//! `time.h` implementation.
//!
//! See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/time.h.html>.

use crate::{
    c_str::{CStr, CString},
    error::ResultExt,
    header::{errno::EOVERFLOW, stdlib::getenv, unistd::readlink},
    platform::{self, types::*, Pal, Sys},
    sync::{Mutex, MutexGuard},
};
use alloc::collections::BTreeSet;
use chrono::{
    offset::MappedLocalTime, DateTime, Datelike, FixedOffset, NaiveDate, NaiveDateTime, Offset,
    TimeZone, Timelike,
};
use chrono_tz::{OffsetComponents, OffsetName, Tz};
use core::{
    cell::OnceCell,
    convert::{TryFrom, TryInto},
    mem, ptr,
};

pub use self::constants::*;

pub mod constants;

mod strftime;
mod strptime;
pub use strptime::strptime;

const YEARS_PER_ERA: time_t = 400;
const DAYS_PER_ERA: time_t = 146097;
const SECS_PER_DAY: time_t = 24 * 60 * 60;

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/time.h.html>.
#[repr(C)]
#[derive(Clone, Copy, Default)]
pub struct timespec {
    pub tv_sec: time_t,
    pub tv_nsec: c_long,
}

impl timespec {
    // TODO: Write test
    pub fn subtract(later: timespec, earlier: timespec) -> Option<timespec> {
        // TODO: Can tv_nsec be negative?
        let later_nsec = c_ulong::try_from(later.tv_nsec).ok()?;
        let earlier_nsec = c_ulong::try_from(earlier.tv_nsec).ok()?;

        Some(if later_nsec > earlier_nsec {
            timespec {
                tv_sec: later.tv_sec.checked_sub(earlier.tv_sec)?,
                tv_nsec: (later_nsec - earlier_nsec) as _,
            }
        } else {
            timespec {
                tv_sec: later.tv_sec.checked_sub(earlier.tv_sec)?.checked_sub(1)?,
                tv_nsec: 1_000_000_000 - (earlier_nsec - later_nsec) as c_long,
            }
        })
    }
}

#[cfg(target_os = "redox")]
impl<'a> From<&'a timespec> for syscall::TimeSpec {
    fn from(tp: &timespec) -> Self {
        Self {
            tv_sec: tp.tv_sec as _,
            tv_nsec: tp.tv_nsec as _,
        }
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/time.h.html>.
#[repr(C)]
pub struct tm {
    pub tm_sec: c_int,          // 0 - 60
    pub tm_min: c_int,          // 0 - 59
    pub tm_hour: c_int,         // 0 - 23
    pub tm_mday: c_int,         // 1 - 31
    pub tm_mon: c_int,          // 0 - 11
    pub tm_year: c_int,         // years since 1900
    pub tm_wday: c_int,         // 0 - 6 (Sunday - Saturday)
    pub tm_yday: c_int,         // 0 - 365
    pub tm_isdst: c_int,        // >0 if DST, 0 if not, <0 if unknown
    pub tm_gmtoff: c_long,      // offset from UTC in seconds
    pub tm_zone: *const c_char, // timezone abbreviation
}

unsafe impl Sync for tm {}

// The C Standard says that localtime and gmtime return the same pointer.
static mut TM: tm = blank_tm();

// The C Standard says that ctime and asctime return the same pointer.
static mut ASCTIME: [c_char; 26] = [0; 26];

#[repr(transparent)]
pub struct TzName([*mut c_char; 2]);

unsafe impl Sync for TzName {}

// Name storage for the `tm_zone` field.
static TIMEZONE_NAMES: Mutex<OnceCell<BTreeSet<CString>>> = Mutex::new(OnceCell::new());

// Hold `TIMEZONE_LOCK` when updating `tzname`, `timezone`, and `daylight`.
static TIMEZONE_LOCK: Mutex<(Option<CString>, Option<CString>)> = Mutex::new((None, None));

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/time.h.html>.
#[allow(non_upper_case_globals)]
#[no_mangle]
pub static mut daylight: c_int = 0;

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/time.h.html>.
#[allow(non_upper_case_globals)]
#[no_mangle]
pub static mut timezone: c_long = 0;

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/time.h.html>.
#[allow(non_upper_case_globals)]
#[no_mangle]
pub static mut tzname: TzName = TzName([ptr::null_mut(); 2]);

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/time.h.html>.
#[repr(C)]
pub struct itimerspec {
    pub it_interval: timespec,
    pub it_value: timespec,
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/time.h.html>.
pub struct sigevent;

