// Copyright 2016 Google Inc. All Rights Reserved.

//

// Licensed under the Apache License, Version 2.0 (the "License");

// you may not use this file except in compliance with the License.

// You may obtain a copy of the License at

//

//   https://www.apache.org/licenses/LICENSE-2.0

//

//   Unless required by applicable law or agreed to in writing, software

//   distributed under the License is distributed on an "AS IS" BASIS,

//   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

//   See the License for the specific language governing permissions and

//   limitations under the License.

// This file implements the TimeZoneIf interface using the "zoneinfo"

// data provided by the IANA Time Zone Database (i.e., the only real game

// in town).

//

// TimeZoneInfo represents the history of UTC-offset changes within a time

// zone. Most changes are due to daylight-saving rules, but occasionally

// shifts are made to the time-zone's base offset. The database only attempts

// to be definitive for times since 1970, so be wary of local-time conversions

// before that. Also, rule and zone-boundary changes are made at the whim

// of governments, so the conversion of future times needs to be taken with

// a grain of salt.

//

// For more information see tzfile(5), http://www.iana.org/time-zones, or

// https://en.wikipedia.org/wiki/Zoneinfo.

//

// Note that we assume the proleptic Gregorian calendar and 60-second

// minutes throughout.

#include "time_zone_info.h"

#include <algorithm>

#include <cassert>

#include <chrono>

#include <cstdint>

#include <cstdio>

#include <cstdlib>

#include <cstring>

#include <fstream>

#include <functional>

#include <memory>

#include <sstream>

#include <string>

#include <utility>

#include "cctz/civil_time.h"

#include "time_zone_fixed.h"

#include "time_zone_posix.h"

namespace cctz {

namespace {

inline bool IsLeap(year_t year) {

  return (year % 4) == 0 && ((year % 100) != 0 || (year % 400) == 0);

}

// The number of days in non-leap and leap years respectively.

const std::int_least32_t kDaysPerYear[2] = {365, 366};

// The day offsets of the beginning of each (1-based) month in non-leap and

// leap years respectively (e.g., 335 days before December in a leap year).

const std::int_least16_t kMonthOffsets[2][1 + 12 + 1] = {

  {-1, 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365},

  {-1, 0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366},

};

// We reject leap-second encoded zoneinfo and so assume 60-second minutes.

const std::int_least32_t kSecsPerDay = 24 * 60 * 60;

// 400-year chunks always have 146097 days (20871 weeks).

const std::int_least64_t kSecsPer400Years = 146097LL * kSecsPerDay;

// Like kDaysPerYear[] but scaled up by a factor of kSecsPerDay.

const std::int_least32_t kSecsPerYear[2] = {

  365 * kSecsPerDay,

  366 * kSecsPerDay,

};

// Convert a cctz::weekday to a POSIX TZ weekday number (0==Sun, ..., 6=Sat).

inline int ToPosixWeekday(weekday wd) {

  switch (wd) {

    case weekday::sunday:

      return 0;

    case weekday::monday:

      return 1;

    case weekday::tuesday:

      return 2;

    case weekday::wednesday:

      return 3;

    case weekday::thursday:

      return 4;

    case weekday::friday:

      return 5;

    case weekday::saturday:

      return 6;

  }

  return 0; /*NOTREACHED*/

}

// Single-byte, unsigned numeric values are encoded directly.

inline std::uint_fast8_t Decode8(const char* cp) {

  return static_cast<std::uint_fast8_t>(*cp) & 0xff;

}

// Multi-byte, numeric values are encoded using a MSB first,

// twos-complement representation. These helpers decode, from

// the given address, 4-byte and 8-byte values respectively.

// Note: If int_fastXX_t == intXX_t and this machine is not

// twos complement, then there will be at least one input value

// we cannot represent.

std::int_fast32_t Decode32(const char* cp) {

  std::uint_fast32_t v = 0;

  for (int i = 0; i != (32 / 8); ++i) v = (v << 8) | Decode8(cp++);

  const std::int_fast32_t s32max = 0x7fffffff;

  const auto s32maxU = static_cast<std::uint_fast32_t>(s32max);

  if (v <= s32maxU) return static_cast<std::int_fast32_t>(v);

  return static_cast<std::int_fast32_t>(v - s32maxU - 1) - s32max - 1;

}

std::int_fast64_t Decode64(const char* cp) {

  std::uint_fast64_t v = 0;

  for (int i = 0; i != (64 / 8); ++i) v = (v << 8) | Decode8(cp++);

  const std::int_fast64_t s64max = 0x7fffffffffffffff;

  const auto s64maxU = static_cast<std::uint_fast64_t>(s64max);

  if (v <= s64maxU) return static_cast<std::int_fast64_t>(v);

  return static_cast<std::int_fast64_t>(v - s64maxU - 1) - s64max - 1;

}

// Does the rule for future transitions call for year-round daylight time?

