/**
  * datefmt provides parsing and formatting for std.datetime objects.
  *
  * The format is taken from strftime:
  *    %a     The abbreviated name of the day of the week.
  *    %A     The full name of the day of the week.
  *    %b     The abbreviated month name.
  *    %B     The full month name.
  *    %C     The century number (year/100) as a 2-digit integer.
  *    %d     The day of the month as a decimal number (range 01 to 31).
  *    %e     Like %d, the day of the month as a decimal number, but space padded.
  *    %f     Fractional seconds. Will parse any precision and emit six decimal places.
  *    %F     Equivalent to %Y-%m-%d (the ISO 8601 date format).
  *    %g     Milliseconds of the second.
  *    %G     Nanoseconds of the second.
  *    %h     The hour as a decimal number using a 12-hour clock (range 01 to 12).
  *    %H     The hour as a decimal number using a 24-hour clock (range 00 to 23).
  *    %I     The hour as a decimal number using a 12-hour clock (range 00 to 23).
  *    %j     The day of the year as a decimal number (range 001 to 366).
  *    %k     The hour (24-hour clock) as a decimal number (range 0 to 23), space padded.
  *    %l     The hour (12-hour clock) as a decimal number (range 1 to 12), space padded.
  *    %m     The month as a decimal number (range 01 to 12).
  *    %M     The minute as a decimal number (range 00 to 59).
  *    %p     "AM" / "PM" (midnight is AM; noon is PM).
  *    %P     "am" / "pm" (midnight is AM; noon is PM).
  *    %r     Equivalent to "%I:%M:%S %p".
  *    %R     Equivalent to "%H:%M".
  *    %s     The number of seconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
  *    %S     The second as a decimal number (range 00 to 60).
  *    %T     Equivalent to "%H:%M:%S".
  *    %u     The day of the week as a decimal, range 1 to 7, Monday being 1 (formatting only).
  *    %V     The ISO 8601 week number (formatting only).
  *    %w     The day of the week as a decimal, range 0 to 6, Sunday being 0 (formatting only).
  *    %y     The year as a decimal number without a century (range 00 to 99).
  *    %Y     The year as a decimal number including the century, minimum 4 digits.
  *    %z     The +hhmm or -hhmm numeric timezone (that is, the hour and minute offset from UTC).
  *    %Z     The timezone name or abbreviation. Formatting only.
  *    %+     The numeric offset, or 'Z' for UTC. This is common with ISO8601 timestamps.
  *    %%     A literal '%' character.
  *
  * Timezone support is awkward. When time formats contain a GMT offset, that is honored. Otherwise,
  * datefmt recognizes a subset of the timezone names defined in RFC1123.
  */
module datefmt;

import core.time;
import std.array;
import std.conv;
import std.datetime;
import std..string;
import std.utf : codeLength;
alias to = std.conv.to;


/**
 * A Format is the platonic ideal of a specific format string.
 *
 * For datetime formats such as RFC1123 or ISO8601, there are many potential formats.
 * Like '2017-01-01' is a valid ISO8601 date. So is '2017-01-01T15:31:00'.
 * A Format object can describe all allomorphs of a format.
 */
struct Format
{
    /// The canonical format, to be used when formatting a datetime.
    string primaryFormat;
    /// Other formats that count as part of this Format, used for parsing.
    string[] formatOptions;
}


/**
 * Format the given datetime with the given Format.
 */
string format(SysTime dt, const Format fmt)
{
    return format(dt, fmt.primaryFormat);
}

/**
 * Format the given datetime with the given format string.
 */
string format(SysTime dt, string formatString)
{
    Appender!string ap;
    bool inPercent;
    foreach (i, c; formatString)
    {
        if (inPercent)
        {
            inPercent = false;
            interpretIntoString(ap, dt, c);
        }
        else if (c == '%')
        {
            inPercent = true;
        }
        else
        {
            ap ~= c;
        }
    }
    return ap.data;
}


