/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

Components.utils.import("resource://gre/modules/AddonManager.jsm");
Components.utils.import("resource://gre/modules/Services.jsm");
Components.utils.import("resource://calendar/modules/calIteratorUtils.jsm");
Components.utils.import("resource://calendar/modules/calUtils.jsm");
Components.utils.import("resource://gre/modules/XPCOMUtils.jsm");
Components.utils.import("resource://gre/modules/Preferences.jsm");
Components.utils.import("resource://calendar/modules/ical.js");
Components.utils.import("resource://gre/modules/NetUtil.jsm");
Components.utils.import("resource://gre/modules/Promise.jsm");

function calStringEnumerator(stringArray) {
    this.mIndex = 0;
    this.mStringArray = stringArray;
}
calStringEnumerator.prototype = {
    // nsIUTF8StringEnumerator:
    hasMore: function calStringEnumerator_hasMore() {
        return (this.mIndex < this.mStringArray.length);
    },
    getNext: function calStringEnumerator_getNext() {
        if (!this.hasMore()) {
            throw Components.results.NS_ERROR_UNEXPECTED;
        }
        return this.mStringArray[this.mIndex++];
    }
};

var g_stringBundle = null;

function calTimezoneService() {
    this.wrappedJSObject = this;

    this.mZones = new Map();

    ICAL.TimezoneService = this.wrappedJSObject;
}
var calTimezoneServiceClassID = Components.ID("{e736f2bd-7640-4715-ab35-887dc866c587}");
var calTimezoneServiceInterfaces = [
    Components.interfaces.calITimezoneService,
    Components.interfaces.calITimezoneProvider,
    Components.interfaces.calIStartupService
];
calTimezoneService.prototype = {
    mDefaultTimezone: null,
    mHasSetupObservers: false,
    mVersion: null,
    mZones: null,

    classID: calTimezoneServiceClassID,
    QueryInterface: XPCOMUtils.generateQI(calTimezoneServiceInterfaces),
    classInfo: XPCOMUtils.generateCI({
        classID: calTimezoneServiceClassID,
        contractID: "@mozilla.org/calendar/timezone-service;1",
        classDescription: "Calendar Timezone Service",
        interfaces: calTimezoneServiceInterfaces,
        flags: Components.interfaces.nsIClassInfo.SINGLETON
    }),

    // ical.js TimezoneService methods
    has: function(id) { return this.getTimezone(id) != null; },
    get: function(id) {
        return id ? unwrapSingle(ICAL.Timezone, this.getTimezone(id)) : null;
    },
    remove: function() {},
    register: function() {},

    // calIStartupService:
    startup: function startup(aCompleteListener) {
        function fetchJSON(aURL) {
            cal.LOG("[calTimezoneService] Loading " + aURL);

            return new Promise((resolve, reject) => {
                let uri = Services.io.newURI(aURL, null, null);
                let channel = Services.io.newChannelFromURI2(uri,
                                                             null,
                                                             Services.scriptSecurityManager.getSystemPrincipal(),
                                                             null,
                                                             Components.interfaces.nsILoadInfo.SEC_REQUIRE_SAME_ORIGIN_DATA_INHERITS,
                                                             Components.interfaces.nsIContentPolicy.TYPE_OTHER);

                NetUtil.asyncFetch(channel, (inputStream, status) => {
                    if (!Components.isSuccessCode(status)) {
                        reject(status);
                        return;
                    }

                    try {
                        let jsonData = NetUtil.readInputStreamToString(inputStream, inputStream.available());
                        let tzData = JSON.parse(jsonData);
                        resolve(tzData);
                    } catch (ex) {
                        reject(ex);
                    }
                });
            });
        }

        let resNamespace = "calendar";
        // Check for presence of the calendar timezones add-on.
        let resProtocol = Services.io.getProtocolHandler('resource')
                                  .QueryInterface(Components.interfaces.nsIResProtocolHandler);
        if (resProtocol.hasSubstitution("calendar-timezones")) {
            resNamespace = "calendar-timezones";
        }

