/**
 * prefs.ts — shared, persisted user preferences (Svelte writable store).
 * ----------------------------------------------------------------------
 * Currently stores the unit system (Metric/Imperial) for distance display.
 * Values are persisted to localStorage so they survive reloads.
 *
 * Usage in runes mode:
 *   import { prefs } from '$lib/prefs';
 *   $prefs.system              // read
 *   $prefs.system = 'imperial' // write (auto-persists)
 */

import { writable } from 'svelte/store';

export type UnitSystem = 'metric' | 'imperial';

export interface Prefs {
	system: UnitSystem;
}

const KEY = 'navigator.prefs.v1';
const DEFAULTS: Prefs = { system: 'metric' };

function isValidSystem(v: unknown): v is UnitSystem {
	return v === 'metric' || v === 'imperial';
}

function load(): Prefs {
	try {
		const raw = localStorage.getItem(KEY);
		if (!raw) return { ...DEFAULTS };
		const parsed = JSON.parse(raw) as Partial<Prefs>;
		return {
			system: isValidSystem(parsed.system) ? parsed.system : DEFAULTS.system
		};
	} catch {
		return { ...DEFAULTS };
	}
}

function createPrefs() {
	const store = writable<Prefs>(load());
	// Persist on every change.
	store.subscribe((p) => {
		try {
			localStorage.setItem(KEY, JSON.stringify(p));
		} catch {
			/* storage unavailable (private mode, etc.) — ignore */
		}
	});
	return store;
}

export const prefs = createPrefs();