using System;
using System.Collections;
using System.Collections.Generic;
using System.IO;
using System.Xml;

namespace System.Xml.Expression
{
  public abstract class Xex
  {
    public struct Name : IEquatable<Name>
    {
      private static NameTable nt = new NameTable ();

      private string name;

      public Name (string str)
	{
	  name = nt.Add (str);
	}

      public static implicit operator Name (string str)
      {
	return new Name (str);
      }

      public static implicit operator string (Name name)
      {
	return name.name;
      }

      public static bool operator== (Name n1, Name n2)
	{
	  return (object) n1.name == (object) n2.name;
	}

      public static bool operator!= (Name n1, Name n2)
	{
	  return (object) n1.name != (object) n2.name;
	}

      public static bool operator== (Name n1, string n2)
	{
	  return (object) n1.name == (object) n2;
	}

      public static bool operator!= (Name n1, string n2)
	{
	  return (object) n1.name != (object) n2;
	}

      public static bool operator== (string n1, Name n2)
	{
	  return (object) n1 == (object) n2.name;
	}

      public static bool operator!= (string n1, Name n2)
	{
	  return (object) n1 != (object) n2.name;
	}

      public bool Equals (Name name)
      {
	return Object.ReferenceEquals (this.name, name.name);
      }

      public override bool Equals (object obj)
      {
	return Object.ReferenceEquals (this.name, obj);
      }

      public override int GetHashCode ()
      {
	return name.GetHashCode ();
      }

      public static NameTable Table { get { return nt; } }

      public override string ToString () { return name; }
    }

    private static Name Nexpr = "expr";

    private static Name Nfuncall = "funcall";
    private static Name Nvariable = "variable";
    private static Name Ninteger = "integer";
    private static Name Nstring = "string";
    private static Name Nboolean = "boolean";
    private static Name Nsymbol = "symbol";
    private static Name Nlist = "list";
    private static Name Nobject = "object";

    private static Name Ndefun = "defun";
    private static Name Nfname = "fname";
    private static Name Nargs = "args";
    private static Name Noptional = "optional";
    private static Name Nrest = "rest";
    private static Name Nbody = "body";

    private static Name Ndefvar = "defvar";
    private static Name Ndescription = "description";
    private static Name Nrange = "range";

    private static Name Nprogn = "progn";

    internal abstract class Function
    {
      public readonly Name name;
      public int min_arg, max_arg;

      public Function (Name name, int min_arg, int max_arg)
      {
	this.name = name;
	this.min_arg = min_arg;
	this.max_arg = max_arg;
      }

      public virtual object Call (Xex[] args, Domain domain)
      {
	object result;

	Console.Write ("calling (" + this);
	foreach (Xex a in args)
	  Console.Write (" " + a);
	Console.Write (") => ");
	result = builtin (args, domain);
	Console.WriteLine (result);
	return result;
      }

      public override string ToString ()
      {
	return str;
      }

      internal class Subroutine : Function
      {
	public Builtin builtin;

	public Subroutine (Builtin builtin, Name name,
			   int min_arg, int max_arg)
	  : base (name, min_arg, max_arg)
	  {
	    this.builtin = builtin;
	  }

	public override object Call (Xex[] args, Domain domain)
	{
	  foreach (Xex a in args)
	    if (a.Eval (domain) == null)
	      throw new Exception (a + ":evaled to null");
	  return base.Call (args, domain);
	}
      }

      internal class SpecialForm : Function
      {
	public Builtin builtin;

	public SpecialForm (Builtin builtin, Name name,
			    int min_arg, int max_arg)
	  : base (name, min_arg, max_arg)
	  {
	    this.builtin = builtin;
	  }

	public override object Call (Xex[] args, Domain domain)
	{
	}
      }

      internal class Lambda : Function
      {
	internal Xex[] args;
	internal Xex[] body;

	public Lambda (Name name, int min_arg, int max_arg)
	  : base (name, min_arg, max_arg)
	  {
	  }