/**
 * Parse the given datetime string with the given format string.
 *
 * This tries rather hard to produce a reasonable result. If the format string doesn't describe an
 * unambiguous point time, the result will be a date that satisfies the inputs and should generally
 * be the earliest such date. However, that is not guaranteed.
 *
 * For instance:
 * ---
 * SysTime time = parse("%d", "21");
 * writeln(time);  // 0000-01-21T00:00:00.000000Z
 * ---
 */
SysTime parse(
        string data,
        const Format fmt,
        immutable(TimeZone) defaultTimeZone = null,
        bool allowTrailingData = false)
{
    SysTime st;
    foreach (f; fmt.formatOptions)
    {
        if (tryParse(data, cast(string)f, st, defaultTimeZone))
        {
            return st;
        }
    }
    return parse(data, fmt.primaryFormat, defaultTimeZone, allowTrailingData);
}


/**
 * Parse the given datetime string with the given format string.
 *
 * This tries rather hard to produce a reasonable result. If the format string doesn't describe an
 * unambiguous point time, the result will be a date that satisfies the inputs and should generally
 * be the earliest such date. However, that is not guaranteed.
 *
 * For instance:
 * ---
 * SysTime time = parse("%d", "21");
 * writeln(time);  // 0000-01-21T00:00:00.000000Z
 * ---
 */
SysTime parse(
        string data,
        string formatString,
        immutable(TimeZone) defaultTimeZone = null,
        bool allowTrailingData = false)
{
    auto a = Interpreter(data);
    auto res = a.parse(formatString, defaultTimeZone);
    if (res.error)
    {
        throw new Exception(res.error ~ " around " ~ res.remaining);
    }
    if (!allowTrailingData && res.remaining.length > 0)
    {
        throw new Exception("trailing data: " ~ res.remaining);
    }
    return res.dt;
}

/**
 * Try to parse the input string according to the given pattern.
 *
 * Return: true to indicate success; false to indicate failure
 */
bool tryParse(
        string data,
        const Format fmt,
        out SysTime dt,
        immutable(TimeZone) defaultTimeZone = null)
{
    foreach (f; fmt.formatOptions)
    {
        if (tryParse(data, cast(string)f, dt, defaultTimeZone))
        {
            return true;
        }
    }
    return false;
}

/**
 * Try to parse the input string according to the given pattern.
 *
 * Return: true to indicate success; false to indicate failure
 */
bool tryParse(
        string data,
        string formatString,
        out SysTime dt,
        immutable(TimeZone) defaultTimeZone = null)
{
    auto a = Interpreter(data);
    auto res = a.parse(formatString, defaultTimeZone);
    if (res.error)
    {
        return false;
    }
    dt = res.dt;
    return true;
}

import std.algorithm : map, cartesianProduct, joiner;
import std.array : array;

/**
  * A Format suitable for RFC1123 dates.
  *
  * For instance, `Sun, 02 Jan 2004 15:31:10 GMT` is a valid RFC1123 date.
  */
immutable Format RFC1123FORMAT = {
    primaryFormat: "%a, %d %b %Y %H:%M:%S %Z",
    formatOptions:
        // According to the spec, day-of-week is optional.
        // Timezone can be specified in a few ways.
        // In the wild, we have a number of variants. Like the day of week can be abbreviated or
        // full. Likewise with the month.
        cartesianProduct(
                ["%a, ", "%A, ", ""],
                ["%d "],
                ["%b", "%B"],
                [" %Y %H:%M:%S "],
                ["%Z", "%z", "%.%.%.", "%.%.", "%."]
                )
        .map!(x => joiner([x.tupleof], "").array.to!string)
        .array
};

/**
  * A Format suitable for ISO8601 dates.
  *
  * For instance, `2010-01-15 06:17:21.015Z` is a valid ISO8601 date.
  */
immutable Format ISO8601FORMAT = {
    primaryFormat: "%Y-%m-%dT%H:%M:%S.%f%+",
    formatOptions: [
        "%Y-%m-%dT%H:%M:%S.%f%+",
        "%Y-%m-%d %H:%M:%S.%f%+",
        "%Y-%m-%dT%H:%M:%S%+",
        "%Y-%m-%d %H:%M:%S%+",
        "%Y-%m-%d %H:%M:%S.%f",
        "%Y-%m-%d %H:%M:%S",
        "%Y-%m-%dT%H:%M:%S",
        "%Y-%m-%d",
    ]
};

/** Parse an RFC1123 date. */
SysTime parseRFC1123(string data, bool allowTrailingData = false)
{
    return parse(data, RFC1123FORMAT, UTC(), allowTrailingData);
}

