var topic_generator = (function () {

var exports = {};

exports.sub_list_generator = function (lst, lower, upper) {
    // lower/upper has Python range semantics so if you pass
    // in lower=5 and upper=8, you get elements 5/6/7
    var i = lower;

    return {
        next: function () {
            if (i >= upper) {
                return;
            }
            var res = lst[i];
            i += 1;
            return res;
        },
    };
};

exports.list_generator = function (lst) {
    return exports.sub_list_generator(lst, 0, lst.length);
};

exports.fchain = function (outer_gen, get_inner_gen) {
    var outer_val = outer_gen.next();
    var inner_gen;

    return {
        next: function () {
            while (outer_val !== undefined) {
                if (inner_gen === undefined) {
                    inner_gen = get_inner_gen(outer_val);
                    if (!inner_gen || !inner_gen.next) {
                        blueslip.error('Invalid generator returned.');
                        return;
                    }
                }
                var inner = inner_gen.next();
                if (inner !== undefined) {
                    return inner;
                }
                outer_val = outer_gen.next();
                inner_gen = undefined;
            }
        },
    };
};

exports.chain = function (gen_lst) {
    function get(which) {
        return which;
    }

    var outer_gen = exports.list_generator(gen_lst);

    return exports.fchain(outer_gen, get);
};

exports.wrap = function (lst, val) {
    if (val === undefined) {
        return exports.list_generator(lst);
    }

    var i = _.indexOf(lst, val);
    if (i < 0) {
        return exports.list_generator(lst);
    }

    var inners = [
         exports.sub_list_generator(lst, i, lst.length),
         exports.sub_list_generator(lst, 0, i),
    ];

    return exports.chain(inners);
};

exports.wrap_exclude = function (lst, val) {
    if (val === undefined) {
        return exports.list_generator(lst);
    }

    var i = _.indexOf(lst, val);
    if (i < 0) {
        return exports.list_generator(lst);
    }

    var inners = [
         exports.sub_list_generator(lst, i+1, lst.length),
         exports.sub_list_generator(lst, 0, i),
    ];

    return exports.chain(inners);
};

exports.filter = function (gen, filter_func) {
    return {
        next: function () {
            while (true) {
                var val = gen.next();
                if (val === undefined) {
                    return;
                }
                if (filter_func(val)) {
                    return val;
                }
            }
        },
    };
};

exports.next_topic = function (streams, get_topics, has_unread_messages, curr_stream, curr_topic) {
    var stream_gen = exports.wrap(streams, curr_stream);

    function get_topic_gen(which_stream) {
        var gen;

        if (which_stream === curr_stream) {
            gen = exports.wrap_exclude(get_topics(which_stream), curr_topic);
        } else {
            gen = exports.list_generator(get_topics(which_stream));
        }
        var has_unread = function (topic) {
            return has_unread_messages(which_stream, topic);
        };

        return exports.filter(gen, has_unread);
    }

    var outer_gen = exports.fchain(stream_gen, get_topic_gen);
    return outer_gen.next();
};


return exports;
}());

if (typeof module !== 'undefined') {
    module.exports = topic_generator;
}