// Distance units.
//
// Everything is COMPUTED in kilometres — the great-circle maths, the backend's
// distance_km on a spot, the map's path lengths — and converted once, here, at
// display time. Storing miles anywhere would mean two sources of truth for the
// same number and a rounding error that grows with every hop.
//
// The preference is portable (see lib/uiPref): an operator who works in miles
// works in miles on every machine they copy their folder to.
import { writeUiPref } from '@/lib/uiPref';

export const KEY_MILES = 'opslog.distanceMiles';
const KM_PER_MILE = 1.609344; // statute miles, the ones a US licence is used in

export function useMiles(): boolean {
  try { return localStorage.getItem(KEY_MILES) === '1'; } catch { return false; }
}

export function setUseMiles(on: boolean): void {
  writeUiPref(KEY_MILES, on ? '1' : '0');
  listeners.forEach((l) => l());
}

// subscribeDistanceUnit notifies on a change. The grids capture the unit inside
// their column definitions (header text and formatter both), so without this a
// toggle would only show up on the next language change or restart.
const listeners = new Set<() => void>();
export function subscribeDistanceUnit(fn: () => void): () => void {
  listeners.add(fn);
  return () => { listeners.delete(fn); };
}

// distanceValue converts a distance in km to the operator's unit, rounded to a
// whole unit — the precision the inputs actually justify (a 4-character grid is
// a square tens of kilometres wide).
export function distanceValue(km: number): number {
  if (!isFinite(km)) return 0;
  return Math.round(useMiles() ? km / KM_PER_MILE : km);
}

// distanceUnit is the short label: "km" or "mi".
export function distanceUnit(): string {
  return useMiles() ? 'mi' : 'km';
}

// formatDistance is value + unit, thousands-separated: "12 345 km".
export function formatDistance(km: number): string {
  return `${distanceValue(km).toLocaleString()} ${distanceUnit()}`;
}