/** Produce an RFC1123 date string from a SysTime. */
string toRFC1123(SysTime date)
{
    return format(date.toUTC(), RFC1123FORMAT);
}

/** Parse an ISO8601 date. */
SysTime parseISO8601(string data, bool allowTrailingData = false)
{
    return parse(data, ISO8601FORMAT, UTC(), allowTrailingData);
}

/** Produce an ISO8601 date string from a SysTime. */
string toISO8601(SysTime date)
{
    return format(date.toUTC(), ISO8601FORMAT);
}

private:


immutable(TimeZone) utc;
static this() { utc = UTC(); }

enum weekdayNames = [
    "Sunday",
    "Monday",
    "Tuesday",
    "Wednesday",
    "Thursday",
    "Friday",
    "Saturday"
];

enum weekdayAbbrev = [
    "Sun",
    "Mon",
    "Tue",
    "Wed",
    "Thu",
    "Fri",
    "Sat"
];

enum monthNames = [
    "January",
    "February",
    "March",
    "April",
    "May",
    "June",
    "July",
    "August",
    "September",
    "October",
    "November",
    "December",
];

enum monthAbbrev = [
    "Jan",
    "Feb",
    "Mar",
    "Apr",
    "May",
    "Jun",
    "Jul",
    "Aug",
    "Sep",
    "Oct",
    "Nov",
    "Dec",
];

struct Result
{
    SysTime dt;
    string error;
    string remaining;
    string remainingFormat;
}

// TODO support wstring, dstring
struct Interpreter
{
    this(string data)
    {
        this.data = data;
    }
    string data;

    int year;
    int century;
    int yearOfCentury;
    Month month;
    int dayOfWeek;
    int dayOfMonth;
    int dayOfYear;
    int isoWeek;
    int hour12;
    int hour24;
    int hour;
    int minute;
    int second;
    int nanosecond;
    int weekNumber;
    Duration tzOffset;
    string tzAbbreviation;
    string tzName;
    long epochSecond;
    enum AMPM { AM, PM, None };
    AMPM amPm = AMPM.None;
    Duration fracSecs;
    immutable(TimeZone)* tz;

    Result parse(string formatString, immutable(TimeZone) defaultTimeZone)
    {
        tz = defaultTimeZone is null ? &utc : &defaultTimeZone;
        bool inPercent;
        foreach (size_t i, dchar c; formatString)
        {
            if (inPercent)
            {
                inPercent = false;
                if (!interpretFromString(c))
                {
                    auto remainder = data;
                    if (remainder.length > 15)
                    {
                        remainder = remainder[0..15];
                    }
                    return Result(SysTime.init, "unexpected value", data, formatString[i..$]);
                }
            }
            else if (c == '%')
            {
                inPercent = true;
            }
            else
            {
                // TODO non-ASCII
                auto b = data;
                bool endedEarly = false;
                foreach (size_t i, dchar dc; b)
                {
                    data = b[i..$];
                    if (i > 0)
                    {
                        endedEarly = true;
                        break;
                    }
                    if (c != dc)
                    {
                        return Result(SysTime.init, "unexpected literal", data, formatString[i..$]);
                    }
                }
                if (!endedEarly) data = "";
            }
        }

        if (!year)
        {
            year = century * 100 + yearOfCentury;
        }
        if (hour12)
        {
            if (amPm == AMPM.PM)
            {
                hour24 = (hour12 + 12) % 24;
            }
            else
            {
                hour24 = hour12;
            }
        }
        auto dt = SysTime(
                DateTime(year, month, dayOfMonth, hour24, minute, second),
                tzOffset ? new immutable SimpleTimeZone(tzOffset) : *tz);
        dt += fracSecs;
        return Result(dt, null, data);
    }