// See tz/zic.c:stringzone() for the details on how such rules are encoded.

bool AllYearDST(const PosixTimeZone& posix) {

  if (posix.dst_start.date.fmt != PosixTransition::N) return false;

  if (posix.dst_start.date.n.day != 0) return false;

  if (posix.dst_start.time.offset != 0) return false;

  if (posix.dst_end.date.fmt != PosixTransition::J) return false;

  if (posix.dst_end.date.j.day != kDaysPerYear[0]) return false;

  const auto offset = posix.std_offset - posix.dst_offset;

  if (posix.dst_end.time.offset + offset != kSecsPerDay) return false;

  return true;

}

// Generate a year-relative offset for a PosixTransition.

std::int_fast64_t TransOffset(bool leap_year, int jan1_weekday,

                              const PosixTransition& pt) {

  std::int_fast64_t days = 0;

  switch (pt.date.fmt) {

    case PosixTransition::J: {

      days = pt.date.j.day;

      if (!leap_year || days < kMonthOffsets[1][3]) days -= 1;

      break;

    }

    case PosixTransition::N: {

      days = pt.date.n.day;

      break;

    }

    case PosixTransition::M: {

      const bool last_week = (pt.date.m.week == 5);

      days = kMonthOffsets[leap_year][pt.date.m.month + last_week];

      const std::int_fast64_t weekday = (jan1_weekday + days) % 7;

      if (last_week) {

        days -= (weekday + 7 - 1 - pt.date.m.weekday) % 7 + 1;

      } else {

        days += (pt.date.m.weekday + 7 - weekday) % 7;

        days += (pt.date.m.week - 1) * 7;

      }

      break;

    }

  }

  return (days * kSecsPerDay) + pt.time.offset;

}

inline time_zone::civil_lookup MakeUnique(const time_point<seconds>& tp) {

  time_zone::civil_lookup cl;

  cl.kind = time_zone::civil_lookup::UNIQUE;

  cl.pre = cl.trans = cl.post = tp;

  return cl;

}

inline time_zone::civil_lookup MakeUnique(std::int_fast64_t unix_time) {

  return MakeUnique(FromUnixSeconds(unix_time));

}

inline time_zone::civil_lookup MakeSkipped(const Transition& tr,

                                           const civil_second& cs) {

  time_zone::civil_lookup cl;

  cl.kind = time_zone::civil_lookup::SKIPPED;

  cl.pre = FromUnixSeconds(tr.unix_time - 1 + (cs - tr.prev_civil_sec));

  cl.trans = FromUnixSeconds(tr.unix_time);

  cl.post = FromUnixSeconds(tr.unix_time - (tr.civil_sec - cs));

  return cl;

}

inline time_zone::civil_lookup MakeRepeated(const Transition& tr,

                                            const civil_second& cs) {

  time_zone::civil_lookup cl;

  cl.kind = time_zone::civil_lookup::REPEATED;

  cl.pre = FromUnixSeconds(tr.unix_time - 1 - (tr.prev_civil_sec - cs));

  cl.trans = FromUnixSeconds(tr.unix_time);

  cl.post = FromUnixSeconds(tr.unix_time + (cs - tr.civil_sec));

  return cl;

}

inline civil_second YearShift(const civil_second& cs, year_t shift) {

  return civil_second(cs.year() + shift, cs.month(), cs.day(),

                      cs.hour(), cs.minute(), cs.second());

}

}  // namespace

// What (no leap-seconds) UTC+seconds zoneinfo would look like.

bool TimeZoneInfo::ResetToBuiltinUTC(const seconds& offset) {

  transition_types_.resize(1);

  TransitionType& tt(transition_types_.back());

  tt.utc_offset = static_cast<std::int_least32_t>(offset.count());

  tt.is_dst = false;

  tt.abbr_index = 0;

  // We temporarily add some redundant, contemporary (2015 through 2025)

  // transitions for performance reasons.  See TimeZoneInfo::LocalTime().