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/asctime.html>.
///
/// # Deprecation
/// The `asctime()` function was marked obsolescent in the Open Group Base
/// Specifications Issue 7.
#[deprecated]
#[no_mangle]
pub unsafe extern "C" fn asctime(timeptr: *const tm) -> *mut c_char {
    asctime_r(timeptr, ASCTIME.as_mut_ptr().cast())
}

/// See <https://pubs.opengroup.org/onlinepubs/9699919799/functions/asctime.html>.
///
/// # Deprecation
/// The `asctime_r()` was marked obsolescent in the Open Group Base
/// Specifications Issue 7, and removed in Issue 8.
#[deprecated]
#[no_mangle]
pub unsafe extern "C" fn asctime_r(tm: *const tm, buf: *mut c_char) -> *mut c_char {
    let tm_sec = (*tm).tm_sec;
    let tm_min = (*tm).tm_min;
    let tm_hour = (*tm).tm_hour;
    let tm_mday = (*tm).tm_mday;
    let tm_mon = (*tm).tm_mon;
    let tm_year = (*tm).tm_year;
    let tm_wday = (*tm).tm_wday;

    /* Panic when we run into undefined behavior.
     *
     * POSIX says (since issue 7) that asctime()/asctime_r() cause UB
     * when the tm member values would cause out-of-bounds array access
     * or overflow the output buffer. This contrasts with ISO C11+,
     * which specifies UB for any tm members being outside their normal
     * ranges. While POSIX explicitly defers to the C standard in case
     * of contradictions, the assertions below follow the interpretation
     * that POSIX simply defines some of C's undefined behavior, rather
     * than conflict with the ISO standard.
     *
     * Note that C's "%.2d" formatting, unlike Rust's "{:02}"
     * formatting, does not count a minus sign against the two digits to
     * print, meaning that we must reject all negative values for
     * seconds, minutes and hours. However, C's "%3d" (for day-of-month)
     * is similar to Rust's "{:3}".
     *
     * To avoid year overflow problems (in Rust, where numeric overflow
     * is considered an error), we subtract 1900 from the endpoints,
     * rather than adding to the tm_year value. POSIX' requirement that
     * tm_year be at most {INT_MAX}-1990 is satisfied for all legal
     * values of {INT_MAX} through the max-4-digit requirement on the
     * year.
     *
     * The tm_mon and tm_wday fields are used for array access and thus
     * will already cause a panic in Rust code when out of range.
     * However, using the assertions below allows a consistent error
     * message for all fields. */
    const OUT_OF_RANGE_MESSAGE: &str = "tm member out of range";

    assert!(0 <= tm_sec && tm_sec <= 99, "{}", OUT_OF_RANGE_MESSAGE);
    assert!(0 <= tm_min && tm_min <= 99, "{}", OUT_OF_RANGE_MESSAGE);
    assert!(0 <= tm_hour && tm_hour <= 99, "{}", OUT_OF_RANGE_MESSAGE);
    assert!(-99 <= tm_mday && tm_mday <= 999, "{}", OUT_OF_RANGE_MESSAGE);
    assert!(0 <= tm_mon && tm_mon <= 11, "{}", OUT_OF_RANGE_MESSAGE);
    assert!(
        -999 - 1900 <= tm_year && tm_year <= 9999 - 1900,
        "{}",
        OUT_OF_RANGE_MESSAGE
    );
    assert!(0 <= tm_wday && tm_wday <= 6, "{}", OUT_OF_RANGE_MESSAGE);

    // At this point, we can safely use the values as given.
    let write_result = core::fmt::write(
        // buf may be either `*mut u8` or `*mut i8`
        &mut platform::UnsafeStringWriter(buf.cast()),
        format_args!(
            "{:.3} {:.3}{:3} {:02}:{:02}:{:02} {}\n",
            DAY_NAMES[usize::try_from(tm_wday).unwrap()],
            MON_NAMES[usize::try_from(tm_mon).unwrap()],
            tm_mday,
            tm_hour,
            tm_min,
            tm_sec,
            1900 + tm_year
        ),
    );
    match write_result {
        Ok(_) => buf,
        Err(_) => {
            /* asctime()/asctime_r() or the equivalent sprintf() call
             * have no defined errno setting */
            ptr::null_mut()
        }
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/clock.html>.
#[no_mangle]
pub extern "C" fn clock() -> clock_t {
    let mut ts = mem::MaybeUninit::<timespec>::uninit();

