function OutputStream(options) {

    options = defaults(options, {
        indent_start  : 0,
        indent_level  : 4,
        quote_keys    : false,
        space_colon   : false,
        ascii_only    : false,
        inline_script : false,
        width         : 80,
        beautify      : true,
        scope_style   : "negate"
    });

    var indentation = 0;
    var current_col = 0;
    var current_line = 0;
    var current_pos = 0;
    var OUTPUT = "";

    function to_ascii(str) {
        return str.replace(/[\u0080-\uffff]/g, function(ch) {
            var code = ch.charCodeAt(0).toString(16);
            while (code.length < 4) code = "0" + code;
            return "\\u" + code;
        });
    };

    function make_string(str) {
        var dq = 0, sq = 0;
        str = str.replace(/[\\\b\f\n\r\t\x22\x27\u2028\u2029\0]/g, function(s){
            switch (s) {
              case "\\": return "\\\\";
              case "\b": return "\\b";
              case "\f": return "\\f";
              case "\n": return "\\n";
              case "\r": return "\\r";
              case "\u2028": return "\\u2028";
              case "\u2029": return "\\u2029";
              case '"': ++dq; return '"';
              case "'": ++sq; return "'";
              case "\0": return "\\0";
            }
            return s;
        });
        if (options.ascii_only) str = to_ascii(str);
        if (dq > sq) return "'" + str.replace(/\x27/g, "\\'") + "'";
        else return '"' + str.replace(/\x22/g, '\\"') + '"';
    };

    function encode_string(str) {
        var ret = make_string(str);
        if (options.inline_script)
            ret = ret.replace(/<\x2fscript([>\/\t\n\f\r ])/gi, "<\\/script$1");
        return ret;
    };

    function make_name(name) {
        name = name.toString();
        if (options.ascii_only)
            name = to_ascii(name);
        return name;
    };

    function make_indent(back) {
        return repeat_string(" ", options.indent_start + indentation - back * options.indent_level);
    };

    /* -----[ beautification/minification ]----- */

    var might_need_space = false;
    var might_need_semicolon = false;
    var last = null;

    function last_char() {
        return last.charAt(last.length - 1);
    };

    function print(str) {
        str = String(str);
        var ch = str.charAt(0);
        if (might_need_semicolon) {
            if (";}".indexOf(ch) < 0 && !/[;]$/.test(last)) {
                OUTPUT += ";";
                current_col++;
                current_pos++;
                if (!options.beautify)
                    might_need_space = false;
            }
            might_need_semicolon = false;
        }
        if (might_need_space) {
            if ((is_identifier_char(last_char())
                 && (is_identifier_char(ch) || ch == "\\"))
                ||
                (/[\+\-]$/.test(last) && /^[\+\-]/.test(str)))
            {
                OUTPUT += " ";
                current_col++;
                current_pos++;
            }
            might_need_space = false;
        }
        var a = str.split(/\r?\n/), n = a.length;
        current_line += n;
        if (n == 1) {
            current_col = a[n - 1].length;
        } else {
            current_col += a[n - 1].length;
        }
        current_pos += str.length;
        last = str;
        OUTPUT += str;
    };

    var space = options.beautify ? function() {
        print(" ");
    } : function() {
        might_need_space = true;
    };

    var indent = options.beautify ? function(half) {
        if (options.beautify) {
            print(make_indent(half ? 0.5 : 0));
        }
    } : noop;

    var with_indent = options.beautify ? function(col, cont) {
        if (col === true) col = next_indent();
        var save_indentation = indentation;
        indentation = col;
        var ret = cont();
        indentation = save_indentation;
        return ret;
    } : function(col, cont) { return cont() };

    var newline = options.beautify ? function() {
        print("\n");
    } : noop;

    var semicolon = options.beautify ? function() {
        print(";");
    } : function() {
        might_need_semicolon = true;
    };

    function next_indent() {
        return indentation + options.indent_level;
    };

    function with_block(cont) {
        var ret;
        print("{");
        newline();
        with_indent(next_indent(), function(){
            ret = cont();
        });
        indent();
        print("}");
        return ret;
    };