        fetchJSON("resource://" + resNamespace + "/timezones/zones.json").then((tzData) => {
            for (let tzid of Object.keys(tzData.aliases)) {
                let data = tzData.aliases[tzid];
                if (typeof data == "object" && data !== null) {
                    this.mZones.set(tzid, data);
                }
            }
            for (let tzid of Object.keys(tzData.zones)) {
                let data = tzData.zones[tzid];
                if (typeof data == "object" && data !== null) {
                    this.mZones.set(tzid, data);
                }
            }

            this.mVersion = tzData.version;
            cal.LOG("[calTimezoneService] Timezones version " + this.version + " loaded");

            let bundleURL = "chrome://" + resNamespace + "/locale/timezones.properties";
            g_stringBundle = ICAL.Timezone.cal_tz_bundle = Services.strings.createBundle(bundleURL);

            // Make sure UTC and floating are cached by calling their getters
            this.UTC;
            this.floating;
        }).then(() => {
            if (aCompleteListener) {
                aCompleteListener.onResult(null, Components.results.NS_OK);
            }
        }, (error) => {
            // We have to give up. Show an error and fail hard!
            let msg = cal.calGetString("calendar", "missingCalendarTimezonesError");
            cal.ERROR(msg);
            cal.showError(msg);
        });
    },

    shutdown: function shutdown(aCompleteListener) {
        Services.prefs.removeObserver("calendar.timezone.local", this);
        aCompleteListener.onResult(null, Components.results.NS_OK);
    },

    get UTC() {
        if (!this.mZones.has("UTC")) {
            let utc;
            if (Preferences.get("calendar.icaljs", false)) {
                utc = new calICALJSTimezone(ICAL.Timezone.utcTimezone);
            } else {
                utc = new calLibicalTimezone("UTC", null, "", "");
                utc.mUTC = true;
            }

            this.mZones.set("UTC", {zone: utc});
        }

        return this.mZones.get("UTC").zone;
    },

    get floating() {
        if (!this.mZones.has("floating")) {
            let floating;
            if (Preferences.get("calendar.icaljs", false)) {
                floating = new calICALJSTimezone(ICAL.Timezone.localTimezone);
            } else {
                floating = new calLibicalTimezone("floating", null, "", "");
                floating.isFloating = true;
            }
            this.mZones.set("floating", {zone: floating});
        }

        return this.mZones.get("floating").zone;
    },

    // calITimezoneProvider:
    getTimezone: function calTimezoneService_getTimezone(tzid) {
        if (!tzid) {
            cal.ERROR("Unknown timezone requested\n" + cal.STACK(10));
            return null;
        }
        if (tzid.startsWith("/mozilla.org/")) {
            // We know that our former tzids look like "/mozilla.org/<dtstamp>/continent/..."
            // The ending of the mozilla prefix is the index of that slash before the
            // continent. Therefore, we start looking for the prefix-ending slash
            // after position 13.
            tzid = tzid.substring(tzid.indexOf("/", 13) + 1);
        }

        let tz = this.mZones.get(tzid);
        if (!tz) {
            cal.ERROR("Couldn't find " + tzid);
            return null;
        }
        if (!tz.zone) {
            if (tz.aliasTo) {
                // This zone is an alias.
                tz.zone = this.getTimezone(tz.aliasTo);
            } else {
                if (Preferences.get("calendar.icaljs", false)) {
                    let parsedComp = ICAL.parse("BEGIN:VCALENDAR\r\n" + tz.ics + "\r\nEND:VCALENDAR");
                    let icalComp = new ICAL.Component(parsedComp);
                    let tzComp = icalComp.getFirstSubcomponent("vtimezone");
                    tz.zone = new calICALJSTimezone(ICAL.Timezone.fromData({
                        tzid: tzid,
                        component: tzComp,
                        latitude: tz.latitude,
                        longitude: tz.longitude
                    }));
                } else {
                    tz.zone = new calLibicalTimezone(tzid, tz.ics, tz.latitude, tz.longitude);
                }
            }
        }
        return tz.zone;
    },

