// Package callwindow enforces the fixed legal window for SIP outbound dialing.
package callwindow

import (
	"fmt"
	"time"
)

const (
	LocationName = "Asia/Shanghai"
	OpenHour     = 9
	CloseHour    = 20
)

var shanghai = time.FixedZone(LocationName, 8*60*60)

// Allowed reports whether SIP outbound dialing is permitted at now. The
// boundary is [09:00, 20:00) in Asia/Shanghai; the input's instant, not its
// presentation timezone, is authoritative.
func Allowed(now time.Time) bool {
	local := now.In(shanghai)
	minutes := local.Hour()*60 + local.Minute()
	return minutes >= OpenHour*60 && minutes < CloseHour*60
}

// Check returns a stable, actionable error when outbound dialing is closed.
func Check(now time.Time) error {
	local := now.In(shanghai)
	if Allowed(now) {
		return nil
	}
	return fmt.Errorf("SIP outbound dialing is closed at %s; allowed window is %02d:00-%02d:00 %s", local.Format("2006-01-02 15:04:05 -0700"), OpenHour, CloseHour, LocationName)
}