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High precision currency in drupal 7 commerce

In Drupal Commerce you can run into rounding errors when only using 2 significant digits, the easiest solution is to inform commerce to work with 4 significant digits

First instruct Drupal Commerce to use 4 decimals.

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/**
 * Implements hook_commerce_currency_info_alter().
 */
function MY_MODULE_commerce_currency_info_alter(&$info) {
  $info['EUR']['decimals'] = 4;
  $info['EUR']['format_callback'] = 'MY_MODULE_format_currency_euro';
}

Implement a custom formatter, so the output uses 2 decimals.

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/**
 * Format EURO currency.
 *
 * @see commerce_currency_format()
 */
function MY_MODULE_format_currency_euro($amount, $currency, $object) {
  $currency['decimals'] = 2;

  // Format the price as a number.
  $price = number_format(commerce_currency_round(abs($amount), $currency), $currency['decimals'], $currency['decimal_separator'], $currency['thousands_separator']);

  // Establish the replacement values to format this price for its currency.
  $replacements = array(
    '@code_before' => $currency['code_placement'] == 'before' ? $currency['code'] : '',
    '@symbol_before' => $currency['symbol_placement'] == 'before' ? $currency['symbol'] : '',
    '@price' => $price,
    '@symbol_after' => $currency['symbol_placement'] == 'after' ? $currency['symbol'] : '',
    '@code_after' => $currency['code_placement'] == 'after' ? $currency['code'] : '',
    '@negative' => $amount < 0 ? '-' : '',
    '@symbol_spacer' => $currency['symbol_spacer'],
    '@code_spacer' => $currency['code_spacer'],
  );

  return trim(t('@code_before@code_spacer@negative@symbol_before@price@symbol_spacer@symbol_after@code_spacer@code_after', $replacements));
}

Implement the hooks to alter the amount before sending it to the payment provider.

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/**
 * Implements hook_commerce_ogone_data_alter().
 */
function MY_MODULE_commerce_ogone_data_alter(&$data, $order, $settings) {
  $wrapper = entity_metadata_wrapper('commerce_order', $order);
  $currency_code = $wrapper->commerce_order_total->currency_code->value();

  // Only update EURO.
  if ($currency_code !== 'EUR') {
    return;
  }

  $amount = $wrapper->commerce_order_total->amount->value();
  $amount = commerce_currency_amount_to_decimal($amount, $currency_code);

  $currency = commerce_currency_load($currency_code);
  $currency['decimals'] = 2;
  $amount = commerce_currency_round($amount, $currency);
  $amount = $amount * 100;

  $data['AMOUNT'] = $amount;
}

/**
 * Implements hook_commerce_stripe_order_charge_alter().
 */
function MY_MODULE_commerce_stripe_order_charge_alter(&$charge, $order) {
  $wrapper = entity_metadata_wrapper('commerce_order', $order);
  $currency_code = $wrapper->commerce_order_total->currency_code->value();

  // Only update EURO.
  if ($currency_code !== 'EUR') {
    return;
  }

  $amount = $wrapper->commerce_order_total->amount->value();
  $amount = commerce_currency_amount_to_decimal($amount, $currency_code);

  $currency = commerce_currency_load($currency_code);
  $currency['decimals'] = 2;
  $amount = commerce_currency_round($amount, $currency);
  $amount = $amount * 100;

  $charge['amount'] = $amount;
}

If you want to do this on an existing site, you need to update all amounts

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/**
 * Update prices to use 4 decimals.
 */
function MY_MODULE_update_7006() {
  // Only update EURO.
  if (variable_get('commerce_default_currency', '') !== 'EUR') {
    return;
  }

  db_query('update {field_data_commerce_price} set commerce_price_amount = 100 * commerce_price_amount');
  db_query('update {field_data_commerce_total} set commerce_total_amount = 100 * commerce_total_amount');
  db_query('update {field_data_commerce_unit_price} set commerce_unit_price_amount = 100 * commerce_unit_price_amount');
  db_query('update {field_data_field_commerce_saleprice} set field_commerce_saleprice_amount = 100 * field_commerce_saleprice_amount');
  db_query('update {field_data_commerce_order_total} set commerce_order_total_amount = 100 * commerce_order_total_amount');

