import 'dart:math' as math;

import '../entities/dose_entry.dart';
import '../value_objects/glp1_compound.dart';

/// Evaluates the accumulated pharmacokinetic serum concentration of GLP-1
/// receptor agonists using the standard single-compartment open decay model:
///
/// ```text
/// C(t) = sum over i of D_i * (0.5) ^ ((t - t_i) / t_half)
/// ```
///
/// Where `D_i` is a dose administered at time `t_i` and `t_half` is the
/// compound's elimination half-life (Semaglutide 168h, Tirzepatide 120h,
/// Liraglutide 13h). Doses recorded with a future [DoseEntry.administeredAt]
/// are ignored, and unrecognized compounds fall back to
/// [defaultHalfLifeHours].
///
/// The use case is pure Dart with no dependencies, so it runs identically in
/// the domain layer, widget tests, and unit tests.
class CalculateSerumLevel {
  const CalculateSerumLevel({this.defaultHalfLifeHours = 168});

  /// Half-life assumed for compounds that are not in [Glp1Compound].
  final double defaultHalfLifeHours;

  /// Convenience entry point mirroring the mathematical notation
  /// `C(t = [at])`.
  double call({
    required Iterable<DoseEntry> doses,
    required DateTime at,
    double? halfLifeHours,
  }) =>
      evaluate(doses: doses, at: at, halfLifeHours: halfLifeHours);

  /// Computes the total active serum concentration (mg) at [at].
  ///
  /// [halfLifeHours] optionally forces a single half-life for every dose,
  /// bypassing per-compound resolution (useful for custom titration studies).
  double evaluate({
    required Iterable<DoseEntry> doses,
    required DateTime at,
    double? halfLifeHours,
  }) {
    var totalMg = 0.0;

    for (final dose in doses) {
      if (dose.doseMg <= 0) continue;

      final elapsed = at.difference(dose.administeredAt);
      // Doses scheduled in the future have not entered circulation yet.
      if (elapsed.isNegative) continue;

      final halfLife = halfLifeHours ?? _halfLifeFor(dose.compoundName);
      if (halfLife <= 0) continue;

      final elapsedHours = elapsed.inMilliseconds / Duration.millisecondsPerHour;
      totalMg += dose.doseMg * math.pow(0.5, elapsedHours / halfLife).toDouble();
    }

    return totalMg;
  }

  /// Resolves the elimination half-life for a compound label, falling back to
  /// [defaultHalfLifeHours] when the name is unknown.
  double _halfLifeFor(String compoundName) =>
      Glp1Compound.tryParse(compoundName)?.halfLifeHours ??
      defaultHalfLifeHours;
}