import { type Accessor, createMemo } from "solid-js";
import { useLabelableContext } from "./labelable-context";

/**
 * Compose an `aria-labelledby` string from:
 *
 * 1. the caller-supplied external `aria-labelledby` value (e.g. from a
 *    consumer prop), and
 * 2. every label id currently registered with the nearest
 *    `<LabelableProvider>`, in **registration order**.
 *
 * Returns an `Accessor<string | undefined>` — `undefined` when neither source
 * contributes anything so the attribute can be omitted entirely (per RFC 07,
 * Solid 2 treats `undefined` as "absent" for presence/absence attributes).
 *
 * ### Why registration order
 *
 * DOM order requires observing the rendered label elements, which would need
 * a MutationObserver or post-mount reconciliation pass. Registration order is
 * a deterministic stand-in: Base UI labels register during setup, which in
 * practice proceeds in source/render order (and therefore in DOM order for
 * the typical "labels before control" layout). If a consumer genuinely needs
 * a different order, they can pass the external `aria-labelledby` string to
 * override — that string wins position-wise (it comes first in the composed
 * result).
 *
 * ### Solid 2 notes
 *
 * - The `external` argument is accepted as either a plain `string | undefined`
 *   (read once during setup, won't react) or an `Accessor<string | undefined>`
 *   (tracks). Most consumers already work with accessors in Solid 2, so this
 *   dual shape is a thin convenience.
 * - Built on `createMemo`, so joining only re-runs when either the external
 *   value or the registered-ids array changes — not on every read.
 */
export function useAriaLabelledBy(
	external?: string | undefined | Accessor<string | undefined>,
): Accessor<string | undefined> {
	const context = useLabelableContext();

	const readExternal = (): string | undefined => {
		if (typeof external === "function") return external();
		return external;
	};

	return createMemo(() => {
		const externalValue = readExternal();
		const registered = context.getLabelIds();

		if (!externalValue && registered.length === 0) {
			return undefined;
		}

		// External wins position-wise — consumers that pass an explicit
		// `aria-labelledby` expect that to come first. Empty/whitespace-only
		// registered ids are skipped to stay tidy.
		const parts: string[] = [];
		if (externalValue) parts.push(externalValue);
		for (const id of registered) {
			if (id) parts.push(id);
		}

		return parts.length > 0 ? parts.join(" ") : undefined;
	});
}