Describes a person and (alcoholic) drink class. When a person drinks a drink, it takes the timestamp into account when calculating the person's BAC (blood alcohol concentration). I made this for 2013 Angelhack in Seattle.

BloodAlcoholConcentration.cpp

// 2013 Nathan Park
// Angelhack in Seattle
#include <iostream>
#include <string>
#include <vector>

typedef float Date;

const float grams_per_lb = 453.592f;
const float lbs_per_gram = 1.0f / grams_per_lb;
const float oz_per_cg = 0.000352739619f;
const float cg_per_oz = 1.0f / oz_per_cg;

struct Drink // stored in a database
{
  Drink(
    const std::string & name,
    float volumeInOunces,
    float calories,
    float percentAlchohol)
  : mName(name)
  , mVolumeInOunces(volumeInOunces)
  , mCalories(calories)
  , mPercentAlchohol(percentAlchohol)
  {}
  std::string mName;
  float mVolumeInOunces;
  float mCalories;
  float mPercentAlchohol; // [0,100] <-- scale is important
};

// made some adjustments, based off 
// http://dogsbody.psych.mun.ca/Tools/BAL/tutorial.html and wikipedia
// to account for the changing time
// mostly the same though
// -Nate
class Person
{
public:
  Person(bool isMale, float weightLbs)
  : mCaloriesDrank(0)
  , mOuncesOfAlchoholInSystem(0)
  , mHasStartedDrinking(false)
  {
    mMetabolismLbs = weightLbs;
    mMetabolismLbs *= isMale ? sMaleGenderConstant : sFemaleGenderConstant;
  }
  void Consume(const Drink & drink, Date currTime)
  {
    mConsumed.push_back(drink);
    mCaloriesDrank += drink.mCalories;
    
    UpdateBAC(currTime);

    // divide the percentage[0,100] by 100 to go to [0,1]
    float drinkAlOz = drink.mVolumeInOunces * drink.mPercentAlchohol / 100.0f;
    mOuncesOfAlchoholInSystem += drinkAlOz;
    
    mHasStartedDrinking = true;
    mLastUpdateTime = currTime;
    // cg                oz                        cg             lbs
    //  /                /                 *       /          *    /
    //  g                lbs                       oz              g
    //mBAC += ( drinkAlOz / mMetabolismLbs ) * ( 1 / oz_per_cg) * (1 / grams_per_lb);
    
  }

  /*
    @param alchohol
      measured in oz
    @return float
      bac, measured in cg alchohol/g metabolism
  */
  float AlchoholToBAC(float alchohol)
  {
    return alchohol * cg_per_oz * lbs_per_gram / mMetabolismLbs;
  }
  /*
    @param bac
      measured in cg alchohol/g metabolism
    @return float
      alchohol, measured in oz
  */
  float BACToAlchohol(float bac)
  {
    return bac * mMetabolismLbs * grams_per_lb * oz_per_cg;
  }
  
  void UpdateBAC(Date currTime)
  {
    if (!mHasStartedDrinking) return;

    Date currentTime = currTime;
    float hourDifference = currentTime - mLastUpdateTime;
    mLastUpdateTime = currentTime;

    // account for the time difference
    float alchoholElimination = hourDifference * sNaturalBACReductionPerHr;

    float bac = AlchoholToBAC(mOuncesOfAlchoholInSystem);
    bac -= alchoholElimination;
    if (bac < 0)
    {
      mOuncesOfAlchoholInSystem = 0;
      return;
    }

    // recalculate the alchohol in system
    mOuncesOfAlchoholInSystem = BACToAlchohol(bac);
  }
  
  float GetBAC(Date currTime)
  {
    UpdateBAC(currTime);
    return AlchoholToBAC(mOuncesOfAlchoholInSystem);
  }

  float GetCaloriesDrunk()const{return mCaloriesDrank;}

private:
  // ( total blood in the body measured in lbs) * genderConstant
  float mMetabolismLbs;
  float mCaloriesDrank;
  float mOuncesOfAlchoholInSystem;

  // http://en.wikipedia.org/wiki/Blood_alcohol_content
  //float mBAC; // measured in mass 1/100 g/g = 1 cg alchohol/g metabolism
  Date mLastUpdateTime;
  bool mHasStartedDrinking;

  std::vector<Drink> mConsumed; // for reference
  
  static const float sNaturalBACReductionPerHr;
  static const float sConversionRate;
  static const float sMaleGenderConstant;
  static const float sFemaleGenderConstant;
};
// how much BAC the body removes per/hr
const float Person::sNaturalBACReductionPerHr = 0.015f; 
const float Person::sConversionRate = 5.14f;
const float Person::sMaleGenderConstant = 0.73f;
const float Person::sFemaleGenderConstant = 0.66f;

int main()
{
  std::cout << std::endl;
  
  //                  name           vol   cal al%
  Drink milk         ("milk"        , 8,   136, 0);
  Drink parrotBay    ("parrot bay"  , 1.5, 87,  20);
  Drink budlightplat ("budlightplat", 12,  137, 6);
  Drink captMorganBlk("captmorgblk" , 1.5, 117, 47.3f);

  float weightInLbs = 190.0f;
  bool isMale = true;
  Person person(isMale, weightInLbs);

  float currTimeInHrs = 0.0f;
  person.Consume(parrotBay    , currTimeInHrs); currTimeInHrs += 5.0f / 14.0f;
  person.Consume(parrotBay    , currTimeInHrs); currTimeInHrs += 5.0f / 14.0f;
  person.Consume(parrotBay    , currTimeInHrs); currTimeInHrs += 5.0f / 14.0f;
  person.Consume(parrotBay    , currTimeInHrs); currTimeInHrs += 5.0f / 14.0f;
  person.Consume(parrotBay    , currTimeInHrs); currTimeInHrs += 5.0f / 14.0f;
  person.Consume(budlightplat , currTimeInHrs); currTimeInHrs += 5.0f / 14.0f;
  person.Consume(budlightplat , currTimeInHrs); currTimeInHrs += 5.0f / 14.0f;
  person.Consume(budlightplat , currTimeInHrs); currTimeInHrs += 5.0f / 14.0f;
  person.Consume(budlightplat , currTimeInHrs); currTimeInHrs += 5.0f / 14.0f;
  person.Consume(captMorganBlk, currTimeInHrs); currTimeInHrs += 5.0f / 14.0f;
  person.Consume(captMorganBlk, currTimeInHrs); currTimeInHrs += 5.0f / 14.0f;
  person.Consume(captMorganBlk, currTimeInHrs); currTimeInHrs += 5.0f / 14.0f;
  person.Consume(captMorganBlk, currTimeInHrs); currTimeInHrs += 5.0f / 14.0f;

  //std::cout << std::endl << person.GetCaloriesDrunk();
  std::cout << std::endl << person.GetBAC(currTimeInHrs);
  return 0;
}