<?php

namespace Rubix\ML\Regressors;

use Tensor\Matrix;
use Tensor\Vector;
use Rubix\ML\Learner;
use Rubix\ML\DataType;
use Rubix\ML\Estimator;
use Rubix\ML\Persistable;
use Rubix\ML\RanksFeatures;
use Rubix\ML\EstimatorType;
use Rubix\ML\Helpers\Params;
use Rubix\ML\Datasets\Dataset;
use Rubix\ML\Traits\AutotrackRevisions;
use Rubix\ML\Specifications\DatasetIsLabeled;
use Rubix\ML\Specifications\DatasetIsNotEmpty;
use Rubix\ML\Specifications\SpecificationChain;
use Rubix\ML\Specifications\DatasetHasDimensionality;
use Rubix\ML\Specifications\LabelsAreCompatibleWithLearner;
use Rubix\ML\Specifications\SamplesAreCompatibleWithEstimator;
use Rubix\ML\Exceptions\InvalidArgumentException;
use Rubix\ML\Exceptions\RuntimeException;

use function is_null;

/**
 * Ridge
 *
 * L2 regularized least squares linear model solved using a closed-form solution. The addition
 * of regularization, controlled by the *alpha* parameter, makes Ridge less prone to overfitting
 * than ordinary linear regression.
 *
 * @category    Machine Learning
 * @package     Rubix/ML
 * @author      Andrew DalPino
 */
class Ridge implements Estimator, Learner, RanksFeatures, Persistable
{
    use AutotrackRevisions;

    /**
     * The strength of the L2 regularization penalty.
     *
     * @var float
     */
    protected float $alpha;

    /**
     * The y intercept i.e. the bias added to the decision function.
     *
     * @var float|null
     */
    protected ?float $bias = null;

    /**
     * The computed coefficients of the regression line.
     *
     * @var \Tensor\Vector|null
     */
    protected ?\Tensor\Vector $coefficients = null;

    /**
     * @param float $alpha
     * @throws \Rubix\ML\Exceptions\InvalidArgumentException
     */
    public function __construct(float $alpha = 1.0)
    {
        if ($alpha < 0.0) {
            throw new InvalidArgumentException('Alpha must be'
                . " greater than 0, $alpha given.");
        }

        $this->alpha = $alpha;
    }

    /**
     * Return the estimator type.
     *
     * @internal
     *
     * @return \Rubix\ML\EstimatorType
     */
    public function type() : EstimatorType
    {
        return EstimatorType::regressor();
    }

    /**
     * Return the data types that the estimator is compatible with.
     *
     * @internal
     *
     * @return list<\Rubix\ML\DataType>
     */
    public function compatibility() : array
    {
        return [
            DataType::continuous(),
        ];
    }

    /**
     * Return the settings of the hyper-parameters in an associative array.
     *
     * @internal
     *
     * @return mixed[]
     */
    public function params() : array
    {
        return [
            'alpha' => $this->alpha,
        ];
    }

    /**
     * Has the learner been trained?
     *
     * @return bool
     */
    public function trained() : bool
    {
        return $this->coefficients and isset($this->bias);
    }

    /**
     * Return the weights of features in the decision function.
     *
     * @return (int|float)[]|null
     */
    public function coefficients() : ?array
    {
        return $this->coefficients ? $this->coefficients->asArray() : null;
    }

    /**
     * Return the bias added to the decision function.
     *
     * @return float|null
     */
    public function bias() : ?float
    {
        return $this->bias;
    }

    /**
     * Train the learner with a dataset.
     *
     * @param \Rubix\ML\Datasets\Labeled $dataset
     */
    public function train(Dataset $dataset) : void
    {
        SpecificationChain::with([
            new DatasetIsLabeled($dataset),
            new DatasetIsNotEmpty($dataset),
            new SamplesAreCompatibleWithEstimator($dataset, $this),
            new LabelsAreCompatibleWithLearner($dataset, $this),
        ])->check();

        $biases = Matrix::ones($dataset->numSamples(), 1);

        $x = Matrix::build($dataset->samples())->augmentLeft($biases);
        $y = Vector::build($dataset->labels());

        /** @var int<0,max> $nHat */
        $nHat = $x->n() - 1;

        $alphas = array_fill(0, $nHat, $this->alpha);

        array_unshift($alphas, 0.0);

        $penalties = Matrix::diagonal($alphas);

        $xT = $x->transpose();

        $coefficients = $xT->matmul($x)
            ->add($penalties)
            ->inverse()
            ->dot($xT->dot($y))
            ->asArray();

        $this->bias = (float) array_shift($coefficients);
        $this->coefficients = Vector::quick($coefficients);
    }

    /**
     * Make a prediction based on the line calculated from the training data.
     *
     * @param \Rubix\ML\Datasets\Dataset $dataset
     * @throws \Rubix\ML\Exceptions\RuntimeException
     * @return list<int|float>
     */
    public function predict(Dataset $dataset) : array
    {
        if (!$this->coefficients or is_null($this->bias)) {
            throw new RuntimeException('Estimator has not been trained.');
        }

        DatasetHasDimensionality::with($dataset, count($this->coefficients))->check();

        return Matrix::build($dataset->samples())
            ->dot($this->coefficients)
            ->add($this->bias)
            ->asArray();
    }

    /**
     * Return the importance scores of each feature column of the training set.
     *
     * @throws \Rubix\ML\Exceptions\RuntimeException
     * @return float[]
     */
    public function featureImportances() : array
    {
        if (is_null($this->coefficients)) {
            throw new RuntimeException('Learner has not been trained.');
        }

        return $this->coefficients->abs()->asArray();
    }

    /**
     * Return the string representation of the object.
     *
     * @internal
     *
     * @return string
     */
    public function __toString() : string
    {
        return 'Ridge (' . Params::stringify($this->params()) . ')';
    }
}
