package main

import (
	"time"
)

type TimezoneConfig struct {
	StandardName   string
	StandardOffset int
	DaylightName   string
	DaylightOffset int
	ObservesUSDST  bool
}

// Map timezone aliases to standard/daylight offsets
var timezones = map[string]TimezoneConfig{
	// Dynamic US zones (switch automatically between Standard and Daylight)
	"US/Eastern":  {"EST", -5 * 3600, "EDT", -4 * 3600, true},
	"ET":          {"EST", -5 * 3600, "EDT", -4 * 3600, true},
	"US/Central":  {"CST", -6 * 3600, "CDT", -5 * 3600, true},
	"CT":          {"CST", -6 * 3600, "CDT", -5 * 3600, true},
	"US/Mountain": {"MST", -7 * 3600, "MDT", -6 * 3600, true},
	"MT":          {"MST", -7 * 3600, "MDT", -6 * 3600, true},
	"US/Pacific":  {"PST", -8 * 3600, "PDT", -7 * 3600, true},
	"PT":          {"PST", -8 * 3600, "PDT", -7 * 3600, true},

	// Fixed non-DST zones
	"UTC": {"UTC", 0, "UTC", 0, false},
	"EST": {"EST", -5 * 3600, "EST", -5 * 3600, false},
	"EDT": {"EDT", -4 * 3600, "EDT", -4 * 3600, false},
	"CST": {"CST", -6 * 3600, "CST", -6 * 3600, false},
	"CDT": {"CDT", -5 * 3600, "CDT", -5 * 3600, false},
	"MST": {"MST", -7 * 3600, "MST", -7 * 3600, false},
	"MDT": {"MDT", -6 * 3600, "MDT", -6 * 3600, false},
	"PST": {"PST", -8 * 3600, "PST", -8 * 3600, false},
	"PDT": {"PDT", -7 * 3600, "PDT", -7 * 3600, false},
}

// IsUSDST checks if the given UTC time falls in US Daylight Saving Time
// (Second Sunday in March at 02:00 local to first Sunday in November at 02:00 local).
func IsUSDST(t time.Time, stdOffsetSec int) bool {
	month := t.Month()
	if month < time.March || month > time.November {
		return false // Dec, Jan, Feb
	}
	if month > time.March && month < time.November {
		return true // Apr through Oct
	}

	day := t.Day()
	weekday := int(t.Weekday())

	if month == time.March {
		// Second Sunday in March is between 8th and 14th
		prevSunday := day - weekday
		if prevSunday < 8 {
			return false
		}
		if prevSunday >= 15 {
			return true
		}
		// Transition occurs at 2:00 AM standard local time -> (2 - stdOffsetHours) in UTC
		stdOffsetHours := stdOffsetSec / 3600
		switchHourUTC := 2 - stdOffsetHours
		return weekday != int(time.Sunday) || t.Hour() >= switchHourUTC
	}

	// November: first Sunday is between 1st and 7th
	prevSunday := day - weekday
	if prevSunday < 1 {
		return true // Before first Sunday
	}
	// Fall-back occurs at 2:00 AM daylight local time -> (2 - (stdOffsetHours + 1)) in UTC
	stdOffsetHours := stdOffsetSec / 3600
	switchHourUTC := 2 - (stdOffsetHours + 1)
	return weekday == int(time.Sunday) && t.Hour() < switchHourUTC
}

// ResolveTimezone dynamically determines the *time.Location and label
// based on whether the zone observes DST at the given UTC time.
func ResolveTimezone(name string, utcNow time.Time) (*time.Location, string) {
	cfg, ok := timezones[name]
	if !ok {
		return time.FixedZone("UTC", 0), "UTC"
	}

	if cfg.ObservesUSDST && IsUSDST(utcNow, cfg.StandardOffset) {
		return time.FixedZone(cfg.DaylightName, cfg.DaylightOffset), cfg.DaylightName
	}
	return time.FixedZone(cfg.StandardName, cfg.StandardOffset), cfg.StandardName
}