rm unused urlparam

src/lib/utils/url-state.ts

@@ -39,6 +39,30 @@ export function syncSearchParams(url: URL, updates: Record<string, string | null
 	}
 }
 
+/**
+ * The value to mirror, or null when it is the view's default. Defaults belong
+ * in the code, not the query string: a shared link should carry only what the
+ * visitor actually changed, and going back to the default has to clear the
+ * parameter again rather than pin the default in place.
+ */
+export function unlessDefault(value: string | null | undefined, fallback: string): string | null {
+	return value && value !== fallback ? value : null;
+}
+
+/**
+ * Same, for the comma-separated list parameters. Order counts - the models are
+ * plotted, coloured and legended in the order they are listed, so a reordered
+ * line-up is a different view even when it holds the same entries.
+ */
+export function listUnlessDefault(
+	values: string[] | null | undefined,
+	fallback: string[]
+): string | null {
+	if (!values?.length) return null;
+	const joined = values.join(',');
+	return joined === fallback.join(',') ? null : joined;
+}
+
 /** Reads a comma-separated list, dropping empties. */
 export function readList(url: URL, key: string): string[] | null {
 	const raw = url.searchParams.get(key);