feat: kata/modulus-11-check-digit

kata/6-kyu/index.md

@@ -234,6 +234,7 @@
 - [Minutes to Midnight](minutes-to-midnight)
 - [Missing Alphabet](missing-alphabet)
 - [Moduli number system](moduli-number-system)
+- [Modulus 11 - Check Digit](modulus-11-check-digit)
 - [More Zeros than Ones](more-zeros-than-ones)
 - [Most Frequent Weekdays](most-frequent-weekdays)
 - [Moves in squared strings (II)](moves-in-squared-strings-ii)

kata/6-kyu/modulus-11-check-digit/README.md

@@ -4,16 +4,20 @@
 
 Some numbers are more important to get right during data entry than others: a common example is product codes.
 
-To reduce the possibility of mistakes, product codes can be crafted in such a way that simple errors are detected. This is done by calculating a single-digit value based on the product number, and then appending that digit to the product number to arrive at the product code.
+To reduce the possibility of mistakes, product codes can be crafted in such a way that simple errors are detected. This is done by
+calculating a single-digit value based on the product number, and then appending that digit to the product number to arrive at the product
+code.
 
-When the product code is checked, the check digit value is stripped off and recalculated. If the supplied value does not match the recalculated value, the product code is rejected.
+When the product code is checked, the check digit value is stripped off and recalculated. If the supplied value does not match the
+recalculated value, the product code is rejected.
 
 A simple scheme for generating self-check digits, described here, is called Modulus 11 Self-Check.
 
-
 ## Calculation method
 
-Each digit in the product number is assigned a multiplication factor. The factors are assigned ***from right to left***, starting at `2` and counting up. For numbers longer than six digits, the factors restart at `2` after `7` is reached. The product of each digit and its factor is calculated, and the products summed. For example:
+Each digit in the product number is assigned a multiplication factor. The factors are assigned ***from right to left***, starting at `2` and
+counting up. For numbers longer than six digits, the factors restart at `2` after `7` is reached. The product of each digit and its factor
+is calculated, and the products summed. For example:
 
 ```
 digit : 1 6 7 7 0 3 6 2 5
@@ -30,12 +34,10 @@ Then the sum of the products is divided by the prime number `11`. The remainder
 
 The result is the **check digit**.
 
-
 ## Your task
 
 Your task is to implement this algorithm and return the input number with the correct check digit appended.
 
-
 ## Examples
 
 ```
@@ -46,4 +48,4 @@ remainder = 81 mod 11 = 4
 check digit = 11 - 4 = 7
 
 output: "0365327"
-```
+```

kata/6-kyu/modulus-11-check-digit/main/Modulus11.java

@@ -0,0 +1,8 @@
+import static java.util.stream.IntStream.range;
+
+interface Modulus11 {
+  static String addCheckDigit(String s) {
+    int rem = range(0, s.length()).reduce(0, (r, i) -> r + (s.charAt(s.length() - i - 1) - '0') * (i % 6 + 2)) % 11;
+    return s + (rem == 1 ? "X" : (11 - rem) % 11);
+  }
+}

kata/6-kyu/modulus-11-check-digit/test/Modulus11Test.java

@@ -0,0 +1,19 @@
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
+
+class Modulus11Test {
+  @ParameterizedTest
+  @CsvSource(textBlock = """
+      2356, 23566
+      6789, 6789X
+      036532, 0365327
+      12388878, 123888782
+      111111111, 1111111118
+      9735597355, 97355973550
+      """)
+  void sample(String number, String check) {
+    assertEquals(check, Modulus11.addCheckDigit(number));
+  }
+}