#include "esphome/core/defines.h"

#ifdef USE_TIME_TIMEZONE

#include "posix_tz.h"
#include <cctype>
#include <cstdio>

namespace esphome::time {

// Global timezone - set once at startup, rarely changes
// NOLINTNEXTLINE(cppcoreguidelines-avoid-non-const-global-variables) - intentional mutable state
static ParsedTimezone global_tz_{};

void set_global_tz(const ParsedTimezone &tz) { global_tz_ = tz; }

const ParsedTimezone &get_global_tz() { return global_tz_; }

namespace internal {

// Remove before 2026.9.0: parse_uint, skip_tz_name, parse_offset, parse_dst_rule,
// and parse_transition_time are only used by parse_posix_tz() (bridge code).
static uint32_t parse_uint(const char *&p) {
  uint32_t value = 0;
  while (std::isdigit(static_cast<unsigned char>(*p))) {
    value = value * 10 + (*p - '0');
    p++;
  }
  return value;
}

bool is_leap_year(int year) { return (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0); }

// Get days in year (avoids duplicate is_leap_year calls)
static inline int days_in_year(int year) { return is_leap_year(year) ? 366 : 365; }

// Count leap years in [1, year] (i.e. up to and including year)
static constexpr int count_leap_years_up_to(int year) { return year / 4 - year / 100 + year / 400; }

constexpr int EPOCH_YEAR = 1970;
constexpr int LEAP_YEARS_BEFORE_EPOCH = count_leap_years_up_to(EPOCH_YEAR - 1);
constexpr int DAYS_PER_YEAR = 365;
constexpr int SECONDS_PER_DAY = 86400;

// Days from epoch (Jan 1 1970) to Jan 1 of given year — O(1)
static inline int64_t days_to_year_start(int year) {
  return static_cast<int64_t>(DAYS_PER_YEAR) * (year - EPOCH_YEAR) +
         (count_leap_years_up_to(year - 1) - LEAP_YEARS_BEFORE_EPOCH);
}

// Convert days since epoch to year, updating days to day-of-year remainder.
// The initial estimate from days/365 can overshoot by multiple years for
// far-future dates (e.g., year 5000+) due to accumulated leap days,
// so we use loops rather than single-step correction.
static int days_to_year(int64_t &days) {
  int year = static_cast<int>(EPOCH_YEAR + days / DAYS_PER_YEAR);
  int64_t year_start = days_to_year_start(year);
  while (days < year_start) {
    year--;
    year_start = days_to_year_start(year);
  }
  while (days >= year_start + days_in_year(year)) {
    year_start += days_in_year(year);
    year++;
  }
  days -= year_start;
  return year;
}

// Extract just the year from a UTC epoch — O(1)
static int epoch_to_year(time_t epoch) {
  int64_t days = epoch / SECONDS_PER_DAY;
  if (epoch < 0 && epoch % SECONDS_PER_DAY != 0)
    days--;
  return days_to_year(days);
}

int days_in_month(int year, int month) {
  switch (month) {
    case 2:
      return is_leap_year(year) ? 29 : 28;
    case 4:
    case 6:
    case 9:
    case 11:
      return 30;
    default:
      return 31;
  }
}

// Zeller-like algorithm for day of week (0 = Sunday)
int __attribute__((noinline)) day_of_week(int year, int month, int day) {
  // Adjust for January/February
  if (month < 3) {
    month += 12;
    year--;
  }
  int k = year % 100;
  int j = year / 100;
  int h = (day + (13 * (month + 1)) / 5 + k + k / 4 + j / 4 - 2 * j) % 7;
  // Convert from Zeller (0=Sat) to standard (0=Sun)
  return ((h + 6) % 7);
}

void __attribute__((noinline)) epoch_to_tm_utc(time_t epoch, struct tm *out_tm) {
  // Days since epoch
  int64_t days = epoch / SECONDS_PER_DAY;
  int32_t remaining_secs = epoch % SECONDS_PER_DAY;
  if (remaining_secs < 0) {
    days--;
    remaining_secs += SECONDS_PER_DAY;
  }

  out_tm->tm_sec = remaining_secs % 60;
  remaining_secs /= 60;
  out_tm->tm_min = remaining_secs % 60;
  out_tm->tm_hour = remaining_secs / 60;

