5.2. Step 2: Birthdays

5.2.1. Model Description

At this step we add a module for birthday events to the model template. Unlike calendar year changes which happen to all actors at the same time and are thus implemented by a common clock, birthdays happen at individual pints in time throughout the year and are therefore implemented on the individual level. Besides the maintenance of a state for age in full years (which could be implemented directly as a self-scheduling state), the explicit modeling of a birthday event prepares the model for routines which typically happen on birthdays, e.g. the update of longitudinal accounts. All new code added at this step is contained in the new module BirthDays.mpp.

5.2.2. The new module: BirthDays.mpp

This module implements birthday events of the person actors. The module showcases the implementation of a clock event: a state integer_age is updated at yearly intervals. The module also demonstrates the typical arrangement of code within a regular simulation module:

• Introduction of new types: here an age range for the allowed life span. It is needed as a dimension of parameters (e.g. a lifetable) and as a table dimension.
• An “actor block” for the declaration of new states (here: integer_age and a variable to store the time of the next birthday) and new events: the birthday.
• The implementation of new functions and events. In our case the implementation of the birthday event. Like all events birthdays have two functions: a timing function returning the next occurrence of the event, and the event function performing all code to happen at the occurrence of the event.

////////////////////////////////////////////////////////////////////////////////////////////////////
// Types
////////////////////////////////////////////////////////////////////////////////////////////////////

range AGE_RANGE { 0, 105 };                                 //EN Age Range

////////////////////////////////////////////////////////////////////////////////////////////////////
// Actor states and functions
////////////////////////////////////////////////////////////////////////////////////////////////////

actor Person
{
    AGE_RANGE   integer_age = { 0 };                        //EN Age

    TIME        time_next_birthday = { TIME_INFINITE };     //EN Time of next birthday
    event       timeBirthdayEvent, BirthdayEvent;           //EN Birthday Event
};

////////////////////////////////////////////////////////////////////////////////////////////////////
// Event Implementation
////////////////////////////////////////////////////////////////////////////////////////////////////

/* NOTE(Person.BirthdayEvent, EN)
   At each birthday integer age is incremented and all code to be performed at birthdays is executed
*/

TIME Person::timeBirthdayEvent()
{
    if ( integer_age == 0 ) return time_of_birth + 1.0;
    else return time_next_birthday;
}

void Person::BirthdayEvent()
{
    // Increment integer age
    if (integer_age < MAX(AGE_RANGE)) integer_age++;

    // Code to be performed at each birthday can be entered here

    // Set clock for next birthday
    time_next_birthday = WAIT(1);
}