jiff_core/tz/

posix.rs

/*!
Implements POSIX time zone string parsing and transition handling.

The `TZ` environment variable is most commonly used to set a time zone. For
example, `TZ=America/New_York`. But it can also be used to tersely define DST
transitions. Moreover, the format is not just used as an environment variable,
but is also included at the end of TZif files (version 2 or greater). The IANA
Time Zone Database project also [documents the `TZ` variable][iana-env] with
a little more commentary.

Note that we (along with pretty much everyone else) don't strictly follow
POSIX here. Namely, `TZ=America/New_York` isn't a POSIX compatible usage,
and I believe it technically should be `TZ=:America/New_York`. Nevertheless,
apparently some group of people (IANA folks?) decided `TZ=America/New_York`
should be fine. From the [IANA `theory.html` documentation][iana-env]:

> It was recognized that allowing the TZ environment variable to take on values
> such as 'America/New_York' might cause "old" programs (that expect TZ to have
> a certain form) to operate incorrectly; consideration was given to using
> some other environment variable (for example, TIMEZONE) to hold the string
> used to generate the TZif file's name. In the end, however, it was decided
> to continue using TZ: it is widely used for time zone purposes; separately
> maintaining both TZ and TIMEZONE seemed a nuisance; and systems where "new"
> forms of TZ might cause problems can simply use legacy TZ values such as
> "EST5EDT" which can be used by "new" programs as well as by "old" programs
> that assume pre-POSIX TZ values.

Indeed, even [musl subscribes to this behavior][musl-env]. So that's what we do
here too.

Note that a POSIX time zone like `EST5` corresponds to the UTC offset `-05:00`,
and `GMT-4` corresponds to the UTC offset `+04:00`. Yes, it's backwards. How
fun.

# IANA v3+ Support

While this module and many of its types are directly associated with POSIX,
this module also plays a supporting role for `TZ` strings in the IANA TZif
binary format for versions 2 and greater. Specifically, for versions 3 and
greater, some minor extensions are supported here via `IanaTz::parse`. But
using `PosixTz::parse` is limited to parsing what is specified by POSIX.
Nevertheless, we generally use `IanaTz::parse` everywhere, even when parsing
the `TZ` environment variable. The reason for this is that it seems to be what
other programs do in practice (for example, GNU date).

# `no-std` and `no-alloc` support

A big part of this module works fine in core-only environments. But because
core-only environments provide means of indirection, and embedding a
`PosixTimeZone` into a `TimeZone` without indirection would use up a lot of
space (and thereby make `Zoned` quite chunky), we provide core-only support
principally through a proc macro. Namely, a `PosixTimeZone` can be parsed by
the proc macro and then turned into static data.

POSIX time zone support isn't explicitly provided directly as a public API
for core-only environments, but is implicitly supported via TZif. (Since TZif
data contains POSIX time zone strings.)

[posix-env]: https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03
[iana-env]: https://data.iana.org/time-zones/tzdb-2024a/theory.html#functions
[musl-env]: https://wiki.musl-libc.org/environment-variables
*/

use crate::{
    civil::{Date, DateTime, Time, TimeSecond, Weekday},
    tz::{
        Abbreviation, AmbiguousOffset, AmbiguousTimestamp, Offset, OffsetInfo,
        TimeZoneId, Transition, REASONABLE_ABBREVIATION_MAX,
    },
    Timestamp,
};

/// The result of parsing the POSIX `TZ` environment variable.
///
/// A `TZ` variable can either be a POSIX time zone string with an optional DST
/// transition rule, or it can begin with a `:` followed by an arbitrary set of
/// bytes that is implementation defined.
///
/// In practice, the content following a `:` is treated as an IANA time zone
/// name. Moreover, even if the `TZ` string doesn't start with a `:` but
/// corresponds to a IANA time zone name, then it is interpreted as such.
/// However, this type only encapsulates the choices strictly provided by
/// POSIX: either a time zone string with an optional DST transition rule,
/// or an implementation defined string with a `:` prefix. If, for example,
/// `TZ="America/New_York"`, then that case isn't encapsulated by this type.
/// Callers needing that functionality will need to handle the error returned
/// by parsing this type and layer their own semantics on top. In general, any
/// valid IANA time zone identifier will be an invalid POSIX time zone string.
#[derive(Debug, Eq, PartialEq)]
pub enum TzEnv {
    /// A valid POSIX time zone with an optional DST transition rule.
    Rule(TimeZone),
    /// An implementation defined string. This occurs when the `TZ` value
    /// starts with a `:`. The string stored here does not include the `:`.
    ///
    /// Typically this is an IANA time zone identifier.
    Implementation(TimeZoneId),
}

impl TzEnv {
    /// Parse a POSIX `TZ` environment variable string from the given bytes.
    pub fn parse<B: AsRef<[u8]>>(bytes: B) -> Result<TzEnv, ParseError> {
        let bytes = bytes.as_ref();
        if bytes.get(0) == Some(&b':') {
            let Ok(string) = core::str::from_utf8(&bytes[1..]) else {
                return Err(ParseErrorKind::TzEnvColonInvalidUtf8.into());
            };
            let Some(smallstr) = TimeZoneId::new(string) else {
                return Err(ParseErrorKind::TzEnvColonTooBig.into());
            };
            Ok(TzEnv::Implementation(smallstr))
        } else {
            TimeZone::parse(bytes).map(TzEnv::Rule)
        }
    }

    /// Parse a POSIX `TZ` environment variable string from the given `OsStr`.
    #[cfg(feature = "std")]
    pub fn parse_os_str<O: AsRef<std::ffi::OsStr>>(
        osstr: O,
    ) -> Result<TzEnv, ParseError> {
        let bytes = crate::util::os_str_bytes(osstr.as_ref())
            .ok_or(ParseError::from(ParseErrorKind::TzEnvInvalidUtf8))?;
        TzEnv::parse(bytes)
    }
}

/// A representation of a POSIX time zone transition rule.
///
/// POSIX time zones are limited in what they can express. Notably, they can't
/// handle historic time zone transitions. They generally can only handle
/// present rules.
///
/// Note that the internals of this type are completely exposed to make writing
/// static data with these types easier.
///
/// # On "reasonable" POSIX time zones
///
/// Jiff only supports "reasonable" POSIX time zones. A "reasonable" POSIX time
/// zone is a POSIX time zone that has a DST transition rule _when_ it has a
/// DST time zone abbreviation. Without the transition rule, it isn't possible
/// to know when DST starts and stops.
///
/// POSIX technically allows a DST time zone abbreviation *without* a
/// transition rule, but the behavior is literally unspecified. So Jiff just
/// rejects them.
///
/// Note that if you're confused as to why Jiff accepts `TZ=EST5EDT` (where
/// `EST5EDT` is an example of an _unreasonable_ POSIX time zone), that's
/// because Jiff rejects `EST5EDT` and instead attempts to use it as an IANA
/// time zone identifier. And indeed, the IANA Time Zone Database contains an
/// entry for `EST5EDT` (presumably for legacy reasons).
///
/// Also, we expect `TZ` strings parsed from IANA v2+ formatted `tzfile`s to
/// also be reasonable or parsing fails. This also seems to be consistent with
/// the [GNU C Library]'s treatment of the `TZ` variable: it only documents
/// support for reasonable POSIX time zone strings.
///
/// Note that a V2 `TZ` string is precisely identical to a POSIX `TZ`
/// environment variable string. A V3 `TZ` string however supports signed DST
/// transition times, and hours in the range `0..=167`. The V2 and V3 here
/// reference how `TZ` strings are defined in the TZif format specified by
/// [RFC 9636]. V2 is the original version of it straight from POSIX, where as
/// V3+ corresponds to an extension added to V3 (and newer versions) of the
/// TZif format. V3 is a superset of V2, so in practice, Jiff just permits
/// V3 everywhere.
///
/// [GNU C Library]: https://www.gnu.org/software/libc/manual/2.25/html_node/TZ-Variable.html
/// [RFC 9636]: https://datatracker.ietf.org/doc/rfc9636/
///
/// # Example
///
/// ```
/// use jiff_core::{tz::{Offset, posix::TimeZone}, Timestamp};
///
/// let tz = TimeZone::parse("EST5EDT,M3.2.0,M11.1.0").unwrap();
///
/// let ts = Timestamp::from_second(1783100574).unwrap();
/// let offset = tz.to_offset(ts);
/// assert_eq!(offset, Offset::from_seconds(-4 * 60 * 60).unwrap());
///
/// // Around 6 months later, we should be out of DST.
/// let ts = Timestamp::from_second(ts.as_second() + 6 * 30 * 86400).unwrap();
/// let offset = tz.to_offset(ts);
/// assert_eq!(offset, Offset::from_seconds(-5 * 60 * 60).unwrap());
/// ```
#[derive(Clone, Debug, Eq, PartialEq)]
// This ensures the alignment of this type is always *at least* 8 bytes. This
// is required for the pointer tagging inside of `TimeZone` to be sound. At
// time of writing (2024-02-24), this explicit `repr` isn't required on 64-bit
// systems since the type definition is such that it will have an alignment
// of at least 8 bytes anyway. But this *is* required for 32-bit systems,
// where the type definition at present only has an alignment of 4 bytes.
// The alignment can also potentially change depending on whether `alloc` is
// enabled (since without `alloc` an `Abbreviation` is always just a fixed size
// array).
#[repr(align(8))]
pub struct TimeZone {
    /// The abbreviation for standard time.
    pub std_abbrev: Abbreviation,
    /// The offset for standard time.
    pub std_offset: Offset,
    /// Whether there is any daylight saving time for this POSIX time zone.
    pub dst: Option<Dst>,
}

/// The time zone transition rule along with its abbreviation and offset.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct Dst {
    /// The abbreviation to assign to civil datetimes when the daylight saving
    /// time rule is satisfied.
    pub abbrev: Abbreviation,
    /// The offset from UTC that civil datetimes get when the daylight saving
    /// time rule is satisfied.
    pub offset: Offset,
    /// The actual rule.
    pub rule: Rule,
}

/// A time zone transition rule.
///
/// This spells out a "range" of datetimes within a calendar year that should
/// get a special daylight saving time offset from UTC.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub struct Rule {
    /// The start of the time zone transition (e.g., daylight saving time).
    pub start: DayTime,
    /// The end of the time zone transition (e.g., daylight saving time).
    pub end: DayTime,
}

/// The day of a year and the time from that day on which a time zone
/// transition occurs.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub struct DayTime {
    /// The way to determine the day of the year.
    pub date: Day,
    /// The civil time on the day when the offset for civil time changes.
    pub time: TransitionCivilTime,
}

/// Represents a day in particular year on which a time zone transition occurs.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum Day {
    /// Julian day in a year, no counting for leap days.
    ///
    /// Valid range is `1..=365`.
    JulianOne(i16),
    /// Julian day in a year, counting for leap days.
    ///
    /// Valid range is `0..=365`.
    JulianZero(i16),
    /// The nth weekday of a month.
    WeekdayOfMonth {
        /// The month.
        ///
        /// Valid range is: `1..=12`.
        month: i8,
        /// The week.
        ///
        /// Valid range is `1..=5`.
        ///
        /// One interesting thing to note here (or my interpretation anyway),
        /// is that a week of `4` means the "4th weekday in a month" where as
        /// a week of `5` means the "last weekday in a month, even if it's the
        /// 4th weekday."
        week: i8,
        /// The weekday.
        weekday: Weekday,
    },
}

/// Represents the civil time at which a time zone transition occurs.
///
/// Note that this does not use `civil::TimeSecond` because this may be longer
/// than a single civil day (i.e., bigger than 86400). It can also be negative.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub struct TransitionCivilTime {
    /// The time in seconds. This may be negative.
    ///
    /// Valid range is `[-604_799, 604_799]`.
    pub second: i32,
}

impl TimeZone {
    /// Parse a POSIX `TZ` environment variable, assuming it's a rule and not
    /// an implementation defined value, from the given byte string.
    ///
    /// # Errors
    ///
    /// This returns an error if the given byte string is not a valid POSIX
    /// time zone transition rule.
    ///
    /// This also returns an error if, after parsing the POSIX time zone
    /// transition rule, there are still bytes remaining in the string given.
    pub fn parse<B: AsRef<[u8]>>(bytes: B) -> Result<TimeZone, ParseError> {
        let bytes = bytes.as_ref();
        // We enable the IANA v3+ extensions here. (Namely, that the time
        // specification hour value has the range `-167..=167` instead of
        // `0..=24`.) Requiring strict POSIX rules doesn't seem necessary
        // since the extension is a strict superset. Plus, GNU tooling
        // seems to accept the extension.
        let parser = Parser { ianav3plus: true, ..Parser::new(bytes) };
        parser.parse()
    }

    /// Like `TimeZone::parse`, but parses a prefix of the input given. In
    /// addition to returning a `TimeZone`, this also returns the offset into
    /// `bytes` pointing at the beginning of any remaining unparsed input.
    ///
    /// # Errors
    ///
    /// This returns an error if the given byte string is not a valid POSIX
    /// time zone transition rule.
    pub fn parse_prefix<'b, B: AsRef<[u8]> + ?Sized>(
        bytes: &'b B,
    ) -> Result<(TimeZone, usize), ParseError> {
        let bytes = bytes.as_ref();
        let parser = Parser { ianav3plus: true, ..Parser::new(bytes) };
        parser.parse_prefix()
    }
}

impl TimeZone {
    /// Returns the appropriate time zone offset to use for the given
    /// timestamp.
    ///
    /// If you need information like whether the offset is in DST or not, or
    /// the time zone abbreviation, then use `TimeZone::to_offset_info`.
    /// But that API may be more expensive to use, so only use it if you need
    /// the additional data.
    pub fn to_offset(&self, timestamp: Timestamp) -> Offset {
        if self.dst.is_none() {
            return self.std_offset;
        }

        let dt = timestamp.to_datetime(Offset::UTC);
        self.dst_info_utc(dt.date().year())
            .filter(|dst_info| dst_info.in_dst(dt))
            .map(|dst_info| dst_info.offset())
            .unwrap_or_else(|| self.std_offset)
    }

    /// Returns the appropriate time zone offset to use for the given
    /// timestamp.
    ///
    /// This also includes whether the offset returned should be considered
    /// to be "DST" or not, along with the time zone abbreviation (e.g., EST
    /// for standard time in New York, and EDT for DST in New York).
    pub fn to_offset_info(&self, timestamp: Timestamp) -> OffsetInfo {
        if self.dst.is_none() {
            return OffsetInfo {
                offset: self.std_offset,
                abbreviation: self.std_abbrev.clone(),
                dst: super::Dst::No,
            };
        }

        let dt = timestamp.to_datetime(Offset::UTC);
        self.dst_info_utc(dt.date().year())
            .filter(|dst_info| dst_info.in_dst(dt))
            .map(|dst_info| OffsetInfo {
                offset: dst_info.offset(),
                abbreviation: dst_info.dst.abbrev.clone(),
                dst: super::Dst::Yes,
            })
            .unwrap_or_else(|| OffsetInfo {
                offset: self.std_offset,
                abbreviation: self.std_abbrev.clone(),
                dst: super::Dst::No,
            })
    }