	public void SetArgs (XmlNode node, int nargs, Domain domain)
	{
	  args = new Xex[nargs];
	  node = node.FirstChild;
	  for (int i = 0; i < nargs; node = node.NextSibling)
	    if (node.Name != Noptional && node.Name != Nrest)
	      args[i++] = New (node, domain);
	}

	public void SetBody (XmlNode node, Domain domain)
	{
	  XmlNodeList nlist = node.ChildNodes;

	  body = new Xex[nlist.Count];
	  for (int i = 0; i < nlist.Count; i++)
	    body[i] = New (nlist[i], domain);
	}

	public void Setup (XmlNode node, Domain domain)
	{
	  lambda = new Lambda ();
	  node = node.FirstChild;
	  if (node.Name == Nargs)
	    {
	      SetArgs (node, max_arg, domain);
	      node = node.NextSibling;
	    }
	  if (node.Name == Nbody)
	    SetBody (node, domain);
	}

	public static Name ParseHead (XmlNode node,
				      out int min_arg, out int max_arg)
	{
	  Name name = node.Attributes[Nfname].Value;
	  int nargs = 0, noptions = 0, nrest = 0;
	  XmlNode n;
	    
	  for (n = node.FirstChild; n != null; n = n.NextSibling)
	    {
	      if (n.Name == Noptional || n.Name == Nrest)
		break;
	      nargs++;
	    }
	  if (n != null && n.Name == Noptional)
	    for (n = n.NextSibling; n != null; n = n.NextSibling)
	      {
		if (n.Name == Nrest)
		  break;
		noptions++;
	      }
	  if (n != null && n.Name == Nrest)
	    for (n = n.NextSibling; n != null; n = n.NextSibling)
	      nrest++;
	  min_arg = nargs;
	  max_arg = nargs + noptions + nrest;
	  if (nrest == 1)
	    max_arg = - max_arg;
	  return name;
	}

	public override object Call (Xex[] args, Domain domain)
	{
	  Bindings current = domain.bindings;
	  object result = false;

	  try {
	    int i;
	    for (i = 0; i < min_arg; i++)
	      {
		Xex a = this.args[i];
		bool isdirect = a is Xex.Const;
		Name name = (isdirect
			     ? (Name) a.val : ((Xex.Varref) a).vari.name);
		Variable var = domain.GetVar (name);

		if (isdirect)
		  domain.Bind (var, args[i].val);
		else
		  domain.Bind (var, args[i].Eval (domain));
	      }
	    Console.Write ("calling (" + this);
	    foreach (Xex e in body)
	      result = e.Eval (domain);
	    Console.WriteLine (result);
	  } finally {
	    domain.UnboundTo (current);
	  }
	  return result;
	}

	public override string ToString ()
	{
	  str = "(" + name;
	  foreach (Xex a in args)
	    str += " " + a;
	  str += ")";
	}
      }
    }

    internal abstract class Variable
    {
      public readonly Name name;
      public readonly Name type;
      internal object val;

      public Variable (Name name, Name type, object value)
      {
	if (value != null)
	  Value = value;
	this.name = name;
	this.type = type;
      }

      public object Value
      {
	get { return val; }
	set
	  {
	    if (! ValueP (value))
	      throw new Exception ("Invalid value type: " + value);
	    val = value;
	  }
      }

      public abstract bool ValueP (object value);

      public override string ToString () { return name + "=" + val; }
    }

    internal class VarInt : Variable
    {
      public struct Range
      {
	public int from, to;
      }

      public Range[] ranges;

      public VarInt (Name name, object value) : base (name, Ninteger, value) { }

      public override bool ValueP (object value)
      {
	int i;

	if (! (value is int))
	  return false;
	if (ranges == null)
	  return true;
	i = (int) value;
	foreach (Range r in ranges)
	  if (i >= r.from && i <= r.to)
	    return true;
	return false;
      }
    }

    internal class VarStr : Variable
    {
      public string[] ranges;

