Function computeReadingTrends

Source

export function computeReadingTrends(
  history: ReadingHistory,
  timeRange: TimeRange,
): Array<{ date: string; chapters: number; count: number }> {
  const filtered = filterHistoryByTimeRange(history, timeRange);
  if (!filtered.length) return [];

  // Establish baseline from pre-range history
  const now = Date.now();
  const ranges = {
    "7d": 7 * 24 * 60 * 60 * 1000,
    "30d": 30 * 24 * 60 * 60 * 1000,
    "90d": 90 * 24 * 60 * 60 * 1000,
  };
  const cutoff = timeRange === "all" ? 0 : now - ranges[timeRange];
  const preRangeBaseline =
    timeRange === "all" ? new Map() : getPreRangeBaseline(history, cutoff);

  // Group by date and calculate chapters read per day
  const dailyMap = new Map<string, { chapters: number; count: number }>();

  // Sort by timestamp to calculate deltas
  const sorted = [...filtered].sort((a, b) => a.timestamp - b.timestamp);

  // Track previous chapters per manga, seeded with pre-range baseline
  const previousChapters = new Map<string | number, number>(preRangeBaseline);

  for (const entry of sorted) {
    const date = getDateKey(entry.timestamp);
    const prev = previousChapters.get(entry.mangaId) ?? 0;
    const delta = Math.max(0, entry.chaptersRead - prev);

    const existing = dailyMap.get(date) ?? { chapters: 0, count: 0 };
    dailyMap.set(date, {
      chapters: existing.chapters + delta,
      count: existing.count + (delta > 0 ? 1 : 0),
    });

    previousChapters.set(entry.mangaId, entry.chaptersRead);
  }

  // Convert to array and sort by date
  return Array.from(dailyMap.entries())
    .map(([date, data]) => ({ date, ...data }))
    .sort((a, b) => a.date.localeCompare(b.date));
}
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