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+# Custom `QROutputInterface`
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+
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+Let's suppose that you want to create your own output interface because there's no built-in output class that supports the format you need for your application.
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+In this example we'll create a string output class that outputs the coordinates for each module, separated by module type.
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+
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+
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+## Class skeleton
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+
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+We'll start with a skeleton that extends `QROutputAbstract` and implements the methods that are required by `QROutputInterface`:
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+
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+```php
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+class MyCustomOutput extends QROutputAbstract{
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+
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+ public static function moduleValueIsValid($value):bool{}
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+
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+ protected function prepareModuleValue($value){}
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+
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+ protected function getDefaultModuleValue(bool $isDark){}
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+
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+ public function dump(string $file = null){}
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+
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+}
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+```
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+
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+
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+## Module values
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+
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+The validator should check whether the given input value and range is valid for the output class and if it can be given to the `QROutputAbstract::prepareModuleValue()` method.
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+For example in the built-in GD output it would check if the value is an array that has a minimum of 3 elements (for RGB), each of which is numeric.
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+
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+In this example we'll accept string values, the characters `a-z` (case-insensitive) and a hyphen `-`:
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+
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+```php
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+ public static function moduleValueIsValid($value):bool{
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+ return is_string($value) && preg_match('/^[a-z-]+$/i', $value) === 1;
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+ }
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+```
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+
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+To prepare the final module substitute, you should transform the given (validated) input value in a way so that it can be accessed without any further calls or transformation.
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+In the built-in output for example this means it would return an `ImagickPixel` instance or the integer value returned by `imagecolorallocate()` on the current `GdImage` instance.
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+
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+For our example, we'll lowercase the validated string:
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+
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+```php
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+ protected function prepareModuleValue($value):string{
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+ return strtolower($value);
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+ }
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+```
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+
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+Finally, we need to provide a default value for dark and light, you can call `prepareModuleValue()` here if necessary.
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+We'll return an empty string `''` here as we're going to use the `QROutputInterface::LAYERNAMES` constant for non-existing values
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+(returning `null` would run into an exception in `QROutputAbstract::getModuleValue()`).
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+
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+```php
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+ protected function getDefaultModuleValue(bool $isDark):string{
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+ return '';
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+ }
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+```
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+
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+
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+## Transform the output
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+
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+In our example, we want to collect the modules by type and have the collections listed under a header for each type.
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+In order to do so, we need to collect the modules per `$M_TYPE` before we can render the final output.
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+
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+```php
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+ public function dump(string $file = null):string{
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+ $collections = [];
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+
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+ // loop over the matrix and collect the modules per layer
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+ foreach($this->matrix->getMatrix() as $y => $row){
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+ foreach($row as $x => $M_TYPE){
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+ $collections[$M_TYPE][] = $this->module($x, $y, $M_TYPE);
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+ }
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+ }
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+
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+ // build the final output
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+ $out = [];
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+
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+ foreach($collections as $M_TYPE => $collection){
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+ $name = ($this->getModuleValue($M_TYPE) ?: $this::LAYERNAMES[$M_TYPE]);
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+ // the section header
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+ $out[] = sprintf("%s (%012b)\n", $name, $M_TYPE);
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+ // the list of modules
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+ $out[] = sprintf("%s\n", implode("\n", $collection));
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+ }
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+
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+ return implode("\n", $out);
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+ }
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+```
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+
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+We've introduced another method that handles the module rendering, which incooperates handling of the `QROptions::$drawLightModules` setting:
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+
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+```php
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+ protected function module(int $x, int $y, int $M_TYPE):string{
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+
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+ if(!$this->drawLightModules && !$this->matrix->isDark($M_TYPE)){
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+ return '';
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+ }
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+
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+ return sprintf('x: %s, y: %s', $x, $y);
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+ }
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+```
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+
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+Speaking of option settings, there's also `QROptions::$connectPaths` which we haven't taken care of yet - the good news is that we don't need to as it is already implemented!
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+We'll modify the above `dump()` method to use `QROutputAbstract::collectModules()` instead.