    function with_parens(cont) {
        print("(");
        //XXX: still nice to have that for argument lists
        //var ret = with_indent(current_col, cont);
        var ret = cont();
        print(")");
        return ret;
    };

    function with_square(cont) {
        print("[");
        //var ret = with_indent(current_col, cont);
        var ret = cont();
        print("]");
        return ret;
    };

    function comma() {
        print(",");
        space();
    };

    function colon() {
        print(":");
        space();
    };

    var stack = [];
    return {
        get          : function() { return OUTPUT },
        indent       : indent,
        newline      : newline,
        print        : print,
        space        : space,
        comma        : comma,
        colon        : colon,
        last         : function() { return last },
        semicolon    : semicolon,
        print_name   : function(name) { print(make_name(name)) },
        print_string : function(str) { print(encode_string(str)) },
        with_indent  : with_indent,
        with_block   : with_block,
        with_parens  : with_parens,
        with_square  : with_square,
        options      : function(opt) { return options[opt] },
        line         : function() { return current_line },
        col          : function() { return current_col },
        pos          : function() { return current_pos },
        push_node    : function(node) { stack.push(node) },
        pop_node     : function() { return stack.pop() },
        stack        : function() { return stack },
        parent       : function(n) {
            return stack[stack.length - 2 - (n || 0)];
        }
    };

};

/* -----[ code generators ]----- */

(function(){

    /* -----[ utils ]----- */

    function DEFPRINT(nodetype, generator) {
        nodetype.DEFMETHOD("print", function(stream){
            var self = this;
            stream.push_node(self);
            //stream.print("«" + self.TYPE + ":" + self.start.line + ":" + self.start.col + "»");
            if (self.needs_parens(stream)) {
                stream.with_parens(function(){
                    generator(self, stream);
                });
            } else {
                generator(self, stream);
            }
            stream.pop_node();
        });
    };

    function PARENS(nodetype, func) {
        nodetype.DEFMETHOD("needs_parens", func);
    };

    /* -----[ PARENTHESES ]----- */

    PARENS(AST_Node, function(){
        return false;
    });

    // a function expression needs parens around it when it's provably
    // the first token to appear in a statement.
    PARENS(AST_Function, function(output){
        return first_in_statement(output);
    });

    // same goes for an object literal, because otherwise it would be
    // interpreted as a block of code.
    PARENS(AST_Object, function(output){
        return first_in_statement(output);
    });

    PARENS(AST_Seq, function(output){
        var p = output.parent();
        return p instanceof AST_Call             // (foo, bar)() —or— foo(1, (2, 3), 4)
            || p instanceof AST_Binary           // 1 + (2, 3) + 4 → 7
            || p instanceof AST_VarDef           // var a = (1, 2), b = a + a; → b = 4
            || p instanceof AST_Dot              // (1, {foo:2}).foo → 2
            || p instanceof AST_Array            // [ 1, (2, 3), 4 ] → [ 1, 3, 4 ]
            || p instanceof AST_ObjectProperty   // { foo: (1, 2) }.foo → 2
            || p instanceof AST_Conditional      /* (false, true) ? (a = 10, b = 20) : (c = 30)
                                                  * → 20 (side effect, set a = 10 and b = 20) */
        ;
    });

    PARENS(AST_Binary, function(output){
        var p = output.parent();
        // (foo && bar)()
        if (p instanceof AST_Call && p.expression === this)
            return true;
        // typeof (foo && bar)
        if (p instanceof AST_Unary)
            return true;
        // (foo && bar)["prop"], (foo && bar).prop
        if (p instanceof AST_PropAccess && p.expression === this)
            return true;
        // this deals with precedence: 3 * (2 + 1)
        if (p instanceof AST_Binary) {
            var po = p.operator, pp = PRECEDENCE[po];
            var so = this.operator, sp = PRECEDENCE[so];
            if (pp > sp
                || (pp == sp
                    && this === p.right
                    && !(so == po &&
                         (so == "*" ||
                          so == "&&" ||
                          so == "||")))) {
                return true;
            }
        }
        // for (var i = (foo in bar);;); ← perhaps useless, but valid syntax
        if (this.operator == "in") {
            // the “NoIn” stuff :-\
            // UglifyJS 1.3.3 misses this one.
            if ((p instanceof AST_For || p instanceof AST_ForIn) && p.init === this)
                return true;
            if (p instanceof AST_VarDef) {
                var v = output.parent(1), p2 = output.parent(2);
                if ((p2 instanceof AST_For || p2 instanceof AST_ForIn) && p2.init === v)
                    return true;
            }
        }
    });