      public VarStr (Name name, object value) : base (name, Nstring, value) { }

      public override bool ValueP (object value)
      {
	string str;

	if (! (value is string))
	  return false;
	if (ranges == null)
	  return true;
	str = (string) value;
	foreach (string s in ranges)
	  if (s == str)
	    return true;
	return false;
      }
    }

    internal class VarBool : Variable
    {
      public VarBool (Name name, object value)
	: base (name, Nboolean, value) { }

      public override bool ValueP (object value)
      {
	if (! (value is bool))
	  return false;
	return true;
      }
    }

    internal class VarMisc : Variable
    {
      public VarMisc (Name name, object value) : base (name, Nobject, value) { }

      public override bool ValueP (object value)
      {
	return true;
      }
    }

    internal class Bindings
    {
      private Variable vari;
      private object old_value;
      private Bindings next;
	
      private Bindings (Variable vari, object value)
      {
	this.vari = vari;
	old_value = value;
      }
	
      public static Bindings Bind (Bindings bindings,
				   Variable vari, object value)
      {
	Bindings b = new Bindings (vari, vari.val);

	b.vari.Value = value;
	b.next = bindings;
	return b;
      }

      internal Bindings UnboundTo (Bindings boundary)
      {
	for (Bindings b = this; b != boundary; b = b.next)
	  vari.val = b.old_value;
	return boundary;
      }

      public override string ToString ()
      {
	string str = "(bindings";
	for (Bindings b = this; b != null; b = b.next)
	  str += " " + vari;
	return str + ")";
      }
    }

#if false
    internal class ThrowException : Exception
    {
      Name tag;
      public object value;

      public ThrowException (Name tag, object value) : base ()
	{
	  this.tag = tag;
	  this.value = value;
	}
    }
#endif

    public class Domain
    {
      public object context;

      internal Dictionary<Name, Function> functions;
      internal Dictionary<Name, Variable> variables;
      internal Bindings bindings;

      internal Domain ()
      {
	functions = new Dictionary<Name, Function> ();
	variables = new Dictionary<Name, Variable> ();
      }

      public Domain (object context) : this (basic, context)
      {
      }

      public Domain (Domain parent, object context)
      {
	functions = new Dictionary<Name, Function> (parent.functions);
	variables = new Dictionary<Name, Variable> (parent.variables);
	this.context = context;
      }

      internal void Bind (Variable vari, object value)
      {
	bindings = Bindings.Bind (bindings, vari, value);
	Console.WriteLine ("binding " + vari);
      }

      internal void UnboundTo (Bindings boundary)
      {
	if (bindings != null)
	  bindings = bindings.UnboundTo (boundary);
      }

      public void DefSubr (Builtin builtin, string str,
			   int min_arg, int max_arg)
      {
	Name name = str;
	functions[name]
	  = new Function.Subroutine (builtin, name, min_arg, max_arg);
      }

      public void DefSpecial (Builtin builtin, string str,
			      int min_arg, int max_arg)
      {
	Name name = str;
	functions[name]
	  = new Function.SpecialForm (builtin, name, min_arg, max_arg);
      }

      internal Function.Lambda RegisterFunction (XmlNode node)
      {
	int min_arg, max_arg;
	Name name = Function.ParseHead (node, out min_arg, out max_arg);
	Lambda lambda = new Function.Lambda (name, min_arg, max_arg);

	functions[name] = lambda;	
	return func;
      }

      internal Function Defun (XmlNode node)
      {
	Name name = node.Attributes[Nfname].Value;
	Function.Lambda lambda;

	if (! functions.TryGetValue (name, out lambda))
	  lambda = RegisterFunction (node);
	lambda.Setup (node, this);
	return func;
      }

      public void Defvar (Name name, XmlNode node)
      {
	Variable vari;

	if (node.Name == Ndescription)
	  node = node.NextSibling;
	if (node != null)
	  {
	    Name type = node.Name;
	    string val = node.Value;
	    XmlNodeList range_list = null;
	    int nranges = 0;

