/**
 * Returns the ISO week number and year for a given date string "YYYY-MM-DD".
 * ISO weeks start on Monday, and week 1 contains the year's first Thursday.
 */
function getISOWeekData(dateString) {
  const date = new Date(dateString + 'T00:00:00');
  const dayOfWeek = date.getDay() || 7; // Convert Sunday=0 to 7
  // Set to nearest Thursday (current date + 4 - current day number)
  const thursday = new Date(date);
  thursday.setDate(date.getDate() + 4 - dayOfWeek);
  const yearStart = new Date(thursday.getFullYear(), 0, 1);
  const weekNumber = Math.ceil(((thursday - yearStart) / 86400000 + 1) / 7);
  return { year: thursday.getFullYear(), week: weekNumber };
}

/**
 * Returns a week key string like "2026-W10" for a given date string.
 */
export function getWeekKey(dateString) {
  const { year, week } = getISOWeekData(dateString);
  return `${year}-W${String(week).padStart(2, '0')}`;
}

/**
 * Groups an array of run entries by their ISO week key.
 * Returns an object like { "2026-W10": [run, run], "2026-W09": [run] }
 */
export function groupByWeek(runs) {
  const groups = {};
  for (const run of runs) {
    const key = getWeekKey(run.date);
    if (!groups[key]) groups[key] = [];
    groups[key].push(run);
  }
  return groups;
}

/**
 * Computes a weekly summary from an array of runs for a single week.
 */
export function computeWeeklySummary(weekRuns) {
  if (!weekRuns || weekRuns.length === 0) {
    return { total_distance_km: 0, total_training_load: 0, run_count: 0, avg_pace: 0 };
  }
  const total_distance_km = weekRuns.reduce((s, r) => s + r.distance_km, 0);
  const total_time = weekRuns.reduce((s, r) => s + r.time_minutes, 0);
  const total_training_load = weekRuns.reduce((s, r) => s + r.distance_km * r.rpe, 0);
  return {
    total_distance_km,
    total_training_load,
    run_count: weekRuns.length,
    avg_pace: total_distance_km > 0 ? total_time / total_distance_km : 0,
  };
}

/**
 * Computes next-week recommendation using the Acute vs Chronic Workload Ratio (ACWR).
 *
 * Takes an array of weekly summaries sorted chronologically (oldest first),
 * representing the most recent weeks of training (up to 4).
 *
 * - Chronic workload = rolling average of the last N weeks (up to 4)
 * - Upper limit = chronic_avg × 1.3  (or × 1.2 if only 1 week of data)
 * - Lower limit = chronic_avg × 0.8
 *
 * Applies to both distance (km) and training load (distance × RPE).
 */
export function computeRecommendation(weeklySummaries) {
  if (!weeklySummaries || weeklySummaries.length === 0) {
    return { min_distance: 0, max_distance: 0, min_load: 0, max_load: 0 };
  }

  const n = weeklySummaries.length;
  const avgDistance = weeklySummaries.reduce((s, w) => s + w.total_distance_km, 0) / n;
  const avgLoad = weeklySummaries.reduce((s, w) => s + w.total_training_load, 0) / n;

  // Week 1 uses a tighter upper multiplier (1.2), weeks 2+ use 1.3
  const upperMultiplier = n === 1 ? 1.2 : 1.3;
  const lowerMultiplier = 0.8;

  return {
    min_distance: +(avgDistance * lowerMultiplier).toFixed(1),
    max_distance: +(avgDistance * upperMultiplier).toFixed(1),
    min_load: +(avgLoad * lowerMultiplier).toFixed(0),
    max_load: +(avgLoad * upperMultiplier).toFixed(0),
  };
}

/**
 * Builds chart-ready data: sorted array of { week, distance, training_load }.
 */
export function buildChartData(runs) {
  const grouped = groupByWeek(runs);
  return Object.entries(grouped)
    .map(([weekKey, weekRuns]) => {
      const summary = computeWeeklySummary(weekRuns);
      return {
        week: weekKey,
        distance: +summary.total_distance_km.toFixed(1),
        training_load: +summary.total_training_load.toFixed(1),
      };
    })
    .sort((a, b) => a.week.localeCompare(b.week));
}

/**
 * Builds pain chart data as a sorted array of { label, during, after, date } per location.
 * Uses a categorical x-axis (date labels) so during/after share exact positions.
 *
 * Returns { locations: { left_knee: { points }, ... } }
 */
export function buildPainChartData(runs) {
  const locations = ['left_knee', 'right_knee'];

  const result = {};
  for (const loc of locations) {
    const painRuns = runs
      .filter((r) => {
        const d = r[`${loc}_during`] ?? (r.injury_location === loc ? r.pain_during : null);
        const a = r[`${loc}_after`] ?? (r.injury_location === loc ? r.pain_after : null);
        return d != null || a != null;
      })
      .sort((a, b) => a.date.localeCompare(b.date));

    const points = painRuns.map((r) => {
      const during = r[`${loc}_during`] ?? (r.injury_location === loc ? r.pain_during : null);
      const after = r[`${loc}_after`] ?? (r.injury_location === loc ? r.pain_after : null);
      const d = new Date(r.date + 'T00:00:00');
      const label = d.toLocaleDateString('en-US', { month: 'short', day: 'numeric' });
      return {
        label,
        during: during ?? undefined,
        after: after ?? undefined,
        date: r.date,
      };
    });

    result[loc] = { points };
  }

  return { locations: result };
}

/**
 * Computes the longest single run (km) in the last 30 days, and
 * the +10% injury threshold based on the BJSM finding that exceeding
 * 10% of the longest run in the past 30 days increases overuse injury risk.
 *
 * Returns { longest_km, threshold_km } or null if no runs in the window.
 */
export function computeLongestRunThreshold(runs, todayStr) {
  const today = new Date(todayStr + 'T00:00:00');
  const cutoff = new Date(today);
  cutoff.setDate(cutoff.getDate() - 30);

  const recentRuns = runs.filter((r) => {
    const d = new Date(r.date + 'T00:00:00');
    return d >= cutoff && d <= today;
  });

  if (recentRuns.length === 0) return null;

  const longest_km = Math.max(...recentRuns.map((r) => r.distance_km));
  return {
    longest_km: +longest_km.toFixed(2),
    threshold_km: +(longest_km * 1.10).toFixed(2),
  };
}

/**
 * Formats pace as M:SS string given time in minutes and distance in km.
 */
export function formatPace(time_minutes, distance_km) {
  if (!distance_km || distance_km === 0) return '--';
  const pace = time_minutes / distance_km;
  const mins = Math.floor(pace);
  const secs = Math.round((pace - mins) * 60);
  return `${mins}:${secs.toString().padStart(2, '0')}`;
}