public function approximate()
{
$correlation = $this->getCorrelation();
$step = $this->getStep();
$this->_tau = 0;
for ($i = 0; $i < count($correlation); $i++) {
if ($correlation[$i] < 0) {
$p1 = new Model_Coordinate($step * ($i - 1), $correlation[$i - 1]);
$p2 = new Model_Coordinate($step * $i, $correlation[$i]);
$k = ($p2->getY() - $p1->getY()) / ($p2->getX() - $p1->getX());
$b = $p2->getY() - $k * $p2->getX();
$this->_tau = -$b / $k;
break;
}
}
$this->_beta = pi() / (2 * $this->_tau);
$s1 = 0;
$s2 = 0;
for ($i = 0; $i * $step < $this->_tau; $i++) {
$tau = $i * $step;
$ro_tau = $correlation[$i];
$s1 += abs($tau * log($ro_tau / cos($this->_beta * $tau)));
$s2 += $tau * $tau;
}
if ($this->_tau < $step) {
$this->_alpha = 1;
} else {
$this->_alpha = $s1 / $s2;
}
}
private function getIntervalPoints($x)
{
$p1 = new Model_Coordinate(0, 0);
$p2 = new Model_Coordinate(0, 0);
foreach ($this->_coordinates as $item) {
if ($x >= $p2->getX() && $p2) {
$p1 = $p2;
$p2 = $item;
// $p2 = next( $this->_coordinates );
}
}
return array($p1, $p2);
}
