var ElmLocalStoragePorts = function() {};

ElmLocalStoragePorts.prototype.subscribe =
    function(app, getPortName, setPortName, clearPortName, responsePortName, listKeysPortName) {
        if (!getPortName) getPortName = "getItem";
        if (!setPortName) setPortName = "setItem";
        if (!clearPortName) clearPortName = "clear";
        if (!listKeysPortName) listKeysPortName = "listKeys";
        if (!responsePortName) responsePortName = "response";

        if (app.ports[responsePortName]) {
            var responsePort = app.ports[responsePortName];

            if (app.ports[getPortName]) {
                app.ports[getPortName].subscribe(function(key) {
                    var val = null;
                    try {
                        val = JSON.parse(localStorage.getItem(key))
                    } catch (e) {}
                    responsePort.send({
                        key:key,
                        value:val
                    })
                });
            } else {
                console.warn(getPortName + ": This port is not connected.");
            }

            if (app.ports[setPortName]) {
                app.ports[setPortName].subscribe(function(kv) {
                    var key = kv[0];
                    var json = kv[1];
                    if (json === null) {
                        localStorage.removeItem(key);
                    } else {
                        localStorage.setItem(key, JSON.stringify(json));
                    }
                });
            } else {
                console.warn(setPortName + ": This port is not connected.");
            }

            if (app.ports[clearPortName]) {
                app.ports[clearPortName].subscribe(function(prefix) {
                    if (prefix) {
                        var cnt = localStorage.length;
                        for (var i = cnt - 1; i >= 0; --i) {
                            var key = localStorage.key(i);
                            if (key && key.startsWith(prefix)) {
                                localStorage.removeItem(key);
                            }
                        }
                    } else {
                        localStorage.clear();
                    }
                });
            } else {
                console.warn(clearPortName + ": This port is not connected.");
            }

            if (app.ports[listKeysPortName]) {
                app.ports[listKeysPortName].subscribe(function(prefix) {
                    var cnt = localStorage.length;
                    var keys = [];
                    for (var i = 0; i < cnt; i++) {
                        var key = localStorage.key(i);
                        if (key && key.startsWith(prefix)) {
                            keys.push(key);
                        }
                    }
                    responsePort.send(keys);
                });
            } else {
                console.warn(listKeysPortName + ": This port is not connected.");
            }
        } else {
            console.warn(responsePortName + ": This port is not connected.");
        }
    };