	    node = node.NextSibling;
	    if (node != null)
	      {
		range_list = node.ChildNodes;
		nranges = range_list.Count;
	      }

	    if (type == Ninteger)
	      {
		VarInt vi = new VarInt (name, parse_integer (val));
		if (range_list != null)
		  {
		    vi.ranges = new VarInt.Range[nranges];

		    for (int i = 0; i < nranges; i++)
		      {
			XmlNode n = range_list[i];

			if (n.Name == Nrange)
			  {
			    vi.ranges[i].from =
			      parse_integer (n.FirstChild.Value);
			    vi.ranges[i].to =
			      parse_integer (n.LastChild.Value);
			  }
			else
			  {
			    int num = parse_integer (n.Value);
			    vi.ranges[i].from = vi.ranges[i].to = num;
			  }
		      }
		  }
		vari = vi;
	      }
	    else if (type == Nstring)
	      {
		VarStr vs = new VarStr (name, val);
		if (range_list != null)
		  vs.ranges = new string[nranges];
		for (int i = 0; i < nranges; i++)
		  vs.ranges[i] = range_list[i].Value;
		vari = vs;
	      }
	    else if (type == Nboolean)
	      {
		vari = new VarBool (name, val == "true");
	      }
	    else
	      throw new Exception ("Unknown type: " + type);
	  }
	else
	  vari = new VarMisc (name, null);
	variables[name] = vari;
      }

      internal Function GetFunc (Name name)
      {
	Function func;

	if (! functions.TryGetValue (name, out func))
	  throw new Exception ("Unknown function: " + name);
	return func;
      }

      public bool CopyFunc (Domain domain, Name name)
      {
	Function func = GetFunc (name);

	domain.functions[name] = func;
	return true;
      }

      public void CopyFunc (Domain domain)
      {
	foreach (KeyValuePair<Name, Function> kv in functions)
	  domain.functions[kv.Key] = kv.Value;
      }

      internal Variable GetVar (Name name)
      {
	Variable vari;

	if (! variables.TryGetValue (name, out vari))
	  variables[name] = vari = new VarMisc (name, null);
	return vari;
      }

      internal Variable GetVar (Xex e)
      {
	if (! (e.val is Name))
	  throw new Exception ("Not a symbol" + e.val);
	return GetVar ((Name) e.val);
      }

      public override string ToString ()
      {
	string str = "<(functions";
	foreach (KeyValuePair<Name, Function> kv in functions)
	  str += " " + kv.Key;
	str += ") (variabls";
	foreach (KeyValuePair<Name, Variable> kv in variables)
	  str += " " + kv.Key;
	str += ")";
	if (bindings != null)
	  str += " " + bindings;
	if (context != null)
	  str += " (" + context + ")";
	str += ">";
	return str;
      }
    }

    public delegate object Builtin (Xex[] args, Domain domain);

    private static Domain basic = new Domain ();

    internal static Function Fprogn;

