsolutePath(final Content to) {
		if (to == null) {
			throw new NullPointerException(
					"Cannot create a path to a null target");
		}

		final StringBuilder sb = new StringBuilder();

		final Element t = (to instanceof Element) ? (Element) to : to
				.getParentElement();
		if (t == null) {
			if (to.getParent() == null) {
				throw new IllegalArgumentException(
						"Cannot create a path to detached target");
			}
			// otherwise it must be at the document level.
			sb.append("/");
			getSingleStep(to, sb);
			return sb.toString();
		}
		Element r = t;
		while (r.getParentElement() != null) {
			r = r.getParentElement();
		}
		sb.append("/");
		getSingleStep(r, sb);
		if (r != t) {
			sb.append("/");
			getRelativeElementPath(r, t, sb);
		}
		if (t != to) {
			sb.append("/");
			getSingleStep(to, sb);
		}
		return sb.toString();
	}

	/**
	 * Returns the absolute path to the specified to Content.
	 * 
	 * @param to
	 *        the Content the generated path shall select.
	 * @return an XPath expression to select the specified node.
	 * @throws IllegalArgumentException
	 *         if <code>to</code> is detached.
	 */
	public static String getAbsolutePath(final Attribute to) {
		if (to == null) {
			throw new NullPointerException(
					"Cannot create a path to a null target");
		}

		final Element t = to.getParent();
		if (t == null) {
			throw new IllegalArgumentException(
					"Cannot create a path to detached target");
		}
		Element r = t;
		while (r.getParentElement() != null) {
			r = r.getParentElement();
		}
		final StringBuilder sb = new StringBuilder();

		sb.append("/");
		getSingleStep(r, sb);
		if (t != r) {
			sb.append("/");
			getRelativeElementPath(r, t, sb);
		}
		sb.append("/");
		getSingleStep(to, sb);
		return sb.toString();
	}

}