5kyu: add ISBN-10 Validation

Signed-off-by: Matej Focko <mfocko@redhat.com>

5kyu/isbn10_validation/solution.c

@@ -0,0 +1,46 @@
+#include <assert.h>
+#include <ctype.h>
+#include <stdbool.h>
+#include <stdlib.h>
+
+static bool check_digit(char digit, int idx, int* sum)
+{
+    if (!isdigit(digit) && (idx != 9 || digit != 'X')) {
+        return false;
+    }
+
+    *sum += (idx + 1) * ((digit == 'X') ? 10 : (digit - '0'));
+    return true;
+}
+
+bool is_valid_ISBN_10(const char* ISBN)
+{
+    if (ISBN == NULL) {
+        return false;
+    }
+
+    int i, sum = 0;
+    for (i = 0; i < 10; i++) {
+        if (!check_digit(ISBN[i], i, &sum)) {
+            return false;
+        }
+    }
+
+    return i == 10 && ISBN[i] == '\0' && sum % 11 == 0;
+}
+
+int main(void)
+{
+    assert(is_valid_ISBN_10("1112223339") == true);
+    assert(is_valid_ISBN_10("048665088X") == true);
+    assert(is_valid_ISBN_10("1293000000") == true);
+    assert(is_valid_ISBN_10("1234554321") == true);
+    assert(is_valid_ISBN_10("1234512345") == false);
+    assert(is_valid_ISBN_10("1293") == false);
+    assert(is_valid_ISBN_10("X123456788") == false);
+    assert(is_valid_ISBN_10("ABCDEFGHIJ") == false);
+    assert(is_valid_ISBN_10("XXXXXXXXXX") == false);
+    assert(is_valid_ISBN_10("048665088XZ") == false);
+
+    return 0;
+}

README.md

@@ -88,6 +88,7 @@
 - [Greed is Good](https://www.codewars.com/kata/5270d0d18625160ada0000e4) - [solution](5kyu/greed_is_good)
 - [Snakes and Ladders](https://www.codewars.com/kata/587136ba2eefcb92a9000027) - [solution](5kyu/snakes_and_ladders)
 - [The Clockwise Spiral](https://www.codewars.com/kata/536a155256eb459b8700077e) - [solution](5kyu/the_clockwise_spiral)
+- [ISBN-10 Validation](https://www.codewars.com/kata/51fc12de24a9d8cb0e000001) - [solution](5kyu/isbn10_validation)
 
 ### Python