    static Xex ()
    {
      basic.DefSubr (set_value, "set", 2, 2);
      basic.DefSubr (set_value, "=", 2, 2);
      basic.DefSubr (and, "and", 1, -1);
      basic.DefSubr (and, "&&", 1, -1);
      basic.DefSubr (or, "or", 1, -1);
      basic.DefSubr (or, "||", 1, -1);
      basic.DefSubr (not, "not", 1, 1);
      basic.DefSubr (not, "!", 1, 1);
      basic.DefSubr (add, "add", 2, -1);
      basic.DefSubr (add, "+", 2, -1);
      basic.DefSubr (mul, "mul", 2, -1);
      basic.DefSubr (mul, "*", 2, -1);
      basic.DefSubr (sub, "sub", 1, -1);
      basic.DefSubr (sub, "-", 1, -1);
      basic.DefSubr (div, "div", 2, -1);
      basic.DefSubr (div, "/", 2, -1);
      basic.DefSubr (mod, "mod", 2, 2);
      basic.DefSubr (mod, "%", 2, 2);
      basic.DefSubr (logior, "logior", 2, -1);
      basic.DefSubr (logior, "|", 2, -1);
      basic.DefSubr (logand, "logand", 2, -1);
      basic.DefSubr (logand, "&", 2, -1);
      basic.DefSubr (add_set, "add-set", 2, -1);
      basic.DefSubr (add_set, "+=", 2, -1);
      basic.DefSubr (mul_set, "mul-set", 2, -1);
      basic.DefSubr (mul_set, "*=", 2, -1);
      basic.DefSubr (sub_set, "sub-set", 2, -1);
      basic.DefSubr (sub_set, "-=", 2, -1);
      basic.DefSubr (div_set, "div-set", 2, -1);
      basic.DefSubr (div_set, "/=", 2, -1);
      basic.DefSubr (mod_set, "mod-set", 2, 2);
      basic.DefSubr (mod_set, "%=", 2, 2);
      basic.DefSubr (logior_set, "logior-set", 2, -1);
      basic.DefSubr (logior_set, "|=", 2, -1);
      basic.DefSubr (logand_set, "logand-set", 2, -1);
      basic.DefSubr (logand_set, "&=", 2, -1);
      basic.DefSubr (lsh, "lsh", 2, 2);
      basic.DefSubr (lsh, "<<", 2, 2);
      basic.DefSubr (rsh, "rsh", 2, 2);
      basic.DefSubr (rsh, ">>", 2, 2);
      basic.DefSubr (lsh_set, "lsh-set", 2, 2);
      basic.DefSubr (lsh_set, "<<=", 2, 2);
      basic.DefSubr (rsh_set, "rsh-set", 2, 2);
      basic.DefSubr (rsh_set, ">>=", 2, 2);
      basic.DefSubr (eq, "eq", 2, -1);
      basic.DefSubr (eq, "==", 2, -1);
      basic.DefSubr (noteq, "noteq", 2, 2);
      basic.DefSubr (noteq, "!=", 2, 2);
      basic.DefSubr (less_than, "lt", 2, -1);
      basic.DefSubr (less_than, "<", 2, -1);
      basic.DefSubr (less_eq, "le", 2, -1);
      basic.DefSubr (less_eq, "<=", 2, -1);
      basic.DefSubr (greater_than, "gt", 2, -1);
      basic.DefSubr (greater_than, ">", 2, -1);
      basic.DefSubr (greater_eq, "ge", 2, -1);
      basic.DefSubr (greater_eq, ">=", 2, -1);
      basic.DefSubr (eval_clause, "eval", 1, 1);
      basic.DefSpecial (progn_clause, "progn", 0, -1);
      basic.DefSpecial (progn_clause, "expr", 0, -1);
      basic.DefSpecial (if_clause, "if", 2, -1);
      basic.DefSpecial (when_clause, "when", 1, -1);
      basic.DefSpecial (while_clause, "while", 1, -1);

      Fprogn = basic.GetFunc (Nprogn);
    }

    private static bool is_true (object val)
    {
      return (val is bool ? (bool) val
	      : val is int ? (int) val == 0
	      : true);
    }

    private static object set_value (Xex[] args, Domain domain)
    {
      Variable vari = domain.GetVar (args[0]);

      vari.Value = args[1].val;
      return vari.val;
    }

    private static object and (Xex[] args, Domain domain)
    {
      foreach (Xex arg in args)
	if (! is_true (arg.val))
	  return false;
      return true;
    }

    private static object or (Xex[] args, Domain domain)
    {
      foreach (Xex arg in args)
	if (is_true (arg.val))
	  return true;
      return false;
    }

    private static object not (Xex[] args, Domain domain)
    {
      return ! is_true (args[0].val);
    }

    private static object add (Xex[] args, Domain domain)
    {
      int n = 0;
      foreach (Xex e in args)
	n += (int) e.val;
      return n;
    }

