/**
 * Mirrors UI state into the query string so a view can be linked and reloaded
 * exactly as it was: which day is open, which model is plotted, which variables
 * are compared.
 *
 * Writes use `replaceState` rather than `goto`, so mirroring state never adds a
 * history entry or re-runs a load - the back button still means "the page
 * before", not "the previous day I clicked".
 */
import { browser } from '$app/environment';
import { replaceState } from '$app/navigation';

export function syncSearchParams(url: URL, updates: Record<string, string | null>): void {
	if (!browser) return;
	const next = new URL(url);
	for (const [key, value] of Object.entries(updates)) {
		if (value == null || value === '') next.searchParams.delete(key);
		else next.searchParams.set(key, value);
	}
	if (next.href === url.href) return;
	try {
		replaceState(next, {});
	} catch {
		// A page whose state settles during mount can get here before the router
		// has taken over. The URL is cosmetic, so retry on the next frame rather
		// than letting it break the page.
		requestAnimationFrame(() => {
			try {
				replaceState(next, {});
			} catch {
				/* give up: the view still works, it just isn't linkable yet */
			}
		});
	}
}

/** Reads a comma-separated list, dropping empties. */
export function readList(url: URL, key: string): string[] | null {
	const raw = url.searchParams.get(key);
	if (!raw) return null;
	const list = raw
		.split(',')
		.map((s) => s.trim())
		.filter(Boolean);
	return list.length > 0 ? list : null;
}