package timeutil

import (
	"fmt"
	"regexp"
	"strconv"
	"strings"
	"time"
)

const DateLayout = "2006-01-02"
const LocalDateTimeLayout = "2006-01-02T15:04"

func LoadLocation(name string) (*time.Location, error) {
	if location, ok := timezoneAliases[name]; ok {
		return location, nil
	}
	if location, ok := parseUTCOffset(name); ok {
		return location, nil
	}
	location, err := time.LoadLocation(name)
	if err == nil {
		return location, nil
	}
	if strings.Contains(name, "/") {
		return nil, fmt.Errorf("load timezone %q: %w", name, err)
	}
	location, chicagoErr := time.LoadLocation("America/" + name)
	if chicagoErr == nil {
		return location, nil
	}
	return nil, fmt.Errorf("load timezone %q: %w", name, err)
}

var timezoneAliases = map[string]*time.Location{
	"Chicago": mustLocation("America/Chicago"),
	"Stl":     mustLocation("America/Chicago"),
	"EST":     time.FixedZone("EST", -5*60*60),
	"EDT":     time.FixedZone("EDT", -4*60*60),
	"CST":     time.FixedZone("CST", -6*60*60),
	"CDT":     time.FixedZone("CDT", -5*60*60),
	"MST":     time.FixedZone("MST", -7*60*60),
	"MDT":     time.FixedZone("MDT", -6*60*60),
	"PST":     time.FixedZone("PST", -8*60*60),
	"PDT":     time.FixedZone("PDT", -7*60*60),
}

func mustLocation(name string) *time.Location {
	location, err := time.LoadLocation(name)
	if err != nil {
		panic(err)
	}
	return location
}

var utcOffsetPattern = regexp.MustCompile(`^([+-])(\d{1,2})(?::?(\d{2}))?$`)

func parseUTCOffset(value string) (*time.Location, bool) {
	matches := utcOffsetPattern.FindStringSubmatch(value)
	if matches == nil {
		return nil, false
	}
	hours, err := strconv.Atoi(matches[2])
	if err != nil || hours > 23 {
		return nil, false
	}
	minutes := 0
	if matches[3] != "" {
		minutes, err = strconv.Atoi(matches[3])
		if err != nil || minutes > 59 {
			return nil, false
		}
	}
	offset := (hours*60 + minutes) * 60
	if matches[1] == "-" {
		offset = -offset
	}
	return time.FixedZone(value, offset), true
}

func LocalDate(now time.Time, location *time.Location) time.Time {
	local := now.In(location)
	return time.Date(local.Year(), local.Month(), local.Day(), 0, 0, 0, 0, location)
}

func ParseLocalDate(value string, location *time.Location) (time.Time, error) {
	parsed, err := time.ParseInLocation(DateLayout, value, location)
	if err != nil {
		return time.Time{}, fmt.Errorf("parse date %q as YYYY-MM-DD: %w", value, err)
	}
	return parsed, nil
}