    get timezoneIds() {
        let zones = [];
        for (let [k, v] of this.mZones.entries()) {
            if (!v.aliasTo && k != "UTC" && k != "floating") {
                zones.push(k);
            }
        }
        return new calStringEnumerator(zones);
    },

    get aliasIds() {
        let zones = [];
        for (let [k, v] of this.mZones.entries()) {
            if (v.aliasTo && k != "UTC" && k != "floating") {
                zones.push(k);
            }
        }
        return new calStringEnumerator(zones);
    },

    get version() {
        return this.mVersion;
    },

    get defaultTimezone() {
        if (!this.mDefaultTimezone) {
            var prefTzid = Preferences.get("calendar.timezone.local", null);
            var tzid = prefTzid;
            if (!tzid) {
                try {
                    tzid = guessSystemTimezone();
                } catch (e) {
                    cal.WARN("An exception occurred guessing the system timezone, trying UTC. Exception: " + e);
                    tzid = "UTC";
                }
            }
            this.mDefaultTimezone = this.getTimezone(tzid);
            cal.ASSERT(this.mDefaultTimezone, "Timezone not found: " + tzid);
            // Update prefs if necessary:
            if (this.mDefaultTimezone && this.mDefaultTimezone.tzid != prefTzid) {
                Preferences.set("calendar.timezone.local", this.mDefaultTimezone.tzid);
            }

            // We need to observe the timezone preference to update the default
            // timezone if needed.
            this.setupObservers();
        }
        return this.mDefaultTimezone;
    },

    setupObservers: function calTimezoneService_setupObservers() {
        if (!this.mHasSetupObservers) {
            // Now set up the observer
            Services.prefs.addObserver("calendar.timezone.local", this, false);
            this.mHasSetupObservers = true;
        }
    },

    observe: function calTimezoneService_observe(aSubject, aTopic, aData) {
        if (aTopic == "nsPref:changed" && aData == "calendar.timezone.local") {
            // Unsetting the default timezone will make the next call to the
            // default timezone getter set up the correct timezone again.
            this.mDefaultTimezone = null;
        }
    }
};

/**
 * We're going to do everything in our power, short of rumaging through the
 * user's actual file-system, to figure out the time-zone they're in.  The
 * deciding factors are the offsets given by (northern-hemisphere) summer and
 * winter JSdates.  However, when available, we also use the name of the
 * timezone in the JSdate, or a string-bundle term from the locale.
 *
 * @return a mozilla ICS timezone string.
*/
function guessSystemTimezone() {
    // Probe JSDates for basic OS timezone offsets and names.
    const dateJun = (new Date(2005, 5,20)).toString();
    const dateDec = (new Date(2005,11,20)).toString();
    const tzNameRegex = /[^(]* ([^ ]*) \(([^)]+)\)/;
    const nameDataJun = dateJun.match(tzNameRegex);
    const nameDataDec = dateDec.match(tzNameRegex);
    const tzNameJun = nameDataJun && nameDataJun[2];
    const tzNameDec = nameDataDec && nameDataDec[2];
    const offsetRegex = /[+-]\d{4}/;
    const offsetJun = dateJun.match(offsetRegex)[0];
    const offsetDec = dateDec.match(offsetRegex)[0];

    const tzSvc = cal.getTimezoneService();

    var continent = "Africa|America|Antarctica|Asia|Australia|Europe";
    var ocean     = "Arctic|Atlantic|Indian|Pacific";
    var tzRegex   = new RegExp(".*((?:"+continent+"|"+ocean+")"+
                               "(?:[/][-A-Z_a-z]+)+)");

    function getIcalString(component, property) {
        var prop = (component && component.getFirstProperty(property));
        return (prop ? prop.valueAsIcalString : null);
    }

    // Check if Olson ZoneInfo timezone matches OS/JSDate timezone properties:
    // * standard offset and daylight/summer offset if present (longitude),
    // * if has summer time, direction of change (northern/southern hemisphere)
    // * if has summer time, dates of next transitions
    // * timezone name (such as "Western European Standard Time").
    // Score is 3 if matches dates and names, 2 if matches dates without names,
    // 1 if matches dates within a week (so changes on different weekday),
    // otherwise 0 if no match.
    function checkTZ(tzId) {
        var tz = tzSvc.getTimezone(tzId);