    private static object mul (Xex[] args, Domain domain)
    {
      int n = 1;
      foreach (Xex e in args)
	n *= (int) e.val;
      return n;
    }

    private static object sub (Xex[] args, Domain domain)
    {
      int n = (int) args[0].val;
      if (args.Length == 1)
	return - n;
      for (int i = 1; i < args.Length; i++)
	n -= (int) args[i].val;
      return n;
    }

    private static object div (Xex[] args, Domain domain)
    {
      int n = (int) args[0].val;
      for (int i = 1; i < args.Length; i++)
	n /= (int) args[i].val;
      return n;
    }

    private static object mod (Xex[] args, Domain domain)
    {
      return ((int) args[0].val % (int) args[1].val);
    }

    private static object logior (Xex[] args, Domain domain)
    {
      int n = 0;
      foreach (Xex e in args)
	n |= (int) e.val;
      return n;
    }

    private static object logand (Xex[] args, Domain domain)
    {
      int n = (int) args[0].val;
      for (int i = 1; i < args.Length; i++)
	n &= (int) args[i].val;
      return n;
    }

    private static object add_set (Xex[] args, Domain domain)
    {
      Variable vari = domain.GetVar (args[0]);
      int n = (int) vari.val;

      for (int i = 1; i < args.Length; i++)
	n += (int) args[i].val;
      vari.val = n;
      return n;
    }

    private static object mul_set (Xex[] args, Domain domain)
    {
      Variable vari = domain.GetVar (args[0]);
      int n = (int) vari.val;

      for (int i = 1; i < args.Length; i++)
	n *= (int) args[i].val;
      vari.val = n;
      return n;
    }

    private static object sub_set (Xex[] args, Domain domain)
    {
      Variable vari = domain.GetVar (args[0]);
      int n = (int) vari.val;

      for (int i = 1; i < args.Length; i++)
	n -= (int) args[i].val;
      vari.val = n;
      return n;
    }

    private static object div_set (Xex[] args, Domain domain)
    {
      Variable vari = domain.GetVar (args[0]);
      int n = (int) vari.val;

      for (int i = 1; i < args.Length; i++)
	n /= (int) args[i].val;
      vari.val = n;
      return n;
    }

    private static object mod_set (Xex[] args, Domain domain)
    {
      Variable vari = domain.GetVar (args[0]);
      int n = (int) vari.val;

      for (int i = 1; i < args.Length; i++)
	n %= (int) args[i].val;
      vari.val = n;
      return n;
    }

    private static object logior_set (Xex[] args, Domain domain)
    {
      Variable vari = domain.GetVar (args[0]);
      int n = (int) vari.val;

      for (int i = 1; i < args.Length; i++)
	n |= (int) args[i].val;
      vari.val = n;
      return n;
    }

    private static object logand_set (Xex[] args, Domain domain)
    {
      Variable vari = domain.GetVar (args[0]);
      int n = (int) vari.val;

      for (int i = 1; i < args.Length; i++)
	n &= (int) args[i].val;
      vari.val = n;
      return n;
    }

    private static object lsh (Xex[] args, Domain domain)
    {
      return (int) args[0].val << (int) args[1].val;
    }

    private static object lsh_set (Xex[] args, Domain domain)
    {
      Variable vari = domain.GetVar (args[0]);
      int n = (int) vari.val;

      n <<= (int) args[1].val;
      vari.val = n;
      return n;
    }

    private static object rsh (Xex[] args, Domain domain)
    {
      return (int) args[0].val >> (int) args[1].val;
    }

    private static object rsh_set (Xex[] args, Domain domain)
    {
      Variable vari = domain.GetVar (args[0]);
      int n = (int) vari.val;

      n >>= (int) args[1].val;
      vari.val = n;
      return n;
    }

    private static object eq (Xex[] args, Domain domain)
    {
      int n = (int) args[0].val;

      for (int i = 1; i < args.Length; i++)
	if (n != (int) args[i].val)
	  return false;
      return true;
    }