  // TODO: Fix the performance issue and remove the extra transitions.

  transitions_.clear();

  transitions_.reserve(12);

  for (const std::int_fast64_t unix_time : {

           -(1LL << 59),  // a "first half" transition

           1420070400LL,  // 2015-01-01T00:00:00+00:00

           1451606400LL,  // 2016-01-01T00:00:00+00:00

           1483228800LL,  // 2017-01-01T00:00:00+00:00

           1514764800LL,  // 2018-01-01T00:00:00+00:00

           1546300800LL,  // 2019-01-01T00:00:00+00:00

           1577836800LL,  // 2020-01-01T00:00:00+00:00

           1609459200LL,  // 2021-01-01T00:00:00+00:00

           1640995200LL,  // 2022-01-01T00:00:00+00:00

           1672531200LL,  // 2023-01-01T00:00:00+00:00

           1704067200LL,  // 2024-01-01T00:00:00+00:00

           1735689600LL,  // 2025-01-01T00:00:00+00:00

       }) {

    Transition& tr(*transitions_.emplace(transitions_.end()));

    tr.unix_time = unix_time;

    tr.type_index = 0;

    tr.civil_sec = LocalTime(tr.unix_time, tt).cs;

    tr.prev_civil_sec = tr.civil_sec - 1;

  }

  default_transition_type_ = 0;

  abbreviations_ = FixedOffsetToAbbr(offset);

  abbreviations_.append(1, '\0');

  future_spec_.clear();  // never needed for a fixed-offset zone

  extended_ = false;

  tt.civil_max = LocalTime(seconds::max().count(), tt).cs;

  tt.civil_min = LocalTime(seconds::min().count(), tt).cs;

  transitions_.shrink_to_fit();

  return true;

}

// Builds the in-memory header using the raw bytes from the file.

bool TimeZoneInfo::Header::Build(const tzhead& tzh) {

  std::int_fast32_t v;

  if ((v = Decode32(tzh.tzh_timecnt)) < 0) return false;

  timecnt = static_cast<std::size_t>(v);

  if ((v = Decode32(tzh.tzh_typecnt)) < 0) return false;

  typecnt = static_cast<std::size_t>(v);

  if ((v = Decode32(tzh.tzh_charcnt)) < 0) return false;

  charcnt = static_cast<std::size_t>(v);

  if ((v = Decode32(tzh.tzh_leapcnt)) < 0) return false;

  leapcnt = static_cast<std::size_t>(v);

  if ((v = Decode32(tzh.tzh_ttisstdcnt)) < 0) return false;

  ttisstdcnt = static_cast<std::size_t>(v);

  if ((v = Decode32(tzh.tzh_ttisutcnt)) < 0) return false;

  ttisutcnt = static_cast<std::size_t>(v);

  return true;

}

// How many bytes of data are associated with this header. The result

// depends upon whether this is a section with 4-byte or 8-byte times.

std::size_t TimeZoneInfo::Header::DataLength(std::size_t time_len) const {

  std::size_t len = 0;

  len += (time_len + 1) * timecnt;  // unix_time + type_index

  len += (4 + 1 + 1) * typecnt;     // utc_offset + is_dst + abbr_index

  len += 1 * charcnt;               // abbreviations

  len += (time_len + 4) * leapcnt;  // leap-time + TAI-UTC

  len += 1 * ttisstdcnt;            // UTC/local indicators

  len += 1 * ttisutcnt;             // standard/wall indicators

  return len;

}

// zic(8) can generate no-op transitions when a zone changes rules at an

// instant when there is actually no discontinuity.  So we check whether

// two transitions have equivalent types (same offset/is_dst/abbr).

bool TimeZoneInfo::EquivTransitions(std::uint_fast8_t tt1_index,

                                    std::uint_fast8_t tt2_index) const {

  if (tt1_index == tt2_index) return true;

  const TransitionType& tt1(transition_types_[tt1_index]);

  const TransitionType& tt2(transition_types_[tt2_index]);

  if (tt1.utc_offset != tt2.utc_offset) return false;

  if (tt1.is_dst != tt2.is_dst) return false;

  if (tt1.abbr_index != tt2.abbr_index) return false;

  return true;

}

// Find/make a transition type with these attributes.

bool TimeZoneInfo::GetTransitionType(std::int_fast32_t utc_offset, bool is_dst,

                                     const std::string& abbr,

                                     std::uint_least8_t* index) {

  std::size_t type_index = 0;

  std::size_t abbr_index = abbreviations_.size();

  for (; type_index != transition_types_.size(); ++type_index) {

    const TransitionType& tt(transition_types_[type_index]);

    const char* tt_abbr = &abbreviations_[tt.abbr_index];

    if (tt_abbr == abbr) abbr_index = tt.abbr_index;

    if (tt.utc_offset == utc_offset && tt.is_dst == is_dst) {

      if (abbr_index == tt.abbr_index) break;  // reuse

    }

  }

  if (type_index > 255 || abbr_index > 255) {

    // No index space (8 bits) available for a new type or abbreviation.

    return false;

  }

  if (type_index == transition_types_.size()) {

    TransitionType& tt(*transition_types_.emplace(transition_types_.end()));

    tt.utc_offset = static_cast<std::int_least32_t>(utc_offset);

    tt.is_dst = is_dst;

    if (abbr_index == abbreviations_.size()) {

      abbreviations_.append(abbr);

      abbreviations_.append(1, '\0');

