Subversion-Projekte lars-tiefland.prado

// Copyright 2005 Google Inc.

// All Rights Reserved

//

// An XPath parser and evaluator written in JavaScript. The

// implementation is complete except for functions handling

// namespaces.

//

// Reference: [XPATH] XPath Specification

// <http://www.w3.org/TR/1999/REC-xpath-19991116>.

//

//

// The API of the parser has several parts:

//

// 1. The parser function xpathParse() that takes a string and returns

// an expession object.

//

// 2. The expression object that has an evaluate() method to evaluate the

// XPath expression it represents. (It is actually a hierarchy of

// objects that resembles the parse tree, but an application will call

// evaluate() only on the top node of this hierarchy.)

//

// 3. The context object that is passed as an argument to the evaluate()

// method, which represents the DOM context in which the expression is

// evaluated.

//

// 4. The value object that is returned from evaluate() and represents

// values of the different types that are defined by XPath (number,

// string, boolean, and node-set), and allows to convert between them.

//

// These parts are near the top of the file, the functions and data

// that are used internally follow after them.

//

//

// TODO(mesch): add jsdoc comments. Use more coherent naming.

//

//

// Author: Steffen Meschkat <mesch@google.com>

// The entry point for the parser.

//

// @param expr a string that contains an XPath expression.

// @return an expression object that can be evaluated with an

// expression context.

function xpathParse(expr) {

  if (xpathdebug) {

    Log.write('XPath parse ' + expr);

  }

  xpathParseInit();

  var cached = xpathCacheLookup(expr);

  if (cached) {

    if (xpathdebug) {

      Log.write(' ... cached');

    }

    return cached;

  }

  // Optimize for a few common cases: simple attribute node tests

  // (@id), simple element node tests (page), variable references

  // ($address), numbers (4), multi-step path expressions where each

  // step is a plain element node test

  // (page/overlay/locations/location).

  if (expr.match(/^(\$|@)?\w+$/i)) {

    var ret = makeSimpleExpr(expr);

    xpathParseCache[expr] = ret;

    if (xpathdebug) {

      Log.write(' ... simple');

    }

    return ret;

  }

  if (expr.match(/^\w+(\/\w+)*$/i)) {

    var ret = makeSimpleExpr2(expr);

    xpathParseCache[expr] = ret;

    if (xpathdebug) {

      Log.write(' ... simple 2');

    }

    return ret;

  }

  var cachekey = expr; // expr is modified during parse

  if (xpathdebug) {

    Timer.start('XPath parse', cachekey);

  }

  var stack = [];

  var ahead = null;

  var previous = null;

  var done = false;

  var parse_count = 0;

  var lexer_count = 0;

  var reduce_count = 0;

  while (!done) {

    parse_count++;

    expr = expr.replace(/^\s*/, '');

    previous = ahead;

    ahead = null;

    var rule = null;

    var match = '';

    for (var i = 0; i < xpathTokenRules.length; ++i) {

      var result = xpathTokenRules[i].re.exec(expr);

      lexer_count++;

      if (result && result.length > 0 && result[0].length > match.length) {

        rule = xpathTokenRules[i];

        match = result[0];

        break;

      }

    }

    // Special case: allow operator keywords to be element and

    // variable names.

    // NOTE(mesch): The parser resolves conflicts by looking ahead,

    // and this is the only case where we look back to

    // disambiguate. So this is indeed something different, and

    // looking back is usually done in the lexer (via states in the

    // general case, called "start conditions" in flex(1)). Also,the

    // conflict resolution in the parser is not as robust as it could

    // be, so I'd like to keep as much off the parser as possible (all

    // these precedence values should be computed from the grammar

    // rules and possibly associativity declarations, as in bison(1),

    // and not explicitly set.

    if (rule &&

        (rule == TOK_DIV ||

         rule == TOK_MOD ||

         rule == TOK_AND ||

         rule == TOK_OR) &&

        (!previous ||

         previous.tag == TOK_AT ||

         previous.tag == TOK_DSLASH ||

         previous.tag == TOK_SLASH ||

         previous.tag == TOK_AXIS ||

         previous.tag == TOK_DOLLAR)) {

      rule = TOK_QNAME;