    PARENS(AST_New, function(output){
        var p = output.parent();
        // (new Date).getTime();
        if (p instanceof AST_Dot && no_constructor_parens(this, output))
            return true;
    });

    function assign_and_conditional_paren_rules(output) {
        var p = output.parent();
        // !(a = false) → true
        if (p instanceof AST_Unary)
            return true;
        // 1 + (a = 2) + 3 → 3, side effect setting a = 2
        if (p instanceof AST_Binary && !(p instanceof AST_Assign))
            return true;
        // (a = func)() —or— new (a = Object)()
        if (p instanceof AST_Call && p.expression === this)
            return true;
        // (a = foo) ? bar : baz
        if (p instanceof AST_Conditional && p.condition === this)
            return true;
        // (a = foo)["prop"] —or— (a = foo).prop
        if (p instanceof AST_PropAccess && p.expression === this)
            return true;
    };

    PARENS(AST_Assign, assign_and_conditional_paren_rules);
    PARENS(AST_Conditional, assign_and_conditional_paren_rules);

    /* -----[ PRINTERS ]----- */

    DEFPRINT(AST_Directive, function(self, output){
        output.print_string(self.value);
    });
    DEFPRINT(AST_Debugger, function(self, output){
        output.print("debugger");
        output.semicolon();
    });

    /* -----[ statements ]----- */

    function display_body(body, is_toplevel, output) {
        var last = body.length - 1;
        body.forEach(function(stmt, i){
            if (!(stmt instanceof AST_EmptyStatement)) {
                output.indent();
                stmt.print(output);
                if (!(i == last && is_toplevel)) {
                    output.newline();
                    if (is_toplevel) output.newline();
                }
            }
        });
    };

    DEFPRINT(AST_Statement, function(self, output){
        self.body.print(output);
        output.semicolon();
    });
    DEFPRINT(AST_Toplevel, function(self, output){
        display_body(self.body, true, output);
    });
    DEFPRINT(AST_LabeledStatement, function(self, output){
        self.label.print(output);
        output.colon();
        self.statement.print(output);
    });
    DEFPRINT(AST_SimpleStatement, function(self, output){
        self.body.print(output);
        output.semicolon();
    });
    DEFPRINT(AST_BlockStatement, function(self, output){
        var body = self.body;
        //if (!(body instanceof Array)) body = [ body ];
        if (body.length > 0) output.with_block(function(){
            display_body(body, false, output);
        });
        else output.print("{}");
    });
    DEFPRINT(AST_EmptyStatement, function(self, output){
        output.semicolon();
    });
    DEFPRINT(AST_Do, function(self, output){
        output.print("do");
        output.space();
        self.body.print(output);
        output.space();
        output.print("while");
        output.space();
        output.with_parens(function(){
            self.condition.print(output);
        });
        output.semicolon();
    });
    DEFPRINT(AST_While, function(self, output){
        output.print("while");
        output.space();
        output.with_parens(function(){
            self.condition.print(output);
        });
        output.space();
        self.body.print(output);
    });
    DEFPRINT(AST_For, function(self, output){
        output.print("for");
        output.space();
        output.with_parens(function(){
            if (self.init) {
                self.init.print(output);
                output.print(";");
                output.space();
            } else {
                output.print(";");
            }
            if (self.condition) {
                self.condition.print(output);
                output.print(";");
                output.space();
            } else {
                output.print(";");
            }
            if (self.step) {
                self.step.print(output);
            }
        });
        output.space();
        self.body.print(output);
    });
    DEFPRINT(AST_ForIn, function(self, output){
        output.print("for");
        output.space();
        output.with_parens(function(){
            if (self.init) {
                self.init.print(output);
            } else {
                self.name.print(output);
            }
            output.space();
            output.print("in");
            output.space();
            self.object.print(output);
        });
        output.space();
        self.body.print(output);
    });
    DEFPRINT(AST_With, function(self, output){
        output.print("with");
        output.space();
        output.with_parens(function(){
            self.expression.print(output);
        });
        output.space();
        self.body.print(output);
    });