        // Have to handle UTC separately because it has no .icalComponent.
        if (tz.isUTC) {
            if (offsetDec == 0 && offsetJun == 0) {
                if (tzNameJun == "UTC" && tzNameDec == "UTC") {
                    return 3;
                } else {
                    return 2;
                }
            } else {
                return 0;
            }
        }

        var subComp = tz.icalComponent;
        // find currently applicable time period, not just first,
        // because offsets of timezone may be changed over the years.
        var standard = findCurrentTimePeriod(tz, subComp, "STANDARD");
        var standardTZOffset = getIcalString(standard, "TZOFFSETTO");
        var standardName     = getIcalString(standard, "TZNAME");
        var daylight = findCurrentTimePeriod(tz, subComp, "DAYLIGHT");
        var daylightTZOffset = getIcalString(daylight, "TZOFFSETTO");
        var daylightName     = getIcalString(daylight, "TZNAME");

        // Try northern hemisphere cases.
        if (offsetDec == standardTZOffset && offsetDec == offsetJun &&
            !daylight) {
            if (standardName && standardName == tzNameJun) {
                return 3;
            } else {
                return 2;
            }
        }

        if (offsetDec == standardTZOffset && offsetJun == daylightTZOffset &&
            daylight) {
            var dateMatchWt = systemTZMatchesTimeShiftDates(tz, subComp);
            if (dateMatchWt > 0) {
                if (standardName && standardName == tzNameJun &&
                    daylightName && daylightName == tzNameDec) {
                    return 3;
                } else {
                    return dateMatchWt;
                }
            }
        }

        // Now flip them and check again, to cover southern hemisphere cases.
        if (offsetJun == standardTZOffset && offsetDec == offsetJun &&
            !daylight) {
            if (standardName && standardName == tzNameDec) {
                return 3;
            } else {
                return 2;
            }
        }

        if (offsetJun == standardTZOffset && offsetDec == daylightTZOffset &&
            daylight) {
            var dateMatchWt = systemTZMatchesTimeShiftDates(tz, subComp);
            if (dateMatchWt > 0) {
                if (standardName && standardName == tzNameJun &&
                    daylightName && daylightName == tzNameDec) {
                    return 3;
                } else {
                    return dateMatchWt;
                }
            }
        }
        return 0;
    }

    // returns 2=match-within-hours, 1=match-within-week, 0=no-match
    function systemTZMatchesTimeShiftDates(tz, subComp) {
        // Verify local autumn and spring shifts also occur in system timezone
        // (jsDate) on correct date in correct direction.
        // (Differs for northern/southern hemisphere.
        //  Local autumn shift is to local winter STANDARD time.
        //  Local spring shift is to local summer DAYLIGHT time.)
        const autumnShiftJSDate =
            findCurrentTimePeriod(tz, subComp, "STANDARD", true);
        const afterAutumnShiftJSDate = new Date(autumnShiftJSDate);
        const beforeAutumnShiftJSDate = new Date(autumnShiftJSDate);
        const springShiftJSDate =
            findCurrentTimePeriod(tz, subComp, "DAYLIGHT", true);
        const beforeSpringShiftJSDate = new Date(springShiftJSDate);
        const afterSpringShiftJSDate = new Date(springShiftJSDate);
        // Try with 6 HOURS fuzz in either direction, since OS and ZoneInfo
        // may disagree on the exact time of shift (midnight, 2am, 4am, etc).
        beforeAutumnShiftJSDate.setHours(autumnShiftJSDate.getHours()-6);
        afterAutumnShiftJSDate.setHours(autumnShiftJSDate.getHours()+6);
        afterSpringShiftJSDate.setHours(afterSpringShiftJSDate.getHours()+6);
        beforeSpringShiftJSDate.setHours(beforeSpringShiftJSDate.getHours()-6);
        if ((beforeAutumnShiftJSDate.getTimezoneOffset() <
             afterAutumnShiftJSDate.getTimezoneOffset()) &&
            (beforeSpringShiftJSDate.getTimezoneOffset() >
             afterSpringShiftJSDate.getTimezoneOffset())) {
            return 2;
        }
        // Try with 7 DAYS fuzz in either direction, so if no other tz found,
        // will have a nearby tz that disagrees only on the weekday of shift
        // (sunday vs. friday vs. calendar day), or off by exactly one week,
        // (e.g., needed to guess Africa/Cairo on w2k in 2006).
        beforeAutumnShiftJSDate.setDate(autumnShiftJSDate.getDate()-7);
        afterAutumnShiftJSDate.setDate(autumnShiftJSDate.getDate()+7);
        afterSpringShiftJSDate.setDate(afterSpringShiftJSDate.getDate()+7);
        beforeSpringShiftJSDate.setDate(beforeSpringShiftJSDate.getDate()-7);
        if ((beforeAutumnShiftJSDate.getTimezoneOffset() <
             afterAutumnShiftJSDate.getTimezoneOffset()) &&
            (beforeSpringShiftJSDate.getTimezoneOffset() >
             afterSpringShiftJSDate.getTimezoneOffset())) {
            return 1;
        }
        // no match
        return 0;
    }