    if unsafe { clock_gettime(CLOCK_PROCESS_CPUTIME_ID, ts.as_mut_ptr()) } != 0 {
        return -1;
    }
    let ts = unsafe { ts.assume_init() };

    let clocks =
        ts.tv_sec * CLOCKS_PER_SEC as i64 + (ts.tv_nsec / (1_000_000_000 / CLOCKS_PER_SEC)) as i64;
    match clock_t::try_from(clocks) {
        Ok(ok) => ok,
        Err(_err) => -1,
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/clock_getres.html>.
#[no_mangle]
pub unsafe extern "C" fn clock_getres(clock_id: clockid_t, tp: *mut timespec) -> c_int {
    Sys::clock_getres(clock_id, tp)
        .map(|()| 0)
        .or_minus_one_errno()
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/clock_getres.html>.
#[no_mangle]
pub unsafe extern "C" fn clock_gettime(clock_id: clockid_t, tp: *mut timespec) -> c_int {
    Sys::clock_gettime(clock_id, tp)
        .map(|()| 0)
        .or_minus_one_errno()
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/clock_getres.html>.
#[no_mangle]
pub unsafe extern "C" fn clock_settime(clock_id: clockid_t, tp: *const timespec) -> c_int {
    Sys::clock_settime(clock_id, tp)
        .map(|()| 0)
        .or_minus_one_errno()
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/ctime.html>.
///
/// # Deprecation
/// The `ctime()` function was marked obsolescent in the Open Group Base
/// Specifications Issue 7.
#[deprecated]
#[no_mangle]
pub unsafe extern "C" fn ctime(clock: *const time_t) -> *mut c_char {
    asctime(localtime(clock))
}

/// See <https://pubs.opengroup.org/onlinepubs/9699919799/functions/ctime.html>.
///
/// # Deprecation
/// The `ctime_r()` function was marked obsolescent in the Open Group Base
/// Specifications Issue 7, and removed in Issue 8.
#[deprecated]
#[no_mangle]
pub unsafe extern "C" fn ctime_r(clock: *const time_t, buf: *mut c_char) -> *mut c_char {
    // Using MaybeUninit<tm> seems to cause a panic during the build process
    let mut tm1 = blank_tm();
    localtime_r(clock, &mut tm1);
    asctime_r(&tm1, buf)
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/difftime.html>.
#[no_mangle]
pub extern "C" fn difftime(time1: time_t, time0: time_t) -> c_double {
    (time1 - time0) as _
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/getdate.html>.
// #[no_mangle]
pub extern "C" fn getdate(string: *const c_char) -> tm {
    unimplemented!();
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/gmtime.html>.
#[no_mangle]
pub unsafe extern "C" fn gmtime(timer: *const time_t) -> *mut tm {
    gmtime_r(timer, &mut TM)
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/gmtime.html>.
#[no_mangle]
pub unsafe extern "C" fn gmtime_r(clock: *const time_t, result: *mut tm) -> *mut tm {
    /* For the details of the algorithm used here, see
     * http://howardhinnant.github.io/date_algorithms.html#civil_from_days
     * Note that we need 0-based months here, though.
     * Overall, this implementation should generate correct results as
     * long as the tm_year value will fit in a c_int. */
    let unix_secs = *clock;

    /* Day number here is possibly negative, remainder will always be
     * nonnegative when using Euclidean division */
    let unix_days: time_t = unix_secs.div_euclid(SECS_PER_DAY);

    /* In range [0, 86399]. Needs a u32 since this is larger (at least
     * theoretically) than the guaranteed range of c_int */
    let secs_of_day: u32 = unix_secs.rem_euclid(SECS_PER_DAY).try_into().unwrap();

    /* Shift origin from 1970-01-01 to 0000-03-01 and find out where we
     * are in terms of 400-year eras since then */
    let days_since_origin = unix_days + 719468;
    let era = days_since_origin.div_euclid(DAYS_PER_ERA);
    let day_of_era = days_since_origin.rem_euclid(DAYS_PER_ERA);
    let year_of_era =
        (day_of_era - day_of_era / 1460 + day_of_era / 36524 - day_of_era / 146096) / 365;