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+
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+The module collector accepts a closure as its only parameter, the closure is called with 4 parameters:
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+
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+- `$x` : current column
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+- `$y` : current row
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+- `$M_TYPE` : field value
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+- `$M_TYPE_LAYER`: (possibly modified) field value that acts as layer id
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+
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+We'll only need the first 3 parameters, so our closure would look as follows:
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+
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+```php
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+$closure = fn(int $x, int $y, int $M_TYPE):string => $this->module($x, $y, $M_TYPE);
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+```
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+
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+As of PHP 8.1+ we can narrow this down with the [first class callable syntax](https://www.php.net/manual/en/functions.first_class_callable_syntax.php):
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+
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+```php
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+$closure = $this->module(...);
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+```
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+
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+This is our final output method then:
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+
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+```php
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+ public function dump(string $file = null):string{
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+ $collections = $this->collectModules($this->module(...));
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+
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+ // build the final output
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+ $out = [];
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+
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+ foreach($collections as $M_TYPE => $collection){
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+ $name = ($this->getModuleValue($M_TYPE) ?: $this::LAYERNAMES[$M_TYPE]);
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+ // the section header
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+ $out[] = sprintf("%s (%012b)\n", $name, $M_TYPE);
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+ // the list of modules
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+ $out[] = sprintf("%s\n", implode("\n", $collection));
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+ }
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+
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+ return implode("\n", $out);
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+ }
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+```
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+
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+
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+## Run the custom output
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+
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+To run the output we just need to set the `QROptions::$outputInterface` to our custom class:
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+
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+```php
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+$options = new QROptions;
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+$options->outputType = QROutputInterface::CUSTOM;
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+$options->outputInterface = MyCustomOutput::class;
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+$options->connectPaths = true;
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+$options->drawLightModules = true;
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+
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+// our custom module values
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+$options->moduleValues = [
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+ QRMatrix::M_DATA => 'these-modules-are-light',
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+ QRMatrix::M_DATA_DARK => 'here-is-a-dark-module',
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+];
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+
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+$qrcode = new QRCode($options);
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+$qrcode->addByteSegment('test');
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+
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+var_dump($qrcode->render());
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+```
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+
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+The output looks similar to the following:
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+```
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+these-modules-are-light (000000000010)
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+
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+x: 0, y: 0
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+x: 1, y: 0
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+x: 2, y: 0
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+...
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+
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+here-is-a-dark-module (100000000010)
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+
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+x: 4, y: 4
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+x: 5, y: 4
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+x: 6, y: 4
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+...
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+```
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+
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+Profit!
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+
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+
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+## Summary
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+
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+We've learned how to create a custom output class for a string based format similar to several of the built-in formats such as SVG or EPS.
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+
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+The full code of our custom class below:
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+
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+```php
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+class MyCustomOutput extends QROutputAbstract{
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+
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+ protected function prepareModuleValue($value):string{
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+ return strtolower($value);
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+ }
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+
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+ protected function getDefaultModuleValue(bool $isDark):string{
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+ return '';
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+ }
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+
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+ public static function moduleValueIsValid($value):bool{
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+ return is_string($value) && preg_match('/^[a-z-]+$/i', $value) === 1;
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+ }
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+
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+ public function dump(string $file = null):string{
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+ $collections = $this->collectModules($this->module(...));
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+
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+ // build the final output
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+ $out = [];
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+
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+ foreach($collections as $M_TYPE => $collection){
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+ $name = ($this->getModuleValue($M_TYPE) ?: $this::LAYERNAMES[$M_TYPE]);
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+ // the section header
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+ $out[] = sprintf("%s (%012b)\n", $name, $M_TYPE);
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+ // the list of modules
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+ $out[] = sprintf("%s\n", implode("\n", $collection));
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+ }
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+
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+ return implode("\n", $out);
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+ }
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+
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+ protected function module(int $x, int $y, int $M_TYPE):string{
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+
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+ if(!$this->drawLightModules && !$this->matrix->isDark($M_TYPE)){
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+ return '';
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+ }
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+
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+ return sprintf('x: %s, y: %s', $x, $y);
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+ }
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+
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+}
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+```
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smiley