    }

    if (rule) {

      expr = expr.substr(match.length);

      if (xpathdebug) {

        Log.write('token: ' + match + ' -- ' + rule.label);

      }

      ahead = {

        tag: rule,

        match: match,

        prec: rule.prec ?  rule.prec : 0, // || 0 is removed by the compiler

        expr: makeTokenExpr(match)

      };

    } else {

      if (xpathdebug) {

        Log.write('DONE');

      }

      done = true;

    }

    while (xpathReduce(stack, ahead)) {

      reduce_count++;

      if (xpathdebug) {

        Log.write('stack: ' + stackToString(stack));

      }

    }

  }

  if (xpathdebug) {

    Log.write(stackToString(stack));

  }

  if (stack.length != 1) {

    throw 'XPath parse error ' + cachekey + ':\n' + stackToString(stack);

  }

  var result = stack[0].expr;

  xpathParseCache[cachekey] = result;

  if (xpathdebug) {

    Timer.end('XPath parse', cachekey);

  }

  if (xpathdebug) {

    Log.write('XPath parse: ' + parse_count + ' / ' +

              lexer_count + ' / ' + reduce_count);

  }

  return result;

}

var xpathParseCache = {};

function xpathCacheLookup(expr) {

  return xpathParseCache[expr];

}

function xpathReduce(stack, ahead) {

  var cand = null;

  if (stack.length > 0) {

    var top = stack[stack.length-1];

    var ruleset = xpathRules[top.tag.key];

    if (ruleset) {

      for (var i = 0; i < ruleset.length; ++i) {

        var rule = ruleset[i];

        var match = xpathMatchStack(stack, rule[1]);

        if (match.length) {

          cand = {

            tag: rule[0],

            rule: rule,

            match: match

          };

          cand.prec = xpathGrammarPrecedence(cand);

          break;

        }

      }

    }

  }

  var ret;

  if (cand && (!ahead || cand.prec > ahead.prec ||

               (ahead.tag.left && cand.prec >= ahead.prec))) {

    for (var i = 0; i < cand.match.matchlength; ++i) {

      stack.pop();

    }

    if (xpathdebug) {

      Log.write('reduce ' + cand.tag.label + ' ' + cand.prec +

                ' ahead ' + (ahead ? ahead.tag.label + ' ' + ahead.prec +

                             (ahead.tag.left ? ' left' : '')

                             : ' none '));

    }

    var matchexpr = mapExpr(cand.match, function(m) { return m.expr; });

    cand.expr = cand.rule[3].apply(null, matchexpr);

    stack.push(cand);

    ret = true;

  } else {

    if (ahead) {

      if (xpathdebug) {

        Log.write('shift ' + ahead.tag.label + ' ' + ahead.prec +

                  (ahead.tag.left ? ' left' : '') +

                  ' over ' + (cand ? cand.tag.label + ' ' +

                              cand.prec : ' none'));

      }

      stack.push(ahead);

    }

    ret = false;

  }

  return ret;

}

function xpathMatchStack(stack, pattern) {

  // NOTE(mesch): The stack matches for variable cardinality are

  // greedy but don't do backtracking. This would be an issue only

  // with rules of the form A* A, i.e. with an element with variable

  // cardinality followed by the same element. Since that doesn't

  // occur in the grammar at hand, all matches on the stack are

  // unambiguous.

  var S = stack.length;

  var P = pattern.length;

  var p, s;

  var match = [];

  match.matchlength = 0;

  var ds = 0;

  for (p = P - 1, s = S - 1; p >= 0 && s >= 0; --p, s -= ds) {

    ds = 0;

    var qmatch = [];

    if (pattern[p] == Q_MM) {

      p -= 1;

      match.push(qmatch);

      while (s - ds >= 0 && stack[s - ds].tag == pattern[p]) {

        qmatch.push(stack[s - ds]);

        ds += 1;

        match.matchlength += 1;

