package main

import (
	"os"
	"regexp"
	"strings"
)

// tzName is the shape of an IANA zone name the console will accept (spec
// §6.2 step 1). Anything else is not sent: the console then falls back to
// the request's IP, then the home.
var tzName = regexp.MustCompile(`^[A-Za-z0-9_+\-]+(/[A-Za-z0-9_+\-]+){0,2}$`)

var notAZone = map[string]bool{"localtime": true, "posixrules": true, "Local": true}

// localTimezone finds this machine's IANA zone name, portably (no build
// tags; the 20-cross gate builds it for darwin, windows and linux-arm64):
// $TZ, else the /etc/localtime symlink target, else /etc/timezone, else "".
// Windows has none of these, so only a valid $TZ is ever sent there.
func localTimezone(getenv func(string) string, readlink func(string) (string, error), readFile func(string) ([]byte, error)) string {
	if tz := clean(strings.TrimPrefix(strings.TrimSpace(getenv("TZ")), ":")); tz != "" {
		return tz
	}
	if target, err := readlink("/etc/localtime"); err == nil {
		if tz := clean(target); tz != "" {
			return tz
		}
	}
	if b, err := readFile("/etc/timezone"); err == nil {
		line, _, _ := strings.Cut(string(b), "\n")
		return clean(line)
	}
	return ""
}

func clean(s string) string {
	s = strings.TrimSpace(s)
	if _, after, ok := strings.Cut(s, "zoneinfo/"); ok {
		s = after
	}
	if s == "" || strings.HasPrefix(s, "/") || notAZone[s] || len(s) > 64 || !tzName.MatchString(s) {
		return ""
	}
	return s
}

func detectTimezone() string { return localTimezone(os.Getenv, os.Readlink, os.ReadFile) }