    /* "transformed" here refers to dates in a calendar where years
     * start on March 1 */
    let year_transformed = year_of_era + 400 * era; // retain large range, don't convert to c_int yet
    let day_of_year_transformed: c_int = (day_of_era
        - (365 * year_of_era + year_of_era / 4 - year_of_era / 100))
        .try_into()
        .unwrap();
    let month_transformed: c_int = (5 * day_of_year_transformed + 2) / 153;

    // Convert back to calendar with year starting on January 1
    let month: c_int = (month_transformed + 2) % 12; // adapted to 0-based months
    let year: time_t = if month < 2 {
        year_transformed + 1
    } else {
        year_transformed
    };

    /* Subtract 1900 *before* converting down to c_int in order to
     * maximize the range of input timestamps that will succeed */
    match c_int::try_from(year - 1900) {
        Ok(year_less_1900) => {
            let mday: c_int = (day_of_year_transformed - (153 * month_transformed + 2) / 5 + 1)
                .try_into()
                .unwrap();

            /* 1970-01-01 was a Thursday. Again, Euclidean division is
             * used to ensure a nonnegative remainder (range [0, 6]). */
            let wday: c_int = ((unix_days + 4).rem_euclid(7)).try_into().unwrap();

            /* Yes, duplicated code for now (to work on non-c_int-values
             * so that we are not constrained by the subtraction of
             * 1900) */
            let is_leap_year: bool = year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);

            /* For dates in January or February, we use the fact that
             * January 1 is always 306 days after March 1 in the
             * previous year. */
            let yday: c_int = if month < 2 {
                day_of_year_transformed - 306
            } else {
                day_of_year_transformed + if is_leap_year { 60 } else { 59 }
            };

            let hour: c_int = (secs_of_day / (60 * 60)).try_into().unwrap();
            let min: c_int = ((secs_of_day / 60) % 60).try_into().unwrap();
            let sec: c_int = (secs_of_day % 60).try_into().unwrap();

            *result = tm {
                tm_sec: sec,
                tm_min: min,
                tm_hour: hour,
                tm_mday: mday,
                tm_mon: month,
                tm_year: year_less_1900,
                tm_wday: wday,
                tm_yday: yday,
                tm_isdst: 0,
                tm_gmtoff: 0,
                tm_zone: UTC,
            };

            result
        }
        Err(_) => {
            platform::ERRNO.set(EOVERFLOW);
            ptr::null_mut()
        }
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/localtime.html>.
#[no_mangle]
pub unsafe extern "C" fn localtime(clock: *const time_t) -> *mut tm {
    localtime_r(clock, &mut TM)
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/localtime.html>.
#[no_mangle]
pub unsafe extern "C" fn localtime_r(clock: *const time_t, t: *mut tm) -> *mut tm {
    let mut lock = TIMEZONE_LOCK.lock();
    clear_timezone(&mut lock);

    let utc_time = *clock;
    let tz = time_zone();

    // Convert UTC time to local time
    let (std_time, dst_time) = match tz.timestamp_opt(utc_time, 0) {
        MappedLocalTime::Single(t) => (t, None),
        // This variant contains the two possible results, in the order (earliest, latest).
        MappedLocalTime::Ambiguous(t1, t2) => (t2, Some(t1)),
        MappedLocalTime::None => return t,
    };

    ptr::write(t, datetime_to_tm(&std_time));
    set_timezone(&mut lock, &std_time, dst_time);
    t
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/mktime.html>.
#[no_mangle]
pub unsafe extern "C" fn mktime(timeptr: *mut tm) -> time_t {
    let mut lock = TIMEZONE_LOCK.lock();
    clear_timezone(&mut lock);

    let year = (*timeptr).tm_year + 1900;
    let month = ((*timeptr).tm_mon + 1) as _;
    let day = (*timeptr).tm_mday as _;
    let hour = (*timeptr).tm_hour as _;
    let minute = (*timeptr).tm_min as _;
    let second = (*timeptr).tm_sec as _;

    let naive_local = match NaiveDate::from_ymd_opt(year, month, day)
        .and_then(|date| date.and_hms_opt(hour, minute, second))
    {
        Some(datetime) => datetime,
        None => {
            platform::ERRNO.set(EOVERFLOW);
            return -1;
        }
    };

    let offset = FixedOffset::east((*timeptr).tm_gmtoff as _);
    let tz = time_zone();

    // Create DateTime<FixedOffset>
    let datetime = match offset.from_local_datetime(&naive_local) {
        MappedLocalTime::Single(datetime) => datetime,
        _ => {
            platform::ERRNO.set(EOVERFLOW);
            return -1;
        }
    };