    const todayUTC = cal.jsDateToDateTime(new Date());
    const oneYrUTC = todayUTC.clone(); oneYrUTC.year += 1;
    const periodStartCalDate = cal.createDateTime();
    const periodUntilCalDate = cal.createDateTime(); // until timezone is UTC
    const periodCalRule = cal.createRecurrenceRule();
    const untilRegex = /UNTIL=(\d{8}T\d{6}Z)/;

    function findCurrentTimePeriod(tz, subComp, standardOrDaylight,
                                   isForNextTransitionDate) {
        // Iterate through 'STANDARD' declarations or 'DAYLIGHT' declarations
        // (periods in history with different settings.
        //  e.g., US changes daylight start in 2007 (from April to March).)
        // Each period is marked by a DTSTART.
        // Find the currently applicable period: has most recent DTSTART
        // not later than today and no UNTIL, or UNTIL is greater than today.
        for (let period of cal.ical.subcomponentIterator(subComp, standardOrDaylight)) {
            periodStartCalDate.icalString = getIcalString(period, "DTSTART");
            periodStartCalDate.timezone = tz;
            if (oneYrUTC.nativeTime < periodStartCalDate.nativeTime) {
                continue; // period starts too far in future
            }
            // Must examine UNTIL date (not next daylight start) because
            // some zones (e.g., Arizona, Hawaii) may stop using daylight
            // time, so there might not be a next daylight start.
            var rrule = period.getFirstProperty("RRULE");
            if (rrule) {
                var match = untilRegex.exec(rrule.valueAsIcalString);
                if (match) {
                    periodUntilCalDate.icalString = match[1];
                    if (todayUTC.nativeTime > periodUntilDate.nativeTime) {
                        continue; // period ends too early
                    }
                } // else forever rule
            } // else no daylight rule

            // found period that covers today.
            if (!isForNextTransitionDate) {
                return period;
            } else /*isForNextTranstionDate*/ {
                if (todayUTC.nativeTime < periodStartCalDate.nativeTime) {
                    // already know periodStartCalDate < oneYr from now,
                    // and transitions are at most once per year, so it is next.
                    return cal.dateTimeToJsDate(periodStartCalDate);
                } else if (rrule) {
                    // find next occurrence after today
                    periodCalRule.icalProperty = rrule;
                    var nextTransitionDate =
                        periodCalRule.getNextOccurrence(periodStartCalDate,
                                                        todayUTC);
                    // make sure rule doesn't end before next transition date.
                    if (nextTransitionDate) {
                        return cal.dateTimeToJsDate(nextTransitionDate);
                    }
                }
            }
        }
        // no such period found
        return null;
    }

    function environmentVariableValue(varName) {
        let envSvc = Components.classes["@mozilla.org/process/environment;1"]
                                .getService(Components.interfaces.nsIEnvironment);
        let value = envSvc.get(varName);
        if (!value) return "";
        if (!value.match(tzRegex)) return "";
        return varName+"="+value;
    }