      }

    } else if (pattern[p] == Q_01) {

      p -= 1;

      match.push(qmatch);

      while (s - ds >= 0 && ds < 2 && stack[s - ds].tag == pattern[p]) {

        qmatch.push(stack[s - ds]);

        ds += 1;

        match.matchlength += 1;

      }

    } else if (pattern[p] == Q_1M) {

      p -= 1;

      match.push(qmatch);

      if (stack[s].tag == pattern[p]) {

        while (s - ds >= 0 && stack[s - ds].tag == pattern[p]) {

          qmatch.push(stack[s - ds]);

          ds += 1;

          match.matchlength += 1;

        }

      } else {

        return [];

      }

    } else if (stack[s].tag == pattern[p]) {

      match.push(stack[s]);

      ds += 1;

      match.matchlength += 1;

    } else {

      return [];

    }

    reverseInplace(qmatch);

    qmatch.expr = mapExpr(qmatch, function(m) { return m.expr; });

  }

  reverseInplace(match);

  if (p == -1) {

    return match;

  } else {

    return [];

  }

}

function xpathTokenPrecedence(tag) {

  return tag.prec || 2;

}

function xpathGrammarPrecedence(frame) {

  var ret = 0;

  if (frame.rule) { /* normal reduce */

    if (frame.rule.length >= 3 && frame.rule[2] >= 0) {

      ret = frame.rule[2];

    } else {

      for (var i = 0; i < frame.rule[1].length; ++i) {

        var p = xpathTokenPrecedence(frame.rule[1][i]);

        ret = Math.max(ret, p);

      }

    }

  } else if (frame.tag) { /* TOKEN match */

    ret = xpathTokenPrecedence(frame.tag);

  } else if (frame.length) { /* Q_ match */

    for (var j = 0; j < frame.length; ++j) {

      var p = xpathGrammarPrecedence(frame[j]);

      ret = Math.max(ret, p);

    }

  }

  return ret;

}

function stackToString(stack) {

  var ret = '';

  for (var i = 0; i < stack.length; ++i) {

    if (ret) {

      ret += '\n';

    }

    ret += stack[i].tag.label;

  }

  return ret;

}

// XPath expression evaluation context. An XPath context consists of a

// DOM node, a list of DOM nodes that contains this node, a number

// that represents the position of the single node in the list, and a

// current set of variable bindings. (See XPath spec.)

//

// The interface of the expression context:

//

//   Constructor -- gets the node, its position, the node set it

//   belongs to, and a parent context as arguments. The parent context

//   is used to implement scoping rules for variables: if a variable

//   is not found in the current context, it is looked for in the

//   parent context, recursively. Except for node, all arguments have

//   default values: default position is 0, default node set is the

//   set that contains only the node, and the default parent is null.

//

//     Notice that position starts at 0 at the outside interface;

//     inside XPath expressions this shows up as position()=1.

//

//   clone() -- creates a new context with the current context as

//   parent. If passed as argument to clone(), the new context has a

//   different node, position, or node set. What is not passed is

//   inherited from the cloned context.

//

//   setVariable(name, expr) -- binds given XPath expression to the

//   name.

//

//   getVariable(name) -- what the name says.

//

//   setNode(node, position) -- sets the context to the new node and

//   its corresponding position. Needed to implement scoping rules for

//   variables in XPath. (A variable is visible to all subsequent

//   siblings, not only to its children.)

function ExprContext(node, position, nodelist, parent) {

  this.node = node;

  this.position = position || 0;

  this.nodelist = nodelist || [ node ];

  this.variables = {};

  this.parent = parent || null;

  this.root = parent ? parent.root : node.ownerDocument;

}

ExprContext.prototype.clone = function(node, position, nodelist) {

  return new

  ExprContext(node || this.node,

              typeof position != 'undefined' ? position : this.position,

              nodelist || this.nodelist, this);

};

ExprContext.prototype.setVariable = function(name, value) {

  this.variables[name] = value;

};

ExprContext.prototype.getVariable = function(name) {

  if (typeof this.variables[name] != 'undefined') {

    return this.variables[name];

  } else if (this.parent) {

    return this.parent.getVariable(name);

  } else {

    return null;

  }

}

ExprContext.prototype.setNode = function(node, position) {

  this.node = node;

  this.position = position;

}

// XPath expression values. They are what XPath expressions evaluate

// to. Strangely, the different value types are not specified in the

// XPath syntax, but only in the semantics, so they don't show up as

// nonterminals in the grammar. Yet, some expressions are required to

// evaluate to particular types, and not every type can be coerced

// into every other type. Although the types of XPath values are

// similar to the types present in JavaScript, the type coercion rules

// are a bit peculiar, so we explicitly model XPath types instead of

// mapping them onto JavaScript types. (See XPath spec.)