  // Day of week (Jan 1, 1970 was Thursday = 4)
  out_tm->tm_wday = static_cast<int>((days + 4) % 7);
  if (out_tm->tm_wday < 0)
    out_tm->tm_wday += 7;

  // Calculate year (updates days to day-of-year)
  int year = days_to_year(days);
  out_tm->tm_year = year - 1900;
  out_tm->tm_yday = static_cast<int>(days);

  // Calculate month and day
  int month = 1;
  int dim;
  while (days >= (dim = days_in_month(year, month))) {
    days -= dim;
    month++;
  }

  out_tm->tm_mon = month - 1;
  out_tm->tm_mday = static_cast<int>(days) + 1;
  out_tm->tm_isdst = 0;
}

bool skip_tz_name(const char *&p) {
  if (*p == '<') {
    // Angle-bracket quoted name: <+07>, <-03>, <AEST>
    p++;  // skip '<'
    while (*p && *p != '>') {
      p++;
    }
    if (*p == '>') {
      p++;  // skip '>'
      return true;
    }
    return false;  // Unterminated
  }

  // Standard name: 3+ letters
  const char *start = p;
  while (*p && std::isalpha(static_cast<unsigned char>(*p))) {
    p++;
  }
  return (p - start) >= 3;
}

int32_t __attribute__((noinline)) parse_offset(const char *&p) {
  int sign = 1;
  if (*p == '-') {
    sign = -1;
    p++;
  } else if (*p == '+') {
    p++;
  }

  int hours = parse_uint(p);
  int minutes = 0;
  int seconds = 0;

  if (*p == ':') {
    p++;
    minutes = parse_uint(p);
    if (*p == ':') {
      p++;
      seconds = parse_uint(p);
    }
  }

  return sign * (hours * 3600 + minutes * 60 + seconds);
}

// Helper to parse the optional /time suffix (reuses parse_offset logic)
static void parse_transition_time(const char *&p, DSTRule &rule) {
  rule.time_seconds = 2 * 3600;  // Default 02:00
  if (*p == '/') {
    p++;
    rule.time_seconds = parse_offset(p);
  }
}

void __attribute__((noinline)) julian_to_month_day(int julian_day, int &out_month, int &out_day) {
  // J format: day 1-365, Feb 29 is NOT counted even in leap years
  // So day 60 is always March 1
  // Iterate forward through months (no array needed)
  int remaining = julian_day;
  out_month = 1;
  while (out_month <= 12) {
    // Days in month for non-leap year (J format ignores leap years)
    int dim = days_in_month(2001, out_month);  // 2001 is non-leap year
    if (remaining <= dim) {
      out_day = remaining;
      return;
    }
    remaining -= dim;
    out_month++;
  }
  out_day = remaining;
}

void __attribute__((noinline)) day_of_year_to_month_day(int day_of_year, int year, int &out_month, int &out_day) {
  // Plain format: day 0-365, Feb 29 IS counted in leap years
  // Day 0 = Jan 1
  int remaining = day_of_year;
  out_month = 1;

  while (out_month <= 12) {
    int days_this_month = days_in_month(year, out_month);
    if (remaining < days_this_month) {
      out_day = remaining + 1;
      return;
    }
    remaining -= days_this_month;
    out_month++;
  }

  // Shouldn't reach here with valid input
  out_month = 12;
  out_day = 31;
}

bool parse_dst_rule(const char *&p, DSTRule &rule) {
  rule = {};  // Zero initialize

  if (*p == 'M' || *p == 'm') {
    // M format: Mm.w.d (month.week.day)
    rule.type = DSTRuleType::MONTH_WEEK_DAY;
    p++;

    rule.month = parse_uint(p);
    if (rule.month < 1 || rule.month > 12)
      return false;

    if (*p++ != '.')
      return false;

    rule.week = parse_uint(p);
    if (rule.week < 1 || rule.week > 5)
      return false;

    if (*p++ != '.')
      return false;

    rule.day_of_week = parse_uint(p);
    if (rule.day_of_week > 6)
      return false;

