Generate a valid BE TAX number (#415)

* Generate valid BE VAT number

* Add comments

Co-authored-by: Rob Van Keilegom <rvk@organi.be>

src/Faker/Provider/nl_BE/Payment.php

@@ -5,7 +5,7 @@
 class Payment extends \Faker\Provider\Payment
 {
     /**
-     * International Bank Account Number (IBAN)
+     * International Bank Account Number (IBAN).
      *
      * @see http://en.wikipedia.org/wiki/International_Bank_Account_Number
      *
@@ -21,7 +21,7 @@ public static function bankAccountNumber($prefix = '', $countryCode = 'BE', $len
     }
 
     /**
-     * Value Added Tax (VAT)
+     * Value Added Tax (VAT).
      *
      * @example 'BE0123456789', ('spaced') 'BE 0123456789'
      *
@@ -37,6 +37,13 @@ public static function vat($spacedNationalPrefix = true)
     {
         $prefix = $spacedNationalPrefix ? 'BE ' : 'BE';
 
-        return sprintf('%s0%d', $prefix, self::randomNumber(9, true));
+        // Generate 7 numbers of vat.
+        $firstSeven = self::randomNumber(7, true);
+
+        // Generate checksum for number
+        $checksum = 97 - fmod($firstSeven, 97);
+
+        // '0' + 7 numbers + checksum
+        return sprintf('%s0%s%s', $prefix, $firstSeven, $checksum);
     }
 }

test/Faker/Provider/nl_BE/PaymentTest.php

@@ -16,6 +16,25 @@ public function testVatIsValid()
         $unspacedVat = $this->faker->vat(false);
         self::assertMatchesRegularExpression('/^(BE 0\d{9})$/', $vat);
         self::assertMatchesRegularExpression('/^(BE0\d{9})$/', $unspacedVat);
+
+        $this->validateChecksum($vat);
+        $this->validateChecksum($unspacedVat);
+    }
+
+    private function validateChecksum($vat)
+    {
+        // Remove the "BE " part from the beginning
+        $numbers = trim(substr($vat, 2));
+
+        $len = strlen($numbers);
+
+        self::assertEquals(10, $len);
+        self::assertStringStartsWith('0', $numbers);
+
+        // Mod97 check on first 8 digits
+        $checksum = 97 - fmod(substr($numbers, 0, 8), 97);
+
+        self::assertEquals((string) $checksum, substr($numbers, 8, 10));
     }
 
     protected function getProviders(): iterable