/**
 * The model of the centre's open documents — the same one VS Code has (owner remark 2).
 * A tab is pinned by a double click or by a meaningful interaction with the content; what counts
 * as meaningful here and why — `Document.tsx`. The functions are pure: the model is checked by a
 * test, not by a frame (documents.test.ts).
 */
export interface Documents {
  /** The pinned tabs, in the order they were opened. */
  pinned: string[];
  /** The preview tab. There is exactly one of it, or none. */
  preview: string | null;
  /** The selected tab; an empty string means "nothing is open". */
  active: string;
}

export const noDocuments: Documents = { pinned: [], preview: null, active: '' };

/** The preview is always the rightmost one: pinning does not move the row. */
export const openedIds = ({ pinned, preview }: Documents): string[] =>
  preview === null ? pinned : [...pinned, preview];

/** A single click. An already pinned tab is simply selected — the preview does not touch it. */
export function previewDocument(documents: Documents, id: string): Documents {
  if (documents.pinned.includes(id)) return { ...documents, active: id };
  return { ...documents, preview: id, active: id };
}

/** A double click or a meaningful interaction: the preview becomes a permanent tab. */
export function pinDocument(documents: Documents, id: string): Documents {
  if (documents.pinned.includes(id)) return { ...documents, active: id };
  return {
    pinned: [...documents.pinned, id],
    preview: documents.preview === id ? null : documents.preview,
    active: id,
  };
}

export function selectDocument(documents: Documents, id: string): Documents {
  return { ...documents, active: id };
}

/**
 * Closing. The selection moves to the neighbouring tab on the RIGHT, and if there is none — on the
 * left: this way the place in the row is not lost, and closing several in a row does not throw the
 * reader out into an empty centre.
 */
export function closeDocument(documents: Documents, id: string): Documents {
  const order = openedIds(documents);
  const index = order.indexOf(id);
  const next: Documents = {
    pinned: documents.pinned.filter((other) => other !== id),
    preview: documents.preview === id ? null : documents.preview,
    active: documents.active,
  };
  if (documents.active !== id) return next;
  const rest = openedIds(next);
  next.active = rest[Math.min(index, rest.length - 1)] ?? '';
  return next;
}

/**
 * What is open while the reader has not touched the tabs: one pinned and one preview — otherwise
 * both forms of a tab are not visible on a static frame, and the VS Code model tells them apart by
 * italics. The second and the third chapters are taken: in the default fixture it is they that
 * have text, and the second of them also has notes. This is a prop of the SHOWCASE and will go
 * away together with it, once a real library appears.
 */
export function firstLook(ids: string[]): Documents {
  const [, pinned, preview] = ids;
  if (pinned === undefined) return noDocuments;
  return { pinned: [pinned], preview: preview ?? null, active: pinned };
}