  } else if (*p == 'J' || *p == 'j') {
    // J format: Jn (Julian day 1-365, not counting Feb 29)
    rule.type = DSTRuleType::JULIAN_NO_LEAP;
    p++;

    rule.day = parse_uint(p);
    if (rule.day < 1 || rule.day > 365)
      return false;

  } else if (std::isdigit(static_cast<unsigned char>(*p))) {
    // Plain number format: n (day 0-365, counting Feb 29)
    rule.type = DSTRuleType::DAY_OF_YEAR;

    rule.day = parse_uint(p);
    if (rule.day > 365)
      return false;

  } else {
    return false;
  }

  // Parse optional /time suffix
  parse_transition_time(p, rule);

  return true;
}

// Calculate days from Jan 1 of given year to given month/day
static int __attribute__((noinline)) days_from_year_start(int year, int month, int day) {
  int days = day - 1;
  for (int m = 1; m < month; m++) {
    days += days_in_month(year, m);
  }
  return days;
}

time_t __attribute__((noinline)) calculate_dst_transition(int year, const DSTRule &rule, int32_t base_offset_seconds) {
  int month, day;

  switch (rule.type) {
    case DSTRuleType::MONTH_WEEK_DAY: {
      // Find the nth occurrence of day_of_week in the given month
      int first_dow = day_of_week(year, rule.month, 1);

      // Days until first occurrence of target day
      int days_until_first = (rule.day_of_week - first_dow + 7) % 7;
      int first_occurrence = 1 + days_until_first;

      if (rule.week == 5) {
        // "Last" occurrence - find the last one in the month
        int dim = days_in_month(year, rule.month);
        day = first_occurrence;
        while (day + 7 <= dim) {
          day += 7;
        }
      } else {
        // nth occurrence
        day = first_occurrence + (rule.week - 1) * 7;
      }
      month = rule.month;
      break;
    }

    case DSTRuleType::JULIAN_NO_LEAP:
      // J format: day 1-365, Feb 29 not counted
      julian_to_month_day(rule.day, month, day);
      break;

    case DSTRuleType::DAY_OF_YEAR:
      // Plain format: day 0-365, Feb 29 counted
      day_of_year_to_month_day(rule.day, year, month, day);
      break;

    case DSTRuleType::NONE:
      // Should never be called with NONE, but handle it gracefully
      month = 1;
      day = 1;
      break;
  }

  // Calculate days from epoch to this date
  int64_t days = days_to_year_start(year) + days_from_year_start(year, month, day);

  // Convert to epoch and add transition time and base offset
  return days * SECONDS_PER_DAY + rule.time_seconds + base_offset_seconds;
}

}  // namespace internal

bool __attribute__((noinline)) is_in_dst(time_t utc_epoch, const ParsedTimezone &tz) {
  if (!tz.has_dst()) {
    return false;
  }

  int year = internal::epoch_to_year(utc_epoch);

  // Calculate DST start and end for this year
  // DST start transition happens in standard time
  time_t dst_start = internal::calculate_dst_transition(year, tz.dst_start, tz.std_offset_seconds);
  // DST end transition happens in daylight time
  time_t dst_end = internal::calculate_dst_transition(year, tz.dst_end, tz.dst_offset_seconds);

  if (dst_start < dst_end) {
    // Northern hemisphere: DST is between start and end
    return (utc_epoch >= dst_start && utc_epoch < dst_end);
  } else {
    // Southern hemisphere: DST is outside the range (wraps around year)
    return (utc_epoch >= dst_start || utc_epoch < dst_end);
  }
}

// Remove before 2026.9.0: This parser is bridge code for backward compatibility with
// older Home Assistant clients that send the timezone as a POSIX TZ string instead of
// the pre-parsed ParsedTimezone protobuf struct. Once all clients send the struct
// directly, this function and the parsing helpers above (skip_tz_name, parse_offset,
// parse_dst_rule, parse_transition_time) can be removed.
// See https://github.com/esphome/backlog/issues/91
bool parse_posix_tz(const char *tz_string, ParsedTimezone &result) {
  if (!tz_string || !*tz_string) {
    return false;
  }

  const char *p = tz_string;