    function symbolicLinkTarget(filepath) {
        try {
            let file = Components.classes["@mozilla.org/file/local;1"]
                                 .createInstance(Components.interfaces.nsILocalFile);
            file.initWithPath(filepath);
            file.QueryInterface(Components.interfaces.nsIFile);
            if (!file.exists()) return "";
            if (!file.isSymlink()) return "";
            if (!file.target.match(tzRegex)) return "";
            return filepath +" -> "+file.target;
        } catch (ex) {
            Components.utils.reportError(filepath+": "+ex);
            return "";
        }
    }

    function fileFirstZoneLineString(filepath) {
        // return first line of file that matches tzRegex (ZoneInfo id),
        // or "" if no file or no matching line.
        try {
            let file = Components.classes["@mozilla.org/file/local;1"]
                                 .createInstance(Components.interfaces.nsILocalFile);
            file.initWithPath(filepath);
            file.QueryInterface(Components.interfaces.nsIFile);
            if (!file.exists()) return "";
            let fileInstream = Components.classes["@mozilla.org/network/file-input-stream;1"]
                                         .createInstance(Components.interfaces.nsIFileInputStream);
            const PR_RDONLY = 0x1;
            fileInstream.init(file, PR_RDONLY, 0, 0);
            fileInstream.QueryInterface(Components.interfaces.nsILineInputStream);
            try {
                let line = {}, hasMore = true, MAXLINES = 10;
                for (let i = 0; hasMore && i < MAXLINES; i++) {
                    hasMore = fileInstream.readLine(line);
                    if (line.value && line.value.match(tzRegex)) {
                        return filepath+": "+line.value;
                    }
                }
                return ""; // not found
            } finally {
                fileInstream.close();
            }
        } catch (ex) {
            Components.utils.reportError(filepath+": "+ex);
            return "";
        }
    }

    function weekday(icsDate, tz) {
        let calDate = cal.createDateTime(icsDate);
        calDate.timezone = tz;
        return cal.dateTimeToJsDate(calDate).toLocaleFormat("%a");
    }

    // Try to find a tz that matches OS/JSDate timezone.  If no name match,
    // will use first of probable timezone(s) with highest score.
    var probableTZId = "floating"; // default fallback tz if no tz matches.
    var probableTZScore = 0;
    var probableTZSource = null;

    const calProperties = Services.strings.createBundle("chrome://calendar/locale/calendar.properties");

    // First, try to detect operating system timezone.
    try {
        var osUserTimeZone = null;
        var zoneInfoIdFromOSUserTimeZone = null;
        var handler = Components.classes["@mozilla.org/network/protocol;1?name=http"]
                                .getService(Components.interfaces.nsIHttpProtocolHandler);

        if (handler.oscpu.match(/^Windows/)) {
            var wrk = (Components
                       .classes["@mozilla.org/windows-registry-key;1"]
                       .createInstance(Components.interfaces.nsIWindowsRegKey));
            wrk.open(wrk.ROOT_KEY_LOCAL_MACHINE,
                     "SYSTEM\\CurrentControlSet\\Control\\TimeZoneInformation",
                     wrk.ACCESS_READ);
            if (wrk.hasValue("TimeZoneKeyName")) {
                // Windows Vista and later have this key.
                // Clear trailing garbage on this key, see bug 1129712.
                osUserTimeZone = wrk.readStringValue("TimeZoneKeyName").split("\0")[0];
            } else {
                // If on Windows XP, current timezone only lists its localized name,
                // so to find its registry key name, match localized name to
                // localized names of each windows timezone listed in registry.
                // Then use the registry key name to see if this timezone has a
                // known ZoneInfo name.
                var currentTZStandardName = wrk.readStringValue("StandardName");
                wrk.close()

                wrk.open(wrk.ROOT_KEY_LOCAL_MACHINE,
                         "SOFTWARE\\Microsoft\\Windows NT\\CurrentVersion\\Time Zones",
                         wrk.ACCESS_READ);

