wikimedia/mediawiki-extensions-CirrusSearch

<?php

namespace CirrusSearch\Search\Rescore;

use CirrusSearch\SearchConfig;
use Elastica\Query\FunctionScore;

/**
 * Normalize values in the [0,1] range
 * Allows to set:
 * - a scale
 * - midpoint
 * It will generate a log scale factor where :
 * - f(0) = 0
 * - f(scale) = 1
 * - f(midpoint) = 0.5
 *
 * Based on log10( a . x + 1 ) / log10( a . M + 1 )
 * a: a factor used to adjust the midpoint
 * M: the max value used to scale
 *
 */
class LogScaleBoostFunctionScoreBuilder extends FunctionScoreBuilder {
    /** @var string */
    private $field;
    /** @var float */
    private $midpoint;
    /** @var float */
    private $scale;

    /**
     * @param SearchConfig $config
     * @param float $weight
     * @param array $profile
     * @throws InvalidRescoreProfileException
     */
    public function __construct( SearchConfig $config, $weight, $profile ) {
        parent::__construct( $config, $weight );

        if ( isset( $profile['midpoint'] ) ) {
            $this->midpoint = $this->getOverriddenFactor( $profile['midpoint'] );
        } else {
            throw new InvalidRescoreProfileException( 'midpoint is mandatory' );
        }

        if ( isset( $profile['scale'] ) ) {
            $this->scale = $this->getOverriddenFactor( $profile['scale'] );
        } else {
            throw new InvalidRescoreProfileException( 'scale is mandatory' );
        }

        if ( isset( $profile['field'] ) ) {
            $this->field = $profile['field'];
        } else {
            throw new InvalidRescoreProfileException( 'field is mandatory' );
        }
    }

    /**
     * find the factor to adjust the scale center,
     * it's like finding the log base to have f(N) = 0.5
     *
     * @param float $M
     * @param float $N
     * @return float
     * @throws InvalidRescoreProfileException
     */
    private function findCenterFactor( $M, $N ) {
        // Neutral point is found by resolving
        // log10( x . N + 1 ) / log10( x . M + 1 ) = 0.5
        // it's equivalent to resolving:
        // N²x² + (2N - M)x + 1 = 0
        // so we we use the quadratic formula:
        // (-(2N-M) + sqrt((2N-M)²-4N²)) / 2N²
        if ( 4 * $N >= $M ) {
            throw new InvalidRescoreProfileException( 'The midpoint point cannot be higher than scale/4' );
        }

        return ( -( 2 * $N - $M ) + sqrt( ( 2 * $N - $M ) * ( 2 * $N - $M ) - 4 * $N * $N ) ) /
               ( 2 * $N * $N );
    }

    public function append( FunctionScore $functionScore ) {
        $formula = $this->getScript();

        $functionScore->addScriptScoreFunction( new \Elastica\Script\Script( $formula, null,
            'expression' ), null, $this->weight );
    }

    /**
     * @return string
     */
    public function getScript() {
        $midFactor = $this->findCenterFactor( $this->scale, $this->midpoint );
        $formula = "log10($midFactor * min(doc['{$this->field}'].value,{$this->scale}) + 1)";
        $formula .= "/log10($midFactor * {$this->scale} + 1)";

        return $formula;
    }
}