    bool interpretFromString(dchar c)
    {
        switch (c)
        {
            case '.':
                // TODO unicodes
                if (data.length >= 1)
                {
                    data = data[1..$];
                    return true;
                }
                return false;
            case 'a':
                foreach (i, m; weekdayAbbrev)
                {
                    if (data.startsWith(m))
                    {
                        data = data[m.length .. $];
                        return true;
                    }
                }
                return false;
            case 'A':
                foreach (i, m; weekdayNames)
                {
                    if (data.startsWith(m))
                    {
                        data = data[m.length .. $];
                        return true;
                    }
                }
                return false;
            case 'b':
                foreach (i, m; monthAbbrev)
                {
                    if (data.startsWith(m))
                    {
                        month = cast(Month)(i + 1);
                        data = data[m.length .. $];
                        return true;
                    }
                }
                return false;
            case 'B':
                foreach (i, m; monthNames)
                {
                    if (data.startsWith(m))
                    {
                        month = cast(Month)(i + 1);
                        data = data[m.length .. $];
                        return true;
                    }
                }
                return false;
            case 'C':
                return parseInt!(x => century = x)(data);
            case 'd':
                return parseInt!(x => dayOfMonth = x)(data);
            case 'e':
                return parseInt!(x => dayOfMonth = x)(data);
            case 'g':
                return parseInt!(x => fracSecs = x.msecs)(data);
            case 'G':
                return parseInt!(x => fracSecs = x.nsecs)(data);
            case 'f':
                size_t end = data.length;
                foreach (i, cc; data)
                {
                    if ('0' > cc || '9' < cc)
                    {
                        end = i;
                        break;
                    }
                }
                auto fss = data[0..end];
                data = data[end..$];
                if (fss.length == 0)
                {
                    return false;
                }
                auto fs = fss.to!ulong;
                while (end < 7)
                {
                    end++;
                    fs *= 10;
                }
                while (end > 7)
                {
                    end--;
                    fs /= 10;
                }
                this.fracSecs = fs.hnsecs;
                return true;
            case 'F':
                auto dash1 = data.indexOf('-');
                if (dash1 <= 0) return false;
                if (dash1 >= data.length - 1) return false;
                auto yearStr = data[0..dash1];
                auto year = yearStr.to!int;
                data = data[dash1 + 1 .. $];

                if (data.length < 5)
                {
                    // Month is 2 digits; day is 2 digits; dash between
                    return false;
                }
                if (data[2] != '-')
                {
                    return false;
                }
                if (!parseInt!(x => month = cast(Month)x)(data)) return false;
                if (!data.startsWith("-")) return false;
                data = data[1..$];
                return parseInt!(x => dayOfMonth = x)(data);
            case 'H':
            case 'k':
                auto h = parseInt!(x => hour24 = x)(data);
                return h;
            case 'h':
            case 'I':
            case 'l':
                return parseInt!(x => hour12 = x)(data);
            case 'j':
                return parseInt!(x => dayOfYear = x, 3)(data);
            case 'm':
                return parseInt!(x => month = cast(Month)x)(data);
            case 'M':
                return parseInt!(x => minute = x)(data);
            case 'p':
                if (data.startsWith("AM"))
                {
                    amPm = AMPM.AM;
                }
                else if (data.startsWith("PM"))
                {
                    amPm = AMPM.PM;
                }
                else
                {
                    return false;
                }
                return true;
            case 'P':
                if (data.startsWith("am"))
                {
                    amPm = AMPM.AM;
                }
                else if (data.startsWith("pm"))
                {
                    amPm = AMPM.PM;
                }
                else
                {
                    return false;
                }
                return true;
            case 'r':
                return interpretFromString('I') &&
                    pop(':') &&
                    interpretFromString('M') &&
                    pop(':') &&
                    interpretFromString('S') &&
                    pop(' ') &&
                    interpretFromString('p');
            case 'R':
                return interpretFromString('H') &&
                    pop(':') &&
                    interpretFromString('M');
            case 's':
                size_t end = 0;
                foreach (i2, c2; data)
                {
                    if (c2 < '0' || c2 > '9')
                    {
                        end = cast()i2;
                        break;
                    }
                }
                if (end == 0) return false;
                epochSecond = data[0..end].to!int;
                data = data[end..$];
                return true;
            case 'S':
                return parseInt!(x => second = x)(data);
            case 'T':
                return interpretFromString('H') &&
                    pop(':') &&
                    interpretFromString('M') &&
                    pop(':') &&
                    interpretFromString('S');
            case 'u':
                return parseInt!(x => dayOfWeek = cast(DayOfWeek)(x % 7))(data);
            case 'V':
                return parseInt!(x => isoWeek = x)(data);
            case 'y':
                return parseInt!(x => yearOfCentury = x)(data);
            case 'Y':
                size_t end = 0;
                foreach (i2, c2; data)
                {
                    if (c2 < '0' || c2 > '9')
                    {
                        end = i2;
                        break;
                    }
                }
                if (end == 0) return false;
                year = data[0..end].to!int;
                data = data[end..$];
                return true;
            case 'z':
            case '+':
                if (pop('Z'))  // for ISO8601
                {
                    tzOffset = 0.seconds;
                    return true;
                }

