class DecimalItem

class DecimalItem extends NumericItemBase {
    
    /**
     * {@inheritdoc}
     */
    public static function defaultStorageSettings() {
        return [
            'precision' => 10,
            'scale' => 2,
        ] + parent::defaultStorageSettings();
    }
    
    /**
     * {@inheritdoc}
     */
    public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
        $properties['value'] = DataDefinition::create('string')->setLabel(t('Decimal value'))
            ->setRequired(TRUE);
        return $properties;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function schema(FieldStorageDefinitionInterface $field_definition) {
        return [
            'columns' => [
                'value' => [
                    'type' => 'numeric',
                    'precision' => $field_definition->getSetting('precision'),
                    'scale' => $field_definition->getSetting('scale'),
                ],
            ],
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function storageSettingsForm(array &$form, FormStateInterface $form_state, $has_data) {
        $element = [];
        $settings = $this->getSettings();
        $element['precision'] = [
            '#type' => 'number',
            '#title' => t('Precision'),
            '#min' => 10,
            '#max' => 32,
            '#default_value' => $settings['precision'],
            '#description' => t('The total number of digits to store in the database, including those to the right of the decimal.'),
            '#disabled' => $has_data,
        ];
        $element['scale'] = [
            '#type' => 'number',
            '#title' => t('Scale', [], [
                'context' => 'decimal places',
            ]),
            '#min' => 0,
            '#max' => 10,
            '#default_value' => $settings['scale'],
            '#description' => t('The number of digits to the right of the decimal.'),
            '#disabled' => $has_data,
        ];
        return $element;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getConstraints() {
        $constraint_manager = \Drupal::typedDataManager()->getValidationConstraintManager();
        $constraints = parent::getConstraints();
        $constraints[] = $constraint_manager->create('ComplexData', [
            'value' => [
                'Regex' => [
                    'pattern' => '/^[+-]?((\\d+(\\.\\d*)?)|(\\.\\d+))$/i',
                ],
            ],
        ]);
        return $constraints;
    }
    
    /**
     * {@inheritdoc}
     */
    public function fieldSettingsForm(array $form, FormStateInterface $form_state) {
        $element = parent::fieldSettingsForm($form, $form_state);
        $settings = $this->getSettings();
        $element['min']['#step'] = pow(0.1, $settings['scale']);
        $element['max']['#step'] = pow(0.1, $settings['scale']);
        return $element;
    }
    
    /**
     * {@inheritdoc}
     */
    public function preSave() {
        $this->value = round($this->value, $this->getSetting('scale'));
    }
    
    /**
     * {@inheritdoc}
     */
    public static function generateSampleValue(FieldDefinitionInterface $field_definition) {
        $settings = $field_definition->getSettings();
        $precision = $settings['precision'] ?: 10;
        $scale = $settings['scale'] ?: 2;
        // $precision - $scale is the number of digits on the left of the decimal
        // point.
        // The maximum number you can get with 3 digits is 10^3 - 1 --> 999.
        // The minimum number you can get with 3 digits is -1 * (10^3 - 1).
        $max = is_numeric($settings['max']) ?: pow(10, $precision - $scale) - 1;
        $min = is_numeric($settings['min']) ?: -pow(10, $precision - $scale) + 1;
        // Get the number of decimal digits for the $max
        $decimal_digits = self::getDecimalDigits($max);
        // Do the same for the min and keep the higher number of decimal digits.
        $decimal_digits = max(self::getDecimalDigits($min), $decimal_digits);
        // If $min = 1.234 and $max = 1.33 then $decimal_digits = 3
        $scale = rand($decimal_digits, $scale);
        // @see "Example #1 Calculate a random floating-point number" in
        // http://php.net/manual/function.mt-getrandmax.php
        $random_decimal = $min + mt_rand() / mt_getrandmax() * ($max - $min);
        $values['value'] = self::truncateDecimal($random_decimal, $scale);
        return $values;
    }
    
    /**
     * Helper method to get the number of decimal digits out of a decimal number.
     *
     * @param int $decimal
     *   The number to calculate the number of decimals digits from.
     *
     * @return int
     *   The number of decimal digits.
     */
    protected static function getDecimalDigits($decimal) {
        $digits = 0;
        while ($decimal - round($decimal)) {
            $decimal *= 10;
            $digits++;
        }
        return $digits;
    }

}