export interface Book {
  ebook: number;
  title: string;
  author: string;
  language: string;
}
/** RFC 4180 records, including quoted commas, escaped quotes, and multiline titles. */
export function parseIndex(text: string): Book[] {
  const rows: string[][] = [];
  let row: string[] = [],
    field = "",
    quoted = false;
  for (let i = 0; i < text.length; i++) {
    const ch = text[i]!;
    if (ch === '"') {
      if (quoted && text[i + 1] === '"') {
        field += '"';
        i++;
      } else if (quoted || field === "") quoted = !quoted;
      else throw Error("Unexpected CSV quote");
    } else if (!quoted && (ch === "," || ch === "\n" || ch === "\r")) {
      row.push(field);
      field = "";
      if (ch !== ",") {
        rows.push(row);
        row = [];
        if (ch === "\r" && text[i + 1] === "\n") i++;
      }
    } else field += ch;
  }
  if (quoted) throw Error("Unclosed CSV quote");
  if (field || row.length) {
    row.push(field);
    rows.push(row);
  }
  const header = rows.shift()!;
  const columns = ["Text#", "Type", "Title", "Language", "Authors"].map((name) => header.indexOf(name));
  if (columns.some((i) => i < 0)) throw Error("Unrecognized Gutenberg CSV header");
  const books: Book[] = [],
    seen = new Set<number>();
  for (const row of rows) {
    if (row.length === 1 && row[0] === "") continue;
    if (row.length !== header.length) throw Error("Invalid Gutenberg CSV row");
    if (row[columns[1]!] !== "Text") continue;
    const ebook = Number(row[columns[0]!]!);
    if (!Number.isSafeInteger(ebook) || ebook < 1 || seen.has(ebook)) throw Error("Invalid or repeated book ID");
    seen.add(ebook);
    books.push({
      ebook,
      title: row[columns[2]!]!.replace(/\s+/g, " ").trim(),
      language: row[columns[3]!]!,
      author: row[columns[4]!]!,
    });
  }
  return books;
}
const normalized = (text: string) =>
  text
    .normalize("NFKD")
    .replace(/\p{M}/gu, "")
    .toLowerCase()
    .replace(/[^\p{L}\p{N}]+/gu, " ")
    .trim();
const grams = (text: string) =>
  new Set(Array.from({ length: Math.max(0, text.length - 2) }, (_, i) => text.slice(i, i + 3)));
export function searchIndex(books: readonly Book[]) {
  const index = books.map((book) => ({
    book,
    title: normalized(book.title),
    words: normalized(book.title + " " + book.author),
    grams: grams(normalized(book.title)),
  }));
  return (query: string): Book[] => {
    const q = normalized(query).slice(0, 120);
    if (q.length < 2) return [];
    const tokens = q.split(" "),
      qgrams = grams(q);
    const matches: { book: Book; score: number }[] = [];
    for (const entry of index) {
      const overlap = [...qgrams].filter((g) => entry.grams.has(g)).length;
      const similarity = (2 * overlap) / Math.max(1, qgrams.size + entry.grams.size);
      const score =
        (entry.title === q ? 10 : entry.title.includes(q) ? 5 : 0) +
        (tokens.every((t) => entry.words.includes(t)) ? 2 : 0) +
        similarity;
      if (score >= 0.24) matches.push({ book: entry.book, score });
    }
    return matches
      .sort((a, b) => b.score - a.score || a.book.ebook - b.book.ebook)
      .slice(0, 20)
      .map((entry) => entry.book);
  };
}
export interface Excerpt {
  text: string;
  words: number;
}
/** The first paragraph boundary at or after roughly 1,000 words. Streaming
 * callers wait for that boundary (or EOF), not the rest of the book. */
export function openingExcerpt(text: string, complete = false): Excerpt | null {
  let body = text.replace(/^\uFEFF/, "").replaceAll("\r", "");
  const start = /\*\*\*\s*START OF (?:THE|THIS) PROJECT GUTENBERG EBOOK[^\n]*\n/i.exec(body);
  if (start) body = body.slice(start.index + start[0].length);
  else if (!complete) return null;
  const end = /\*\*\*\s*END OF (?:THE|THIS) PROJECT GUTENBERG EBOOK/i.exec(body);
  if (end) {
    body = body.slice(0, end.index);
    complete = true;
  }
  const paragraphs = body.split(/\n[ \t]*\n+/);
  if (!complete) paragraphs.pop();
  const result: string[] = [];
  let words = 0;
  for (const paragraph of paragraphs) {
    const part = paragraph.trim();
    if (!part) continue;
    result.push(part);
    words += part.split(/\s+/u).length;
    if (words >= 1000) break;
  }
  // An unusually large single paragraph must still fit the document contract.
  // Prefer its last word boundary; ordinary excerpts always keep paragraphs whole.
  if (result.join("\n\n").length > 56000 || (body.length >= 56000 && words < 1000)) {
    const prefix = body.slice(0, 56000),
      at = prefix.lastIndexOf(" ");
    const bounded = (at > 0 ? prefix.slice(0, at) : prefix).replace(/[\uD800-\uDBFF]$/, "").trim();
    return { text: bounded, words: bounded ? bounded.split(/\s+/u).length : 0 };
  }
  if (words < 1000 && !complete) return null;
  return { text: result.join("\n\n"), words };
}