Autocorrelation demo in Processing

autocorrelation.pde

float mysteryLag; 
float signalA[];
float signalB[];
float autoCorr[];
int nData;

void setup() {
  size(512, 700);
  nData = 512;

  signalA = new float[nData];
  signalB = new float[nData];
  autoCorr = new float[nData];
}


void draw() {
  background(200); 
  noiseSeed(55); 

  mysteryLag = 0.96*mysteryLag + 0.04*mouseX;
  for (int i=0; i<nData; i++) {
    float tA = map(i, 0, nData, 0, TWO_PI*5.5); 
    float tB = map(i-mysteryLag, 0, nData, 0, TWO_PI*5.5);
    signalA[i] = 40 * (sin(tA + 3.0*(noise(tA*0.6)-0.5)) + 2.5*(noise(tA*0.4)-0.5));
    signalB[i] = 40 * (sin(tB + random(2.0,4.0)*(noise(tB*random(0.5,0.7) + 10.0*random(-1,1))-0.5)) + 2.5*(noise(tB*0.4)-0.5) + 0.2*random(-1,1));
    if (random(1.0) < 0.07) {
      signalB[i] = 40 * random(-1, 1);
    }
  }


  for (int lag=0; lag<nData; lag++) {
    float sum = 0; 
    for (int i=0; i<nData; i++) {
      if ((lag+i) < nData) {
        sum += signalA[i] * signalB[lag+i];
      }
    }
    sum /= nData; 
    autoCorr[lag] = sum;
  }
  
  // find max of autocorr. The index of the peak is our mysterylag!
  float autocorrMax = 0; 
  int indexOfAutocorrMax =0; 
  for (int i=0; i<nData; i++){
    if (autoCorr[i] > autocorrMax){
      autocorrMax = autoCorr[i];
      indexOfAutocorrMax = i;
    }
  }



  noFill(); 
  stroke(0); 
  beginShape(); 
  for (int i=0; i<nData; i++) {
    vertex(i, 400 + signalA[i]);
  }
  endShape();
  line(0, 400, width, 400); 

  beginShape();
  for (int i=0; i<nData; i++) {
    vertex(i, 550 + signalB[i]);
  }
  endShape();
  line(0, 550, width, 550); 

  float autoCorrScale = -10.0;
  beginShape();
  for (int i=0; i<nData; i++) {
    vertex(i, 200 + autoCorr[i]/autoCorrScale);
  }
  endShape();
  
  fill(0); 
  line(0, 200, width, 200);
  line(0, 200+ autocorrMax/autoCorrScale, width, 200+ autocorrMax/autoCorrScale); 
  line(indexOfAutocorrMax, 200, indexOfAutocorrMax, 200+ autocorrMax/autoCorrScale);
  text("Computed Lag = " + indexOfAutocorrMax, indexOfAutocorrMax, 200 + autocorrMax/autoCorrScale - 7); 
  
  text("Actual lag = " + mysteryLag, 20, 20); 
}