    /// Returns a possibly ambiguous timestamp for the given civil datetime.
    ///
    /// The given datetime should correspond to the "wall" clock time of what
    /// humans use to tell time for this time zone.
    ///
    /// Note that "ambiguous timestamp" is represented by the possible
    /// selection of offsets that could be applied to the given datetime. In
    /// general, it is only ambiguous around transitions to-and-from DST. The
    /// ambiguity can arise as a "fold" (when a particular wall clock time is
    /// repeated) or as a "gap" (when a particular wall clock time is skipped
    /// entirely).
    pub fn to_ambiguous_timestamp(&self, dt: DateTime) -> AmbiguousTimestamp {
        let year = dt.date().year();
        let std_offset = self.std_offset;
        let Some(dst_info) = self.dst_info_wall(year) else {
            return AmbiguousOffset::Unambiguous { offset: std_offset }
                .into_ambiguous_timestamp(dt);
        };
        let dst_offset = dst_info.offset();
        let diff = std_offset.until(dst_offset);
        // When the difference between DST and standard is positive, that means
        // STD->DST results in a gap while DST->STD results in a fold. However,
        // when the difference is negative, that means STD->DST results in a
        // fold while DST->STD results in a gap. The former is by far the most
        // common. The latter is a bit weird, but real cases do exist. For
        // example, Dublin has DST in winter (UTC+01) and STD in the summer
        // (UTC+00).
        //
        // When the difference is zero, then we have a weird POSIX time zone
        // where a DST transition rule was specified, but was set to explicitly
        // be the same as STD. In this case, there can be no ambiguity. (The
        // zero case is strictly redundant. Both the diff < 0 and diff > 0
        // cases handle the zero case correctly. But we write it out for
        // clarity.)
        let ambiguous_offset = if diff == 0 {
            debug_assert_eq!(std_offset, dst_offset);
            AmbiguousOffset::Unambiguous { offset: std_offset }
        } else if diff.is_negative() {
            // For DST transitions that always move behind one hour, ambiguous
            // timestamps only occur when the given civil datetime falls in the
            // standard time range.
            if dst_info.in_dst(dt) {
                AmbiguousOffset::Unambiguous { offset: dst_offset }
            } else {
                let fold_start = dst_info.start.saturating_add_seconds(diff);
                let gap_end =
                    dst_info.end.saturating_add_seconds(diff.saturating_neg());
                if fold_start <= dt && dt < dst_info.start {
                    AmbiguousOffset::Fold {
                        before: std_offset,
                        after: dst_offset,
                    }
                } else if dst_info.end <= dt && dt < gap_end {
                    AmbiguousOffset::Gap {
                        before: dst_offset,
                        after: std_offset,
                    }
                } else {
                    AmbiguousOffset::Unambiguous { offset: std_offset }
                }
            }
        } else {
            // For DST transitions that always move ahead one hour, ambiguous
            // timestamps only occur when the given civil datetime falls in the
            // DST range.
            if !dst_info.in_dst(dt) {
                AmbiguousOffset::Unambiguous { offset: std_offset }
            } else {
                // PERF: I wonder if it makes sense to pre-compute these?
                // Probably not, because we have to do it based on year of
                // datetime given. But if we ever add a "caching" layer for
                // POSIX time zones, then it might be worth adding these to it.
                let gap_end = dst_info.start.saturating_add_seconds(diff);
                let fold_start =
                    dst_info.end.saturating_add_seconds(diff.saturating_neg());
                if dst_info.start <= dt && dt < gap_end {
                    AmbiguousOffset::Gap {
                        before: std_offset,
                        after: dst_offset,
                    }
                } else if fold_start <= dt && dt < dst_info.end {
                    AmbiguousOffset::Fold {
                        before: dst_offset,
                        after: std_offset,
                    }
                } else {
                    AmbiguousOffset::Unambiguous { offset: dst_offset }
                }
            }
        };
        ambiguous_offset.into_ambiguous_timestamp(dt)
    }

    /// Returns the timestamp of the most recent time zone transition prior
    /// to the timestamp given. If one doesn't exist, `None` is returned.
    pub fn previous_transition(
        &self,
        timestamp: Timestamp,
    ) -> Option<Transition> {
        let dt = timestamp.to_datetime(Offset::UTC);
        let dst_info = self.dst_info_utc(dt.date().year())?;
        let (earlier, later) = dst_info.ordered();
        let (prev, dst_info) = if dt > later {
            (later, dst_info)
        } else if dt > earlier {
            (earlier, dst_info)
        } else {
            let prev_year = if dt.date().year() == Date::MIN.year() {
                return None;
            } else {
                dt.date().year() - 1
            };
            let dst_info = self.dst_info_utc(prev_year)?;
            let (_, later) = dst_info.ordered();
            (later, dst_info)
        };

        let timestamp = prev.to_timestamp(Offset::UTC).ok()?;
        let dt = timestamp.to_datetime(Offset::UTC);
        let (offset, abbreviation, dst) = if dst_info.in_dst(dt) {
            (dst_info.offset(), dst_info.dst.abbrev.clone(), super::Dst::Yes)
        } else {
            (self.std_offset, self.std_abbrev.clone(), super::Dst::No)
        };
        let info = OffsetInfo { offset, abbreviation, dst };
        Some(Transition { timestamp, info })
    }

    /// Returns the timestamp of the soonest time zone transition after the
    /// timestamp given. If one doesn't exist, `None` is returned.
    pub fn next_transition(&self, timestamp: Timestamp) -> Option<Transition> {
        let dt = timestamp.to_datetime(Offset::UTC);
        let dst_info = self.dst_info_utc(dt.date().year())?;
        let (earlier, later) = dst_info.ordered();
        let (next, dst_info) = if dt < earlier {
            (earlier, dst_info)
        } else if dt < later {
            (later, dst_info)
        } else {
            let next_year = if dt.date().year() == Date::MAX.year() {
                return None;
            } else {
                dt.date().year() + 1
            };
            let dst_info = self.dst_info_utc(next_year)?;
            let (earlier, _) = dst_info.ordered();
            (earlier, dst_info)
        };

        let timestamp = next.to_timestamp(Offset::UTC).ok()?;
        let dt = timestamp.to_datetime(Offset::UTC);
        let (offset, abbreviation, dst) = if dst_info.in_dst(dt) {
            (dst_info.offset(), dst_info.dst.abbrev.clone(), super::Dst::Yes)
        } else {
            (self.std_offset, self.std_abbrev.clone(), super::Dst::No)
        };
        let info = OffsetInfo { offset, abbreviation, dst };
        Some(Transition { timestamp, info })
    }

    /// Returns the range in which DST occurs.
    ///
    /// The civil datetimes returned are in UTC. This is useful for determining
    /// whether a timestamp is in DST or not.
    fn dst_info_utc(&self, year: i16) -> Option<DstInfo<'_>> {
        let dst = self.dst.as_ref()?;
        // DST time starts with respect to standard time, so offset it by the
        // standard offset.
        let start = dst.rule.start.to_datetime(year, self.std_offset);
        // DST time ends with respect to DST time, so offset it by the DST
        // offset.
        let mut end = dst.rule.end.to_datetime(year, dst.offset);
        // This is a whacky special case when DST is permanent, but the math
        // used to calculate the start/end datetimes ends up leaving a gap
        // for standard time to appear. In which case, it's possible for a
        // timestamp at the end of a calendar year to get standard time when
        // it really should be DST.
        //
        // We detect this case by re-interpreting the end of the boundary using
        // the standard offset. If we get a datetime that is in a different
        // year, then it follows that standard time is actually impossible to
        // occur.
        //
        // These weird POSIX time zones can occur as the TZ strings in
        // a TZif file compiled using rearguard semantics. For example,
        // `Africa/Casablanca` has:
        //
        //     XXX-2<+01>-1,0/0,J365/23
        //
        // Notice here that DST is actually one hour *behind* (it is usually
        // one hour *ahead*) _and_ it ends at 23:00:00 on the last day of the
        // year. But if it ends at 23:00, then jumping to standard time moves
        // the clocks *forward*. Which would bring us to 00:00:00 on the first
        // of the next year... but that is when DST begins! Hence, DST is
        // permanent.
        //
        // Ideally, this could just be handled by our math automatically. But
        // I couldn't figure out how to make it work. In particular, in the
        // above example for year 2087, we get
        //
        //     start == 2087-01-01T00:00:00Z
        //     end == 2087-12-31T22:00:00Z
        //
        // Which leaves a two hour gap for a timestamp to get erroneously
        // categorized as standard time.
        //
        // ... so we special case this. We could pre-compute whether a POSIX
        // time zone is in permanent DST at construction time, but it's not
        // obvious to me that it's worth it. Especially since this is an
        // exceptionally rare case.
        //
        // Note that I did try to consult tzcode's (incredibly inscrutable)
        // `localtime` implementation to figure out how they deal with it. At
        // first, it looks like they don't have any special handling for this
        // case. But looking more closely, they skip any time zone transitions
        // generated by POSIX time zones whose rule spans more than 1 year:
        //
        //     https://github.com/eggert/tz/blob/8d65db9786753f3b263087e31c59d191561d63e3/localtime.c#L1717-L1735
        //
        // By just ignoring them, I think it achieves the desired effect of
        // permanent DST. But I'm not 100% confident in my understanding of
        // the code.
        if start.date().month() == 1
            && start.date().day() == 1
            && start.time() == Time::MIN
            // NOTE: This should come last because it is potentially expensive.
            && year
                != end.saturating_add_seconds(self.std_offset.seconds()).date().year()
        {
            end = DateTime::from_parts(
                Date::new(year, 12, 31)
                    .expect("12/31 is valid for all valid years"),
                Time::MAX,
            );
        }
        Some(DstInfo { dst, start, end })
    }

    /// Returns the range in which DST occurs.
    ///
    /// The civil datetimes returned are in "wall clock time." That is, they
    /// represent the transitions as they are seen from humans reading a clock
    /// within the geographic location of that time zone.
    fn dst_info_wall(&self, year: i16) -> Option<DstInfo<'_>> {
        let dst = self.dst.as_ref()?;
        // POSIX time zones express their DST transitions in terms of wall
        // clock time. Since this method specifically is returning wall
        // clock times, we don't want to offset our datetimes at all.
        let start = dst.rule.start.to_datetime(year, Offset::UTC);
        let end = dst.rule.end.to_datetime(year, Offset::UTC);
        Some(DstInfo { dst, start, end })
    }

    /// Returns the DST transition rule. This panics if this time zone doesn't
    /// have DST.
    #[cfg(test)]
    fn rule(&self) -> &Rule {
        &self.dst.as_ref().unwrap().rule
    }
}

impl DayTime {
    /// Turns this POSIX datetime spec into a civil datetime in the year given
    /// with the given offset. The datetimes returned are offset by the given
    /// offset. For wall clock time, an offset of `0` should be given. For
    /// UTC time, the offset (standard or DST) corresponding to this time
    /// spec should be given.
    ///
    /// The datetime returned is guaranteed to have a year component equal
    /// to the year given. This guarantee is upheld even when the datetime
    /// specification (combined with the offset) would extend past the end of
    /// the year (or before the start of the year). In this case, the maximal
    /// (or minimal) datetime for the given year is returned.
    fn to_datetime(&self, year: i16, offset: Offset) -> DateTime {
        let mkmin =
            || DateTime::from_parts(Date::new(year, 1, 1).unwrap(), Time::MIN);
        let mkmax = || {
            DateTime::from_parts(Date::new(year, 12, 31).unwrap(), Time::MAX)
        };
        let Some(date) = self.date.to_date(year) else { return mkmax() };
        // The range on `self.time` is `-604799..=604799`, and the range
        // on `offset.second` is `-93599..=93599`. Therefore, subtracting
        // them can never overflow an `i32`.
        let offset = self.time.second - offset.seconds();
        // If the time goes negative or above 86400, then we might have
        // to adjust our date.
        let days = offset.div_euclid(86400);
        let second = offset.rem_euclid(86400);

        let Ok(date) = date.checked_add(days) else {
            return if offset < 0 { mkmin() } else { mkmax() };
        };
        if date.year() < year {
            mkmin()
        } else if date.year() > year {
            mkmax()
        } else {
            // OK because we just did modulo 86400 above.
            let time = TimeSecond::new(second).unwrap().to_time();
            DateTime::from_parts(date, time)
        }
    }
}

impl Day {
    /// Convert this date specification to a civil date in the year given.
    ///
    /// If this date specification couldn't be turned into a date in the year
    /// given, then `None` is returned. This happens when `366` is given as
    /// a day, but the year given is not a leap year. In this case, callers may
    /// want to assume a datetime that is maximal for the year given.
    fn to_date(&self, year: i16) -> Option<Date> {
        match *self {
            Day::JulianOne(day) => {
                // Parsing validates that our day is 1-365 which will always
                // succeed for all possible year values. That is, every valid
                // year has a December 31.
                Some(
                    Date::from_day_of_year_no_leap(year, day)
                        .expect("Julian `J day` should be in bounds"),
                )
            }
            Day::JulianZero(day) => {
                // OK because our value for `day` is validated to be `0..=365`,
                // and since it is an `i16`, it is always valid to add 1.
                //
                // Also, while `day+1` is guaranteed to be in `1..=366`, it is
                // possible that `366` is invalid, for when `year` is not a
                // leap year. In this case, we throw our hands up, and ask the
                // caller to make a decision for how to deal with it. Why does
                // POSIX go out of its way to specifically not specify behavior
                // in error cases?
                Date::from_day_of_year(year, day + 1).ok()
            }
            Day::WeekdayOfMonth { month, week, weekday } => {
                let first = Date::new(year, month, 1)
                    .expect("all valid year/month combinations support day 1");
                let week = if week == 5 { -1 } else { week };
                debug_assert!(week == -1 || (1..=4).contains(&week));
                // This is maybe non-obvious, but this will always succeed
                // because it can only fail when the week number is one of
                // {-5, 0, 5}. Since we've validated that 'week' is in 1..=5,
                // we know it can't be 0. Moreover, because of the conditional
                // above and since `5` actually means "last weekday of month,"
                // that case will always translate to `-1`.
                //
                // Also, I looked at how other libraries deal with this case,
                // and almost all of them just do a bunch of inline hairy
                // arithmetic here. I suppose I could be reduced to such
                // things if perf called for it, but we have a nice civil date
                // abstraction. So use it, god damn it. (Well, we did, and now
                // we have a lower level IDate abstraction. But it's still
                // an abstraction!)
                Some(
                    first
                        .nth_weekday_of_month(week, weekday)
                        .expect("nth weekday always exists"),
                )
            }
        }
    }
}

impl TransitionCivilTime {
    /// The "default" time for a time zone transition to occur.
    ///
    /// This is used, as specified by POSIX, whenever a specific time of day
    /// is omitted from a POSIX time zone transition rule string.
    pub const DEFAULT: TransitionCivilTime =
        TransitionCivilTime { second: 2 * 60 * 60 };
}

/// The daylight saving time (DST) info for a POSIX time zone in a particular
/// year.
#[derive(Debug, Eq, PartialEq)]
struct DstInfo<'a> {
    /// The DST transition rule that generated this info.
    dst: &'a Dst,
    /// The start time (inclusive) that DST begins.
    ///
    /// Note that this may be greater than `end`. This tends to happen in the
    /// southern hemisphere.
    ///
    /// Note also that this may be in UTC or in wall clock civil
    /// time. It depends on whether `TimeZone::dst_info_utc` or
    /// `TimeZone::dst_info_wall` was used.
    start: DateTime,
    /// The end time (exclusive) that DST ends.
    ///
    /// Note that this may be less than `start`. This tends to happen in the
    /// southern hemisphere.
    ///
    /// Note also that this may be in UTC or in wall clock civil
    /// time. It depends on whether `TimeZone::dst_info_utc` or
    /// `TimeZone::dst_info_wall` was used.
    end: DateTime,
}

impl<'a> DstInfo<'a> {
    /// Returns true if and only if the given civil datetime ought to be
    /// considered in DST.
    fn in_dst(&self, utc_dt: DateTime) -> bool {
        if self.start <= self.end {
            self.start <= utc_dt && utc_dt < self.end
        } else {
            !(self.end <= utc_dt && utc_dt < self.start)
        }
    }

