export type NewsletterConfig = {
  threshold: number;
  headerKeys: string[];
  subjectTokens: string[];
  fromTokens: string[];
  weightHeader: number;
  weightPrecedence: number;
  weightSubject: number;
  weightFrom: number;
};

const DEFAULT_CONFIG: NewsletterConfig = {
  threshold: 2,
  headerKeys: [
    "list-unsubscribe",
    "list-id",
    "list-help",
    "list-archive",
    "list-post",
    "list-owner",
    "list-subscribe",
    "list-unsubscribe-post"
  ],
  subjectTokens: ["newsletter", "unsubscribe", "update", "news", "digest"],
  fromTokens: ["newsletter", "no-reply", "noreply", "news", "updates"],
  weightHeader: 1,
  weightPrecedence: 1,
  weightSubject: 1,
  weightFrom: 1
};

const headerValue = (headers: Map<string, string>, key: string) =>
  headers.get(key.toLowerCase()) ?? "";

const containsAny = (value: string, tokens: string[]) =>
  tokens.some((token) => value.includes(token));

const normalizeList = (items: string[]) =>
  items.map((item) => item.trim().toLowerCase()).filter(Boolean);

export const detectNewsletter = (params: {
  headers: Map<string, string>;
  subject?: string | null;
  from?: string | null;
  config?: Partial<NewsletterConfig>;
}) => {
  const subject = (params.subject ?? "").toLowerCase();
  const from = (params.from ?? "").toLowerCase();
  const headers = params.headers;
  const config: NewsletterConfig = {
    threshold: params.config?.threshold ?? DEFAULT_CONFIG.threshold,
    headerKeys: normalizeList(params.config?.headerKeys ?? DEFAULT_CONFIG.headerKeys),
    subjectTokens: normalizeList(params.config?.subjectTokens ?? DEFAULT_CONFIG.subjectTokens),
    fromTokens: normalizeList(params.config?.fromTokens ?? DEFAULT_CONFIG.fromTokens),
    weightHeader: params.config?.weightHeader ?? DEFAULT_CONFIG.weightHeader,
    weightPrecedence: params.config?.weightPrecedence ?? DEFAULT_CONFIG.weightPrecedence,
    weightSubject: params.config?.weightSubject ?? DEFAULT_CONFIG.weightSubject,
    weightFrom: params.config?.weightFrom ?? DEFAULT_CONFIG.weightFrom
  };

  const matchedHeaderKeys = config.headerKeys.filter((key) => headers.has(key));
  const precedence = headerValue(headers, "precedence").toLowerCase();
  const bulkHeader = headerValue(headers, "x-precedence").toLowerCase();
  const precedenceHint = containsAny(precedence, ["bulk", "list"]) || containsAny(bulkHeader, ["bulk", "list"]);

  const subjectMatches = config.subjectTokens.filter((token) => subject.includes(token));
  const fromMatches = config.fromTokens.filter((token) => from.includes(token));

  const headerScore = matchedHeaderKeys.length * config.weightHeader;
  const precedenceScore = precedenceHint ? config.weightPrecedence : 0;
  const subjectScore = subjectMatches.length ? config.weightSubject : 0;
  const fromScore = fromMatches.length ? config.weightFrom : 0;
  const score = headerScore + precedenceScore + subjectScore + fromScore;

  return {
    isNewsletter: score >= config.threshold,
    score,
    signals: {
      headerKeys: matchedHeaderKeys,
      precedenceHint,
      subjectTokens: subjectMatches,
      fromTokens: fromMatches,
      scoreBreakdown: {
        headerMatches: matchedHeaderKeys.length,
        headerWeight: config.weightHeader,
        headerScore,
        precedenceWeight: config.weightPrecedence,
        precedenceScore,
        subjectMatches: subjectMatches.length,
        subjectWeight: config.weightSubject,
        subjectScore,
        fromMatches: fromMatches.length,
        fromWeight: config.weightFrom,
        fromScore
      }
    }
  };
};