                // Linear search matching localized name of standard timezone
                // to find the non-localized registry key.
                // (Registry keys are sorted by subkeyName, not by localized name
                //  nor offset, so cannot use binary search.)
                for (var i = 0; i < wrk.childCount; i++) {
                    var subkeyName  = wrk.getChildName(i);
                    var subkey = wrk.openChild(subkeyName, wrk.ACCESS_READ);
                    var std = subkey.readStringValue("Std");
                    subkey.close();
                    if (std == currentTZStandardName) {
                        osUserTimeZone = subkeyName;
                        break;
                    }
                }
            }
            wrk.close();

            if (osUserTimeZone != null) {
                // Lookup timezone registry key in table of known tz keys
                // to convert to ZoneInfo timezone id.
                const regKeyToZoneInfoBundle = Services.strings.createBundle(
                    "chrome://calendar/content/WindowsNTToZoneInfoTZId.properties");
                zoneInfoIdFromOSUserTimeZone =
                    regKeyToZoneInfoBundle.GetStringFromName(osUserTimeZone);
            }
        } else {
            // Else look for ZoneInfo timezone id in
            // - TZ environment variable value
            // - /etc/localtime symbolic link target path
            // - /etc/TIMEZONE or /etc/timezone file content
            // - /etc/sysconfig/clock file line content.
            // The timezone is set per user via the TZ environment variable.
            // TZ may contain a path that may start with a colon and ends with
            // a ZoneInfo timezone identifier, such as ":America/New_York" or
            // ":/share/lib/zoneinfo/America/New_York".  The others are
            // in the filesystem so they give one timezone for the system;
            // the values are similar (but cannot have a leading colon).
            // (Note: the OS ZoneInfo database may be a different version from
            // the one we use, so still need to check that DST dates match.)
            osUserTimeZone = (environmentVariableValue("TZ") ||
                              symbolicLinkTarget("/etc/localtime") ||
                              fileFirstZoneLineString("/etc/TIMEZONE") ||
                              fileFirstZoneLineString("/etc/timezone") ||
                              fileFirstZoneLineString("/etc/sysconfig/clock"));
            let results = osUserTimeZone.match(tzRegex);
            if (results) {
                zoneInfoIdFromOSUserTimeZone = results[1];
            }
        }

        // check how well OS tz matches tz defined in our version of zoneinfo db
        if (zoneInfoIdFromOSUserTimeZone != null) {
            var tzId = zoneInfoIdFromOSUserTimeZone;
            var score = checkTZ(tzId);
            switch(score) {
            case 0:
                // Did not match.
                // Maybe OS or Application is old, and the timezone changed.
                // Or maybe user turned off DST in Date/Time control panel.
                // Will look for a better matching tz, or fallback to floating.
                // (Match OS so alarms go off at time indicated by OS clock.)
                cal.WARN(calProperties.formatStringFromName(
                         "WarningOSTZNoMatch", [osUserTimeZone, zoneInfoIdFromOSUserTimeZone], 2));
                break;
            case 1: case 2:
                // inexact match: OS TZ and our ZoneInfo TZ matched imperfectly.
                // Will keep looking, will use tzId unless another is better.
                // (maybe OS TZ has changed to match a nearby TZ, so maybe
                // another ZoneInfo TZ matches it better).
                probableTZId = tzId;
                probableTZScore = score;
                probableTZSource = (calProperties.formatStringFromName
                                    ("TZFromOS", [osUserTimeZone], 1));
                break;
            case 3:
                // exact match
                return tzId;
            }
        }
    } catch (ex) {
        // zoneInfo id given was not recognized by our ZoneInfo database
        var errMsg = (calProperties.formatStringFromName
                      ("SkippingOSTimezone",
                       [zoneInfoIdFromOSUserTimeZone || osUserTimeZone], 1));
        Components.utils.reportError(errMsg+" "+ex);
    }