    /// Returns the earlier and later times for this DST info.
    fn ordered(&self) -> (DateTime, DateTime) {
        if self.start <= self.end {
            (self.start, self.end)
        } else {
            (self.end, self.start)
        }
    }

    /// Returns the DST offset.
    fn offset(&self) -> Offset {
        self.dst.offset
    }
}

/// A parser for POSIX time zones.
#[derive(Debug)]
struct Parser<'s> {
    /// The `TZ` string that we're parsing.
    tz: &'s [u8],
    /// The parser's current position in `tz`.
    pos: core::cell::Cell<usize>,
    /// Whether to use IANA rules, i.e., when parsing a TZ string in a TZif
    /// file of version 3 or greater. From `tzfile(5)`:
    ///
    /// > First, the hours part of its transition times may be signed and range
    /// > from `-167` through `167` instead of the POSIX-required unsigned
    /// > values from `0` through `24`. Second, DST is in effect all year if
    /// > it starts January 1 at 00:00 and ends December 31 at 24:00 plus the
    /// > difference between daylight saving and standard time.
    ///
    /// At time of writing, I don't think I understand the significance of
    /// the second part above. (RFC 8536 elaborates that it is meant to be an
    /// explicit clarification of something that POSIX itself implies.) But the
    /// first part is clear: it permits the hours to be a bigger range.
    ianav3plus: bool,
}

impl<'s> Parser<'s> {
    /// Create a new parser for extracting a POSIX time zone from the given
    /// bytes.
    fn new<B: ?Sized + AsRef<[u8]>>(tz: &'s B) -> Parser<'s> {
        Parser {
            tz: tz.as_ref(),
            pos: core::cell::Cell::new(0),
            ianav3plus: false,
        }
    }

    /// Parses a POSIX time zone from the current position of the parser and
    /// ensures that the entire TZ string corresponds to a single valid POSIX
    /// time zone.
    fn parse(&self) -> Result<TimeZone, ParseError> {
        let (time_zone, len) = self.parse_prefix()?;
        if !self.tz[len..].is_empty() {
            return Err(ParseErrorKind::FoundRemaining.into());
        }
        Ok(time_zone)
    }

    /// Parses a POSIX time zone from the current position of the parser and
    /// returns the remaining input.
    fn parse_prefix(&self) -> Result<(TimeZone, usize), ParseError> {
        let time_zone = self.parse_posix_time_zone()?;
        Ok((time_zone, self.pos()))
    }

    /// Parse a POSIX time zone from the current position of the parser.
    ///
    /// Upon success, the parser will be positioned immediately following the
    /// TZ string.
    #[inline(never)] // avoid making multiple copies of the parser
    fn parse_posix_time_zone(&self) -> Result<TimeZone, ParseError> {
        if self.is_done() {
            return Err(ParseErrorKind::Empty.into());
        }
        let std_abbrev = self
            .parse_abbreviation()
            .map_err(ParseErrorKind::AbbreviationStd)?;
        let std_offset =
            self.parse_posix_offset().map_err(ParseErrorKind::OffsetStd)?;
        let mut dst = None;
        if !self.is_done()
            && (self.byte().is_ascii_alphabetic() || self.byte() == b'<')
        {
            dst = Some(self.parse_posix_dst(std_offset)?);
        }
        Ok(TimeZone { std_abbrev, std_offset, dst })
    }

    /// Parse a DST zone with an optional explicit transition rule.
    ///
    /// This assumes the parser is positioned at the first byte of the DST
    /// abbreviation.
    ///
    /// Upon success, the parser will be positioned immediately after the end
    /// of the DST transition rule (which might just be the abbreviation, but
    /// might also include explicit start/end datetime specifications).
    fn parse_posix_dst(&self, std_offset: Offset) -> Result<Dst, ParseError> {
        let abbrev = self
            .parse_abbreviation()
            .map_err(ParseErrorKind::AbbreviationDst)?;
        if self.is_done() {
            return Err(ParseErrorKind::FoundDstNoRule.into());
        }
        // This is the default: one hour ahead of standard time. We may
        // override this if the DST portion specifies an offset. (But it
        // usually doesn't.)
        //
        // This unwrap is okay, but in a subtle way. We ensure that all PARSED
        // offsets are 24:59:59 or less. But the maximum offset is 25:59:59.
        // It was specifically setup that way so as to make this addition work.
        let mut offset = std_offset.checked_add(3600).unwrap();
        if self.byte() != b',' {
            offset = self
                .parse_posix_offset()
                .map_err(ParseErrorKind::OffsetDst)?;
            if self.is_done() {
                return Err(ParseErrorKind::FoundDstNoRuleWithOffset.into());
            }
        }
        if self.byte() != b',' {
            return Err(ParseErrorKind::ExpectedCommaAfterDst.into());
        }
        if !self.bump() {
            return Err(ParseErrorKind::FoundEndAfterComma.into());
        }
        let rule = self.parse_rule().map_err(ParseErrorKind::Rule)?;
        Ok(Dst { abbrev, offset, rule })
    }

    /// Parse a time zone abbreviation.
    ///
    /// This assumes the parser is positioned at the first byte of
    /// the abbreviation. This is either the first character in the
    /// abbreviation, or the opening quote of a quoted abbreviation.
    ///
    /// Upon success, the parser will be positioned immediately following
    /// the abbreviation name.
    ///
    /// The string returned is guaranteed to be no more than 30 bytes.
    /// (This restriction is somewhat arbitrary, but it's so we can put
    /// the abbreviation in a fixed capacity array.)
    fn parse_abbreviation(&self) -> Result<Abbreviation, AbbreviationError> {
        if self.byte() == b'<' {
            if !self.bump() {
                return Err(AbbreviationError::Quoted(
                    QuotedAbbreviationError::UnexpectedEndAfterOpening,
                ));
            }
            self.parse_quoted_abbreviation().map_err(AbbreviationError::Quoted)
        } else {
            self.parse_unquoted_abbreviation()
                .map_err(AbbreviationError::Unquoted)
        }
    }

    /// Parses an unquoted time zone abbreviation.
    ///
    /// This assumes the parser is position at the first byte in the
    /// abbreviation.
    ///
    /// Upon success, the parser will be positioned immediately after the
    /// last byte in the abbreviation.
    ///
    /// The string returned is guaranteed to be no more than 30 bytes.
    /// (This restriction is somewhat arbitrary, but it's so we can put
    /// the abbreviation in a fixed capacity array.)
    fn parse_unquoted_abbreviation(
        &self,
    ) -> Result<Abbreviation, UnquotedAbbreviationError> {
        let start = self.pos();
        for _ in 0.. {
            if !self.byte().is_ascii_alphabetic() {
                break;
            }
            if self.pos() - start >= REASONABLE_ABBREVIATION_MAX {
                return Err(UnquotedAbbreviationError::TooLong);
            }
            if !self.bump() {
                break;
            }
        }
        let end = self.pos();
        let abbrev =
            core::str::from_utf8(&self.tz[start..end]).map_err(|_| {
                // NOTE: I believe this error is technically impossible
                // since the loop above restricts letters in an
                // abbreviation to ASCII. So everything from `start` to
                // `end` is ASCII and thus should be UTF-8. But it doesn't
                // cost us anything to report an error here in case the
                // code above evolves somehow.
                UnquotedAbbreviationError::InvalidUtf8
            })?;
        if abbrev.len() < 3 {
            return Err(UnquotedAbbreviationError::TooShort);
        }
        Abbreviation::new(abbrev).ok_or(UnquotedAbbreviationError::TooLong)
    }

    /// Parses a quoted time zone abbreviation.
    ///
    /// This assumes the parser is positioned immediately after the opening
    /// `<` quote. That is, at the first byte in the abbreviation.
    ///
    /// Upon success, the parser will be positioned immediately after the
    /// closing `>` quote.
    ///
    /// The string returned is guaranteed to be no more than 30 bytes.
    /// (This restriction is somewhat arbitrary, but it's so we can put
    /// the abbreviation in a fixed capacity array.)
    fn parse_quoted_abbreviation(
        &self,
    ) -> Result<Abbreviation, QuotedAbbreviationError> {
        let start = self.pos();
        for _ in 0.. {
            if !self.byte().is_ascii_alphanumeric()
                && self.byte() != b'+'
                && self.byte() != b'-'
            {
                break;
            }
            if self.pos() - start >= REASONABLE_ABBREVIATION_MAX {
                return Err(QuotedAbbreviationError::TooLong);
            }
            if !self.bump() {
                break;
            }
        }
        let end = self.pos();
        let abbrev =
            core::str::from_utf8(&self.tz[start..end]).map_err(|_| {
                // NOTE: I believe this error is technically impossible
                // since the loop above restricts letters in an
                // abbreviation to ASCII. So everything from `start` to
                // `end` is ASCII and thus should be UTF-8. But it doesn't
                // cost us anything to report an error here in case the
                // code above evolves somehow.
                QuotedAbbreviationError::InvalidUtf8
            })?;
        if self.is_done() {
            return Err(QuotedAbbreviationError::UnexpectedEnd);
        }
        if self.byte() != b'>' {
            return Err(QuotedAbbreviationError::UnexpectedLastByte);
        }
        self.bump();
        if abbrev.len() < 3 {
            return Err(QuotedAbbreviationError::TooShort);
        }
        Abbreviation::new(abbrev).ok_or(QuotedAbbreviationError::TooLong)
    }

    /// Parse a POSIX time offset.
    ///
    /// This assumes the parser is positioned at the first byte of the
    /// offset. This can either be a digit (for a positive offset) or the
    /// sign of the offset (which must be either `-` or `+`).
    ///
    /// Upon success, the parser will be positioned immediately after the
    /// end of the offset.
    fn parse_posix_offset(&self) -> Result<Offset, OffsetError> {
        let sign = self.parse_optional_sign()?.unwrap_or(1);
        let hour = self.parse_hour_posix()?;
        let (mut minute, mut second) = (0, 0);
        if self.maybe_byte() == Some(b':') {
            if !self.bump() {
                return Err(OffsetError::IncompleteMinutes);
            }
            minute = self.parse_minute()?;
            if self.maybe_byte() == Some(b':') {
                if !self.bump() {
                    return Err(OffsetError::IncompleteSeconds);
                }
                second = self.parse_second()?;
            }
        }
        let mut offset = Offset::from_hours(hour).expect("hours are valid");
        offset += i32::from(minute) * 60;
        offset += i32::from(second);
        // Yes, we flip the sign, because POSIX is backwards.
        // For example, `EST5` corresponds to `-05:00`.
        if sign.is_positive() {
            offset = -offset;
        }
        // Must be true because the parsing routines for hours, minutes
        // and seconds enforce they are in the ranges -24..=24, 0..=59
        // and 0..=59, respectively.
        assert!(
            -89999 <= offset.seconds() && offset.seconds() <= 89999,
            "POSIX offset seconds {} is out of range",
            offset.seconds(),
        );
        Ok(offset)
    }

    /// Parses a POSIX DST transition rule.
    ///
    /// This assumes the parser is positioned at the first byte in the
    /// rule. That is, it comes immediately after the DST abbreviation or
    /// its optional offset.
    ///
    /// Upon success, the parser will be positioned immediately after the
    /// DST transition rule. In typical cases, this corresponds to the end
    /// of the TZ string.
    fn parse_rule(&self) -> Result<Rule, RuleError> {
        let start =
            self.parse_posix_datetime().map_err(RuleError::DateTimeStart)?;
        if self.maybe_byte() != Some(b',') || !self.bump() {
            return Err(RuleError::ExpectedEnd);
        }
        let end =
            self.parse_posix_datetime().map_err(RuleError::DateTimeEnd)?;
        Ok(Rule { start, end })
    }

    /// Parses a POSIX datetime specification.
    ///
    /// This assumes the parser is position at the first byte where a
    /// datetime specification is expected to occur.
    ///
    /// Upon success, the parser will be positioned after the datetime
    /// specification. This will either be immediately after the date, or
    /// if it's present, the time part of the specification.
    fn parse_posix_datetime(&self) -> Result<DayTime, DateTimeError> {
        let mut daytime = DayTime {
            date: self.parse_posix_date()?,
            time: TransitionCivilTime::DEFAULT,
        };
        if self.maybe_byte() != Some(b'/') {
            return Ok(daytime);
        }
        if !self.bump() {
            return Err(DateTimeError::ExpectedTime);
        }
        daytime.time = self.parse_posix_time()?;
        Ok(daytime)
    }

    /// Parses a POSIX date specification.
    ///
    /// This assumes the parser is positioned at the first byte of the date
    /// specification. This can be `J` (for one based Julian day without
    /// leap days), `M` (for "weekday of month") or a digit starting the
    /// zero based Julian day with leap days. This routine will validate
    /// that the position points to one of these possible values. That is,
    /// the caller doesn't need to parse the `M` or the `J` or the leading
    /// digit. The caller should just call this routine when it *expect* a
    /// date specification to follow.
    ///
    /// Upon success, the parser will be positioned immediately after the
    /// date specification.
    fn parse_posix_date(&self) -> Result<Day, DateError> {
        match self.byte() {
            b'J' => {
                if !self.bump() {
                    return Err(DateError::ExpectedJulianNoLeap);
                }
                Ok(Day::JulianOne(self.parse_posix_julian_day_no_leap()?))
            }
            b'0'..=b'9' => {
                Ok(Day::JulianZero(self.parse_posix_julian_day_with_leap()?))
            }
            b'M' => {
                if !self.bump() {
                    return Err(DateError::ExpectedMonthWeekWeekday);
                }
                let (month, week, weekday) = self.parse_weekday_of_month()?;
                Ok(Day::WeekdayOfMonth { month, week, weekday })
            }
            _ => Err(DateError::UnexpectedByte),
        }
    }

    /// Parses a POSIX Julian day that does not include leap days
    /// (`1 <= n <= 365`).
    ///
    /// This assumes the parser is positioned just after the `J` and at the
    /// first digit of the Julian day. Upon success, the parser will be
    /// positioned immediately following the day number.
    fn parse_posix_julian_day_no_leap(
        &self,
    ) -> Result<i16, JulianNoLeapError> {
        let number = self
            .parse_number_with_upto_n_digits(3)
            .map_err(JulianNoLeapError::Parse)?;
        let number =
            i16::try_from(number).map_err(|_| JulianNoLeapError::Range)?;
        if !(1 <= number && number <= 365) {
            return Err(JulianNoLeapError::Range);
        }
        Ok(number)
    }

    /// Parses a POSIX Julian day that includes leap days (`0 <= n <=
    /// 365`).
    ///
    /// This assumes the parser is positioned at the first digit of the
    /// Julian day. Upon success, the parser will be positioned immediately
    /// following the day number.
    fn parse_posix_julian_day_with_leap(
        &self,
    ) -> Result<i16, JulianLeapError> {
        let number = self
            .parse_number_with_upto_n_digits(3)
            .map_err(JulianLeapError::Parse)?;
        let number =
            i16::try_from(number).map_err(|_| JulianLeapError::Range)?;
        if !(0 <= number && number <= 365) {
            return Err(JulianLeapError::Range);
        }
        Ok(number)
    }