    }

    tt.abbr_index = static_cast<std::uint_least8_t>(abbr_index);

  }

  *index = static_cast<std::uint_least8_t>(type_index);

  return true;

}

// Use the POSIX-TZ-environment-variable-style string to handle times

// in years after the last transition stored in the zoneinfo data.

bool TimeZoneInfo::ExtendTransitions() {

  extended_ = false;

  if (future_spec_.empty()) return true;  // last transition prevails

  PosixTimeZone posix;

  if (!ParsePosixSpec(future_spec_, &posix)) return false;

  // Find transition type for the future std specification.

  std::uint_least8_t std_ti;

  if (!GetTransitionType(posix.std_offset, false, posix.std_abbr, &std_ti))

    return false;

  if (posix.dst_abbr.empty()) {  // std only

    // The future specification should match the last transition, and

    // that means that handling the future will fall out naturally.

    return EquivTransitions(transitions_.back().type_index, std_ti);

  }

  // Find transition type for the future dst specification.

  std::uint_least8_t dst_ti;

  if (!GetTransitionType(posix.dst_offset, true, posix.dst_abbr, &dst_ti))

    return false;

  if (AllYearDST(posix)) {  // dst only

    // The future specification should match the last transition, and

    // that means that handling the future will fall out naturally.

    return EquivTransitions(transitions_.back().type_index, dst_ti);

  }

  // Extend the transitions for an additional 400 years using the

  // future specification. Years beyond those can be handled by

  // mapping back to a cycle-equivalent year within that range.

  // We may need two additional transitions for the current year.

  transitions_.reserve(transitions_.size() + 400 * 2 + 2);

  extended_ = true;

  const Transition& last(transitions_.back());

  const std::int_fast64_t last_time = last.unix_time;

  const TransitionType& last_tt(transition_types_[last.type_index]);

  last_year_ = LocalTime(last_time, last_tt).cs.year();

  bool leap_year = IsLeap(last_year_);

  const civil_second jan1(last_year_);

  std::int_fast64_t jan1_time = jan1 - civil_second();

  int jan1_weekday = ToPosixWeekday(get_weekday(jan1));

  Transition dst = {0, dst_ti, civil_second(), civil_second()};

  Transition std = {0, std_ti, civil_second(), civil_second()};

  for (const year_t limit = last_year_ + 400;; ++last_year_) {

    auto dst_trans_off = TransOffset(leap_year, jan1_weekday, posix.dst_start);

    auto std_trans_off = TransOffset(leap_year, jan1_weekday, posix.dst_end);

    dst.unix_time = jan1_time + dst_trans_off - posix.std_offset;

    std.unix_time = jan1_time + std_trans_off - posix.dst_offset;

    const auto* ta = dst.unix_time < std.unix_time ? &dst : &std;

    const auto* tb = dst.unix_time < std.unix_time ? &std : &dst;

    if (last_time < tb->unix_time) {

      if (last_time < ta->unix_time) transitions_.push_back(*ta);

      transitions_.push_back(*tb);

    }

    if (last_year_ == limit) break;

    jan1_time += kSecsPerYear[leap_year];

    jan1_weekday = (jan1_weekday + kDaysPerYear[leap_year]) % 7;

    leap_year = !leap_year && IsLeap(last_year_ + 1);

  }

  return true;

}

bool TimeZoneInfo::Load(ZoneInfoSource* zip) {

  // Read and validate the header.

  tzhead tzh;

  if (zip->Read(&tzh, sizeof(tzh)) != sizeof(tzh))

    return false;

  if (strncmp(tzh.tzh_magic, TZ_MAGIC, sizeof(tzh.tzh_magic)) != 0)

    return false;

  Header hdr;

  if (!hdr.Build(tzh))

    return false;

  std::size_t time_len = 4;

  if (tzh.tzh_version[0] != '\0') {

    // Skip the 4-byte data.

    if (zip->Skip(hdr.DataLength(time_len)) != 0)

      return false;

    // Read and validate the header for the 8-byte data.

    if (zip->Read(&tzh, sizeof(tzh)) != sizeof(tzh))

      return false;

    if (strncmp(tzh.tzh_magic, TZ_MAGIC, sizeof(tzh.tzh_magic)) != 0)

      return false;

    if (tzh.tzh_version[0] == '\0')

      return false;

    if (!hdr.Build(tzh))

      return false;

    time_len = 8;

  }

  if (hdr.typecnt == 0)

    return false;

  if (hdr.leapcnt != 0) {

    // This code assumes 60-second minutes so we do not want

    // the leap-second encoded zoneinfo. We could reverse the

    // compensation, but the "right" encoding is rarely used

    // so currently we simply reject such data.