//

// The four types are:

//

//   StringValue

//

//   NumberValue

//

//   BooleanValue

//

//   NodeSetValue

//

// The common interface of the value classes consists of methods that

// implement the XPath type coercion rules:

//

//   stringValue() -- returns the value as a JavaScript String,

//

//   numberValue() -- returns the value as a JavaScript Number,

//

//   booleanValue() -- returns the value as a JavaScript Boolean,

//

//   nodeSetValue() -- returns the value as a JavaScript Array of DOM

//   Node objects.

//

function StringValue(value) {

  this.value = value;

  this.type = 'string';

}

StringValue.prototype.stringValue = function() {

  return this.value;

}

StringValue.prototype.booleanValue = function() {

  return this.value.length > 0;

}

StringValue.prototype.numberValue = function() {

  return this.value - 0;

}

StringValue.prototype.nodeSetValue = function() {

  throw this + ' ' + Error().stack;

}

function BooleanValue(value) {

  this.value = value;

  this.type = 'boolean';

}

BooleanValue.prototype.stringValue = function() {

  return '' + this.value;

}

BooleanValue.prototype.booleanValue = function() {

  return this.value;

}

BooleanValue.prototype.numberValue = function() {

  return this.value ? 1 : 0;

}

BooleanValue.prototype.nodeSetValue = function() {

  throw this + ' ' + Error().stack;

}

function NumberValue(value) {

  this.value = value;

  this.type = 'number';

}

NumberValue.prototype.stringValue = function() {

  return '' + this.value;

}

NumberValue.prototype.booleanValue = function() {

  return !!this.value;

}

NumberValue.prototype.numberValue = function() {

  return this.value - 0;

}

NumberValue.prototype.nodeSetValue = function() {

  throw this + ' ' + Error().stack;

}

function NodeSetValue(value) {

  this.value = value;

  this.type = 'node-set';

}

NodeSetValue.prototype.stringValue = function() {

  if (this.value.length == 0) {

    return '';

  } else {

    return xmlValue(this.value[0]);

  }

}

NodeSetValue.prototype.booleanValue = function() {

  return this.value.length > 0;

}

NodeSetValue.prototype.numberValue = function() {

  return this.stringValue() - 0;

}

NodeSetValue.prototype.nodeSetValue = function() {

  return this.value;

};

// XPath expressions. They are used as nodes in the parse tree and

// possess an evaluate() method to compute an XPath value given an XPath

// context. Expressions are returned from the parser. Teh set of

// expression classes closely mirrors the set of non terminal symbols

// in the grammar. Every non trivial nonterminal symbol has a

// corresponding expression class.

//

// The common expression interface consists of the following methods:

//

// evaluate(context) -- evaluates the expression, returns a value.

//

// toString() -- returns the XPath text representation of the

// expression (defined in xsltdebug.js).

//

// parseTree(indent) -- returns a parse tree representation of the

// expression (defined in xsltdebug.js).

function TokenExpr(m) {

  this.value = m;

}

TokenExpr.prototype.evaluate = function() {

  return new StringValue(this.value);

};

function LocationExpr() {

  this.absolute = false;

  this.steps = [];

}

LocationExpr.prototype.appendStep = function(s) {

  this.steps.push(s);

}

LocationExpr.prototype.prependStep = function(s) {

  var steps0 = this.steps;

  this.steps = [ s ];

  for (var i = 0; i < steps0.length; ++i) {

    this.steps.push(steps0[i]);

  }

};

LocationExpr.prototype.evaluate = function(ctx) {

  var start;

  if (this.absolute) {

    start = ctx.root;

