php-ai / php-ml

         * 2) k(x) mode: only x is given

         */

        switch ($this->kernel) {

            case self::KERNEL_RBF:

                // k(x,y)=exp(-γ.|x-y|) where |..| is Euclidean distance

                $dist = new Euclidean();

                return function ($x, $y = null) use ($dist) {

                    if ($y === null) {

                        foreach ($x as $i => $element) {

                            $x[$i] = exp(-$this->gamma * $element);

                        }

                        return $x;

                    }

                    return exp(-$this->gamma *    $dist->distance($x, $y));

                };

            case self::KERNEL_SIGMOID:

                // k(x,y)=tanh(γ.xT.y+c0) where c0=0

                    return tanh($this->gamma * $res->toScalar());

                };

            case self::KERNEL_LAPLACIAN:

                // k(x,y)=exp(-γ.|x-y|) where |..| is Manhattan distance

                $dist = new Manhattan();

                return function ($x, $y = null) use ($dist) {

                    if ($y === null) {

                        foreach ($x as $i => $element) {

                            $x[$i] = exp(-$this->gamma * $element);

                        }

                        return $x;

                    }

                    return exp(-$this->gamma * $dist->distance($x, $y));

                };

        }

    }