    // Second, give priority to "likelyTimezone"s if provided by locale.
    try {
        // The likelyTimezone property is a comma-separated list of
        // ZoneInfo timezone ids.
        const bundleTZString =
            calProperties.GetStringFromName("likelyTimezone");
        const bundleTZIds = bundleTZString.split(/\s*,\s*/);
        for (var bareTZId of bundleTZIds) {
            var tzId = bareTZId;
            try {
                var score = checkTZ(tzId);

                switch (score) {
                case 0:
                    break;
                case 1: case 2:
                    if (score > probableTZScore) {
                        probableTZId = tzId;
                        probableTZScore = score;
                        probableTZSource = (calProperties.GetStringFromName
                                            ("TZFromLocale"));
                    }
                    break;
                case 3:
                    return tzId;
                }
            } catch (ex) {
                var errMsg = (calProperties.formatStringFromName
                              ("SkippingLocaleTimezone", [bareTZId], 1));
                Components.utils.reportError(errMsg+" "+ex);
            }
        }
    } catch (ex) { // Oh well, this didn't work, next option...
        Components.utils.reportError(ex);
    }

    // Third, try all known timezones.
    const tzIDs = tzSvc.timezoneIds;
    while (tzIDs.hasMore()) {
        var tzId = tzIDs.getNext();
        try {
            var score = checkTZ(tzId);
            switch(score) {
            case 0: break;
            case 1: case 2:
                if (score > probableTZScore) {
                    probableTZId = tzId;
                    probableTZScore = score;
                    probableTZSource = (calProperties.GetStringFromName
                                        ("TZFromKnownTimezones"));
                }
                break;
            case 3:
                return tzId;
            }
        } catch (ex) { // bug if ics service doesn't recognize own tzid!
            var msg = ("ics-service doesn't recognize own tzid: "+tzId+"\n"+
                       ex);
            Components.utils.reportError(msg);
        }
    }

    // If reach here, there were no score=3 matches, so Warn in console.
    try {
        switch(probableTZScore) {
        case 0:
            cal.WARN(calProperties.GetStringFromName("warningUsingFloatingTZNoMatch"));
            break;
        case 1: case 2:
            var tzId = probableTZId;
            var tz = tzSvc.getTimezone(tzId);
            var subComp = tz.icalComponent;
            var standard = findCurrentTimePeriod(tz, subComp, "STANDARD");
            var standardTZOffset = getIcalString(standard, "TZOFFSETTO");
            var daylight = findCurrentTimePeriod(tz, subComp, "DAYLIGHT");
            var daylightTZOffset = getIcalString(daylight, "TZOFFSETTO");
            var warningDetail;
            if (probableTZScore == 1) {
                // score 1 means has daylight time,
                // but transitions start on different weekday from os timezone.
                var standardStart = getIcalString(standard, "DTSTART");
                var standardStartWeekday = weekday(standardStart, tz);
                var standardRule  = getIcalString(standard, "RRULE");
                var standardText =
                    ("  Standard: "+standardStart+" "+standardStartWeekday+"\n"+
                     "            "+standardRule+"\n");
                var daylightStart = getIcalString(daylight, "DTSTART");
                var daylightStartWeekday = weekday(daylightStart, tz);
                var daylightRule  = getIcalString(daylight, "RRULE");
                var daylightText =
                    ("  Daylight: "+daylightStart+" "+daylightStartWeekday+"\n"+
                     "            "+daylightRule+"\n");
                warningDetail =
                    ((standardStart < daylightStart
                      ? standardText + daylightText
                      : daylightText + standardText)+
                     (calProperties.GetStringFromName
                      ("TZAlmostMatchesOSDifferAtMostAWeek")));
            } else {
                warningDetail =
                    (calProperties.GetStringFromName("TZSeemsToMatchOS"));
            }
            var offsetString = (standardTZOffset+
                                 (!daylightTZOffset? "": "/"+daylightTZOffset));
            var warningMsg = (calProperties.formatStringFromName
                              ("WarningUsingGuessedTZ",
                               [tzId, offsetString, warningDetail,
                                probableTZSource], 4));
            cal.WARN(warningMsg);
            break;
        }
    } catch (ex) { // don't abort if error occurs warning user
        Components.utils.reportError(ex);
    }

    // return the guessed timezone
    return probableTZId;
}

var NSGetFactory = cal.loadingNSGetFactory(["calTimezone.js"], [calTimezoneService], this);