    /* -----[ functions ]----- */
    AST_Lambda.DEFMETHOD("_do_print", function(output, nokeyword){
        var self = this;
        if (!nokeyword) {
            output.print("function");
        }
        if (self.name) {
            output.space();
            self.name.print(output);
        }
        output.with_parens(function(){
            self.argnames.forEach(function(arg, i){
                if (i) output.comma();
                arg.print(output);
            });
        });
        output.space();
        self.body.print(output);
    });
    DEFPRINT(AST_Lambda, function(self, output){
        self._do_print(output);
    });

    /* -----[ exits ]----- */
    AST_Exit.DEFMETHOD("_do_print", function(output, kind){
        output.print(kind);
        if (this.value) {
            output.space();
            this.value.print(output);
        }
        output.semicolon();
    });
    DEFPRINT(AST_Return, function(self, output){
        self._do_print(output, "return");
    });
    DEFPRINT(AST_Throw, function(self, output){
        self._do_print(output, "throw");
    });

    /* -----[ loop control ]----- */
    AST_LoopControl.DEFMETHOD("_do_print", function(output, kind){
        output.print(kind);
        if (this.label) {
            output.space();
            this.label.print(output);
        }
        output.semicolon();
    });
    DEFPRINT(AST_Break, function(self, output){
        self._do_print(output, "break");
    });
    DEFPRINT(AST_Continue, function(self, output){
        self._do_print(output, "continue");
    });

    /* -----[ if ]----- */
    function make_then(self, output) {
        // The squeezer replaces "block"-s that contain only a single
        // statement with the statement itself; technically, the AST
        // is correct, but this can create problems when we output an
        // IF having an ELSE clause where the THEN clause ends in an
        // IF *without* an ELSE block (then the outer ELSE would refer
        // to the inner IF).  This function checks for this case and
        // adds the block brackets if needed.
        if (!self.consequent)
            return output.semicolon();
        if (self.consequent instanceof AST_Do) {
            // https://github.com/mishoo/UglifyJS/issues/#issue/57 IE
            // croaks with "syntax error" on code like this: if (foo)
            // do ... while(cond); else ...  we need block brackets
            // around do/while
            make_block(self.consequent, output);
            return;
        }
        var b = self.consequent;
        while (true) {
            if (b instanceof AST_If) {
                if (!b.alternative) {
                    make_block(self.consequent, output);
                    return;
                }
                b = b.alternative;
            }
            else if (b instanceof AST_While ||
                     b instanceof AST_Do ||
                     b instanceof AST_For ||
                     b instanceof AST_ForIn) {
                b = b.body;
            }
            else break;
        }
        self.consequent.print(output);
    };
    DEFPRINT(AST_If, function(self, output){
        output.print("if");
        output.space();
        output.with_parens(function(){
            self.condition.print(output);
        });
        output.space();
        if (self.alternative) {
            make_then(self, output);
            output.space();
            output.print("else");
            output.space();
            self.alternative.print(output);
        } else {
            self.consequent.print(output);
        }
    });

    /* -----[ switch ]----- */
    DEFPRINT(AST_Switch, function(self, output){
        output.print("switch");
        output.space();
        output.with_parens(function(){
            self.expression.print(output);
        });
        output.space();
        self.body.print(output);
    });
    DEFPRINT(AST_SwitchBlock, function(self, output){
        if (self.body.length > 0) output.with_block(function(){
            self.body.forEach(function(stmt, i){
                if (i) output.newline();
                output.indent(true);
                stmt.print(output);
            });
        });
        else output.print("{}");
    });
    AST_SwitchBranch.DEFMETHOD("_do_print_body", function(output){
        if (this.body.length > 0) {
            output.newline();
            this.body.forEach(function(stmt){
                output.indent();
                stmt.print(output);
                output.newline();
            });
        }
    });
    DEFPRINT(AST_Default, function(self, output){
        output.print("default:");
        self._do_print_body(output);
    });
    DEFPRINT(AST_Case, function(self, output){
        output.print("case");
        output.space();
        self.expression.print(output);
        output.print(":");
        self._do_print_body(output);
    });