    private static object noteq (Xex[] args, Domain domain)
    {
      return ((int) args[0].val != (int) args[1].val);
    }

    private static object less_than (Xex[] args, Domain domain)
    {
      int n = (int) args[0].val;

      for (int i = 1; i < args.Length; i++)
	{
	  int n1 = (int) args[i].val;
	  if (n >= n1)
	    return false;
	  n = n1;
	}
      return true;
    }

    private static object less_eq (Xex[] args, Domain domain)
    {
      int n = (int) args[0].val;
      for (int i = 1; i < args.Length; i++)
	{
	  int n1 = (int) args[i].val;
	  if (n > n1)
	    return false;
	  n = n1;
	}
      return true;
    }

    private static object greater_than (Xex[] args, Domain domain)
    {
      int n = (int) args[0].val;
      for (int i = 1; i < args.Length; i++)
	{
	  int n1 = (int) args[i].val;
	  if (n <= n1)
	    return false;
	  n = n1;
	}
      return true;
    }

    private static object greater_eq (Xex[] args, Domain domain)
    {
      int n = (int) args[0].val;
      for (int i = 1; i < args.Length; i++)
	{
	  int n1 = (int) args[i].val;
	  if (n < n1)
	    return false;
	  n = n1;
	}
      return true;
    }

    private static object eval_clause (Xex[] args, Domain domain)
    {
      return args[0].Eval (domain);
    }

    private static object progn_clause (Xex[] args, Domain domain)
    {
      object result = true;

      foreach (Xex e in args)
	result = e.Eval (domain);
      return result;
    }

    private static object if_clause (Xex[] args, Domain domain)
    {
      object result;

      if (is_true (args[0].Eval (domain)))
	result = args[1].Eval (domain);
      else
	{
	  result = false;
	  for (int i = 2; i < args.Length; i++)
	    result = args[i].Eval (domain);
	}
      return result;
    }

    private static object when_clause (Xex[] args, Domain domain)
    {
      if (! is_true (args[0].Eval (domain)))
	return false;

      object result = true;
      for (int i = 1; i < args.Length; i++)
	result = args[i].Eval (domain);
      return result;
    }

    private static object while_clause (Xex[] args, Domain domain)
    {
      while (is_true (args[0].Eval (domain)))
	for (int i = 1; i < args.Length; i++)
	  args[i].Eval (domain);
      return false;
    }

    // FUNCALL: function != null
    // VARREF:  function == null, args[0] = DIRECT-SYMBOL
    // DIRECT:  function == null, args == null

    private object val;
    public abstract object Eval (Domain domain);
    public object Val { get { return val; } }

    private class Funcall : Xex
    {
      internal Function func;
      internal Xex[] args;

      public Funcall (Function func, Xex[] args)
	{
	  this.func = func;
	  this.args = args;
	}

      public override object Eval (Domain domain)
      {
	val = func.Call (args, domain);
	return val;
      }

      public override string ToString ()
      {
	string str = "(" + func.name;
	if (args != null)
	  foreach (Xex e in args)
	    str += " " + e.ToString ();
	return (str + ")");
      }
    }

    private class Varref : Xex
    {
      internal Variable vari;

      public Varref (Variable vari)
	{
	  this.vari = vari;
	}

      public override object Eval (Domain domain)
      {
	val = vari.val;
	return val;
      }

      public override string ToString ()
      {
	return "$" + vari.name + "/" + vari.val;
      }
    }

    private class Const : Xex
    {
      public Const (object val)
	{
	  this.val = val;
	}

      public override object Eval (Domain domain)
      {
	return val;
      }

      public override string ToString ()
      {
	return val.ToString ();
      }
    }

    internal static int parse_integer (string str)
    {
      int len = str.Length;
      bool negative = false;

      if (len <= 1)
	return (len == 0 ? 0 : str[0] - '0');

      int c = str[0];
      int i;

