export interface HistoryMetadata {
  keywords: string[];
  topics: string[];
}

export interface HistoryIndexerConfig {
  maxKeywords: number;
}

const STOPWORDS = new Set([
  'a', 'an', 'and', 'are', 'as', 'at', 'be', 'by', 'for', 'from', 'has', 'he', 'in', 'is', 'it', 'its',
  'of', 'on', 'or', 'that', 'the', 'to', 'was', 'were', 'will', 'with', 'you', 'your', 'we', 'this',
  'they', 'their', 'our', 'but', 'not', 'can', 'just', 'into', 'about', 'after', 'before', 'than', 'then',
]);

export function tokenize(text: string): string[] {
  return text
    .toLowerCase()
    .split(/[^a-z0-9]+/)
    .filter(token => token.length >= 3 && !STOPWORDS.has(token));
}

export class SessionIndexer {
  private readonly maxKeywords: number;

  constructor(config: HistoryIndexerConfig) {
    this.maxKeywords = config.maxKeywords;
  }

  indexText(text: string): HistoryMetadata {
    const tokens = tokenize(text);
    const frequencies = new Map<string, number>();

    for (const token of tokens) {
      frequencies.set(token, (frequencies.get(token) ?? 0) + 1);
    }

    const sorted = Array.from(frequencies.entries())
      .sort((a, b) => b[1] - a[1] || a[0].localeCompare(b[0]))
      .slice(0, this.maxKeywords)
      .map(([token]) => token);

    return {
      keywords: sorted,
      topics: sorted.slice(0, Math.min(3, sorted.length)),
    };
  }
}