/// Viability status of a seed lot, derived purely from its age versus the
/// species' typical seed longevity (bundled reference data). Turns passive
/// storage into active stewardship: it surfaces which lots to sow or regenerate
/// before they lapse, instead of just accumulating them.
enum SeedViability {
  /// Comfortably within the species' typical viability window.
  fresh,

  /// In the final year of the window — sow or reproduce this season.
  expiringSoon,

  /// Past the typical viability window — germination is likely dropping fast.
  expired,

  /// Not enough data to judge (no harvest year, or no reference figure).
  unknown,
}

/// Computes the viability status of a seed lot harvested in [harvestYear] for a
/// species whose typical longevity is [viabilityYears], as of [currentYear].
///
/// A conservative, age-based signal: the bundled figure is a single number, not
/// a decay curve, so this only distinguishes "fine / use soon / past it".
/// [expiringSoon] flags the last year of the window so the grower can prioritise
/// what to reproduce before it lapses. Any actual [GerminationTest] a grower
/// records remains the ground truth and is shown alongside this estimate.
SeedViability seedViability({
  required int? harvestYear,
  required int? viabilityYears,
  required int currentYear,
}) {
  if (harvestYear == null || viabilityYears == null || viabilityYears <= 0) {
    return SeedViability.unknown;
  }
  final age = currentYear - harvestYear;
  // A harvest stamped in the future is treated as fresh, not expired.
  if (age < 0) return SeedViability.fresh;
  if (age >= viabilityYears) return SeedViability.expired;
  if (age >= viabilityYears - 1) return SeedViability.expiringSoon;
  return SeedViability.fresh;
}