      if (c == '0' && str[1] == 'x')
	{
	  i = 0;
	  for (int idx = 2; idx < len; idx++)
	    {
	      c = str[idx];
	      if (c < '0')
		break;
	      else if (c <= '9')
		i = i * 16 + (c - '0');
	      else if (c < 'A')
		break;
	      else if (c <= 'F')
		i = i * 16 + (c - 'A');
	      else if (c < 'a')
		break;
	      else if (c <= 'f')
		i = i * 16 + (c - 'a');
	      else
		break;
	    }
	  return i;
	}
      if (c == '-')
	negative = true;
      i = c - '0';
      for (int idx = 1; idx < len; idx++)
	{
	  c = str[idx];
	  if (c < '0' || c > '9')
	    break;
	  i = i * 10 + (c - '0');
	}
      return negative ? - i : i;
    }

    private static int pre_parse (XmlNodeList nlist, Domain domain)
    {
      int len = 0;
      foreach (XmlNode node in nlist)
	{
	  if (node.Name == Ndefun)
	    domain.RegisterFunction (node);
	  else if (node.Name == Ndefvar)
	    domain.Defvar ((Name) node.Attributes[0].Value, node.FirstChild);
	  else
	    len++;
	}
      return len;
    }

    private static void post_parse (XmlNodeList nlist, Xex[] args,
				    Domain domain)
    {
      for (int i = 0, j = 0; i < nlist.Count; i++)
	{
	  XmlNode node = nlist[i];

	  if (node.Name == Ndefun)
	    domain.Defun (node);
	  else if (node.Name != Ndefvar)
	    args[j++] = New (node, domain);
	}
    }

    public static Xex New (string url, Domain domain)
    {
      XmlDocument doc = new XmlDocument (Name.Table);
      XmlNode node;

      using (XmlTextReader reader = new XmlTextReader (url, Name.Table))
	{
	  do {
	    reader.Read ();
	  } while (reader.NodeType != XmlNodeType.None
		   && (reader.NodeType != XmlNodeType.Element
		       || Nexpr != reader.Name));
	  if (reader.NodeType == XmlNodeType.None)
	    throw new Exception ("Node <expr> not found");
	  node = doc.ReadNode (reader);
	}

      return New (node, domain);
    }

    // EXPR = SYMBOL | MTEXT | INTEGER | FUNCALL | PROGN
    // FUNCALL = '(' SYMBOL EXPR* ')'
    // PROGN = '(' EXPR * ')'
    public static Xex New (XmlNode node, Domain domain)
    {
      Name name = node.Name;
      Xex xex;
      if (name == Nvariable)
	{
	  Variable vari = domain.GetVar ((Name) node.Attributes[0].Value);
	  xex = new Xex.Varref (vari);
	}
      else if (name == Ninteger)
	xex = new Xex.Const (parse_integer (node.InnerText));
      else if (name == Nstring)
	xex = new Xex.Const (node.InnerText);
      else if (name == Nsymbol)
	xex = new Xex.Const ((Name) node.InnerText);
      else if (name == Nboolean)
	xex = new Xex.Const (node.InnerText == "true");
      else if (name == Nlist)
	{
	  List<Xex> list = new List<Xex> ();
	  for (XmlNode n = node.FirstChild; n != null; n = n.NextSibling)
	    list.Add (New (n, domain));
	  xex = new Xex.Const (list);
	}
      else
	{
	  if (name == Nfuncall)
	    name = node.Attributes[0].Value;
	  Function func = domain.GetFunc (name);

	  XmlNodeList nlist = node.ChildNodes;
	  int nargs = nlist.Count;

	  if (nargs < func.min_arg
	      || (func.max_arg >= 0 && nargs > func.max_arg))
	    throw new Exception ("Invalid number of arguments to: "
				 + name + " " + nargs);
	  nargs =  pre_parse (nlist, domain);
	  Xex[] args = new Xex[nargs];
	  post_parse (nlist, args, domain);
	  xex = new Xex.Funcall (func, args);
	}
      return xex;
    }
  }
}