How to Build a ETG Calculator

interface EtGInput {
  drinks: number;           // number of standard drinks (14g ethanol each)
  bodyWeightKg: number;
  gender: 'male' | 'female';
  hoursSinceFirstDrink: number;
  hoursSinceLastDrink: number;
  }

  interface EtGResult {
  estimatedBAC: number;
  isSober: boolean;
  urineDetectionHoursRemaining: number;
  bloodDetectionHoursRemaining: number;
  hairDetectable: boolean;
  cutoffUsed: number;
  }

  function calculateEtGWindow(input: EtGInput): EtGResult {
  const METABOLISM_RATE = 0.015; // BAC per hour
  const GRAMS_PER_DRINK = 14;
  const r = input.gender === 'male' ? 0.68 : 0.55;

  // Widmark BAC estimation
  const alcoholGrams = input.drinks * GRAMS_PER_DRINK;
  const bodyWeightGrams = input.bodyWeightKg * 1000;
  const peakBAC = alcoholGrams / (bodyWeightGrams * r);
  const currentBAC = Math.max(0, peakBAC - (METABOLISM_RATE * input.hoursSinceFirstDrink));

  // Hours until BAC reaches 0
  const hoursToSober = currentBAC > 0 ? currentBAC / METABOLISM_RATE : 0;

  // EtG persists in urine 24-80 hrs AFTER sobriety depending on amount
  const etgPersistenceHours = Math.min(80, 24 + (input.drinks * 4));
  const totalUrineWindow = hoursToSober + etgPersistenceHours;
  const urineRemaining = Math.max(0, totalUrineWindow - input.hoursSinceLastDrink);

  // Blood EtG: up to 36 hours after last drink
  const bloodRemaining = Math.max(0, 36 - input.hoursSinceLastDrink);

  return {
    estimatedBAC: Math.round(currentBAC * 1000) / 1000,
    isSober: currentBAC <= 0,
    urineDetectionHoursRemaining: Math.round(urineRemaining),
    bloodDetectionHoursRemaining: Math.round(bloodRemaining),
    hairDetectable: true, // always true within 90 days
    cutoffUsed: 500 // ng/mL standard cutoff
  };
  }