    /// Parses a POSIX "weekday of month" specification.
    ///
    /// This assumes the parser is positioned just after the `M` byte and
    /// at the first digit of the month. Upon success, the parser will be
    /// positioned immediately following the "weekday of the month" that
    /// was parsed.
    ///
    /// The tuple returned is month (1..=12), week (1..=5) and weekday
    /// (0..=6 with 0=Sunday).
    fn parse_weekday_of_month(
        &self,
    ) -> Result<(i8, i8, Weekday), WeekdayOfMonthError> {
        let month = self.parse_month()?;
        if self.maybe_byte() != Some(b'.') {
            return Err(WeekdayOfMonthError::ExpectedDotAfterMonth);
        }
        if !self.bump() {
            return Err(WeekdayOfMonthError::ExpectedWeekAfterMonth);
        }
        let week = self.parse_week()?;
        if self.maybe_byte() != Some(b'.') {
            return Err(WeekdayOfMonthError::ExpectedDotAfterWeek);
        }
        if !self.bump() {
            return Err(WeekdayOfMonthError::ExpectedDayOfWeekAfterWeek);
        }
        let weekday = self.parse_weekday()?;
        Ok((month, week, weekday))
    }

    /// This parses a POSIX time specification in the format
    /// `[+/-]hh?[:mm[:ss]]`.
    ///
    /// This assumes the parser is positioned at the first `h` (or the
    /// sign, if present). Upon success, the parser will be positioned
    /// immediately following the end of the time specification.
    fn parse_posix_time(&self) -> Result<TransitionCivilTime, TimeError> {
        let (sign, hour) = if self.ianav3plus {
            let sign = self.parse_optional_sign()?.unwrap_or(1);
            let hour = self.parse_hour_ianav3plus()?;
            (sign, hour)
        } else {
            (1, i16::from(self.parse_hour_posix()?))
        };
        let (mut minute, mut second) = (0, 0);
        if self.maybe_byte() == Some(b':') {
            if !self.bump() {
                return Err(TimeError::IncompleteMinutes);
            }
            minute = self.parse_minute()?;
            if self.maybe_byte() == Some(b':') {
                if !self.bump() {
                    return Err(TimeError::IncompleteSeconds);
                }
                second = self.parse_second()?;
            }
        }
        let mut time = TransitionCivilTime { second: i32::from(hour) * 3600 };
        time.second += i32::from(minute) * 60;
        time.second += i32::from(second);
        time.second *= i32::from(sign);
        // Must be true because the parsing routines for hours, minutes
        // and seconds enforce they are in the ranges -167..=167, 0..=59
        // and 0..=59, respectively.
        assert!(
            -604799 <= time.second && time.second <= 604799,
            "POSIX time seconds {} is out of range",
            time.second
        );
        Ok(time)
    }

    /// Parses a month.
    ///
    /// This is expected to be positioned at the first digit. Upon success,
    /// the parser will be positioned after the month (which may contain
    /// two digits).
    fn parse_month(&self) -> Result<i8, MonthError> {
        let number = self
            .parse_number_with_upto_n_digits(2)
            .map_err(MonthError::Parse)?;
        let number = i8::try_from(number).map_err(|_| MonthError::Range)?;
        if !(1 <= number && number <= 12) {
            return Err(MonthError::Range);
        }
        Ok(number)
    }

    /// Parses a week-of-month number.
    ///
    /// This is expected to be positioned at the first digit. Upon success,
    /// the parser will be positioned after the week digit.
    fn parse_week(&self) -> Result<i8, WeekOfMonthError> {
        let number = self
            .parse_number_with_exactly_n_digits(1)
            .map_err(WeekOfMonthError::Parse)?;
        let number =
            i8::try_from(number).map_err(|_| WeekOfMonthError::Range)?;
        if !(1 <= number && number <= 5) {
            return Err(WeekOfMonthError::Range);
        }
        Ok(number)
    }

    /// Parses a weekday number.
    ///
    /// This is expected to be positioned at the first digit. Upon success,
    /// the parser will be positioned after the week digit.
    ///
    /// The weekday returned is guaranteed to be in the range `0..=6`, with
    /// `0` corresponding to Sunday.
    fn parse_weekday(&self) -> Result<Weekday, WeekdayError> {
        let number = self
            .parse_number_with_exactly_n_digits(1)
            .map_err(WeekdayError::Parse)?;
        let number = i8::try_from(number).map_err(|_| WeekdayError::Range)?;

        Weekday::from_sunday_zero_offset(number)
            .map_err(|_| WeekdayError::Range)
    }

    /// Parses an hour from a POSIX time specification with the IANA
    /// v3+ extension. That is, the hour may be in the range `0..=167`.
    /// (Callers should parse an optional sign preceding the hour digits
    /// when IANA V3+ parsing is enabled.)
    ///
    /// The hour is allowed to be a single digit (unlike minutes or
    /// seconds).
    ///
    /// This assumes the parser is positioned at the position where the
    /// first hour digit should occur. Upon success, the parser will be
    /// positioned immediately after the last hour digit.
    fn parse_hour_ianav3plus(&self) -> Result<i16, HourIanaError> {
        // Callers should only be using this method when IANA v3+ parsing
        // is enabled.
        assert!(self.ianav3plus);
        let number = self
            .parse_number_with_upto_n_digits(3)
            .map_err(HourIanaError::Parse)?;
        let number =
            i16::try_from(number).map_err(|_| HourIanaError::Range)?;
        if !(0 <= number && number <= 167) {
            // The error message says -167 but the check above uses 0.
            // This is because the caller is responsible for parsing
            // the sign.
            return Err(HourIanaError::Range);
        }
        Ok(number)
    }

    /// Parses an hour from a POSIX time specification, with the allowed
    /// range being `0..=24`.
    ///
    /// The hour is allowed to be a single digit (unlike minutes or
    /// seconds).
    ///
    /// This assumes the parser is positioned at the position where the
    /// first hour digit should occur. Upon success, the parser will be
    /// positioned immediately after the last hour digit.
    fn parse_hour_posix(&self) -> Result<i8, HourPosixError> {
        let number = self
            .parse_number_with_upto_n_digits(2)
            .map_err(HourPosixError::Parse)?;
        let number =
            i8::try_from(number).map_err(|_| HourPosixError::Range)?;
        if !(0 <= number && number <= 24) {
            return Err(HourPosixError::Range);
        }
        Ok(number)
    }

    /// Parses a minute from a POSIX time specification.
    ///
    /// The minute must be exactly two digits.
    ///
    /// This assumes the parser is positioned at the position where the
    /// first minute digit should occur. Upon success, the parser will be
    /// positioned immediately after the second minute digit.
    fn parse_minute(&self) -> Result<i8, MinuteError> {
        let number = self
            .parse_number_with_exactly_n_digits(2)
            .map_err(MinuteError::Parse)?;
        let number = i8::try_from(number).map_err(|_| MinuteError::Range)?;
        if !(0 <= number && number <= 59) {
            return Err(MinuteError::Range);
        }
        Ok(number)
    }

    /// Parses a second from a POSIX time specification.
    ///
    /// The second must be exactly two digits.
    ///
    /// This assumes the parser is positioned at the position where the
    /// first second digit should occur. Upon success, the parser will be
    /// positioned immediately after the second second digit.
    fn parse_second(&self) -> Result<i8, SecondError> {
        let number = self
            .parse_number_with_exactly_n_digits(2)
            .map_err(SecondError::Parse)?;
        let number = i8::try_from(number).map_err(|_| SecondError::Range)?;
        if !(0 <= number && number <= 59) {
            return Err(SecondError::Range);
        }
        Ok(number)
    }

    /// Parses a signed 64-bit integer expressed in exactly `n` digits.
    ///
    /// If `n` digits could not be found (or if the `TZ` string ends before
    /// `n` digits could be found), then this returns an error.
    ///
    /// This assumes that `n >= 1` and that the parser is positioned at the
    /// first digit. Upon success, the parser is positioned immediately
    /// after the `n`th digit.
    fn parse_number_with_exactly_n_digits(
        &self,
        n: usize,
    ) -> Result<i32, NumberError> {
        assert!(n >= 1, "numbers must have at least 1 digit");
        let mut number: i32 = 0;
        for _ in 0..n {
            if self.is_done() {
                return Err(NumberError::ExpectedLength);
            }
            let byte = self.byte();
            let digit = match byte.checked_sub(b'0') {
                None => {
                    return Err(NumberError::InvalidDigit);
                }
                Some(digit) if digit > 9 => {
                    return Err(NumberError::InvalidDigit);
                }
                Some(digit) => {
                    debug_assert!((0..=9).contains(&digit));
                    i32::from(digit)
                }
            };
            number = number
                .checked_mul(10)
                .and_then(|n| n.checked_add(digit))
                .ok_or(NumberError::TooBig)?;
            self.bump();
        }
        Ok(number)
    }

    /// Parses a signed 64-bit integer expressed with up to `n` digits and
    /// at least 1 digit.
    ///
    /// This assumes that `n >= 1` and that the parser is positioned at the
    /// first digit. Upon success, the parser is position immediately after
    /// the last digit (which can be at most `n`).
    fn parse_number_with_upto_n_digits(
        &self,
        n: usize,
    ) -> Result<i32, NumberError> {
        assert!(n >= 1, "numbers must have at least 1 digit");
        let mut number: i32 = 0;
        for i in 0..n {
            if self.is_done() || !self.byte().is_ascii_digit() {
                if i == 0 {
                    return Err(NumberError::Empty);
                }
                break;
            }
            let digit = i32::from(self.byte() - b'0');
            number = number
                .checked_mul(10)
                .and_then(|n| n.checked_add(digit))
                .ok_or(NumberError::TooBig)?;
            self.bump();
        }
        Ok(number)
    }

    /// Parses an optional sign.
    ///
    /// This assumes the parser is positioned at the position where a
    /// positive or negative sign is permitted. If one exists, then it
    /// is consumed and returned. Moreover, if one exists, then this
    /// guarantees that it is not the last byte in the input. That is, upon
    /// success, it is valid to call `self.byte()`.
    fn parse_optional_sign(&self) -> Result<Option<i8>, OptionalSignError> {
        if self.is_done() {
            return Ok(None);
        }
        Ok(match self.byte() {
            b'-' => {
                if !self.bump() {
                    return Err(OptionalSignError::ExpectedDigitAfterMinus);
                }
                Some(-1)
            }
            b'+' => {
                if !self.bump() {
                    return Err(OptionalSignError::ExpectedDigitAfterPlus);
                }
                Some(1)
            }
            _ => None,
        })
    }
}

/// Helper routines for parsing a POSIX `TZ` string.
impl<'s> Parser<'s> {
    /// Bump the parser to the next byte.
    ///
    /// If the end of the input has been reached, then `false` is returned.
    fn bump(&self) -> bool {
        if self.is_done() {
            return false;
        }
        self.pos.set(
            self.pos().checked_add(1).expect("pos cannot overflow usize"),
        );
        !self.is_done()
    }

    /// Returns true if the next call to `bump` would return false.
    fn is_done(&self) -> bool {
        self.pos() == self.tz.len()
    }

    /// Return the byte at the current position of the parser.
    ///
    /// This panics if the parser is positioned at the end of the TZ
    /// string.
    fn byte(&self) -> u8 {
        self.tz[self.pos()]
    }

    /// Return the byte at the current position of the parser. If the TZ
    /// string has been exhausted, then this returns `None`.
    fn maybe_byte(&self) -> Option<u8> {
        self.tz.get(self.pos()).copied()
    }

    /// Return the current byte offset of the parser.
    ///
    /// The offset starts at `0` from the beginning of the TZ string.
    fn pos(&self) -> usize {
        self.pos.get()
    }
}

/// An error that can occur when parsing a POSIX time zone string.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
pub struct ParseError {
    kind: ParseErrorKind,
}

#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
enum ParseErrorKind {
    AbbreviationDst(AbbreviationError),
    AbbreviationStd(AbbreviationError),
    Empty,
    ExpectedCommaAfterDst,
    FoundDstNoRule,
    FoundDstNoRuleWithOffset,
    FoundEndAfterComma,
    FoundRemaining,
    OffsetDst(OffsetError),
    OffsetStd(OffsetError),
    Rule(RuleError),
    TzEnvColonTooBig,
    TzEnvColonInvalidUtf8,
    #[allow(dead_code)] // not used when std is disabled
    TzEnvInvalidUtf8,
}

impl core::fmt::Display for ParseError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        use self::ParseErrorKind::*;
        match self.kind {
            AbbreviationDst(ref err) => {
                f.write_str("failed to parse DST time zone abbreviation: ")?;
                core::fmt::Display::fmt(err, f)
            }
            AbbreviationStd(ref err) => {
                f.write_str(
                    "failed to parse standard time zone abbreviation: ",
                )?;
                core::fmt::Display::fmt(err, f)
            }
            Empty => f.write_str(
                "an empty string is not a valid POSIX time zone \
                 transition rule",
            ),
            ExpectedCommaAfterDst => f.write_str(
                "expected `,` after parsing DST offset \
                 in POSIX time zone string",
            ),
            FoundDstNoRule => f.write_str(
                "found DST abbreviation in POSIX time zone string, \
                 but no transition rule \
                 (this is technically allowed by POSIX, but has \
                 unspecified behavior)",
            ),
            FoundDstNoRuleWithOffset => f.write_str(
                "found DST abbreviation and offset in POSIX time zone string, \
                 but no transition rule \
                 (this is technically allowed by POSIX, but has \
                 unspecified behavior)",
            ),
            FoundEndAfterComma => f.write_str(
                "after parsing DST offset in POSIX time zone string, \
                 found end of string after a trailing `,`",
            ),
            FoundRemaining => f.write_str(
                "expected entire POSIX TZ string to be a valid \
                 time zone transition rule, but found data after \
                 parsing a valid time zone transition rule",
            ),
            OffsetDst(ref err) => {
                f.write_str("failed to parse DST offset: ")?;
                core::fmt::Display::fmt(err, f)
            }
            OffsetStd(ref err) => {
                f.write_str("failed to parse standard offset: ")?;
                core::fmt::Display::fmt(err, f)
            }
            Rule(ref err) => core::fmt::Display::fmt(err, f),
            TzEnvColonTooBig => {
                f.write_str(
                    "IANA time zone identifier is too big for core-only \
                     environments \
                     (must be less than or equal to ",
                )?;
                core::fmt::Display::fmt(&TimeZoneId::array_capacity_max(), f)?;
                f.write_str(" bytes)")
            }
            TzEnvColonInvalidUtf8 => f.write_str(
                "IANA time zone identifier is invalid UTF-8",
            ),
            TzEnvInvalidUtf8 => f.write_str(
                "POSIX transition rule or \
                 IANA time zone identifier is invalid UTF-8",
            ),
        }
    }
}

impl From<ParseErrorKind> for ParseError {
    fn from(kind: ParseErrorKind) -> ParseError {
        ParseError { kind }
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
enum OffsetError {
    HourPosix(HourPosixError),
    IncompleteMinutes,
    IncompleteSeconds,
    Minute(MinuteError),
    OptionalSign(OptionalSignError),
    Second(SecondError),
}

impl core::fmt::Display for OffsetError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        use self::OffsetError::*;
        match *self {
            HourPosix(ref err) => core::fmt::Display::fmt(err, f),
            IncompleteMinutes => f.write_str(
                "incomplete time in \
                 POSIX time zone string (missing minutes)",
            ),
            IncompleteSeconds => f.write_str(
                "incomplete time in \
                 POSIX time zone string (missing seconds)",
            ),
            Minute(ref err) => core::fmt::Display::fmt(err, f),
            Second(ref err) => core::fmt::Display::fmt(err, f),
            OptionalSign(ref err) => {
                f.write_str(
                    "failed to parse sign for time offset \
                     POSIX time zone string",
                )?;
                core::fmt::Display::fmt(err, f)
            }
        }
    }
}

impl From<HourPosixError> for OffsetError {
    fn from(err: HourPosixError) -> OffsetError {
        OffsetError::HourPosix(err)
    }
}

impl From<MinuteError> for OffsetError {
    fn from(err: MinuteError) -> OffsetError {
        OffsetError::Minute(err)
    }
}