                int sign = 0;
                if (pop('-'))
                {
                    sign = -1;
                }
                else if (pop('+'))
                {
                    sign = 1;
                }
                else
                {
                    return false;
                }
                int hour, minute;
                parseInt!(x => hour = x)(data);
                parseInt!(x => minute = x)(data);
                tzOffset = dur!"minutes"(sign * (minute + 60 * hour));
                return true;
            case 'Z':
                foreach (i, v; canonicalZones)
                {
                    if (data.startsWith(v.name))
                    {
                        tz = &canonicalZones[i].zone;
                        break;
                    }
                }
                return false;
            default:
                throw new Exception("unrecognized control character %" ~ c.to!string);
        }
    }

    bool pop(dchar c)
    {
        if (data.startsWith(c))
        {
            data = data[c.codeLength!char .. $];
            return true;
        }
        return false;
    }
}

struct CanonicalZone
{
    string name;
    immutable(TimeZone) zone;
}
// array so we can control iteration order -- longest first
CanonicalZone[] canonicalZones;
shared static this()
{
    version (Posix)
    {
        auto utc = PosixTimeZone.getTimeZone("Etc/UTC");
        auto est = PosixTimeZone.getTimeZone("America/New_York");
        auto cst = PosixTimeZone.getTimeZone("America/Boise");
        auto mst = PosixTimeZone.getTimeZone("America/Chicago");
        auto pst = PosixTimeZone.getTimeZone("America/Los_Angeles");
    }
    else
    {
        auto utc = WindowsTimeZone.getTimeZone("UTC");
        auto est = WindowsTimeZone.getTimeZone("Eastern Standard Time");
        auto cst = WindowsTimeZone.getTimeZone("Central Standard Time");
        auto mst = WindowsTimeZone.getTimeZone("Mountain Standard Time");
        auto pst = WindowsTimeZone.getTimeZone("Pacific Standard Time");
    }
    canonicalZones =
    [
        // TODO ensure the MDT style variants prefer the daylight time version of the date
        CanonicalZone("UTC", utc),
        CanonicalZone("UT", utc),
        CanonicalZone("Z", utc),
        CanonicalZone("GMT", utc),
        CanonicalZone("EST", est),
        CanonicalZone("EDT", est),
        CanonicalZone("CST", cst),
        CanonicalZone("CDT", cst),
        CanonicalZone("MST", mst),
        CanonicalZone("MDT", mst),
        CanonicalZone("PST", pst),
        CanonicalZone("PDT", pst),
    ];
}

bool parseInt(alias setter, int length = 2)(ref string data)
{
    if (data.length < length)
    {
        return false;
    }
    auto c = data[0..length].strip;
    data = data[length..$];
    int v;
    try
    {
        v = c.to!int;
    }
    catch (ConvException e)
    {
        return false;
    }
    cast(void)setter(c.to!int);
    return true;
}

