// Plain-language summary of today's forecast ("Scattered showers today",
// "Rain likely later", "Don't forget your umbrella"...). Neither Met Office's Global
// Spot API nor Open-Meteo return anything like this — both are purely numeric/coded
// (temps, weather codes, precip%) — so this is generated from today's already-fetched
// hourly points (icon + precip_prob), rule-based, no extra API call and no LLM.
//
// Computed fresh per-request in routes/forecast.js (not stored/cached at fetch time),
// since "later" is relative to whoever's looking at the screen right now, not to when
// the hourly data was last fetched.
const RAIN_ICONS = new Set(['drizzle', 'rain-light', 'rain-heavy', 'sleet', 'hail'])
const SNOW_ICONS = new Set(['snow-light', 'snow-heavy'])

// summarizeToday(points, nowHour) — `points` is today's hourly forecast_points rows
// (already icon-normalized), `nowHour` is 0-23 in Europe/London. Returns null if
// there's nothing to summarize (no data yet).
export function summarizeToday(points, nowHour) {
  if (!points.length) return null

  const isRain = p => RAIN_ICONS.has(p.icon)
  const isSnow = p => SNOW_ICONS.has(p.icon)
  const isThunder = p => p.icon === 'thunder'
  const isHeavyRain = p => p.icon === 'rain-heavy'

  const rainPoints = points.filter(isRain)
  const snowPoints = points.filter(isSnow)
  const thunderPoints = points.filter(isThunder)
  const rainFraction = rainPoints.length / points.length
  const snowFraction = snowPoints.length / points.length

  const laterPoints = points.filter(p => hourOf(p) >= nowHour)
  const earlierPoints = points.filter(p => hourOf(p) < nowHour)
  const laterWetFraction = laterPoints.length
    ? laterPoints.filter(p => isRain(p) || isThunder(p)).length / laterPoints.length
    : 0
  const earlierWetCount = earlierPoints.filter(p => isRain(p) || isThunder(p)).length

  let headline
  if (snowFraction > 0.3) {
    headline = 'Snow expected today'
  } else if (rainPoints.length === 0 && thunderPoints.length === 0) {
    headline = 'Dry today'
  } else if (rainPoints.some(isHeavyRain) && rainFraction > 0.5) {
    headline = 'Heavy rain today'
  } else if (rainFraction > 0.6) {
    headline = 'Rain likely for much of the day'
  } else if (laterWetFraction > 0.4 && earlierWetCount === 0) {
    headline = 'Rain likely later'
  } else if (rainFraction > 0) {
    headline = 'Scattered showers today'
  } else {
    headline = 'Thunderstorms possible'
  }

  // Append rather than replace the headline when thunder shows up alongside a
  // rain pattern that already earned its own headline above.
  const note = thunderPoints.length > 0 && headline !== 'Thunderstorms possible'
    ? 'Thunderstorms possible'
    : null

  // Umbrella nudge: real rain chance either right now or coming up later today.
  const umbrella = laterWetFraction > 0.25 || rainFraction > 0.25

  return { headline, note, umbrella }
}

function hourOf(point) {
  return parseInt(new Date(point.ts).toLocaleString('en-GB', { timeZone: 'Europe/London', hour: '2-digit', hour12: false }))
}