impl From<OptionalSignError> for OffsetError {
    fn from(err: OptionalSignError) -> OffsetError {
        OffsetError::OptionalSign(err)
    }
}

impl From<SecondError> for OffsetError {
    fn from(err: SecondError) -> OffsetError {
        OffsetError::Second(err)
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
enum RuleError {
    DateTimeEnd(DateTimeError),
    DateTimeStart(DateTimeError),
    ExpectedEnd,
}

impl core::fmt::Display for RuleError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        use self::RuleError::*;
        match *self {
            DateTimeEnd(ref err) => {
                f.write_str("failed to parse end of DST transition rule: ")?;
                core::fmt::Display::fmt(err, f)
            }
            DateTimeStart(ref err) => {
                f.write_str("failed to parse start of DST transition rule: ")?;
                core::fmt::Display::fmt(err, f)
            }
            ExpectedEnd => f.write_str(
                "expected end of DST rule after parsing the start \
                 of the DST rule",
            ),
        }
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
enum DateTimeError {
    Date(DateError),
    ExpectedTime,
    Time(TimeError),
}

impl core::fmt::Display for DateTimeError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        use self::DateTimeError::*;
        match *self {
            Date(ref err) => core::fmt::Display::fmt(err, f),
            ExpectedTime => f.write_str(
                "expected time specification after `/` following a date
                 specification in a POSIX time zone DST transition rule",
            ),
            Time(ref err) => core::fmt::Display::fmt(err, f),
        }
    }
}

impl From<DateError> for DateTimeError {
    fn from(err: DateError) -> DateTimeError {
        DateTimeError::Date(err)
    }
}

impl From<TimeError> for DateTimeError {
    fn from(err: TimeError) -> DateTimeError {
        DateTimeError::Time(err)
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
enum DateError {
    ExpectedJulianNoLeap,
    ExpectedMonthWeekWeekday,
    JulianLeap(JulianLeapError),
    JulianNoLeap(JulianNoLeapError),
    UnexpectedByte,
    WeekdayOfMonth(WeekdayOfMonthError),
}

impl core::fmt::Display for DateError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        use self::DateError::*;
        match *self {
            ExpectedJulianNoLeap => f.write_str(
                "expected one-based Julian day after `J` in date \
                 specification of a POSIX time zone DST \
                 transition rule, but found the end of input",
            ),
            ExpectedMonthWeekWeekday => f.write_str(
                "expected month-week-weekday after `M` in date \
                 specification of a POSIX time zone DST \
                 transition rule, but found the end of input",
            ),
            JulianLeap(ref err) => core::fmt::Display::fmt(err, f),
            JulianNoLeap(ref err) => core::fmt::Display::fmt(err, f),
            UnexpectedByte => f.write_str(
                "expected `J`, a digit or `M` at the beginning of a date \
                 specification of a POSIX time zone DST transition rule",
            ),
            WeekdayOfMonth(ref err) => core::fmt::Display::fmt(err, f),
        }
    }
}

impl From<JulianLeapError> for DateError {
    fn from(err: JulianLeapError) -> DateError {
        DateError::JulianLeap(err)
    }
}

impl From<JulianNoLeapError> for DateError {
    fn from(err: JulianNoLeapError) -> DateError {
        DateError::JulianNoLeap(err)
    }
}

impl From<WeekdayOfMonthError> for DateError {
    fn from(err: WeekdayOfMonthError) -> DateError {
        DateError::WeekdayOfMonth(err)
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
enum JulianNoLeapError {
    Parse(NumberError),
    Range,
}

impl core::fmt::Display for JulianNoLeapError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        use self::JulianNoLeapError::*;
        match *self {
            Parse(ref err) => {
                f.write_str("invalid one-based Julian day digits: ")?;
                core::fmt::Display::fmt(err, f)
            }
            Range => f.write_str(
                "parsed one-based Julian day, but it's not in supported \
                 range of `1..=365`",
            ),
        }
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
enum JulianLeapError {
    Parse(NumberError),
    Range,
}

impl core::fmt::Display for JulianLeapError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        use self::JulianLeapError::*;
        match *self {
            Parse(ref err) => {
                f.write_str("invalid zero-based Julian day digits: ")?;
                core::fmt::Display::fmt(err, f)
            }
            Range => f.write_str(
                "parsed zero-based Julian day, but it's not in supported \
                 range of `0..=365`",
            ),
        }
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
enum AbbreviationError {
    Quoted(QuotedAbbreviationError),
    Unquoted(UnquotedAbbreviationError),
}

impl core::fmt::Display for AbbreviationError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        use self::AbbreviationError::*;
        match *self {
            Quoted(ref err) => core::fmt::Display::fmt(err, f),
            Unquoted(ref err) => core::fmt::Display::fmt(err, f),
        }
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
enum UnquotedAbbreviationError {
    InvalidUtf8,
    TooLong,
    TooShort,
}

impl core::fmt::Display for UnquotedAbbreviationError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        use self::UnquotedAbbreviationError::*;
        match *self {
            InvalidUtf8 => f.write_str(
                "unquoted time zone abbreviation must be valid UTF-8",
            ),
            TooLong => write!(
                f,
                "expected unquoted time zone abbreviation with at most \
                 {} bytes, but found an abbreviation that is longer",
                REASONABLE_ABBREVIATION_MAX,
            ),
            TooShort => f.write_str(
                "expected unquoted time zone abbreviation to have length of \
                 3 or more bytes, but an abbreviation that is shorter",
            ),
        }
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
enum QuotedAbbreviationError {
    InvalidUtf8,
    TooLong,
    TooShort,
    UnexpectedEnd,
    UnexpectedEndAfterOpening,
    UnexpectedLastByte,
}

impl core::fmt::Display for QuotedAbbreviationError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        use self::QuotedAbbreviationError::*;
        match *self {
            InvalidUtf8 => f.write_str(
                "quoted time zone abbreviation must be valid UTF-8",
            ),
            TooLong => write!(
                f,
                "expected quoted time zone abbreviation with at most \
                 {} bytes, but found an abbreviation that is longer",
                REASONABLE_ABBREVIATION_MAX,
            ),
            TooShort => f.write_str(
                "expected quoted time zone abbreviation to have length of \
                 3 or more bytes, but an abbreviation that is shorter",
            ),
            UnexpectedEnd => f.write_str(
                "found non-empty quoted time zone abbreviation, but \
                 found end of input before an end-of-quoted abbreviation \
                 `>` character",
            ),
            UnexpectedEndAfterOpening => f.write_str(
                "found opening `<` quote for time zone abbreviation in \
                 POSIX time zone transition rule, and expected a name \
                 following it, but found the end of input instead",
            ),
            UnexpectedLastByte => f.write_str(
                "found non-empty quoted time zone abbreviation, but \
                 found did not find end-of-quoted abbreviation `>` \
                 character",
            ),
        }
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
enum WeekdayOfMonthError {
    ExpectedDayOfWeekAfterWeek,
    ExpectedDotAfterMonth,
    ExpectedDotAfterWeek,
    ExpectedWeekAfterMonth,
    Month(MonthError),
    WeekOfMonth(WeekOfMonthError),
    Weekday(WeekdayError),
}

impl core::fmt::Display for WeekdayOfMonthError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        use self::WeekdayOfMonthError::*;
        match *self {
            ExpectedDayOfWeekAfterWeek => f.write_str(
                "expected day-of-week after week in POSIX time zone rule",
            ),
            ExpectedDotAfterMonth => {
                f.write_str("expected `.` after month in POSIX time zone rule")
            }
            ExpectedWeekAfterMonth => f.write_str(
                "expected week after month in POSIX time zone rule",
            ),
            ExpectedDotAfterWeek => {
                f.write_str("expected `.` after week in POSIX time zone rule")
            }
            Month(ref err) => core::fmt::Display::fmt(err, f),
            WeekOfMonth(ref err) => core::fmt::Display::fmt(err, f),
            Weekday(ref err) => core::fmt::Display::fmt(err, f),
        }
    }
}

impl From<MonthError> for WeekdayOfMonthError {
    fn from(err: MonthError) -> WeekdayOfMonthError {
        WeekdayOfMonthError::Month(err)
    }
}

impl From<WeekOfMonthError> for WeekdayOfMonthError {
    fn from(err: WeekOfMonthError) -> WeekdayOfMonthError {
        WeekdayOfMonthError::WeekOfMonth(err)
    }
}

impl From<WeekdayError> for WeekdayOfMonthError {
    fn from(err: WeekdayError) -> WeekdayOfMonthError {
        WeekdayOfMonthError::Weekday(err)
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
enum TimeError {
    HourIana(HourIanaError),
    HourPosix(HourPosixError),
    IncompleteMinutes,
    IncompleteSeconds,
    Minute(MinuteError),
    OptionalSign(OptionalSignError),
    Second(SecondError),
}

impl core::fmt::Display for TimeError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        use self::TimeError::*;
        match *self {
            HourIana(ref err) => core::fmt::Display::fmt(err, f),
            HourPosix(ref err) => core::fmt::Display::fmt(err, f),
            IncompleteMinutes => f.write_str(
                "incomplete time zone transition time in \
                 POSIX time zone string (missing minutes)",
            ),
            IncompleteSeconds => f.write_str(
                "incomplete time zone transition time in \
                 POSIX time zone string (missing seconds)",
            ),
            Minute(ref err) => core::fmt::Display::fmt(err, f),
            Second(ref err) => core::fmt::Display::fmt(err, f),
            OptionalSign(ref err) => {
                f.write_str(
                    "failed to parse sign for time zone transition time",
                )?;
                core::fmt::Display::fmt(err, f)
            }
        }
    }
}

impl From<HourIanaError> for TimeError {
    fn from(err: HourIanaError) -> TimeError {
        TimeError::HourIana(err)
    }
}

impl From<HourPosixError> for TimeError {
    fn from(err: HourPosixError) -> TimeError {
        TimeError::HourPosix(err)
    }
}

impl From<MinuteError> for TimeError {
    fn from(err: MinuteError) -> TimeError {
        TimeError::Minute(err)
    }
}

impl From<OptionalSignError> for TimeError {
    fn from(err: OptionalSignError) -> TimeError {
        TimeError::OptionalSign(err)
    }
}

impl From<SecondError> for TimeError {
    fn from(err: SecondError) -> TimeError {
        TimeError::Second(err)
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
enum MonthError {
    Parse(NumberError),
    Range,
}

impl core::fmt::Display for MonthError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        use self::MonthError::*;
        match *self {
            Parse(ref err) => {
                f.write_str("invalid month digits: ")?;
                core::fmt::Display::fmt(err, f)
            }
            Range => f.write_str(
                "parsed month, but it's not in supported \
                 range of `1..=12`",
            ),
        }
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
enum WeekOfMonthError {
    Parse(NumberError),
    Range,
}

impl core::fmt::Display for WeekOfMonthError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        use self::WeekOfMonthError::*;
        match *self {
            Parse(ref err) => {
                f.write_str("invalid week-of-month digits: ")?;
                core::fmt::Display::fmt(err, f)
            }
            Range => f.write_str(
                "parsed week-of-month, but it's not in supported \
                 range of `1..=5`",
            ),
        }
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
enum WeekdayError {
    Parse(NumberError),
    Range,
}

impl core::fmt::Display for WeekdayError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        use self::WeekdayError::*;
        match *self {
            Parse(ref err) => {
                f.write_str("invalid weekday digits: ")?;
                core::fmt::Display::fmt(err, f)
            }
            Range => f.write_str(
                "parsed weekday, but it's not in supported \
                 range of `0..=6` (with `0` corresponding to Sunday)",
            ),
        }
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
enum HourIanaError {
    Parse(NumberError),
    Range,
}

impl core::fmt::Display for HourIanaError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        use self::HourIanaError::*;
        match *self {
            Parse(ref err) => {
                f.write_str("invalid hour digits: ")?;
                core::fmt::Display::fmt(err, f)
            }
            Range => f.write_str(
                "parsed hours, but it's not in supported \
                 range of `-167..=167`",
            ),
        }
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
enum HourPosixError {
    Parse(NumberError),
    Range,
}

impl core::fmt::Display for HourPosixError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        use self::HourPosixError::*;
        match *self {
            Parse(ref err) => {
                f.write_str("invalid hour digits: ")?;
                core::fmt::Display::fmt(err, f)
            }
            Range => f.write_str(
                "parsed hours, but it's not in supported \
                 range of `0..=24`",
            ),
        }
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
enum MinuteError {
    Parse(NumberError),
    Range,
}

impl core::fmt::Display for MinuteError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        use self::MinuteError::*;
        match *self {
            Parse(ref err) => {
                f.write_str("invalid minute digits: ")?;
                core::fmt::Display::fmt(err, f)
            }
            Range => f.write_str(
                "parsed minutes, but it's not in supported \
                 range of `0..=59`",
            ),
        }
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
enum SecondError {
    Parse(NumberError),
    Range,
}

impl core::fmt::Display for SecondError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        use self::SecondError::*;
        match *self {
            Parse(ref err) => {
                f.write_str("invalid second digits: ")?;
                core::fmt::Display::fmt(err, f)
            }
            Range => f.write_str(
                "parsed seconds, but it's not in supported \
                 range of `0..=59`",
            ),
        }
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
enum NumberError {
    Empty,
    ExpectedLength,
    InvalidDigit,
    TooBig,
}

impl core::fmt::Display for NumberError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        use self::NumberError::*;
        match *self {
            Empty => f.write_str("invalid number, no digits found"),
            ExpectedLength => f.write_str(
                "expected a fixed number of digits, \
                 but found incorrect number",
            ),
            InvalidDigit => f.write_str("expected digit in range `0..=9`"),
            TooBig => f.write_str(
                "parsed number too big to fit into a 32-bit signed integer",
            ),
        }
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
enum OptionalSignError {
    ExpectedDigitAfterMinus,
    ExpectedDigitAfterPlus,
}

impl core::fmt::Display for OptionalSignError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        use self::OptionalSignError::*;
        match *self {
            ExpectedDigitAfterMinus => f.write_str(
                "expected digit after `-` sign, \
                 but got end of input",
            ),
            ExpectedDigitAfterPlus => f.write_str(
                "expected digit after `+` sign, \
                 but got end of input",
            ),
        }
    }
}

#[cfg(test)]
mod tests {
    use crate::civil::date;

    use super::*;

    fn posix_time_zone(input: impl AsRef<[u8]>) -> TimeZone {
        let input = input.as_ref();
        let tz = TimeZone::parse(input).unwrap();
        tz
    }

    fn parser(s: &str) -> Parser<'_> {
        Parser::new(s)
    }

    fn astr(s: &'static str) -> Abbreviation {
        Abbreviation::array(s)
    }

    fn off(seconds: i32) -> Offset {
        Offset::from_seconds(seconds).unwrap()
    }

    #[test]
    fn parse() {
        let p = parser("NZST-12NZDT,J60,J300");
        assert_eq!(
            p.parse().unwrap(),
            TimeZone {
                std_abbrev: astr("NZST"),
                std_offset: off(12 * 60 * 60),
                dst: Some(Dst {
                    abbrev: astr("NZDT"),
                    offset: off(13 * 60 * 60),
                    rule: Rule {
                        start: DayTime {
                            date: Day::JulianOne(60),
                            time: TransitionCivilTime { second: 2 * 60 * 60 },
                        },
                        end: DayTime {
                            date: Day::JulianOne(300),
                            time: TransitionCivilTime { second: 2 * 60 * 60 },
                        },
                    },
                }),
            },
        );

        let p = Parser::new("NZST-12NZDT,J60,J300WAT");
        assert!(p.parse().is_err());
    }