void interpretIntoString(ref Appender!string ap, SysTime dt, char c)
{
    static import std.format;
    switch (c)
    {
        case 'a':
            ap ~= weekdayAbbrev[cast(size_t)dt.dayOfWeek];
            return;
        case 'A':
            ap ~= weekdayNames[cast(size_t)dt.dayOfWeek];
            return;
        case 'b':
            ap ~= monthAbbrev[cast(size_t)dt.month];
            return;
        case 'B':
            ap ~= monthNames[cast(size_t)dt.month];
            return;
        case 'C':
            ap ~= (dt.year / 100).to!string;
            return;
        case 'd':
            auto s = dt.day.to!string;
            if (s.length == 1)
            {
                ap ~= "0";
            }
            ap ~= s;
            return;
        case 'e':
            auto s = dt.day.to!string;
            if (s.length == 1)
            {
                ap ~= " ";
            }
            ap ~= s;
            return;
        case 'f':
            ap ~= std.format.format("%06d", dt.fracSecs.total!"usecs");
            return;
        case 'g':
            ap ~= std.format.format("%03d", dt.fracSecs.total!"msecs");
            return;
        case 'G':
            ap ~= std.format.format("%09d", dt.fracSecs.total!"nsecs");
            return;
        case 'F':
            interpretIntoString(ap, dt, 'Y');
            ap ~= '-';
            interpretIntoString(ap, dt, 'm');
            ap ~= '-';
            interpretIntoString(ap, dt, 'd');
            return;
        case 'h':
        case 'I':
            auto h = dt.hour;
            if (h == 0)
            {
                h = 12;
            }
            else if (h > 12)
            {
                h -= 12;
            }
            ap.pad(h.to!string, '0', 2);
            return;
        case 'H':
            ap.pad(dt.hour.to!string, '0', 2);
            return;
        case 'j':
            ap.pad(dt.dayOfYear.to!string, '0', 3);
            return;
        case 'k':
            ap.pad(dt.hour.to!string, ' ', 2);
            return;
        case 'l':
            auto h = dt.hour;
            if (h == 0)
            {
                h = 12;
            }
            else if (h > 12)
            {
                h -= 12;
            }
            ap.pad(h.to!string, ' ', 2);
            return;
        case 'm':
            uint m = cast(uint)dt.month;
            ap.pad(m.to!string, '0', 2);
            return;
        case 'M':
            ap.pad(dt.minute.to!string, '0', 2);
            return;
        case 'p':
            if (dt.hour >= 12)
            {
                ap ~= "PM";
            }
            else
            {
                ap ~= "AM";
            }
            return;
        case 'P':
            if (dt.hour >= 12)
            {
                ap ~= "pm";
            }
            else
            {
                ap ~= "am";
            }
            return;
        case 'r':
            interpretIntoString(ap, dt, 'I');
            ap ~= ':';
            interpretIntoString(ap, dt, 'M');
            ap ~= ':';
            interpretIntoString(ap, dt, 'S');
            ap ~= ' ';
            interpretIntoString(ap, dt, 'p');
            return;
        case 'R':
            interpretIntoString(ap, dt, 'H');
            ap ~= ':';
            interpretIntoString(ap, dt, 'M');
            return;
        case 's':
            auto delta = dt - SysTime(DateTime(1970, 1, 1), UTC());
            ap ~= delta.total!"seconds"().to!string;
            return;
        case 'S':
            ap.pad(dt.second.to!string, '0', 2);
            return;
        case 'T':
            interpretIntoString(ap, dt, 'H');
            ap ~= ':';
            interpretIntoString(ap, dt, 'M');
            ap ~= ':';
            interpretIntoString(ap, dt, 'S');
            return;
        case 'u':
            auto dow = cast(uint)dt.dayOfWeek;
            if (dow == 0) dow = 7;
            ap ~= dow.to!string;
            return;
        case 'w':
            ap ~= (cast(uint)dt.dayOfWeek).to!string;
            return;
        case 'y':
            ap.pad((dt.year % 100).to!string, '0', 2);
            return;
        case 'Y':
            ap.pad(dt.year.to!string, '0', 4);
            return;
        case '+':
            if (dt.utcOffset == dur!"seconds"(0))
            {
                ap ~= 'Z';
                return;
            }
            // If it's not UTC, format as +HHMM
            goto case;
        case 'z':
            import std.math : abs;
            auto d = dt.utcOffset;
            if (d < dur!"seconds"(0))
            {
                ap ~= '-';
            }
            else
            {
                ap ~= '+';
            }
            auto minutes = abs(d.total!"minutes");
            ap.pad((minutes / 60).to!string, '0', 2);
            ap.pad((minutes % 60).to!string, '0', 2);
            return;
        case 'Z':
            if (dt.timezone is null || dt.timezone == UTC())
            {
                ap ~= 'Z';
            }
            if (dt.dstInEffect)
            {
                ap ~= dt.timezone.stdName;
            }
            else
            {
                ap ~= dt.timezone.dstName;
            }
            return;
        case '%':
            ap ~= '%';
            return;
        default:
            throw new Exception("format element %" ~ c ~ " not recognized");
    }
}