    /* -----[ exceptions ]----- */
    DEFPRINT(AST_Try, function(self, output){
        output.print("try");
        output.space();
        self.btry.print(output);
        if (self.bcatch) {
            output.space();
            self.bcatch.print(output);
        }
        if (self.bfinally) {
            output.space();
            self.bfinally.print(output);
        }
    });
    DEFPRINT(AST_Catch, function(self, output){
        output.print("catch");
        output.space();
        output.with_parens(function(){
            self.argname.print(output);
        });
        output.space();
        self.body.print(output);
    });
    DEFPRINT(AST_Finally, function(self, output){
        output.print("finally");
        output.space();
        self.body.print(output);
    });

    /* -----[ var/const ]----- */
    AST_Definitions.DEFMETHOD("_do_print", function(output, kind){
        output.print(kind);
        output.space();
        this.definitions.forEach(function(def, i){
            if (i) output.comma();
            def.print(output);
        });
        var p = output.parent();
        var in_for = p instanceof AST_For || p instanceof AST_ForIn;
        var avoid_semicolon = in_for && p.init === this;
        if (!avoid_semicolon)
            output.semicolon();
    });
    DEFPRINT(AST_Var, function(self, output){
        self._do_print(output, "var");
    });
    DEFPRINT(AST_Const, function(self, output){
        self._do_print(output, "const");
    });
    DEFPRINT(AST_VarDef, function(self, output){
        self.name.print(output);
        if (self.value) {
            output.space();
            output.print("=");
            output.space();
            self.value.print(output);
        }
    });

    /* -----[ other expressions ]----- */
    DEFPRINT(AST_Call, function(self, output){
        self.expression.print(output);
        if (self instanceof AST_New && no_constructor_parens(self, output))
            return;
        output.with_parens(function(){
            self.args.forEach(function(expr, i){
                if (i) output.comma();
                expr.print(output);
            });
        });
    });
    DEFPRINT(AST_New, function(self, output){
        output.print("new");
        output.space();
        AST_Call.prototype.print.call(self, output);
    });
    DEFPRINT(AST_Seq, function(self, output){
        self.first.print(output);
        output.comma();
        self.second.print(output);
    });
    DEFPRINT(AST_Dot, function(self, output){
        var expr = self.expression;
        expr.print(output);
        if (expr instanceof AST_Number) {
            if (!/[xa-f.]/i.test(output.last())) {
                output.print(".");
            }
        }
        output.print(".");
        output.print_name(self.property);
    });
    DEFPRINT(AST_Sub, function(self, output){
        self.expression.print(output);
        output.print("[");
        self.property.print(output);
        output.print("]");
    });
    DEFPRINT(AST_UnaryPrefix, function(self, output){
        var op = self.operator;
        output.print(op);
        if (/^[a-z]/i.test(op))
            output.space();
        self.expression.print(output);
    });
    DEFPRINT(AST_UnaryPostfix, function(self, output){
        self.expression.print(output);
        output.print(self.operator);
    });
    DEFPRINT(AST_Binary, function(self, output){
        self.left.print(output);
        output.space();
        output.print(self.operator);
        output.space();
        self.right.print(output);
    });
    DEFPRINT(AST_Conditional, function(self, output){
        self.condition.print(output);
        output.space();
        output.print("?");
        output.space();
        self.consequent.print(output);
        output.space();
        output.colon();
        self.alternative.print(output);
    });