    return false;

  }

  if (hdr.ttisstdcnt != 0 && hdr.ttisstdcnt != hdr.typecnt)

    return false;

  if (hdr.ttisutcnt != 0 && hdr.ttisutcnt != hdr.typecnt)

    return false;

  // Read the data into a local buffer.

  std::size_t len = hdr.DataLength(time_len);

  std::vector<char> tbuf(len);

  if (zip->Read(tbuf.data(), len) != len)

    return false;

  const char* bp = tbuf.data();

  // Decode and validate the transitions.

  transitions_.reserve(hdr.timecnt + 2);

  transitions_.resize(hdr.timecnt);

  for (std::size_t i = 0; i != hdr.timecnt; ++i) {

    transitions_[i].unix_time = (time_len == 4) ? Decode32(bp) : Decode64(bp);

    bp += time_len;

    if (i != 0) {

      // Check that the transitions are ordered by time (as zic guarantees).

      if (!Transition::ByUnixTime()(transitions_[i - 1], transitions_[i]))

        return false;  // out of order

    }

  }

  bool seen_type_0 = false;

  for (std::size_t i = 0; i != hdr.timecnt; ++i) {

    transitions_[i].type_index = Decode8(bp++);

    if (transitions_[i].type_index >= hdr.typecnt)

      return false;

    if (transitions_[i].type_index == 0)

      seen_type_0 = true;

  }

  // Decode and validate the transition types.

  transition_types_.reserve(hdr.typecnt + 2);

  transition_types_.resize(hdr.typecnt);

  for (std::size_t i = 0; i != hdr.typecnt; ++i) {

    transition_types_[i].utc_offset =

        static_cast<std::int_least32_t>(Decode32(bp));

    if (transition_types_[i].utc_offset >= kSecsPerDay ||

        transition_types_[i].utc_offset <= -kSecsPerDay)

      return false;

    bp += 4;

    transition_types_[i].is_dst = (Decode8(bp++) != 0);

    transition_types_[i].abbr_index = Decode8(bp++);

    if (transition_types_[i].abbr_index >= hdr.charcnt)

      return false;

  }

  // Determine the before-first-transition type.

  default_transition_type_ = 0;

  if (seen_type_0 && hdr.timecnt != 0) {

    std::uint_fast8_t index = 0;

    if (transition_types_[0].is_dst) {

      index = transitions_[0].type_index;

      while (index != 0 && transition_types_[index].is_dst)

        --index;

    }

    while (index != hdr.typecnt && transition_types_[index].is_dst)

      ++index;

    if (index != hdr.typecnt)

      default_transition_type_ = index;

  }

  // Copy all the abbreviations.

  abbreviations_.reserve(hdr.charcnt + 10);

  abbreviations_.assign(bp, hdr.charcnt);

  bp += hdr.charcnt;

  // Skip the unused portions. We've already dispensed with leap-second

  // encoded zoneinfo. The ttisstd/ttisgmt indicators only apply when

  // interpreting a POSIX spec that does not include start/end rules, and

  // that isn't the case here (see "zic -p").

  bp += (time_len + 4) * hdr.leapcnt;  // leap-time + TAI-UTC

  bp += 1 * hdr.ttisstdcnt;            // UTC/local indicators

  bp += 1 * hdr.ttisutcnt;             // standard/wall indicators

  assert(bp == tbuf.data() + tbuf.size());

  future_spec_.clear();

  if (tzh.tzh_version[0] != '\0') {

    // Snarf up the NL-enclosed future POSIX spec. Note

    // that version '3' files utilize an extended format.

    auto get_char = [](ZoneInfoSource* azip) -> int {

      unsigned char ch;  // all non-EOF results are positive

      return (azip->Read(&ch, 1) == 1) ? ch : EOF;

    };

    if (get_char(zip) != '\n')

      return false;

    for (int c = get_char(zip); c != '\n'; c = get_char(zip)) {

      if (c == EOF)

        return false;

      future_spec_.push_back(static_cast<char>(c));

    }

  }

  // We don't check for EOF so that we're forwards compatible.