void pad(ref Appender!string ap, string s, char pad, uint length)
{
    if (s.length >= length)
    {
        ap ~= s;
        return;
    }
    for (uint i = 0; i < length - s.length; i++)
    {
        ap ~= pad;
    }
    ap ~= s;
}

unittest
{
    import std.stdio;
    auto dt = SysTime(
            DateTime(2017, 5, 3, 14, 31, 57),
            UTC());
    auto isoishFmt = "%Y-%m-%d %H:%M:%S %z";
    auto isoish = dt.format(isoishFmt);
    assert(isoish == "2017-05-03 14:31:57 +0000", isoish);
    auto parsed = isoish.parse(isoishFmt);
    assert(parsed.timezone !is null);
    assert(parsed.timezone == UTC());
    assert(parsed == dt, parsed.format(isoishFmt));
}

unittest
{
    SysTime st;
    assert(tryParse("2013-10-09T14:56:33.050-06:00", ISO8601FORMAT, st, UTC()));
    assert(st.year == 2013);
    assert(st.month == 10);
    assert(st.day == 9);
    assert(st.hour == 14, st.hour.to!string);
    assert(st.minute == 56);
    assert(st.second == 33);
    assert(st.fracSecs == 50.msecs);
    assert(st.timezone !is null);
    assert(st.timezone != UTC());
    assert(st.timezone.utcOffsetAt(st.stdTime) == -6.hours);
}

unittest
{
    import std.stdio;
    auto dt = SysTime(
            DateTime(2017, 5, 3, 14, 31, 57),
            UTC());
    auto isoishFmt = ISO8601FORMAT;
    auto isoish = "2017-05-03T14:31:57.000000Z";
    auto parsed = isoish.parse(isoishFmt);
    assert(parsed.timezone !is null);
    assert(parsed.timezone == UTC());
    assert(parsed == dt, parsed.format(isoishFmt));
}

unittest
{
    import std.stdio;
    auto dt = SysTime(
            DateTime(2017, 5, 3, 14, 31, 57),
            UTC()) + 10.msecs;
    assert(dt.fracSecs == 10.msecs, "can't add dates");
    auto isoishFmt = ISO8601FORMAT;
    auto isoish = "2017-05-03T14:31:57.010000Z";
    auto parsed = isoish.parse(isoishFmt);
    assert(parsed.fracSecs == 10.msecs, "can't parse millis");
    assert(parsed.timezone !is null);
    assert(parsed.timezone == UTC());
    assert(parsed == dt, parsed.format(isoishFmt));

}

unittest
{
    auto formatted = "Thu, 04 Sep 2014 06:42:22 GMT";
    auto dt = parseRFC1123(formatted);
    assert(dt == SysTime(DateTime(2014, 9, 4, 6, 42, 22), UTC()), dt.toISOString());
}

unittest
{
    // RFC1123 dates
    /*
    // Uncomment to list out the generated format strings (for debugging)
    foreach (fmt; exhaustiveDateFormat.formatOptions)
    {
        import std.stdio : writeln;
        writeln(fmt);
    }
    */
    void test(string date)
    {
        SysTime st;
        assert(tryParse(date, RFC1123FORMAT, st, UTC()),
                "failed to parse date " ~ date);
        assert(st.year == 2017);
        assert(st.month == 6);
        assert(st.day == 11);
        assert(st.hour == 22);
        assert(st.minute == 15);
        assert(st.second == 47);
        assert(st.timezone == UTC());
    }

    // RFC1123-ish
    test("Sun, 11 Jun 2017 22:15:47 UTC");
    test("11 Jun 2017 22:15:47 UTC");
    test("Sunday, 11 Jun 2017 22:15:47 UTC");
    test("Sun, 11 June 2017 22:15:47 UTC");
    test("11 June 2017 22:15:47 UTC");
    test("Sunday, 11 June 2017 22:15:47 UTC");
    test("Sun, 11 Jun 2017 22:15:47 +0000");
    test("11 Jun 2017 22:15:47 +0000");
    test("Sunday, 11 Jun 2017 22:15:47 +0000");
    test("Sun, 11 June 2017 22:15:47 +0000");
    test("11 June 2017 22:15:47 +0000");
    test("Sunday, 11 June 2017 22:15:47 +0000");
}

unittest
{
    SysTime st;
    assert(!tryParse("", RFC1123FORMAT, st, UTC()));
}