    // Convert to UTC and get timestamp
    let tz_datetime = datetime.with_timezone(&tz);
    let timestamp = tz_datetime.timestamp();

    ptr::write(timeptr, datetime_to_tm(&tz_datetime));

    // Convert UTC time to local time
    if let (Some(std_time), dst_time) = match tz.timestamp_opt(timestamp, 0) {
        MappedLocalTime::Single(t) => (Some(t), None),
        // This variant contains the two possible results, in the order (earliest, latest).
        MappedLocalTime::Ambiguous(t1, t2) => (Some(t2), Some(t1)),
        MappedLocalTime::None => (None, None),
    } {
        set_timezone(&mut lock, &std_time, dst_time);
    }

    timestamp
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/nanosleep.html>.
#[no_mangle]
pub unsafe extern "C" fn nanosleep(rqtp: *const timespec, rmtp: *mut timespec) -> c_int {
    Sys::nanosleep(rqtp, rmtp).map(|()| 0).or_minus_one_errno()
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/strftime.html>.
#[no_mangle]
pub unsafe extern "C" fn strftime(
    s: *mut c_char,
    maxsize: size_t,
    format: *const c_char,
    timeptr: *const tm,
) -> size_t {
    let ret = strftime::strftime(
        &mut platform::StringWriter(s as *mut u8, maxsize),
        format,
        timeptr,
    );
    if ret < maxsize {
        ret
    } else {
        0
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/time.html>.
#[no_mangle]
pub unsafe extern "C" fn time(tloc: *mut time_t) -> time_t {
    let mut ts = timespec::default();
    Sys::clock_gettime(CLOCK_REALTIME, &mut ts);
    if !tloc.is_null() {
        *tloc = ts.tv_sec
    };
    ts.tv_sec
}

/// Non-POSIX, see <https://www.man7.org/linux/man-pages/man3/timegm.3.html>.
#[no_mangle]
pub unsafe extern "C" fn timegm(tm: *mut tm) -> time_t {
    mktime(tm)
}

/// Non-POSIX, see <https://www.man7.org/linux/man-pages/man3/timegm.3.html>.
#[deprecated]
#[no_mangle]
pub unsafe extern "C" fn timelocal(tm: *mut tm) -> time_t {
    //TODO: timezone
    timegm(tm)
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/timer_create.html>.
// #[no_mangle]
pub extern "C" fn timer_create(
    clock_id: clockid_t,
    evp: *mut sigevent,
    timerid: *mut timer_t,
) -> c_int {
    unimplemented!();
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/timer_delete.html>.
// #[no_mangle]
pub extern "C" fn timer_delete(timerid: timer_t) -> c_int {
    unimplemented!();
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/timer_getoverrun.html>.
// #[no_mangle]
pub extern "C" fn timer_getoverrun(timerid: timer_t) -> c_int {
    unimplemented!();
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/timer_getoverrun.html>.
// #[no_mangle]
pub extern "C" fn timer_gettime(timerid: timer_t, value: *mut itimerspec) -> c_int {
    unimplemented!();
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/timer_getoverrun.html>.
// #[no_mangle]
pub extern "C" fn timer_settime(
    timerid: timer_t,
    flags: c_int,
    value: *const itimerspec,
    ovalue: *mut itimerspec,
) -> c_int {
    unimplemented!();
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/tzset.html>.
#[no_mangle]
pub unsafe extern "C" fn tzset() {
    let mut lock = TIMEZONE_LOCK.lock();
    unsafe { clear_timezone(&mut lock) };

    let tz = time_zone();
    let datetime = now();
    let (std_time, dst_time) = match tz.from_local_datetime(&datetime) {
        MappedLocalTime::Single(t) => (t, None),
        // This variant contains the two possible results, in the order (earliest, latest).
        MappedLocalTime::Ambiguous(t1, t2) => (t2, Some(t1)),
        MappedLocalTime::None => return,
    };

    set_timezone(&mut lock, &std_time, dst_time)
}

fn clear_timezone(guard: &mut MutexGuard<'_, (Option<CString>, Option<CString>)>) {
    guard.0 = None;
    guard.1 = None;
    unsafe {
        tzname.0[0] = ptr::null_mut();
        tzname.0[1] = ptr::null_mut();
        timezone = 0;
        daylight = 0;
    }
}

fn get_system_time_zone<'a>() -> Option<&'a str> {
    // Resolve the symlink for localtime
    const BSIZE: size_t = 100;
    let mut buffer: [u8; BSIZE] = [0; BSIZE];