  // If we did not find version information during the standard loading

  // process (as of tzh_version '3' that is unsupported), then ask the

  // ZoneInfoSource for any out-of-bound version string it may be privy to.

  if (version_.empty()) {

    version_ = zip->Version();

  }

  // Trim redundant transitions. zic may have added these to work around

  // differences between the glibc and reference implementations (see

  // zic.c:dontmerge) or to avoid bugs in old readers. For us, they just

  // get in the way when we do future_spec_ extension.

  while (hdr.timecnt > 1) {

    if (!EquivTransitions(transitions_[hdr.timecnt - 1].type_index,

                          transitions_[hdr.timecnt - 2].type_index)) {

      break;

    }

    hdr.timecnt -= 1;

  }

  transitions_.resize(hdr.timecnt);

  // Ensure that there is always a transition in the first half of the

  // time line (the second half is handled below) so that the signed

  // difference between a civil_second and the civil_second of its

  // previous transition is always representable, without overflow.

  if (transitions_.empty() || transitions_.front().unix_time >= 0) {

    Transition& tr(*transitions_.emplace(transitions_.begin()));

    tr.unix_time = -(1LL << 59);  // -18267312070-10-26T17:01:52+00:00

    tr.type_index = default_transition_type_;

  }

  // Extend the transitions using the future specification.

  if (!ExtendTransitions()) return false;

  // Ensure that there is always a transition in the second half of the

  // time line (the first half is handled above) so that the signed

  // difference between a civil_second and the civil_second of its

  // previous transition is always representable, without overflow.

  const Transition& last(transitions_.back());

  if (last.unix_time < 0) {

    const std::uint_fast8_t type_index = last.type_index;

    Transition& tr(*transitions_.emplace(transitions_.end()));

    tr.unix_time = 2147483647;  // 2038-01-19T03:14:07+00:00

    tr.type_index = type_index;

  }

  // Compute the local civil time for each transition and the preceding

  // second. These will be used for reverse conversions in MakeTime().

  const TransitionType* ttp = &transition_types_[default_transition_type_];

  for (std::size_t i = 0; i != transitions_.size(); ++i) {

    Transition& tr(transitions_[i]);

    tr.prev_civil_sec = LocalTime(tr.unix_time, *ttp).cs - 1;

    ttp = &transition_types_[tr.type_index];

    tr.civil_sec = LocalTime(tr.unix_time, *ttp).cs;

    if (i != 0) {

      // Check that the transitions are ordered by civil time. Essentially

      // this means that an offset change cannot cross another such change.

      // No one does this in practice, and we depend on it in MakeTime().

      if (!Transition::ByCivilTime()(transitions_[i - 1], tr))

        return false;  // out of order

    }

  }

  // Compute the maximum/minimum civil times that can be converted to a

  // time_point<seconds> for each of the zone's transition types.

  for (auto& tt : transition_types_) {

    tt.civil_max = LocalTime(seconds::max().count(), tt).cs;

    tt.civil_min = LocalTime(seconds::min().count(), tt).cs;

  }

  transitions_.shrink_to_fit();

  return true;

}

namespace {

using FilePtr = std::unique_ptr<FILE, int(*)(FILE*)>;

// fopen(3) adaptor.

inline FilePtr FOpen(const char* path, const char* mode) {

#if defined(_MSC_VER)

  FILE* fp;

  if (fopen_s(&fp, path, mode) != 0) fp = nullptr;

  return FilePtr(fp, fclose);

#else

  // TODO: Enable the close-on-exec flag.

  return FilePtr(fopen(path, mode), fclose);

#endif

}

// A stdio(3)-backed implementation of ZoneInfoSource.

class FileZoneInfoSource : public ZoneInfoSource {

 public:

  static std::unique_ptr<ZoneInfoSource> Open(const std::string& name);

  std::size_t Read(void* ptr, std::size_t size) override {

    size = std::min(size, len_);

    std::size_t nread = fread(ptr, 1, size, fp_.get());

    len_ -= nread;

    return nread;

  }

  int Skip(std::size_t offset) override {

    offset = std::min(offset, len_);

    int rc = fseek(fp_.get(), static_cast<long>(offset), SEEK_CUR);

    if (rc == 0) len_ -= offset;

    return rc;

  }

  std::string Version() const override {

    // TODO: It would nice if the zoneinfo data included the tzdb version.

    return std::string();

  }

 protected:

  explicit FileZoneInfoSource(

      FilePtr fp, std::size_t len = std::numeric_limits<std::size_t>::max())

      : fp_(std::move(fp)), len_(len) {}

 private:

  FilePtr fp_;

  std::size_t len_;

};

std::unique_ptr<ZoneInfoSource> FileZoneInfoSource::Open(

    const std::string& name) {

  // Use of the "file:" prefix is intended for testing purposes only.

  const std::size_t pos = (name.compare(0, 5, "file:") == 0) ? 5 : 0;

  // Map the time-zone name to a path name.

  std::string path;

  if (pos == name.size() || name[pos] != '/') {

    const char* tzdir = "/usr/share/zoneinfo";

    char* tzdir_env = nullptr;

#if defined(_MSC_VER)

    _dupenv_s(&tzdir_env, nullptr, "TZDIR");

#else

    tzdir_env = std::getenv("TZDIR");