  } else {

    start = ctx.node;

  }

  var nodes = [];

  xPathStep(nodes, this.steps, 0, start, ctx);

  return new NodeSetValue(nodes);

};

function xPathStep(nodes, steps, step, input, ctx) {

  var s = steps[step];

  var ctx2 = ctx.clone(input);

  var nodelist = s.evaluate(ctx2).nodeSetValue();

  for (var i = 0; i < nodelist.length; ++i) {

    if (step == steps.length - 1) {

      nodes.push(nodelist[i]);

    } else {

      xPathStep(nodes, steps, step + 1, nodelist[i], ctx);

    }

  }

}

function StepExpr(axis, nodetest, predicate) {

  this.axis = axis;

  this.nodetest = nodetest;

  this.predicate = predicate || [];

}

StepExpr.prototype.appendPredicate = function(p) {

  this.predicate.push(p);

}

StepExpr.prototype.evaluate = function(ctx) {

  var input = ctx.node;

  var nodelist = [];

  // NOTE(mesch): When this was a switch() statement, it didn't work

  // in Safari/2.0. Not sure why though; it resulted in the JavaScript

  // console output "undefined" (without any line number or so).

  if (this.axis ==  xpathAxis.ANCESTOR_OR_SELF) {

    nodelist.push(input);

    for (var n = input.parentNode; n; n = input.parentNode) {

      nodelist.push(n);

    }

  } else if (this.axis == xpathAxis.ANCESTOR) {

    for (var n = input.parentNode; n; n = input.parentNode) {

      nodelist.push(n);

    }

  } else if (this.axis == xpathAxis.ATTRIBUTE) {

    copyArray(nodelist, input.attributes);

  } else if (this.axis == xpathAxis.CHILD) {

    copyArray(nodelist, input.childNodes);

  } else if (this.axis == xpathAxis.DESCENDANT_OR_SELF) {

    nodelist.push(input);

    xpathCollectDescendants(nodelist, input);

  } else if (this.axis == xpathAxis.DESCENDANT) {

    xpathCollectDescendants(nodelist, input);

  } else if (this.axis == xpathAxis.FOLLOWING) {

    for (var n = input.parentNode; n; n = n.parentNode) {

      for (var nn = n.nextSibling; nn; nn = nn.nextSibling) {

        nodelist.push(nn);

        xpathCollectDescendants(nodelist, nn);

      }

    }

  } else if (this.axis == xpathAxis.FOLLOWING_SIBLING) {

    for (var n = input.nextSibling; n; n = input.nextSibling) {

      nodelist.push(n);

    }

  } else if (this.axis == xpathAxis.NAMESPACE) {

    alert('not implemented: axis namespace');

  } else if (this.axis == xpathAxis.PARENT) {

    if (input.parentNode) {

      nodelist.push(input.parentNode);

    }

  } else if (this.axis == xpathAxis.PRECEDING) {

    for (var n = input.parentNode; n; n = n.parentNode) {

      for (var nn = n.previousSibling; nn; nn = nn.previousSibling) {

        nodelist.push(nn);

        xpathCollectDescendantsReverse(nodelist, nn);

      }

    }

  } else if (this.axis == xpathAxis.PRECEDING_SIBLING) {

    for (var n = input.previousSibling; n; n = input.previousSibling) {

      nodelist.push(n);

    }

  } else if (this.axis == xpathAxis.SELF) {

    nodelist.push(input);

  } else {

    throw 'ERROR -- NO SUCH AXIS: ' + this.axis;

  }

  // process node test

  var nodelist0 = nodelist;

  nodelist = [];

  for (var i = 0; i < nodelist0.length; ++i) {

    var n = nodelist0[i];

    if (this.nodetest.evaluate(ctx.clone(n, i, nodelist0)).booleanValue()) {

      nodelist.push(n);

    }

  }

  // process predicates

  for (var i = 0; i < this.predicate.length; ++i) {

    var nodelist0 = nodelist;

    nodelist = [];

    for (var ii = 0; ii < nodelist0.length; ++ii) {

      var n = nodelist0[ii];

      if (this.predicate[i].evaluate(ctx.clone(n, ii, nodelist0)).booleanValue()) {

        nodelist.push(n);

