source: Dev/trunk/src/client/dojox/lang/functional/tailrec.js @ 532

dojo.provide("dojox.lang.functional.tailrec");

dojo.require("dojox.lang.functional.lambda");
dojo.require("dojox.lang.functional.util");

// This module provides recursion combinators:
//      - a tail recursion combinator.

// Acknowledgements:
//      - recursion combinators are inspired by Manfred von Thun's article
//              "Recursion Theory and Joy"
//              (http://www.latrobe.edu.au/philosophy/phimvt/joy/j05cmp.html)

// Notes:
//      - recursion combinators produce a function, which implements
//      their respective recusion patterns. String lambdas are inlined, if possible.

(function(){
        var df = dojox.lang.functional, inline = df.inlineLambda, _x ="_x";

        df.tailrec = function(
                                        /*Function|String|Array*/ cond,
                                        /*Function|String|Array*/ then,
                                        /*Function|String|Array*/ before){
                // summary:
                //              Generates a function for the tail recursion pattern. This is the simplified
                //              version of the linear recursive combinator without the "after" function,
                //              and with the modified "before" function. All parameter functions are called
                //              in the context of "this" object.
                // cond:
                //              The lambda expression, which is used to detect the termination of recursion.
                //              It accepts the same parameter as the generated recursive function itself.
                //              This function should return "true", if the recursion should be stopped,
                //              and the "then" part should be executed. Otherwise the recursion will proceed.
                // then:
                //              The lambda expression, which is called upon termination of the recursion.
                //              It accepts the same parameters as the generated recursive function itself.
                //              The returned value will be returned as the value of the generated function.
                // before:
                //              The lambda expression, which is called before the recursive step.
                //              It accepts the same parameter as the generated recursive function itself,
                //              and returns an array of arguments for the next recursive call of
                //              the generated function.

                var c, t, b, cs, ts, bs, dict1 = {}, dict2 = {},
                        add2dict = function(x){ dict1[x] = 1; };
                if(typeof cond == "string"){
                        cs = inline(cond, _x, add2dict);
                }else{
                        c = df.lambda(cond);
                        cs = "_c.apply(this, _x)";
                        dict2["_c=_t.c"] = 1;
                }
                if(typeof then == "string"){
                        ts = inline(then, _x, add2dict);
                }else{
                        t = df.lambda(then);
                        ts = "_t.t.apply(this, _x)";
                }
                if(typeof before == "string"){
                        bs = inline(before, _x, add2dict);
                }else{
                        b = df.lambda(before);
                        bs = "_b.apply(this, _x)";
                        dict2["_b=_t.b"] = 1;
                }
                var locals1 = df.keys(dict1), locals2 = df.keys(dict2),
                        f = new Function([], "var _x=arguments,_t=_x.callee,_c=_t.c,_b=_t.b".concat(    // Function
                                locals1.length ? "," + locals1.join(",") : "",
                                locals2.length ? ",_t=_x.callee," + locals2.join(",") : t ? ",_t=_x.callee" : "",
                                ";for(;!",
                                cs,
                                ";_x=",
                                bs,
                                ");return ",
                                ts
                        ));
                if(c){ f.c = c; }
                if(t){ f.t = t; }
                if(b){ f.b = b; }
                return f;
        };
})();

/*
For documentation only:

1) The original recursive version:

var tailrec1 = function(cond, then, before){
        var cond   = df.lambda(cond),
                then   = df.lambda(then),
                before = df.lambda(before);
        return function(){
                if(cond.apply(this, arguments)){
                        return then.apply(this, arguments);
                }
                var args = before.apply(this, arguments);
                return arguments.callee.apply(this, args);
        };
};

2) The original iterative version (before minification and inlining):

var tailrec2 = function(cond, then, before){
        var cond   = df.lambda(cond),
                then   = df.lambda(then),
                before = df.lambda(before);
        return function(){
                var args = arguments;
                for(; !cond.apply(this, args); args = before.apply(this, args));
                return then.apply(this, args);
        };
};

*/