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package org.apache.catalina.ssi;
import java.text.ParseException;
import java.util.LinkedList;
import java.util.List;
/**
* Represents a parsed expression.
*
* @version $Revision: 1.2 $
* @author Paul Speed
*/
public class ExpressionParseTree {
/**
* Contains the current set of completed nodes. This is a workspace for the
* parser.
*/
private LinkedList<Node> nodeStack = new LinkedList<Node>();
/**
* Contains operator nodes that don't yet have values. This is a workspace
* for the parser.
*/
private LinkedList<OppNode> oppStack = new LinkedList<OppNode>();
/**
* The root node after the expression has been parsed.
*/
private Node root;
/**
* The SSIMediator to use when evaluating the expressions.
*/
private SSIMediator ssiMediator;
/**
* Creates a new parse tree for the specified expression.
*/
public ExpressionParseTree(String expr, SSIMediator ssiMediator)
throws ParseException {
this.ssiMediator = ssiMediator;
parseExpression(expr);
}
/**
* Evaluates the tree and returns true or false. The specified SSIMediator
* is used to resolve variable references.
*/
public boolean evaluateTree() {
return root.evaluate();
}
/**
* Pushes a new operator onto the opp stack, resolving existing opps as
* needed.
*/
private void pushOpp(OppNode node) {
// If node is null then it's just a group marker
if (node == null) {
oppStack.add(0, null);
return;
}
while (true) {
if (oppStack.size() == 0) break;
OppNode top = oppStack.get(0);
// If the top is a spacer then don't pop
// anything
if (top == null) break;
// If the top node has a lower precedence then
// let it stay
if (top.getPrecedence() < node.getPrecedence()) break;
// Remove the top node
oppStack.remove(0);
// Let it fill its branches
top.popValues(nodeStack);
// Stick it on the resolved node stack
nodeStack.add(0, top);
}
// Add the new node to the opp stack
oppStack.add(0, node);
}
/**
* Resolves all pending opp nodes on the stack until the next group marker
* is reached.
*/
private void resolveGroup() {
OppNode top = null;
while ((top = oppStack.remove(0)) != null) {
// Let it fill its branches
top.popValues(nodeStack);
// Stick it on the resolved node stack
nodeStack.add(0, top);
}
}
/**
* Parses the specified expression into a tree of parse nodes.
*/
private void parseExpression(String expr) throws ParseException {
StringNode currStringNode = null;
// We cheat a little and start an artificial
// group right away. It makes finishing easier.
pushOpp(null);
ExpressionTokenizer et = new ExpressionTokenizer(expr);
while (et.hasMoreTokens()) {
int token = et.nextToken();
if (token != ExpressionTokenizer.TOKEN_STRING)
currStringNode = null;
switch (token) {
case ExpressionTokenizer.TOKEN_STRING :
if (currStringNode == null) {
currStringNode = new StringNode(et.getTokenValue());
nodeStack.add(0, currStringNode);
} else {
// Add to the existing
currStringNode.value.append(" ");
currStringNode.value.append(et.getTokenValue());
}
break;
case ExpressionTokenizer.TOKEN_AND :
pushOpp(new AndNode());
break;
case ExpressionTokenizer.TOKEN_OR :
pushOpp(new OrNode());
break;
case ExpressionTokenizer.TOKEN_NOT :
pushOpp(new NotNode());
break;
case ExpressionTokenizer.TOKEN_EQ :
pushOpp(new EqualNode());
break;
case ExpressionTokenizer.TOKEN_NOT_EQ :
pushOpp(new NotNode());
// Sneak the regular node in. The NOT will
// be resolved when the next opp comes along.