      }

    }

  }

  return new NodeSetValue(nodelist);

};

function NodeTestAny() {

  this.value = new BooleanValue(true);

}

NodeTestAny.prototype.evaluate = function(ctx) {

  return this.value;

};

function NodeTestElement() {}

NodeTestElement.prototype.evaluate = function(ctx) {

  return new BooleanValue(ctx.node.nodeType == DOM_ELEMENT_NODE);

}

function NodeTestText() {}

NodeTestText.prototype.evaluate = function(ctx) {

  return new BooleanValue(ctx.node.nodeType == DOM_TEXT_NODE);

}

function NodeTestComment() {}

NodeTestComment.prototype.evaluate = function(ctx) {

  return new BooleanValue(ctx.node.nodeType == DOM_COMMENT_NODE);

}

function NodeTestPI(target) {

  this.target = target;

}

NodeTestPI.prototype.evaluate = function(ctx) {

  return new

  BooleanValue(ctx.node.nodeType == DOM_PROCESSING_INSTRUCTION_NODE &&

               (!this.target || ctx.node.nodeName == this.target));

}

function NodeTestNC(nsprefix) {

  this.regex = new RegExp("^" + nsprefix + ":");

  this.nsprefix = nsprefix;

}

NodeTestNC.prototype.evaluate = function(ctx) {

  var n = ctx.node;

  return new BooleanValue(this.regex.match(n.nodeName));

}

function NodeTestName(name) {

  this.name = name;

}

NodeTestName.prototype.evaluate = function(ctx) {

  var n = ctx.node;

  // NOTE (Patrick Lightbody): this change allows node selection to be case-insensitive

  return new BooleanValue(n.nodeName.toUpperCase() == this.name.toUpperCase());

}

function PredicateExpr(expr) {

  this.expr = expr;

}

PredicateExpr.prototype.evaluate = function(ctx) {

  var v = this.expr.evaluate(ctx);

  if (v.type == 'number') {

    // NOTE(mesch): Internally, position is represented starting with

    // 0, however in XPath position starts with 1. See functions

    // position() and last().

    return new BooleanValue(ctx.position == v.numberValue() - 1);

  } else {

    return new BooleanValue(v.booleanValue());

  }

};

function FunctionCallExpr(name) {

  this.name = name;

  this.args = [];

}

FunctionCallExpr.prototype.appendArg = function(arg) {

  this.args.push(arg);

};

FunctionCallExpr.prototype.evaluate = function(ctx) {

  var fn = '' + this.name.value;

  var f = this.xpathfunctions[fn];

  if (f) {

    return f.call(this, ctx);

  } else {

    Log.write('XPath NO SUCH FUNCTION ' + fn);

    return new BooleanValue(false);

  }

};

FunctionCallExpr.prototype.xpathfunctions = {

  'last': function(ctx) {

    assert(this.args.length == 0);

    // NOTE(mesch): XPath position starts at 1.

    return new NumberValue(ctx.nodelist.length);

  },

  'position': function(ctx) {

    assert(this.args.length == 0);

    // NOTE(mesch): XPath position starts at 1.

    return new NumberValue(ctx.position + 1);

  },

  'count': function(ctx) {

    assert(this.args.length == 1);

    var v = this.args[0].evaluate(ctx);

    return new NumberValue(v.nodeSetValue().length);

  },

  'id': function(ctx) {

    assert(this.args.length == 1);

    var e = this.args.evaluate(ctx);

    var ret = [];

    var ids;

    if (e.type == 'node-set') {

      ids = [];

      for (var i = 0; i < e.length; ++i) {

        var v = xmlValue(e[i]).split(/\s+/);

        for (var ii = 0; ii < v.length; ++ii) {

          ids.push(v[ii]);

        }

      }

    } else {

      ids = e.split(/\s+/);

    }

    var d = ctx.node.ownerDocument;

    for (var i = 0; i < ids.length; ++i) {

      var n = d.getElementById(ids[i]);

      if (n) {

        ret.push(n);