    #[test]
    fn parse_posix_time_zone() {
        let p = Parser::new("NZST-12NZDT,M9.5.0,M4.1.0/3");
        assert_eq!(
            p.parse_posix_time_zone().unwrap(),
            TimeZone {
                std_abbrev: astr("NZST"),
                std_offset: off(12 * 60 * 60),
                dst: Some(Dst {
                    abbrev: astr("NZDT"),
                    offset: off(13 * 60 * 60),
                    rule: Rule {
                        start: DayTime {
                            date: Day::WeekdayOfMonth {
                                month: 9,
                                week: 5,
                                weekday: Weekday::Sunday,
                            },
                            time: TransitionCivilTime { second: 2 * 60 * 60 },
                        },
                        end: DayTime {
                            date: Day::WeekdayOfMonth {
                                month: 4,
                                week: 1,
                                weekday: Weekday::Sunday,
                            },
                            time: TransitionCivilTime { second: 3 * 60 * 60 },
                        },
                    },
                }),
            },
        );

        let p = Parser::new("NZST-12NZDT,M9.5.0,M4.1.0/3WAT");
        assert_eq!(
            p.parse_posix_time_zone().unwrap(),
            TimeZone {
                std_abbrev: astr("NZST"),
                std_offset: off(12 * 60 * 60),
                dst: Some(Dst {
                    abbrev: astr("NZDT"),
                    offset: off(13 * 60 * 60),
                    rule: Rule {
                        start: DayTime {
                            date: Day::WeekdayOfMonth {
                                month: 9,
                                week: 5,
                                weekday: Weekday::Sunday,
                            },
                            time: TransitionCivilTime { second: 2 * 60 * 60 },
                        },
                        end: DayTime {
                            date: Day::WeekdayOfMonth {
                                month: 4,
                                week: 1,
                                weekday: Weekday::Sunday,
                            },
                            time: TransitionCivilTime { second: 3 * 60 * 60 },
                        },
                    },
                }),
            },
        );

        let p = Parser::new("NZST-12NZDT,J60,J300");
        assert_eq!(
            p.parse_posix_time_zone().unwrap(),
            TimeZone {
                std_abbrev: astr("NZST"),
                std_offset: off(12 * 60 * 60),
                dst: Some(Dst {
                    abbrev: astr("NZDT"),
                    offset: off(13 * 60 * 60),
                    rule: Rule {
                        start: DayTime {
                            date: Day::JulianOne(60),
                            time: TransitionCivilTime { second: 2 * 60 * 60 },
                        },
                        end: DayTime {
                            date: Day::JulianOne(300),
                            time: TransitionCivilTime { second: 2 * 60 * 60 },
                        },
                    },
                }),
            },
        );

        let p = Parser::new("NZST-12NZDT,J60,J300WAT");
        assert_eq!(
            p.parse_posix_time_zone().unwrap(),
            TimeZone {
                std_abbrev: astr("NZST"),
                std_offset: off(12 * 60 * 60),
                dst: Some(Dst {
                    abbrev: astr("NZDT"),
                    offset: off(13 * 60 * 60),
                    rule: Rule {
                        start: DayTime {
                            date: Day::JulianOne(60),
                            time: TransitionCivilTime { second: 2 * 60 * 60 },
                        },
                        end: DayTime {
                            date: Day::JulianOne(300),
                            time: TransitionCivilTime { second: 2 * 60 * 60 },
                        },
                    },
                }),
            },
        );
    }

    #[test]
    fn parse_posix_dst() {
        let p = Parser::new("NZDT,M9.5.0,M4.1.0/3");
        assert_eq!(
            p.parse_posix_dst(off(12 * 60 * 60)).unwrap(),
            Dst {
                abbrev: astr("NZDT"),
                offset: off(13 * 60 * 60),
                rule: Rule {
                    start: DayTime {
                        date: Day::WeekdayOfMonth {
                            month: 9,
                            week: 5,
                            weekday: Weekday::Sunday,
                        },
                        time: TransitionCivilTime { second: 2 * 60 * 60 },
                    },
                    end: DayTime {
                        date: Day::WeekdayOfMonth {
                            month: 4,
                            week: 1,
                            weekday: Weekday::Sunday,
                        },
                        time: TransitionCivilTime { second: 3 * 60 * 60 },
                    },
                },
            },
        );

        let p = Parser::new("NZDT,J60,J300");
        assert_eq!(
            p.parse_posix_dst(off(12 * 60 * 60)).unwrap(),
            Dst {
                abbrev: astr("NZDT"),
                offset: off(13 * 60 * 60),
                rule: Rule {
                    start: DayTime {
                        date: Day::JulianOne(60),
                        time: TransitionCivilTime { second: 2 * 60 * 60 },
                    },
                    end: DayTime {
                        date: Day::JulianOne(300),
                        time: TransitionCivilTime { second: 2 * 60 * 60 },
                    },
                },
            },
        );

        let p = Parser::new("NZDT-7,J60,J300");
        assert_eq!(
            p.parse_posix_dst(off(12 * 60 * 60)).unwrap(),
            Dst {
                abbrev: astr("NZDT"),
                offset: off(7 * 60 * 60),
                rule: Rule {
                    start: DayTime {
                        date: Day::JulianOne(60),
                        time: TransitionCivilTime { second: 2 * 60 * 60 },
                    },
                    end: DayTime {
                        date: Day::JulianOne(300),
                        time: TransitionCivilTime { second: 2 * 60 * 60 },
                    },
                },
            },
        );

        let p = Parser::new("NZDT+7,J60,J300");
        assert_eq!(
            p.parse_posix_dst(off(12 * 60 * 60)).unwrap(),
            Dst {
                abbrev: astr("NZDT"),
                offset: off(-7 * 60 * 60),
                rule: Rule {
                    start: DayTime {
                        date: Day::JulianOne(60),
                        time: TransitionCivilTime { second: 2 * 60 * 60 },
                    },
                    end: DayTime {
                        date: Day::JulianOne(300),
                        time: TransitionCivilTime { second: 2 * 60 * 60 },
                    },
                },
            },
        );

        let p = Parser::new("NZDT7,J60,J300");
        assert_eq!(
            p.parse_posix_dst(off(12 * 60 * 60)).unwrap(),
            Dst {
                abbrev: astr("NZDT"),
                offset: off(-7 * 60 * 60),
                rule: Rule {
                    start: DayTime {
                        date: Day::JulianOne(60),
                        time: TransitionCivilTime { second: 2 * 60 * 60 },
                    },
                    end: DayTime {
                        date: Day::JulianOne(300),
                        time: TransitionCivilTime { second: 2 * 60 * 60 },
                    },
                },
            },
        );

        let p = Parser::new("NZDT7,");
        assert!(p.parse_posix_dst(off(12 * 60 * 60)).is_err());

        let p = Parser::new("NZDT7!");
        assert!(p.parse_posix_dst(off(12 * 60 * 60)).is_err());
    }

    #[test]
    fn parse_abbreviation() {
        let p = Parser::new("ABC");
        assert_eq!(p.parse_abbreviation().unwrap(), "ABC");

        let p = Parser::new("<ABC>");
        assert_eq!(p.parse_abbreviation().unwrap(), "ABC");

        let p = Parser::new("<+09>");
        assert_eq!(p.parse_abbreviation().unwrap(), "+09");

        let p = Parser::new("+09");
        assert!(p.parse_abbreviation().is_err());
    }

    #[test]
    fn parse_unquoted_abbreviation() {
        let p = Parser::new("ABC");
        assert_eq!(p.parse_unquoted_abbreviation().unwrap(), "ABC");

        let p = Parser::new("ABCXYZ");
        assert_eq!(p.parse_unquoted_abbreviation().unwrap(), "ABCXYZ");

        let p = Parser::new("ABC123");
        assert_eq!(p.parse_unquoted_abbreviation().unwrap(), "ABC");

        let tz = "a".repeat(6);
        let p = Parser::new(&tz);
        assert_eq!(p.parse_unquoted_abbreviation().unwrap(), &*tz);

        let p = Parser::new("a");
        assert!(p.parse_unquoted_abbreviation().is_err());

        let p = Parser::new("ab");
        assert!(p.parse_unquoted_abbreviation().is_err());

        let p = Parser::new("ab1");
        assert!(p.parse_unquoted_abbreviation().is_err());

        let tz = "a".repeat(31);
        #[cfg(feature = "alloc")]
        {
            let p = Parser::new(&tz);
            assert_eq!(p.parse_unquoted_abbreviation().unwrap(), &*tz);
        }
        #[cfg(not(feature = "alloc"))]
        {
            let p = Parser::new(&tz);
            assert!(p.parse_unquoted_abbreviation().is_err());
        }

        let p = Parser::new(b"ab\xFFcd");
        assert!(p.parse_unquoted_abbreviation().is_err());
    }

    #[test]
    fn parse_quoted_abbreviation() {
        // The inputs look a little funny here, but that's because
        // 'parse_quoted_abbreviation' starts after the opening quote
        // has been parsed.

        let p = Parser::new("ABC>");
        assert_eq!(p.parse_quoted_abbreviation().unwrap(), "ABC");

        let p = Parser::new("ABCXYZ>");
        assert_eq!(p.parse_quoted_abbreviation().unwrap(), "ABCXYZ");

        let p = Parser::new("ABC>123");
        assert_eq!(p.parse_quoted_abbreviation().unwrap(), "ABC");

        let p = Parser::new("ABC123>");
        assert_eq!(p.parse_quoted_abbreviation().unwrap(), "ABC123");

        let p = Parser::new("ab1>");
        assert_eq!(p.parse_quoted_abbreviation().unwrap(), "ab1");

        let p = Parser::new("+09>");
        assert_eq!(p.parse_quoted_abbreviation().unwrap(), "+09");

        let p = Parser::new("-09>");
        assert_eq!(p.parse_quoted_abbreviation().unwrap(), "-09");

        let tz = alloc::format!("{}>", "a".repeat(6));
        let p = Parser::new(&tz);
        assert_eq!(
            p.parse_quoted_abbreviation().unwrap(),
            tz.trim_end_matches(">")
        );

        let p = Parser::new("a>");
        assert!(p.parse_quoted_abbreviation().is_err());

        let p = Parser::new("ab>");
        assert!(p.parse_quoted_abbreviation().is_err());

        let tz = alloc::format!("{}>", "a".repeat(31));
        #[cfg(feature = "alloc")]
        {
            let p = Parser::new(&tz);
            assert_eq!(
                p.parse_quoted_abbreviation().unwrap(),
                tz.trim_end_matches(">")
            );
        }
        #[cfg(not(feature = "alloc"))]
        {
            let p = Parser::new(&tz);
            assert!(p.parse_quoted_abbreviation().is_err());
        }

        let p = Parser::new(b"ab\xFFcd>");
        assert!(p.parse_quoted_abbreviation().is_err());

        let p = Parser::new("ABC");
        assert!(p.parse_quoted_abbreviation().is_err());

        let p = Parser::new("ABC!>");
        assert!(p.parse_quoted_abbreviation().is_err());
    }

    #[test]
    fn parse_posix_offset() {
        let p = Parser::new("5");
        assert_eq!(p.parse_posix_offset().unwrap().seconds(), -5 * 60 * 60);

        let p = Parser::new("+5");
        assert_eq!(p.parse_posix_offset().unwrap().seconds(), -5 * 60 * 60);

        let p = Parser::new("-5");
        assert_eq!(p.parse_posix_offset().unwrap().seconds(), 5 * 60 * 60);

        let p = Parser::new("-12:34:56");
        assert_eq!(
            p.parse_posix_offset().unwrap().seconds(),
            12 * 60 * 60 + 34 * 60 + 56,
        );

        let p = Parser::new("a");
        assert!(p.parse_posix_offset().is_err());

        let p = Parser::new("-");
        assert!(p.parse_posix_offset().is_err());

        let p = Parser::new("+");
        assert!(p.parse_posix_offset().is_err());

        let p = Parser::new("-a");
        assert!(p.parse_posix_offset().is_err());

        let p = Parser::new("+a");
        assert!(p.parse_posix_offset().is_err());

        let p = Parser::new("-25");
        assert!(p.parse_posix_offset().is_err());

        let p = Parser::new("+25");
        assert!(p.parse_posix_offset().is_err());

        // This checks that we don't accidentally permit IANA rules for
        // offset parsing. Namely, the IANA tzfile v3+ extension only applies
        // to transition times. But since POSIX says that the "time" for the
        // offset and transition is the same format, it would be an easy
        // implementation mistake to implement the more flexible rule for
        // IANA and have it accidentally also apply to the offset. So we check
        // that it doesn't here.
        let p = Parser { ianav3plus: true, ..Parser::new("25") };
        assert!(p.parse_posix_offset().is_err());
        let p = Parser { ianav3plus: true, ..Parser::new("+25") };
        assert!(p.parse_posix_offset().is_err());
        let p = Parser { ianav3plus: true, ..Parser::new("-25") };
        assert!(p.parse_posix_offset().is_err());
    }

    #[test]
    fn parse_rule() {
        let p = Parser::new("M9.5.0,M4.1.0/3");
        assert_eq!(
            p.parse_rule().unwrap(),
            Rule {
                start: DayTime {
                    date: Day::WeekdayOfMonth {
                        month: 9,
                        week: 5,
                        weekday: Weekday::Sunday,
                    },
                    time: TransitionCivilTime { second: 2 * 60 * 60 },
                },
                end: DayTime {
                    date: Day::WeekdayOfMonth {
                        month: 4,
                        week: 1,
                        weekday: Weekday::Sunday,
                    },
                    time: TransitionCivilTime { second: 3 * 60 * 60 },
                },
            },
        );

        let p = Parser::new("M9.5.0");
        assert!(p.parse_rule().is_err());

        let p = Parser::new(",M9.5.0,M4.1.0/3");
        assert!(p.parse_rule().is_err());

        let p = Parser::new("M9.5.0/");
        assert!(p.parse_rule().is_err());

        let p = Parser::new("M9.5.0,M4.1.0/");
        assert!(p.parse_rule().is_err());
    }

    #[test]
    fn parse_posix_datetime() {
        let p = Parser::new("J1");
        assert_eq!(
            p.parse_posix_datetime().unwrap(),
            DayTime {
                date: Day::JulianOne(1),
                time: TransitionCivilTime { second: 2 * 60 * 60 }
            },
        );

        let p = Parser::new("J1/3");
        assert_eq!(
            p.parse_posix_datetime().unwrap(),
            DayTime {
                date: Day::JulianOne(1),
                time: TransitionCivilTime { second: 3 * 60 * 60 }
            },
        );

        let p = Parser::new("M4.1.0/3");
        assert_eq!(
            p.parse_posix_datetime().unwrap(),
            DayTime {
                date: Day::WeekdayOfMonth {
                    month: 4,
                    week: 1,
                    weekday: Weekday::Sunday
                },
                time: TransitionCivilTime { second: 3 * 60 * 60 },
            },
        );

        let p = Parser::new("1/3:45:05");
        assert_eq!(
            p.parse_posix_datetime().unwrap(),
            DayTime {
                date: Day::JulianZero(1),
                time: TransitionCivilTime {
                    second: 3 * 60 * 60 + 45 * 60 + 5
                },
            },
        );

        let p = Parser::new("a");
        assert!(p.parse_posix_datetime().is_err());

        let p = Parser::new("J1/");
        assert!(p.parse_posix_datetime().is_err());

        let p = Parser::new("1/");
        assert!(p.parse_posix_datetime().is_err());

        let p = Parser::new("M4.1.0/");
        assert!(p.parse_posix_datetime().is_err());
    }