oppStack.add(0, new EqualNode());
break;
case ExpressionTokenizer.TOKEN_RBRACE :
// Closeout the current group
resolveGroup();
break;
case ExpressionTokenizer.TOKEN_LBRACE :
// Push a group marker
pushOpp(null);
break;
case ExpressionTokenizer.TOKEN_GE :
pushOpp(new NotNode());
// Similar strategy to NOT_EQ above, except this
// is NOT less than
oppStack.add(0, new LessThanNode());
break;
case ExpressionTokenizer.TOKEN_LE :
pushOpp(new NotNode());
// Similar strategy to NOT_EQ above, except this
// is NOT greater than
oppStack.add(0, new GreaterThanNode());
break;
case ExpressionTokenizer.TOKEN_GT :
pushOpp(new GreaterThanNode());
break;
case ExpressionTokenizer.TOKEN_LT :
pushOpp(new LessThanNode());
break;
case ExpressionTokenizer.TOKEN_END :
break;
}
}
// Finish off the rest of the opps
resolveGroup();
if (nodeStack.size() == 0) {
throw new ParseException("No nodes created.", et.getIndex());
}
if (nodeStack.size() > 1) {
throw new ParseException("Extra nodes created.", et.getIndex());
}
if (oppStack.size() != 0) {
throw new ParseException("Unused opp nodes exist.", et.getIndex());
}
root = nodeStack.get(0);
}
/**
* A node in the expression parse tree.
*/
private abstract class Node {
/**
* Return true if the node evaluates to true.
*/
public abstract boolean evaluate();
}
/**
* A node the represents a String value
*/
private class StringNode extends Node {
StringBuilder value;
String resolved = null;
public StringNode(String value) {
this.value = new StringBuilder(value);
}
/**
* Resolves any variable references and returns the value string.
*/
public String getValue() {
if (resolved == null)
resolved = ssiMediator.substituteVariables(value.toString());
return resolved;
}
/**
* Returns true if the string is not empty.
*/
public boolean evaluate() {
return !(getValue().length() == 0);
}
public String toString() {
return value.toString();
}
}
private static final int PRECEDENCE_NOT = 5;
private static final int PRECEDENCE_COMPARE = 4;
private static final int PRECEDENCE_LOGICAL = 1;
/**
* A node implementation that represents an operation.
*/
private abstract class OppNode extends Node {
/**
* The left branch.
*/
Node left;
/**
* The right branch.
*/
Node right;
/**
* Returns a preference level suitable for comparison to other OppNode
* preference levels.
*/
public abstract int getPrecedence();
/**
* Lets the node pop its own branch nodes off the front of the
* specified list. The default pulls two.
*/
public void popValues(List<Node> values) {
right = values.remove(0);
left = values.remove(0);
}
}
private final class NotNode extends OppNode {
public boolean evaluate() {
return !left.evaluate();
}
public int getPrecedence() {
return PRECEDENCE_NOT;
}
/**
* Overridden to pop only one value.
*/
public void popValues(List<Node> values) {
left = values.remove(0);
}
public String toString() {
return left + " NOT";
}
}
private final class AndNode extends OppNode {
public boolean evaluate() {
if (!left.evaluate()) // Short circuit
return false;
return right.evaluate();
}
public int getPrecedence() {
return PRECEDENCE_LOGICAL;
}
public String toString() {
return left + " " + right + " AND";
}
}
private final class OrNode extends OppNode {
public boolean evaluate() {
if (left.evaluate()) // Short circuit
return true;
return right.evaluate();
}
public int getPrecedence() {
return PRECEDENCE_LOGICAL;
}
public String toString() {
return left + " " + right + " OR";
}
}
private abstract class CompareNode extends OppNode {
protected int compareBranches() {
String val1 = ((StringNode)left).getValue();
String val2 = ((StringNode)right).getValue();
return val1.compareTo(val2);
}
}
private final class EqualNode extends CompareNode {
public boolean evaluate() {
return (compareBranches() == 0);
}
public int getPrecedence() {
return PRECEDENCE_COMPARE;
}
public String toString() {
return left + " " + right + " EQ";
}
}
private final class GreaterThanNode extends CompareNode {
public boolean evaluate() {
return (compareBranches() > 0);
}
public int getPrecedence() {
return PRECEDENCE_COMPARE;
}
public String toString() {
return left + " " + right + " GT";
}
}
private final class LessThanNode extends CompareNode {
public boolean evaluate() {
return (compareBranches() < 0);
}
public int getPrecedence() {
return PRECEDENCE_COMPARE;
}
public String toString() {
return left + " " + right + " LT";
}
}
}