    #[cfg(not(target_os = "redox"))]
    let (localtime, prefix) = (c"/etc/localtime", "/usr/share/zoneinfo/");

    #[cfg(target_os = "redox")]
    let (localtime, prefix) = (c"/etc/localtime", "/usr/share/zoneinfo/");

    if unsafe { readlink(localtime.as_ptr().cast(), buffer.as_mut_ptr().cast(), BSIZE) } == -1 {
        return None;
    }

    let path = unsafe { CStr::from_ptr(buffer.as_mut_ptr().cast()) };

    if let Ok(tz_name) = path.to_str() {
        if let Some(stripped) = tz_name.strip_prefix(prefix) {
            return Some(stripped);
        }
    }

    None
}

fn get_current_time_zone<'a>() -> &'a str {
    // Check the `TZ` environment variable
    let tz_env = unsafe { getenv(b"TZ\0".as_ptr() as _) };
    if !tz_env.is_null() {
        if let Ok(tz) = unsafe { CStr::from_ptr(tz_env) }.to_str() {
            return tz;
        }
    }

    // Fallback to the system's default time zone
    if let Some(tz) = get_system_time_zone() {
        return tz;
    }

    // If all else fails, use UTC
    "UTC"
}

#[inline(always)]
fn time_zone() -> Tz {
    get_current_time_zone().parse().unwrap_or(Tz::UTC)
}

#[inline(always)]
fn now() -> NaiveDateTime {
    let mut now = timespec::default();
    unsafe {
        Sys::clock_gettime(CLOCK_REALTIME, &mut now);
    }
    NaiveDateTime::from_timestamp(now.tv_sec, now.tv_nsec as _)
}

unsafe fn datetime_to_tm(local_time: &DateTime<Tz>) -> tm {
    let tz = local_time.timezone().name();
    let mut t = blank_tm();
    // Populate the `tm` structure
    t.tm_sec = local_time.second() as _;
    t.tm_min = local_time.minute() as _;
    t.tm_hour = local_time.hour() as _;
    t.tm_mday = local_time.day() as _;
    t.tm_mon = local_time.month0() as _; // 0-based month
    t.tm_year = (local_time.year() - 1900) as _; // Years since 1900
    t.tm_wday = local_time.weekday().num_days_from_sunday() as _;
    t.tm_yday = local_time.ordinal0() as _; // 0-based day of year

    let offset = local_time.offset();
    t.tm_isdst = offset.dst_offset().num_hours() as _;
    // Get the UTC offset in seconds
    t.tm_gmtoff = offset.fix().local_minus_utc() as _;

    let tm_zone = {
        let mut timezone_names = TIMEZONE_NAMES.lock();
        timezone_names.get_or_init(BTreeSet::new);
        let cstr = CString::new(tz).unwrap();
        timezone_names.get_mut().unwrap().insert(cstr.clone());
        timezone_names.get().unwrap().get(&cstr).unwrap().as_ptr()
    };

    t.tm_zone = tm_zone.cast();
    t
}

unsafe fn set_timezone(
    guard: &mut MutexGuard<'_, (Option<CString>, Option<CString>)>,
    std: &DateTime<Tz>,
    dst: Option<DateTime<Tz>>,
) {
    let ut_offset = std.offset();

    guard.0 = Some(CString::new(ut_offset.abbreviation().expect("Wrong timezone")).unwrap());
    tzname.0[0] = guard.0.as_ref().unwrap().as_ptr().cast_mut();

    match dst {
        Some(dst) => {
            guard.1 =
                Some(CString::new(dst.offset().abbreviation().expect("Wrong timezone")).unwrap());
            tzname.0[1] = guard.1.as_ref().unwrap().as_ptr().cast_mut();
            daylight = 1;
        }
        None => {
            guard.1 = None;
            tzname.0[1] = guard.0.as_ref().unwrap().as_ptr().cast_mut();
            daylight = 0;
        }
    }

    timezone = -c_long::from(ut_offset.fix().local_minus_utc());
}

const fn blank_tm() -> tm {
    tm {
        tm_year: 0,
        tm_mon: 0,
        tm_mday: 0,
        tm_hour: 0,
        tm_min: 0,
        tm_sec: 0,
        tm_wday: 0,
        tm_yday: 0,
        tm_isdst: -1,
        tm_gmtoff: 0,
        tm_zone: ptr::null_mut(),
    }
}