export function exportSession(s) {
  return JSON.stringify({
    version: 2,
    savedAt: new Date().toISOString(),
    fileName: s.fileName,
    pdfHash: s.pdfHash || '',
    screen: 'reader',
    noteModes: s.noteModes,
    commentary: s.commentary,
    paper: s.paper,
    litPapers: s.litPapers,
    litByPara: s.litByPara,
    figures: s.figures || [],
    litByFig: s.litByFig || {},
    equations: s.equations || [],
    references: s.references || [],
    citeIndex: s.citeIndex || {},
    userNotes: s.userNotes || {},
    paperNote: s.paperNote || '',
  }, null, 2);
}

export function importSession(json) {
  const s = JSON.parse(json);
  if ((s.version !== 1 && s.version !== 2) || !s.paper || !s.paper.paragraphs) {
    throw new Error('could not read that session file');
  }
  // Tolerant defaults for fields absent in older sessions
  if (!s.figures) s.figures = [];
  if (!s.litByFig) s.litByFig = {};
  if (!s.equations) s.equations = [];
  if (!s.references) s.references = [];
  if (!s.citeIndex) s.citeIndex = {};
  if (!s.userNotes) s.userNotes = {};
  if (!s.paperNote) s.paperNote = '';
  return s;
}

export async function hashPdf(arrayBuffer) {
  const digest = await crypto.subtle.digest('SHA-256', arrayBuffer);
  return [...new Uint8Array(digest)].map((b) => b.toString(16).padStart(2, '0')).join('');
}