snipe-it/app/Models/Depreciable.php

<?php
namespace App\Models;

class Depreciable extends SnipeModel
{
    /**
    * Depreciation Relation, and associated helper methods
    */

    //REQUIRES a purchase_date field
    //     and a purchase_cost field

    //REQUIRES a get_depreciation method,
    //which will return the deprecation.
    //this is needed because assets get
    //their depreciation from a model,
    //whereas licenses have deprecations
    //directly associated with them.

    //assets will override the following
    //two methods in order to inherit from
    //their model instead of directly (like
    //here)

    public function depreciation()
    {
        return $this->belongsTo('\App\Models\Depreciation', 'depreciation_id');
    }

    public function get_depreciation()
    {
        return $this->depreciation;
    }

    /**
     * @return float|int
     */

    public function getDepreciatedValue()
    {
        if (!$this->get_depreciation()) { // will never happen
            return $this->purchase_cost;
        }

        if ($this->get_depreciation()->months <= 0) {
            return $this->purchase_cost;
        }
        $depreciation = 0;
        $setting = Setting::getSettings();
        switch($setting->depreciation_method) {
            case 'half_1':
            $depreciation = $this->getHalfYearDepreciatedValue(true);
            break;
            
            case 'half_2':
            $depreciation = $this->getHalfYearDepreciatedValue(false);
            break;

            default:
            $depreciation = $this->getLinearDepreciatedValue();
        }
        return $depreciation;
    }

    /**
     * @return float|int
     */

    public function getLinearDepreciatedValue()
    {
        // fraction of value left
        $months_remaining = $this->time_until_depreciated()->m + 12*$this->time_until_depreciated()->y; //UGlY
        $current_value = round(($months_remaining/ $this->get_depreciation()->months) * $this->purchase_cost, 2);

        if ($current_value < 0) {
            $current_value = 0;
        }
        return $current_value;
    }

    /**
     * @param onlyHalfFirstYear Boolean always applied only second half of the first year
     * @return float|int
     */
    public function getHalfYearDepreciatedValue($onlyHalfFirstYear = false) 
    {
        // @link http://www.php.net/manual/en/class.dateinterval.php
        $current_date = $this->getDateTime();
        $purchase_date = date_create($this->purchase_date);
        $currentYear = $this->get_fiscal_year( $current_date );
        $purchaseYear = $this->get_fiscal_year( $purchase_date );
        $yearsPast = $currentYear - $purchaseYear;
        $deprecationYears = ceil($this->get_depreciation()->months / 12);
        if( $onlyHalfFirstYear ) {
            $yearsPast -= 0.5;
        }
        else if( !$this->is_first_half_of_year($purchase_date) ) {
            $yearsPast -= 0.5;
        }
        if( !$this->is_first_half_of_year($current_date) ) {
            $yearsPast += 0.5;
        }

        if($yearsPast >= $deprecationYears) {
            $yearsPast = $deprecationYears;
        }
        else if($yearsPast < 0) {
            $yearsPast = 0;
        }
        return round($yearsPast / $deprecationYears * $this->purchase_cost, 2);
    }
    
    /**
    * @param \DateTime $date
    * @return int
    */
    protected function get_fiscal_year($date) {
        $year = intval($date->format('Y'));
        // also, maybe it'll have to set fiscal year date
        if($date->format('nj') === '1231') {
            return $year;
        }
        else {
            return $year - 1;
        }
    }

    /**
    * @param \DateTime $date
    * @return bool
    */
    protected function is_first_half_of_year($date) {
        $date0m0d = intval($date->format('md'));
        return ($date0m0d < 601) || ($date0m0d >= 1231);
    }

    public function time_until_depreciated()
    {
        // @link http://www.php.net/manual/en/class.datetime.php
        $d1 = new \DateTime();
        $d2 = $this->depreciated_date();

        // @link http://www.php.net/manual/en/class.dateinterval.php
        $interval = $d1->diff($d2);
        if (!$interval->invert) {
            return $interval;
        } else {
            return new \DateInterval("PT0S"); //null interval (zero seconds from now)
        }
    }

    public function depreciated_date()
    {
        $date = date_create($this->purchase_date);
        date_add($date, date_interval_create_from_date_string($this->get_depreciation()->months . ' months'));
        return $date; //date_format($date, 'Y-m-d'); //don't bake-in format, for internationalization
    }

    // it's necessary for unit tests
    protected function getDateTime($time = null) 
    {
        return new \DateTime($time);
    }
}