more fluent texts

src/lib/utils/url-state.ts

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+/**
+ * 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;
+}