#endif

    if (tzdir_env && *tzdir_env) tzdir = tzdir_env;

    path += tzdir;

    path += '/';

#if defined(_MSC_VER)

    free(tzdir_env);

#endif

  }

  path.append(name, pos, std::string::npos);

  // Open the zoneinfo file.

  auto fp = FOpen(path.c_str(), "rb");

  if (fp == nullptr) return nullptr;

  return std::unique_ptr<ZoneInfoSource>(new FileZoneInfoSource(std::move(fp)));

}

class AndroidZoneInfoSource : public FileZoneInfoSource {

 public:

  static std::unique_ptr<ZoneInfoSource> Open(const std::string& name);

  std::string Version() const override { return version_; }

 private:

  explicit AndroidZoneInfoSource(FilePtr fp, std::size_t len,

                                 std::string version)

      : FileZoneInfoSource(std::move(fp), len), version_(std::move(version)) {}

  std::string version_;

};

std::unique_ptr<ZoneInfoSource> AndroidZoneInfoSource::Open(

    const std::string& name) {

  // Use of the "file:" prefix is intended for testing purposes only.

  const std::size_t pos = (name.compare(0, 5, "file:") == 0) ? 5 : 0;

  // See Android's libc/tzcode/bionic.cpp for additional information.

  for (const char* tzdata : {"/data/misc/zoneinfo/current/tzdata",

                             "/system/usr/share/zoneinfo/tzdata"}) {

    auto fp = FOpen(tzdata, "rb");

    if (fp == nullptr) continue;

    char hbuf[24];  // covers header.zonetab_offset too

    if (fread(hbuf, 1, sizeof(hbuf), fp.get()) != sizeof(hbuf)) continue;

    if (strncmp(hbuf, "tzdata", 6) != 0) continue;

    const char* vers = (hbuf[11] == '\0') ? hbuf + 6 : "";

    const std::int_fast32_t index_offset = Decode32(hbuf + 12);

    const std::int_fast32_t data_offset = Decode32(hbuf + 16);

    if (index_offset < 0 || data_offset < index_offset) continue;

    if (fseek(fp.get(), static_cast<long>(index_offset), SEEK_SET) != 0)

      continue;

    char ebuf[52];  // covers entry.unused too

    const std::size_t index_size =

        static_cast<std::size_t>(data_offset - index_offset);

    const std::size_t zonecnt = index_size / sizeof(ebuf);

    if (zonecnt * sizeof(ebuf) != index_size) continue;

    for (std::size_t i = 0; i != zonecnt; ++i) {

      if (fread(ebuf, 1, sizeof(ebuf), fp.get()) != sizeof(ebuf)) break;

      const std::int_fast32_t start = data_offset + Decode32(ebuf + 40);

      const std::int_fast32_t length = Decode32(ebuf + 44);

      if (start < 0 || length < 0) break;

      ebuf[40] = '\0';  // ensure zone name is NUL terminated

      if (strcmp(name.c_str() + pos, ebuf) == 0) {

        if (fseek(fp.get(), static_cast<long>(start), SEEK_SET) != 0) break;

        return std::unique_ptr<ZoneInfoSource>(new AndroidZoneInfoSource(

            std::move(fp), static_cast<std::size_t>(length), vers));

      }

    }

  }

  return nullptr;

}

// A zoneinfo source for use inside Fuchsia components. This attempts to

// read zoneinfo files from one of several known paths in a component's

// incoming namespace. [Config data][1] is preferred, but package-specific

// resources are also supported.

//

// Fuchsia's implementation supports `FileZoneInfoSource::Version()`.

//

// [1]: https://fuchsia.dev/fuchsia-src/development/components/data#using_config_data_in_your_component

class FuchsiaZoneInfoSource : public FileZoneInfoSource {

 public:

  static std::unique_ptr<ZoneInfoSource> Open(const std::string& name);

  std::string Version() const override { return version_; }

 private:

  explicit FuchsiaZoneInfoSource(FilePtr fp, std::string version)

      : FileZoneInfoSource(std::move(fp)), version_(std::move(version)) {}

  std::string version_;

};

std::unique_ptr<ZoneInfoSource> FuchsiaZoneInfoSource::Open(

    const std::string& name) {

  // Use of the "file:" prefix is intended for testing purposes only.

  const std::size_t pos = (name.compare(0, 5, "file:") == 0) ? 5 : 0;

  // Prefixes where a Fuchsia component might find zoneinfo files,

  // in descending order of preference.

  const auto kTzdataPrefixes = {

      "/config/data/tzdata/",

      "/pkg/data/tzdata/",

      "/data/tzdata/",

  };

  const auto kEmptyPrefix = {""};

  const bool name_absolute = (pos != name.size() && name[pos] == '/');

  const auto prefixes = name_absolute ? kEmptyPrefix : kTzdataPrefixes;