    #[test]
    fn parse_posix_date() {
        let p = Parser::new("J1");
        assert_eq!(p.parse_posix_date().unwrap(), Day::JulianOne(1));
        let p = Parser::new("J365");
        assert_eq!(p.parse_posix_date().unwrap(), Day::JulianOne(365));

        let p = Parser::new("0");
        assert_eq!(p.parse_posix_date().unwrap(), Day::JulianZero(0));
        let p = Parser::new("1");
        assert_eq!(p.parse_posix_date().unwrap(), Day::JulianZero(1));
        let p = Parser::new("365");
        assert_eq!(p.parse_posix_date().unwrap(), Day::JulianZero(365));

        let p = Parser::new("M9.5.0");
        assert_eq!(
            p.parse_posix_date().unwrap(),
            Day::WeekdayOfMonth {
                month: 9,
                week: 5,
                weekday: Weekday::Sunday
            },
        );
        let p = Parser::new("M9.5.6");
        assert_eq!(
            p.parse_posix_date().unwrap(),
            Day::WeekdayOfMonth {
                month: 9,
                week: 5,
                weekday: Weekday::Saturday
            },
        );
        let p = Parser::new("M09.5.6");
        assert_eq!(
            p.parse_posix_date().unwrap(),
            Day::WeekdayOfMonth {
                month: 9,
                week: 5,
                weekday: Weekday::Saturday
            },
        );
        let p = Parser::new("M12.1.1");
        assert_eq!(
            p.parse_posix_date().unwrap(),
            Day::WeekdayOfMonth {
                month: 12,
                week: 1,
                weekday: Weekday::Monday
            },
        );

        let p = Parser::new("a");
        assert!(p.parse_posix_date().is_err());

        let p = Parser::new("j");
        assert!(p.parse_posix_date().is_err());

        let p = Parser::new("m");
        assert!(p.parse_posix_date().is_err());

        let p = Parser::new("n");
        assert!(p.parse_posix_date().is_err());

        let p = Parser::new("J366");
        assert!(p.parse_posix_date().is_err());

        let p = Parser::new("366");
        assert!(p.parse_posix_date().is_err());
    }

    #[test]
    fn parse_posix_julian_day_no_leap() {
        let p = Parser::new("1");
        assert_eq!(p.parse_posix_julian_day_no_leap().unwrap(), 1);

        let p = Parser::new("001");
        assert_eq!(p.parse_posix_julian_day_no_leap().unwrap(), 1);

        let p = Parser::new("365");
        assert_eq!(p.parse_posix_julian_day_no_leap().unwrap(), 365);

        let p = Parser::new("3655");
        assert_eq!(p.parse_posix_julian_day_no_leap().unwrap(), 365);

        let p = Parser::new("0");
        assert!(p.parse_posix_julian_day_no_leap().is_err());

        let p = Parser::new("366");
        assert!(p.parse_posix_julian_day_no_leap().is_err());
    }

    #[test]
    fn parse_posix_julian_day_with_leap() {
        let p = Parser::new("0");
        assert_eq!(p.parse_posix_julian_day_with_leap().unwrap(), 0);

        let p = Parser::new("1");
        assert_eq!(p.parse_posix_julian_day_with_leap().unwrap(), 1);

        let p = Parser::new("001");
        assert_eq!(p.parse_posix_julian_day_with_leap().unwrap(), 1);

        let p = Parser::new("365");
        assert_eq!(p.parse_posix_julian_day_with_leap().unwrap(), 365);

        let p = Parser::new("3655");
        assert_eq!(p.parse_posix_julian_day_with_leap().unwrap(), 365);

        let p = Parser::new("366");
        assert!(p.parse_posix_julian_day_with_leap().is_err());
    }

    #[test]
    fn parse_weekday_of_month() {
        let p = Parser::new("9.5.0");
        assert_eq!(
            p.parse_weekday_of_month().unwrap(),
            (9, 5, Weekday::Sunday)
        );

        let p = Parser::new("9.1.6");
        assert_eq!(
            p.parse_weekday_of_month().unwrap(),
            (9, 1, Weekday::Saturday)
        );

        let p = Parser::new("09.1.6");
        assert_eq!(
            p.parse_weekday_of_month().unwrap(),
            (9, 1, Weekday::Saturday)
        );

        let p = Parser::new("9");
        assert!(p.parse_weekday_of_month().is_err());

        let p = Parser::new("9.");
        assert!(p.parse_weekday_of_month().is_err());

        let p = Parser::new("9.5");
        assert!(p.parse_weekday_of_month().is_err());

        let p = Parser::new("9.5.");
        assert!(p.parse_weekday_of_month().is_err());

        let p = Parser::new("0.5.0");
        assert!(p.parse_weekday_of_month().is_err());

        let p = Parser::new("13.5.0");
        assert!(p.parse_weekday_of_month().is_err());

        let p = Parser::new("9.0.0");
        assert!(p.parse_weekday_of_month().is_err());

        let p = Parser::new("9.6.0");
        assert!(p.parse_weekday_of_month().is_err());

        let p = Parser::new("9.5.7");
        assert!(p.parse_weekday_of_month().is_err());
    }

    #[test]
    fn parse_posix_time() {
        let p = Parser::new("5");
        assert_eq!(p.parse_posix_time().unwrap().second, 5 * 60 * 60);

        let p = Parser::new("22");
        assert_eq!(p.parse_posix_time().unwrap().second, 22 * 60 * 60);

        let p = Parser::new("02");
        assert_eq!(p.parse_posix_time().unwrap().second, 2 * 60 * 60);

        let p = Parser::new("5:45");
        assert_eq!(
            p.parse_posix_time().unwrap().second,
            5 * 60 * 60 + 45 * 60
        );

        let p = Parser::new("5:45:12");
        assert_eq!(
            p.parse_posix_time().unwrap().second,
            5 * 60 * 60 + 45 * 60 + 12
        );

        let p = Parser::new("5:45:129");
        assert_eq!(
            p.parse_posix_time().unwrap().second,
            5 * 60 * 60 + 45 * 60 + 12
        );

        let p = Parser::new("5:45:12:");
        assert_eq!(
            p.parse_posix_time().unwrap().second,
            5 * 60 * 60 + 45 * 60 + 12
        );

        let p = Parser { ianav3plus: true, ..Parser::new("+5:45:12") };
        assert_eq!(
            p.parse_posix_time().unwrap().second,
            5 * 60 * 60 + 45 * 60 + 12
        );

        let p = Parser { ianav3plus: true, ..Parser::new("-5:45:12") };
        assert_eq!(
            p.parse_posix_time().unwrap().second,
            -(5 * 60 * 60 + 45 * 60 + 12)
        );

        let p = Parser { ianav3plus: true, ..Parser::new("-167:45:12") };
        assert_eq!(
            p.parse_posix_time().unwrap().second,
            -(167 * 60 * 60 + 45 * 60 + 12),
        );

        let p = Parser::new("25");
        assert!(p.parse_posix_time().is_err());

        let p = Parser::new("12:2");
        assert!(p.parse_posix_time().is_err());

        let p = Parser::new("12:");
        assert!(p.parse_posix_time().is_err());

        let p = Parser::new("12:23:5");
        assert!(p.parse_posix_time().is_err());

        let p = Parser::new("12:23:");
        assert!(p.parse_posix_time().is_err());

        let p = Parser { ianav3plus: true, ..Parser::new("168") };
        assert!(p.parse_posix_time().is_err());

        let p = Parser { ianav3plus: true, ..Parser::new("-168") };
        assert!(p.parse_posix_time().is_err());

        let p = Parser { ianav3plus: true, ..Parser::new("+168") };
        assert!(p.parse_posix_time().is_err());
    }

    #[test]
    fn parse_month() {
        let p = Parser::new("1");
        assert_eq!(p.parse_month().unwrap(), 1);

        // Should this be allowed? POSIX spec is unclear.
        // We allow it because our parse does stop at 2
        // digits, so this seems harmless. Namely, '001'
        // results in an error.
        let p = Parser::new("01");
        assert_eq!(p.parse_month().unwrap(), 1);

        let p = Parser::new("12");
        assert_eq!(p.parse_month().unwrap(), 12);

        let p = Parser::new("0");
        assert!(p.parse_month().is_err());

        let p = Parser::new("00");
        assert!(p.parse_month().is_err());

        let p = Parser::new("001");
        assert!(p.parse_month().is_err());

        let p = Parser::new("13");
        assert!(p.parse_month().is_err());
    }

    #[test]
    fn parse_week() {
        let p = Parser::new("1");
        assert_eq!(p.parse_week().unwrap(), 1);

        let p = Parser::new("5");
        assert_eq!(p.parse_week().unwrap(), 5);

        let p = Parser::new("55");
        assert_eq!(p.parse_week().unwrap(), 5);

        let p = Parser::new("0");
        assert!(p.parse_week().is_err());

        let p = Parser::new("6");
        assert!(p.parse_week().is_err());

        let p = Parser::new("00");
        assert!(p.parse_week().is_err());

        let p = Parser::new("01");
        assert!(p.parse_week().is_err());

        let p = Parser::new("05");
        assert!(p.parse_week().is_err());
    }

    #[test]
    fn parse_weekday() {
        let p = Parser::new("0");
        assert_eq!(p.parse_weekday().unwrap(), Weekday::Sunday);

        let p = Parser::new("1");
        assert_eq!(p.parse_weekday().unwrap(), Weekday::Monday);

        let p = Parser::new("6");
        assert_eq!(p.parse_weekday().unwrap(), Weekday::Saturday);

        let p = Parser::new("00");
        assert_eq!(p.parse_weekday().unwrap(), Weekday::Sunday);

        let p = Parser::new("06");
        assert_eq!(p.parse_weekday().unwrap(), Weekday::Sunday);

        let p = Parser::new("60");
        assert_eq!(p.parse_weekday().unwrap(), Weekday::Saturday);

        let p = Parser::new("7");
        assert!(p.parse_weekday().is_err());
    }

    #[test]
    fn parse_hour_posix() {
        let p = Parser::new("5");
        assert_eq!(p.parse_hour_posix().unwrap(), 5);

        let p = Parser::new("0");
        assert_eq!(p.parse_hour_posix().unwrap(), 0);

        let p = Parser::new("00");
        assert_eq!(p.parse_hour_posix().unwrap(), 0);

        let p = Parser::new("24");
        assert_eq!(p.parse_hour_posix().unwrap(), 24);

        let p = Parser::new("100");
        assert_eq!(p.parse_hour_posix().unwrap(), 10);

        let p = Parser::new("25");
        assert!(p.parse_hour_posix().is_err());

        let p = Parser::new("99");
        assert!(p.parse_hour_posix().is_err());
    }

    #[test]
    fn parse_hour_ianav3plus() {
        let new = |input| Parser { ianav3plus: true, ..Parser::new(input) };

        let p = new("5");
        assert_eq!(p.parse_hour_ianav3plus().unwrap(), 5);

        let p = new("0");
        assert_eq!(p.parse_hour_ianav3plus().unwrap(), 0);

        let p = new("00");
        assert_eq!(p.parse_hour_ianav3plus().unwrap(), 0);

        let p = new("000");
        assert_eq!(p.parse_hour_ianav3plus().unwrap(), 0);

        let p = new("24");
        assert_eq!(p.parse_hour_ianav3plus().unwrap(), 24);

        let p = new("100");
        assert_eq!(p.parse_hour_ianav3plus().unwrap(), 100);

        let p = new("1000");
        assert_eq!(p.parse_hour_ianav3plus().unwrap(), 100);

        let p = new("167");
        assert_eq!(p.parse_hour_ianav3plus().unwrap(), 167);

        let p = new("168");
        assert!(p.parse_hour_ianav3plus().is_err());

        let p = new("999");
        assert!(p.parse_hour_ianav3plus().is_err());
    }

    #[test]
    fn parse_minute() {
        let p = Parser::new("00");
        assert_eq!(p.parse_minute().unwrap(), 0);

        let p = Parser::new("24");
        assert_eq!(p.parse_minute().unwrap(), 24);

        let p = Parser::new("59");
        assert_eq!(p.parse_minute().unwrap(), 59);

        let p = Parser::new("599");
        assert_eq!(p.parse_minute().unwrap(), 59);

        let p = Parser::new("0");
        assert!(p.parse_minute().is_err());

        let p = Parser::new("1");
        assert!(p.parse_minute().is_err());

        let p = Parser::new("9");
        assert!(p.parse_minute().is_err());

        let p = Parser::new("60");
        assert!(p.parse_minute().is_err());
    }

    #[test]
    fn parse_second() {
        let p = Parser::new("00");
        assert_eq!(p.parse_second().unwrap(), 0);

        let p = Parser::new("24");
        assert_eq!(p.parse_second().unwrap(), 24);

        let p = Parser::new("59");
        assert_eq!(p.parse_second().unwrap(), 59);

        let p = Parser::new("599");
        assert_eq!(p.parse_second().unwrap(), 59);

        let p = Parser::new("0");
        assert!(p.parse_second().is_err());

        let p = Parser::new("1");
        assert!(p.parse_second().is_err());

        let p = Parser::new("9");
        assert!(p.parse_second().is_err());

        let p = Parser::new("60");
        assert!(p.parse_second().is_err());
    }

    #[test]
    fn parse_number_with_exactly_n_digits() {
        let p = Parser::new("1");
        assert_eq!(p.parse_number_with_exactly_n_digits(1).unwrap(), 1);

        let p = Parser::new("12");
        assert_eq!(p.parse_number_with_exactly_n_digits(2).unwrap(), 12);

        let p = Parser::new("123");
        assert_eq!(p.parse_number_with_exactly_n_digits(2).unwrap(), 12);

        let p = Parser::new("");
        assert!(p.parse_number_with_exactly_n_digits(1).is_err());

        let p = Parser::new("1");
        assert!(p.parse_number_with_exactly_n_digits(2).is_err());

        let p = Parser::new("12");
        assert!(p.parse_number_with_exactly_n_digits(3).is_err());
    }

    #[test]
    fn parse_number_with_upto_n_digits() {
        let p = Parser::new("1");
        assert_eq!(p.parse_number_with_upto_n_digits(1).unwrap(), 1);

        let p = Parser::new("1");
        assert_eq!(p.parse_number_with_upto_n_digits(2).unwrap(), 1);

        let p = Parser::new("12");
        assert_eq!(p.parse_number_with_upto_n_digits(2).unwrap(), 12);

        let p = Parser::new("12");
        assert_eq!(p.parse_number_with_upto_n_digits(3).unwrap(), 12);

        let p = Parser::new("123");
        assert_eq!(p.parse_number_with_upto_n_digits(2).unwrap(), 12);

        let p = Parser::new("");
        assert!(p.parse_number_with_upto_n_digits(1).is_err());

        let p = Parser::new("a");
        assert!(p.parse_number_with_upto_n_digits(1).is_err());
    }

    #[test]
    fn to_dst_civil_datetime_utc_range() {
        let tz = posix_time_zone("WART4WARST,J1/-3,J365/20");
        let dst_info = DstInfo {
            // We test this in other places. It's too annoying to write this
            // out here, and I didn't adopt snapshot testing until I had
            // written out these tests by hand.
            dst: tz.dst.as_ref().unwrap(),
            start: date(2024, 1, 1).at(1, 0, 0, 0),
            end: date(2024, 12, 31).at(23, 0, 0, 0),
        };
        assert_eq!(tz.dst_info_utc(2024), Some(dst_info));

        let tz = posix_time_zone("WART4WARST,J1/-4,J365/21");
        let dst_info = DstInfo {
            dst: tz.dst.as_ref().unwrap(),
            start: date(2024, 1, 1).at(0, 0, 0, 0),
            end: date(2024, 12, 31).at(23, 59, 59, 999_999_999),
        };
        assert_eq!(tz.dst_info_utc(2024), Some(dst_info));

        let tz = posix_time_zone("EST5EDT,M3.2.0,M11.1.0");
        let dst_info = DstInfo {
            dst: tz.dst.as_ref().unwrap(),
            start: date(2024, 3, 10).at(7, 0, 0, 0),
            end: date(2024, 11, 3).at(6, 0, 0, 0),
        };
        assert_eq!(tz.dst_info_utc(2024), Some(dst_info));
    }