      }

    }

    return new NodeSetValue(ret);

  },

  'local-name': function(ctx) {

    alert('not implmented yet: XPath function local-name()');

  },

  'namespace-uri': function(ctx) {

    alert('not implmented yet: XPath function namespace-uri()');

  },

  'name': function(ctx) {

    assert(this.args.length == 1 || this.args.length == 0);

    var n;

    if (this.args.length == 0) {

      n = [ ctx.node ];

    } else {

      n = this.args[0].evaluate(ctx).nodeSetValue();

    }

    if (n.length == 0) {

      return new StringValue('');

    } else {

      return new StringValue(n[0].nodeName);

    }

  },

  'string':  function(ctx) {

    assert(this.args.length == 1 || this.args.length == 0);

    if (this.args.length == 0) {

      return new StringValue(new NodeSetValue([ ctx.node ]).stringValue());

    } else {

      return new StringValue(this.args[0].evaluate(ctx).stringValue());

    }

  },

  'concat': function(ctx) {

    var ret = '';

    for (var i = 0; i < this.args.length; ++i) {

      ret += this.args[i].evaluate(ctx).stringValue();

    }

    return new StringValue(ret);

  },

  'starts-with': function(ctx) {

    assert(this.args.length == 2);

    var s0 = this.args[0].evaluate(ctx).stringValue();

    var s1 = this.args[1].evaluate(ctx).stringValue();

    return new BooleanValue(s0.indexOf(s1) == 0);

  },

  'contains': function(ctx) {

    assert(this.args.length == 2);

    var s0 = this.args[0].evaluate(ctx).stringValue();

    var s1 = this.args[1].evaluate(ctx).stringValue();

    return new BooleanValue(s0.indexOf(s1) != -1);

  },

  'substring-before': function(ctx) {

    assert(this.args.length == 2);

    var s0 = this.args[0].evaluate(ctx).stringValue();

    var s1 = this.args[1].evaluate(ctx).stringValue();

    var i = s0.indexOf(s1);

    var ret;

    if (i == -1) {

      ret = '';

    } else {

      ret = s0.substr(0,i);

    }

    return new StringValue(ret);

  },

  'substring-after': function(ctx) {

    assert(this.args.length == 2);

    var s0 = this.args[0].evaluate(ctx).stringValue();

    var s1 = this.args[1].evaluate(ctx).stringValue();

    var i = s0.indexOf(s1);

    var ret;

    if (i == -1) {

      ret = '';

    } else {

      ret = s0.substr(i + s1.length);

    }

    return new StringValue(ret);

  },

  'substring': function(ctx) {

    // NOTE: XPath defines the position of the first character in a

    // string to be 1, in JavaScript this is 0 ([XPATH] Section 4.2).

    assert(this.args.length == 2 || this.args.length == 3);

    var s0 = this.args[0].evaluate(ctx).stringValue();

    var s1 = this.args[1].evaluate(ctx).numberValue();

    var ret;

    if (this.args.length == 2) {

      var i1 = Math.max(0, Math.round(s1) - 1);

      ret = s0.substr(i1);

    } else {

      var s2 = this.args[2].evaluate(ctx).numberValue();

      var i0 = Math.round(s1) - 1;

      var i1 = Math.max(0, i0);

      var i2 = Math.round(s2) - Math.max(0, -i0);

      ret = s0.substr(i1, i2);

    }

    return new StringValue(ret);

  },

  'string-length': function(ctx) {

    var s;

    if (this.args.length > 0) {

      s = this.args[0].evaluate(ctx).stringValue();

    } else {

      s = new NodeSetValue([ ctx.node ]).stringValue();

    }

    return new NumberValue(s.length);

  },

  'normalize-space': function(ctx) {

    var s;

    if (this.args.length > 0) {

      s = this.args[0].evaluate(ctx).stringValue();

    } else {

      s = new NodeSetValue([ ctx.node ]).stringValue();

    }

    s = s.replace(/^\s*/,'').replace(/\s*$/,'').replace(/\s+/g, ' ');

    return new StringValue(s);

  },