    /* -----[ literals ]----- */
    DEFPRINT(AST_Array, function(self, output){
        output.with_square(function(){
            var a = self.elements, len = a.length;
            if (len > 0) output.space();
            a.forEach(function(exp, i){
                if (i) output.comma();
                exp.print(output);
            });
            if (len > 0) output.space();
        });
    });
    DEFPRINT(AST_Object, function(self, output){
        if (self.properties.length > 0) output.with_block(function(){
            self.properties.forEach(function(prop, i){
                if (i) {
                    output.print(",");
                    output.newline();
                }
                output.indent();
                prop.print(output);
            });
            output.newline();
        });
        else output.print("{}");
    });
    DEFPRINT(AST_ObjectKeyVal, function(self, output){
        var key = self.key;
        if (output.options("quote_keys")) {
            output.print_string(key);
        } else if ((typeof key == "number"
                    || !output.options("beautify")
                    && +key + "" == key)
                   && parseFloat(key) >= 0) {
            output.print(make_num(key));
        } else if (!is_identifier(key)) {
            output.print_string(key);
        } else {
            output.print_name(key);
        }
        output.colon();
        self.value.print(output);
    });
    DEFPRINT(AST_ObjectSetter, function(self, output){
        output.print("set");
        output.space();
        self.value._do_print(output, true);
    });
    DEFPRINT(AST_ObjectGetter, function(self, output){
        output.print("get");
        output.space();
        self.value._do_print(output, true);
    });
    DEFPRINT(AST_Symbol, function(self, output){
        output.print_name(self.name);
    });
    DEFPRINT(AST_SymbolDeclaration, function(self, output){
        if (self.uniq) {
            self.uniq.print(output);
        } else {
            output.print_name(self.mangled_name || self.name);
        }
    });
    DEFPRINT(AST_SymbolRef, function(self, output){
        var def = self.symbol;
        if (def) {
            def.print(output);
        } else {
            output.print_name(self.name);
        }
    });
    DEFPRINT(AST_This, function(self, output){
        output.print("this");
    });
    DEFPRINT(AST_Constant, function(self, output){
        output.print(self.getValue());
    });
    DEFPRINT(AST_String, function(self, output){
        output.print_string(self.getValue());
    });
    DEFPRINT(AST_Number, function(self, output){
        output.print(make_num(self.getValue()));
    });
    DEFPRINT(AST_RegExp, function(self, output){
        output.print("/");
        output.print(self.pattern);
        output.print("/");
        if (self.mods) output.print(self.mods);
    });

    // return true if the node at the top of the stack (that means the
    // innermost node in the current output) is lexically the first in
    // a statement.
    function first_in_statement(output) {
        var a = output.stack(), i = a.length, node = a[--i], p = a[--i];
        while (i > 0) {
            if (p instanceof AST_Statement)
                return true;
            if ((p instanceof AST_Seq           && p.first === node      ) ||
                (p instanceof AST_Call          && p.expression === node ) ||
                (p instanceof AST_Dot           && p.expression === node ) ||
                (p instanceof AST_Sub           && p.expression === node ) ||
                (p instanceof AST_Conditional   && p.condition === node  ) ||
                (p instanceof AST_Binary        && p.first === node      ) ||
                (p instanceof AST_Assign        && p.first === node      ) ||
                (p instanceof AST_UnaryPostfix  && p.expression === node ))
            {
                node = p;
                p = a[--i];
            } else {
                return false;
            }
        }
    };

    // self should be AST_New.  decide if we want to show parens or not.
    function no_constructor_parens(self, output) {
        return self.args.length == 0 && !output.options("beautify");
    };

    function best_of(a) {
        var best = a[0], len = best.length;
        for (var i = 1; i < a.length; ++i) {
            if (a[i].length < len) {
                best = a[i];
                len = best.length;
            }
        }
        return best;
    };

    function make_num(num) {
        var str = num.toString(10), a = [ str.replace(/^0\./, ".").replace('e+', 'e') ], m;
        if (Math.floor(num) === num) {
            if (num >= 0) {
                a.push("0x" + num.toString(16).toLowerCase(), // probably pointless
                       "0" + num.toString(8)); // same.
            } else {
                a.push("-0x" + (-num).toString(16).toLowerCase(), // probably pointless
                       "-0" + (-num).toString(8)); // same.
            }
            if ((m = /^(.*?)(0+)$/.exec(num))) {
                a.push(m[1] + "e" + m[2].length);
            }
        } else if ((m = /^0?\.(0+)(.*)$/.exec(num))) {
            a.push(m[2] + "e-" + (m[1].length + m[2].length),
                   str.substr(str.indexOf(".")));
        }
        return best_of(a);
    };

    function make_block(stmt, output) {
        if (stmt instanceof AST_BlockStatement) {
            stmt.print(output);
            return;
        }
        output.with_block(function(){
            output.indent();
            stmt.print(output);
            output.newline();
        });
    };

})();