  // Fuchsia builds place zoneinfo files at "<prefix><format><name>".

  for (const std::string prefix : prefixes) {

    std::string path = prefix;

    if (!prefix.empty()) path += "zoneinfo/tzif2/";  // format

    path.append(name, pos, std::string::npos);

    auto fp = FOpen(path.c_str(), "rb");

    if (fp == nullptr) continue;

    std::string version;

    if (!prefix.empty()) {

      // Fuchsia builds place the version in "<prefix>revision.txt".

      std::ifstream version_stream(prefix + "revision.txt");

      if (version_stream.is_open()) {

        // revision.txt should contain no newlines, but to be

        // defensive we read just the first line.

        std::getline(version_stream, version);

      }

    }

    return std::unique_ptr<ZoneInfoSource>(

        new FuchsiaZoneInfoSource(std::move(fp), std::move(version)));

  }

  return nullptr;

}

}  // namespace

bool TimeZoneInfo::Load(const std::string& name) {

  // We can ensure that the loading of UTC or any other fixed-offset

  // zone never fails because the simple, fixed-offset state can be

  // internally generated. Note that this depends on our choice to not

  // accept leap-second encoded ("right") zoneinfo.

  auto offset = seconds::zero();

  if (FixedOffsetFromName(name, &offset)) {

    return ResetToBuiltinUTC(offset);

  }

  // Find and use a ZoneInfoSource to load the named zone.

  auto zip = cctz_extension::zone_info_source_factory(

      name, [](const std::string& n) -> std::unique_ptr<ZoneInfoSource> {

        if (auto z = FileZoneInfoSource::Open(n)) return z;

        if (auto z = AndroidZoneInfoSource::Open(n)) return z;

        if (auto z = FuchsiaZoneInfoSource::Open(n)) return z;

        return nullptr;

      });

  return zip != nullptr && Load(zip.get());

}

// BreakTime() translation for a particular transition type.

time_zone::absolute_lookup TimeZoneInfo::LocalTime(

    std::int_fast64_t unix_time, const TransitionType& tt) const {

  // A civil time in "+offset" looks like (time+offset) in UTC.

  // Note: We perform two additions in the civil_second domain to

  // sidestep the chance of overflow in (unix_time + tt.utc_offset).

  return {(civil_second() + unix_time) + tt.utc_offset,

          tt.utc_offset, tt.is_dst, &abbreviations_[tt.abbr_index]};

}

// BreakTime() translation for a particular transition.

time_zone::absolute_lookup TimeZoneInfo::LocalTime(

    std::int_fast64_t unix_time, const Transition& tr) const {

  const TransitionType& tt = transition_types_[tr.type_index];

  // Note: (unix_time - tr.unix_time) will never overflow as we

  // have ensured that there is always a "nearby" transition.

  return {tr.civil_sec + (unix_time - tr.unix_time),  // TODO: Optimize.

          tt.utc_offset, tt.is_dst, &abbreviations_[tt.abbr_index]};

}

// MakeTime() translation with a conversion-preserving +N * 400-year shift.

time_zone::civil_lookup TimeZoneInfo::TimeLocal(const civil_second& cs,

                                                year_t c4_shift) const {

  assert(last_year_ - 400 < cs.year() && cs.year() <= last_year_);

  time_zone::civil_lookup cl = MakeTime(cs);

  if (c4_shift > seconds::max().count() / kSecsPer400Years) {

    cl.pre = cl.trans = cl.post = time_point<seconds>::max();

  } else {

    const auto offset = seconds(c4_shift * kSecsPer400Years);

    const auto limit = time_point<seconds>::max() - offset;

    for (auto* tp : {&cl.pre, &cl.trans, &cl.post}) {

      if (*tp > limit) {

        *tp = time_point<seconds>::max();

      } else {

        *tp += offset;

      }

    }

  }

  return cl;

}

time_zone::absolute_lookup TimeZoneInfo::BreakTime(

    const time_point<seconds>& tp) const {

  std::int_fast64_t unix_time = ToUnixSeconds(tp);

  const std::size_t timecnt = transitions_.size();

  assert(timecnt != 0);  // We always add a transition.

  if (unix_time < transitions_[0].unix_time) {

    return LocalTime(unix_time, transition_types_[default_transition_type_]);

  }

  if (unix_time >= transitions_[timecnt - 1].unix_time) {

    // After the last transition. If we extended the transitions using

    // future_spec_, shift back to a supported year using the 400-year

    // cycle of calendaric equivalence and then compensate accordingly.

    if (extended_) {

      const std::int_fast64_t diff =

          unix_time - transitions_[timecnt - 1].unix_time;