  // Initialize result (dst_start/dst_end default to type=NONE, so has_dst() returns false)
  result.std_offset_seconds = 0;
  result.dst_offset_seconds = 0;
  result.dst_start = {};
  result.dst_end = {};

  // Skip standard timezone name
  if (!internal::skip_tz_name(p)) {
    return false;
  }

  // Parse standard offset (required)
  if (!*p || (!std::isdigit(static_cast<unsigned char>(*p)) && *p != '+' && *p != '-')) {
    return false;
  }
  result.std_offset_seconds = internal::parse_offset(p);

  // Check for DST name
  if (!*p) {
    return true;  // No DST
  }

  // If next char is comma, there's no DST name but there are rules (invalid)
  if (*p == ',') {
    return false;
  }

  // Check if there's something that looks like a DST name start
  // (letter or angle bracket). If not, treat as trailing garbage and return success.
  if (!std::isalpha(static_cast<unsigned char>(*p)) && *p != '<') {
    return true;  // No DST, trailing characters ignored
  }

  if (!internal::skip_tz_name(p)) {
    return false;  // Invalid DST name (started but malformed)
  }

  // Optional DST offset (default is std - 1 hour)
  if (*p && *p != ',' && (std::isdigit(static_cast<unsigned char>(*p)) || *p == '+' || *p == '-')) {
    result.dst_offset_seconds = internal::parse_offset(p);
  } else {
    result.dst_offset_seconds = result.std_offset_seconds - 3600;
  }

  // Parse DST rules (required when DST name is present)
  if (*p != ',') {
    // DST name without rules - treat as no DST since we can't determine transitions
    return true;
  }

  p++;
  if (!internal::parse_dst_rule(p, result.dst_start)) {
    return false;
  }

  // Second rule is required per POSIX
  if (*p != ',') {
    return false;
  }
  p++;
  // has_dst() now returns true since dst_start.type was set by parse_dst_rule
  return internal::parse_dst_rule(p, result.dst_end);
}

// Format a POSIX offset (positive = west) as "+HHMM" / "-HHMM" for display.
// Convention: negate POSIX sign so east-of-UTC is positive (ISO 8601 / RFC 2822).
void format_designation(int32_t posix_offset, char *buf, size_t buf_size) {
  int32_t display = -posix_offset;
  char sign = display >= 0 ? '+' : '-';
  if (display < 0)
    display = -display;
  int h = display / 3600;
  int m = (display % 3600) / 60;
  snprintf(buf, buf_size, "%c%02d%02d", sign, h, m);
}

bool epoch_to_local_tm(time_t utc_epoch, const ParsedTimezone &tz, struct tm *out_tm) {
  if (!out_tm) {
    return false;
  }

  // Determine DST status once (avoids duplicate is_in_dst calculation)
  bool in_dst = is_in_dst(utc_epoch, tz);
  int32_t offset = in_dst ? tz.dst_offset_seconds : tz.std_offset_seconds;

  // Apply offset (POSIX offset is positive west, so subtract to get local)
  time_t local_epoch = utc_epoch - offset;

  internal::epoch_to_tm_utc(local_epoch, out_tm);
  out_tm->tm_isdst = in_dst ? 1 : 0;

  return true;
}

}  // namespace esphome::time

#ifndef USE_HOST
// Override libc's localtime functions to use our timezone on embedded platforms.
// This allows user lambdas calling ::localtime() to get correct local time
// without needing the TZ environment variable (which pulls in scanf bloat).
// On host, we use the normal TZ mechanism since there's no memory constraint.

// Thread-safe version
extern "C" struct tm *localtime_r(const time_t *timer, struct tm *result) {
  if (timer == nullptr || result == nullptr) {
    return nullptr;
  }
  esphome::time::epoch_to_local_tm(*timer, esphome::time::get_global_tz(), result);
  return result;
}

// Non-thread-safe version (uses static buffer, standard libc behavior)
extern "C" struct tm *localtime(const time_t *timer) {
  // NOLINTNEXTLINE(cppcoreguidelines-avoid-non-const-global-variables)
  static struct tm localtime_buf;
  return localtime_r(timer, &localtime_buf);
}
#endif  // !USE_HOST

#endif  // USE_TIME_TIMEZONE