    // See: https://github.com/BurntSushi/jiff/issues/386
    #[test]
    fn regression_permanent_dst() {
        let tz = posix_time_zone("XXX-2<+01>-1,0/0,J365/23");
        let dst_info = DstInfo {
            dst: tz.dst.as_ref().unwrap(),
            start: date(2087, 1, 1).at(0, 0, 0, 0),
            end: date(2087, 12, 31).at(23, 59, 59, 999_999_999),
        };
        assert_eq!(tz.dst_info_utc(2087), Some(dst_info));
    }

    #[test]
    fn reasonable() {
        assert!(TimeZone::parse(b"EST5").is_ok());
        assert!(TimeZone::parse(b"EST5EDT").is_err());
        assert!(TimeZone::parse(b"EST5EDT,J1,J365").is_ok());

        let tz = posix_time_zone("EST24EDT,J1,J365");
        assert_eq!(
            tz,
            TimeZone {
                std_abbrev: astr("EST"),
                std_offset: off(-24 * 60 * 60),
                dst: Some(Dst {
                    abbrev: astr("EDT"),
                    offset: off(-23 * 60 * 60),
                    rule: Rule {
                        start: DayTime {
                            date: Day::JulianOne(1),
                            time: TransitionCivilTime::DEFAULT,
                        },
                        end: DayTime {
                            date: Day::JulianOne(365),
                            time: TransitionCivilTime::DEFAULT,
                        },
                    },
                }),
            },
        );

        let tz = posix_time_zone("EST-24EDT,J1,J365");
        assert_eq!(
            tz,
            TimeZone {
                std_abbrev: astr("EST"),
                std_offset: off(24 * 60 * 60),
                dst: Some(Dst {
                    abbrev: astr("EDT"),
                    offset: off(25 * 60 * 60),
                    rule: Rule {
                        start: DayTime {
                            date: Day::JulianOne(1),
                            time: TransitionCivilTime::DEFAULT,
                        },
                        end: DayTime {
                            date: Day::JulianOne(365),
                            time: TransitionCivilTime::DEFAULT,
                        },
                    },
                }),
            },
        );
    }

    #[test]
    fn posix_date_time_spec_to_datetime() {
        // For this test, we just keep the offset to zero to simplify things
        // a bit. We get coverage for non-zero offsets in higher level tests.
        let to_datetime = |daytime: &DayTime, year: i16| {
            daytime.to_datetime(year, Offset::UTC)
        };

        let tz = posix_time_zone("EST5EDT,J1,J365/5:12:34");
        assert_eq!(
            to_datetime(&tz.rule().start, 2023),
            date(2023, 1, 1).at(2, 0, 0, 0),
        );
        assert_eq!(
            to_datetime(&tz.rule().end, 2023),
            date(2023, 12, 31).at(5, 12, 34, 0),
        );

        let tz = posix_time_zone("EST+5EDT,M3.2.0/2,M11.1.0/2");
        assert_eq!(
            to_datetime(&tz.rule().start, 2024),
            date(2024, 3, 10).at(2, 0, 0, 0),
        );
        assert_eq!(
            to_datetime(&tz.rule().end, 2024),
            date(2024, 11, 3).at(2, 0, 0, 0),
        );

        let tz = posix_time_zone("EST+5EDT,M1.1.1,M12.5.2");
        assert_eq!(
            to_datetime(&tz.rule().start, 2024),
            date(2024, 1, 1).at(2, 0, 0, 0),
        );
        assert_eq!(
            to_datetime(&tz.rule().end, 2024),
            date(2024, 12, 31).at(2, 0, 0, 0),
        );

        let tz = posix_time_zone("EST5EDT,0/0,J365/25");
        assert_eq!(
            to_datetime(&tz.rule().start, 2024),
            date(2024, 1, 1).at(0, 0, 0, 0),
        );
        assert_eq!(
            to_datetime(&tz.rule().end, 2024),
            date(2024, 12, 31).at(23, 59, 59, 999_999_999),
        );

        let tz = posix_time_zone("XXX3EDT4,0/0,J365/23");
        assert_eq!(
            to_datetime(&tz.rule().start, 2024),
            date(2024, 1, 1).at(0, 0, 0, 0),
        );
        assert_eq!(
            to_datetime(&tz.rule().end, 2024),
            date(2024, 12, 31).at(23, 0, 0, 0),
        );

        let tz = posix_time_zone("XXX3EDT4,0/0,365");
        assert_eq!(
            to_datetime(&tz.rule().end, 2023),
            date(2023, 12, 31).at(23, 59, 59, 999_999_999),
        );
        assert_eq!(
            to_datetime(&tz.rule().end, 2024),
            date(2024, 12, 31).at(2, 0, 0, 0),
        );

        let tz = posix_time_zone("XXX3EDT4,J1/-167:59:59,J365/167:59:59");
        assert_eq!(
            to_datetime(&tz.rule().start, 2024),
            date(2024, 1, 1).at(0, 0, 0, 0),
        );
        assert_eq!(
            to_datetime(&tz.rule().end, 2024),
            date(2024, 12, 31).at(23, 59, 59, 999_999_999),
        );
    }

    #[test]
    fn posix_date_time_spec_time() {
        let tz = posix_time_zone("EST5EDT,J1,J365/5:12:34");
        assert_eq!(tz.rule().start.time, TransitionCivilTime::DEFAULT);
        assert_eq!(
            tz.rule().end.time,
            TransitionCivilTime { second: 5 * 60 * 60 + 12 * 60 + 34 },
        );
    }

    #[test]
    fn posix_date_spec_to_date() {
        let tz = posix_time_zone("EST+5EDT,M3.2.0/2,M11.1.0/2");
        let start = tz.rule().start.date.to_date(2023);
        assert_eq!(start, Some(date(2023, 3, 12)));
        let end = tz.rule().end.date.to_date(2023);
        assert_eq!(end, Some(date(2023, 11, 5)));
        let start = tz.rule().start.date.to_date(2024);
        assert_eq!(start, Some(date(2024, 3, 10)));
        let end = tz.rule().end.date.to_date(2024);
        assert_eq!(end, Some(date(2024, 11, 3)));

        let tz = posix_time_zone("EST+5EDT,J60,J365");
        let start = tz.rule().start.date.to_date(2023);
        assert_eq!(start, Some(date(2023, 3, 1)));
        let end = tz.rule().end.date.to_date(2023);
        assert_eq!(end, Some(date(2023, 12, 31)));
        let start = tz.rule().start.date.to_date(2024);
        assert_eq!(start, Some(date(2024, 3, 1)));
        let end = tz.rule().end.date.to_date(2024);
        assert_eq!(end, Some(date(2024, 12, 31)));

        let tz = posix_time_zone("EST+5EDT,59,365");
        let start = tz.rule().start.date.to_date(2023);
        assert_eq!(start, Some(date(2023, 3, 1)));
        let end = tz.rule().end.date.to_date(2023);
        assert_eq!(end, None);
        let start = tz.rule().start.date.to_date(2024);
        assert_eq!(start, Some(date(2024, 2, 29)));
        let end = tz.rule().end.date.to_date(2024);
        assert_eq!(end, Some(date(2024, 12, 31)));

        let tz = posix_time_zone("EST+5EDT,M1.1.1,M12.5.2");
        let start = tz.rule().start.date.to_date(2024);
        assert_eq!(start, Some(date(2024, 1, 1)));
        let end = tz.rule().end.date.to_date(2024);
        assert_eq!(end, Some(date(2024, 12, 31)));
    }

    #[test]
    fn posix_time_spec_to_civil_time() {
        let tz = posix_time_zone("EST5EDT,J1,J365/5:12:34");
        assert_eq!(
            tz.dst.as_ref().unwrap().rule.start.time.second,
            2 * 60 * 60,
        );
        assert_eq!(
            tz.dst.as_ref().unwrap().rule.end.time.second,
            5 * 60 * 60 + 12 * 60 + 34,
        );

        let tz = posix_time_zone("EST5EDT,J1/23:59:59,J365/24:00:00");
        assert_eq!(
            tz.dst.as_ref().unwrap().rule.start.time.second,
            23 * 60 * 60 + 59 * 60 + 59,
        );
        assert_eq!(
            tz.dst.as_ref().unwrap().rule.end.time.second,
            24 * 60 * 60,
        );

        let tz = posix_time_zone("EST5EDT,J1/-1,J365/167:00:00");
        assert_eq!(
            tz.dst.as_ref().unwrap().rule.start.time.second,
            -1 * 60 * 60,
        );
        assert_eq!(
            tz.dst.as_ref().unwrap().rule.end.time.second,
            167 * 60 * 60,
        );
    }

    #[test]
    fn parse_iana() {
        // Ref: https://github.com/chronotope/chrono/issues/1153
        let p = TimeZone::parse(b"CRAZY5SHORT,M12.5.0/50,0/2").unwrap();
        assert_eq!(
            p,
            TimeZone {
                std_abbrev: astr("CRAZY"),
                std_offset: off(-5 * 60 * 60),
                dst: Some(Dst {
                    abbrev: astr("SHORT"),
                    offset: off(-4 * 60 * 60),
                    rule: Rule {
                        start: DayTime {
                            date: Day::WeekdayOfMonth {
                                month: 12,
                                week: 5,
                                weekday: Weekday::Sunday,
                            },
                            time: TransitionCivilTime { second: 50 * 60 * 60 },
                        },
                        end: DayTime {
                            date: Day::JulianZero(0),
                            time: TransitionCivilTime { second: 2 * 60 * 60 },
                        },
                    },
                }),
            },
        );

        assert!(TimeZone::parse(b"America/New_York").is_err());
        assert!(TimeZone::parse(b":America/New_York").is_err());
    }

    // See: https://github.com/BurntSushi/jiff/issues/407
    #[test]
    fn parse_empty_is_err() {
        assert!(TimeZone::parse(b"").is_err());
    }

    // See: https://github.com/BurntSushi/jiff/issues/407
    #[test]
    fn parse_weird_is_err() {
        let s =
            b"AAAAAAAAAAAAAAACAAAAAAAAAAAAQA8AACAAAAAAAAAAAAAAAAACAAAAAAAAAAA";
        assert!(TimeZone::parse(s).is_err());

        let s =
            b"<AAAAAAAAAAAAAAACAAAAAAAAAAAAQA>8<AACAAAAAAAAAAAAAAAAACAAAAAAAAAAA>";
        assert!(TimeZone::parse(s).is_err());

        let s = b"PPPPPPPPPPPPPPPPPPPPnoofPPPAAA6DaPPPPPPPPPPPPPPPPPPPPPnoofPPPPP,n";
        assert!(TimeZone::parse(s).is_err());

        let s = b"oooooooooovooooooooooooooooool9<ooooo2o-o-oooooookoorooooooooroo8";
        assert!(TimeZone::parse(s).is_err());
    }

    #[test]
    fn parse_long() {
        let ts = Timestamp::new(1782939639, 0).unwrap();

        let p = posix_time_zone("EST5EDT,M3.2.0,M11.1.0");
        assert_eq!(p.next_transition(ts).unwrap().abbreviation(), "EST");
        assert_eq!(p.previous_transition(ts).unwrap().abbreviation(), "EDT");

        let p = posix_time_zone("ABCDEF5abcdef,M3.2.0,M11.1.0");
        assert_eq!(p.next_transition(ts).unwrap().abbreviation(), "ABCDEF");
        assert_eq!(
            p.previous_transition(ts).unwrap().abbreviation(),
            "abcdef"
        );

        #[cfg(not(feature = "alloc"))]
        {
            let core_only_over_max = "ABCDEFG5abcdefg,M3.2.0,M11.1.0";
            assert!(TimeZone::parse(core_only_over_max).is_err());
        }

        #[cfg(feature = "alloc")]
        {
            let alloc_max = "\
                ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWX\
                ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWX\
                ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWX\
                ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWX\
                ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWX\
                ABCDE\
                5\
                abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwx\
                abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwx\
                abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwx\
                abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwx\
                abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwx\
                abcde\
                ,M3.2.0,M11.1.0\
            ";
            let p = posix_time_zone(alloc_max);
            assert_eq!(
                p.next_transition(ts).unwrap().abbreviation(),
                "ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWX\
                 ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWX\
                 ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWX\
                 ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWX\
                 ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWX\
                 ABCDE\
                ",
            );
            assert_eq!(
                p.previous_transition(ts).unwrap().abbreviation(),
                "abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwx\
                 abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwx\
                 abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwx\
                 abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwx\
                 abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwx\
                 abcde\
                ",
            );

            let alloc_over_max = "\
                ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWX\
                ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWX\
                ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWX\
                ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWX\
                ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWX\
                ABCDEF\
                5\
                abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwx\
                abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwx\
                abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwx\
                abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwx\
                abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwx\
                abcdef\
                ,M3.2.0,M11.1.0\
            ";
            assert!(TimeZone::parse(alloc_over_max).is_err());
        }
    }

    #[test]
    fn extremes() {
        assert!(TimeZone::parse(b"XXX25YYY,M3.2.0,M11.1.0").is_err());
        assert!(TimeZone::parse(b"XXX-25YYY,M3.2.0,M11.1.0").is_err());
        assert!(
            TimeZone::parse(b"XXX24:59:59YYY25:59:59,M3.2.0,M11.1.0").is_err()
        );
        // Weird, but actually possible if you omit the DST offset!
        assert!(TimeZone::parse(b"XXX-24:59:59YYY-25:59:59,M3.2.0,M11.1.0")
            .is_err());

        let p = TimeZone::parse(b"XXX24:59:59YYY,M3.2.0,M11.1.0").unwrap();
        // in std
        let ts = Timestamp::from_second(1783116302 + 6 * 30 * 86400).unwrap();
        assert_eq!(p.to_offset(ts), Offset::from_seconds(-89_999).unwrap());
        // in dst
        let ts = Timestamp::from_second(1783116302).unwrap();
        assert_eq!(p.to_offset(ts), Offset::from_seconds(-86_399).unwrap());

        let p = TimeZone::parse(b"XXX-24:59:59YYY,M3.2.0,M11.1.0").unwrap();
        // in std
        let ts = Timestamp::from_second(1783116302 + 6 * 30 * 86400).unwrap();
        assert_eq!(p.to_offset(ts), Offset::from_seconds(89_999).unwrap());
        // in dst
        let ts = Timestamp::from_second(1783116302).unwrap();
        // And this is why `Offset::MAX` is set to the value it's at!
        assert_eq!(p.to_offset(ts), Offset::MAX);
    }

    #[test]
    fn parse_posix_tz() {
        // We used to parse this and then error when we tried to
        // convert to a "reasonable" POSIX time zone with a DST
        // transition rule. We never actually used unreasonable POSIX
        // time zones and it was complicating the type definitions, so
        // now we just reject it outright.
        assert!(TzEnv::parse("EST5EDT").is_err());

        let tz = TzEnv::parse(":EST5EDT").unwrap();
        assert_eq!(
            tz,
            TzEnv::Implementation(TimeZoneId::new("EST5EDT").unwrap())
        );

        // We require implementation strings to be UTF-8, because we're
        // sensible.
        assert!(TzEnv::parse(b":EST5\xFFEDT").is_err());
    }
}