  db_query('update {field_revision_commerce_price} set commerce_price_amount = 100 * commerce_price_amount');
  db_query('update {field_revision_commerce_total} set commerce_total_amount = 100 * commerce_total_amount');
  db_query('update {field_revision_commerce_unit_price} set commerce_unit_price_amount = 100 * commerce_unit_price_amount');
  db_query('update {field_revision_field_commerce_saleprice} set field_commerce_saleprice_amount = 100 * field_commerce_saleprice_amount');
  db_query('update {field_revision_commerce_order_total} set commerce_order_total_amount = 100 * commerce_order_total_amount');

  db_query('update {commerce_payment_transaction} set amount = 100 * amount');
  db_query('update {commerce_payment_transaction_revision} set amount = 100 * amount');

  db_query('update {commerce_flat_rate_service} set amount = 100 * amount');

  db_query('update {field_data_commerce_fixed_amount} set commerce_fixed_amount_amount = 100 * commerce_fixed_amount_amount');
  db_query('update {field_revision_commerce_fixed_amount} set commerce_fixed_amount_amount = 100 * commerce_fixed_amount_amount');
}

/**
 * Update prices of completed orders.
 */
function MY_MODULE_update_7007() {
  // Only update EURO.
  if (variable_get('commerce_default_currency', '') !== 'EUR') {
    return;
  }

  $result = db_query('SELECT * FROM {field_data_commerce_total}');
  foreach ($result as $record) {
    $data = unserialize($record->commerce_total_data);
    if (isset($data['components'])) {
      $needs_update = FALSE;
      foreach ($data['components'] as &$component) {
        if (isset($component['price']['amount'])) {
          $component['price']['amount'] = 100 * $component['price']['amount'];
          $needs_update = TRUE;
        }
      }
      if ($needs_update) {
        $record->commerce_total_data = $data;
        drupal_write_record('field_data_commerce_total', $record, array(
          'entity_type',
          'entity_id',
          'delta',
        ));
      }
    }
  }

  $result = db_query('SELECT * FROM {field_revision_commerce_total}');
  foreach ($result as $record) {
    $data = unserialize($record->commerce_total_data);
    if (isset($data['components'])) {
      $needs_update = FALSE;
      foreach ($data['components'] as &$component) {
        if (isset($component['price']['amount'])) {
          $component['price']['amount'] = 100 * $component['price']['amount'];
          $needs_update = TRUE;
        }
      }
      if ($needs_update) {
        $record->commerce_total_data = $data;
        drupal_write_record('field_revision_commerce_total', $record, array(
          'entity_type',
          'entity_id',
          'revision_id',
          'delta',
        ));
      }
    }
  }

  $result = db_query('SELECT * FROM {field_data_commerce_order_total}');
  foreach ($result as $record) {
    $data = unserialize($record->commerce_order_total_data);
    if (isset($data['components'])) {
      $needs_update = FALSE;
      foreach ($data['components'] as &$component) {
        if (isset($component['price']['amount'])) {
          $component['price']['amount'] = 100 * $component['price']['amount'];
          $needs_update = TRUE;
        }
      }
      if ($needs_update) {
        $record->commerce_order_total_data = $data;
        drupal_write_record('field_data_commerce_order_total', $record, array(
          'entity_type',
          'entity_id',
          'delta',
        ));
      }
    }
  }

  $result = db_query('SELECT * FROM {field_revision_commerce_order_total}');
  foreach ($result as $record) {
    $data = unserialize($record->commerce_order_total_data);
    if (isset($data['components'])) {
      $needs_update = FALSE;
      foreach ($data['components'] as &$component) {
        if (isset($component['price']['amount'])) {
          $component['price']['amount'] = 100 * $component['price']['amount'];
          $needs_update = TRUE;
        }
      }
      if ($needs_update) {
        $record->commerce_order_total_data = $data;
        drupal_write_record('field_revision_commerce_order_total', $record, array(
          'entity_type',
          'entity_id',
          'revision_id',
          'delta',
        ));
      }
    }
  }
}