/*
 Demand Forecasting using Simple Exponential Smoothing.
 By: Muhammad Faruq Nuruddinsyah @ March 18, 2014.
*/
 
public class SES{
    public static void main(String[] args){
        new SES();
    }
 
    public SES(){
        final double ALPHA = 0.1;
        double[] demand = {8000, 13000, 23000, 34000, 10000, 18000, 23000, 38000, 12000, 13000, 32000, 41000};
        int periodCycle = 4;
 
        double forecast = estimateDemand(13, demand, periodCycle, ALPHA); // Estimate demand for next period (13)
        System.out.println(forecast);
    }
 
    double estimateDemand(int period, double[] demand, int periodCycle, double alpha){
        if(period > demand.length + 1){
            return level(demand.length, demand, periodCycle, alpha);
        }else{
            return level(period - 1, demand, periodCycle, alpha);
        }
    }
 
    double level(int period, double[] demand, int periodCycle, double alpha){
        if(period == 0){
            return levelZero(demand, periodCycle);
        }else{
            return alpha * demand[period - 1] + (1.0 - alpha) * level(period - 1, demand, periodCycle, alpha);
        }
    }
 
    double levelZero(double[] demand, int periodCycle){
        int result = 0;
 
        for(int i = 0; i < periodCycle; i++){
            result += demand[i];
        }
 
        return result / periodCycle;
    }
}