commit a7997c4636c5af44d02b2ba8b9567f757d43f204
Author: bot50 <bot@cs50.harvard.edu>
Date:   Wed Feb 21 20:47:37 2024 +0000

    kukemuna-cs50/problems/2024/x/credit@20240221T204737.334922147Z

diff --git a/credit.c b/credit.c
new file mode 100644
index 0000000..882037d
--- /dev/null
+++ b/credit.c
@@ -0,0 +1,133 @@
+#include <cs50.h>
+#include <stdio.h>
+
+// Prototypes
+long long get_card_number(void);
+void find_issuer(long long card_number);
+int validate(long long card_number);
+int get_id(long long card_number);
+
+int main(void)
+{
+    long long card_number = get_card_number();
+
+    if (validate(card_number))
+    {
+        find_issuer(card_number);
+    }
+    return 0;
+}
+
+long long get_card_number()
+{
+    long long number;
+    do
+    {
+        number = get_long("Insert card number: ");
+    }
+    while (number < 0);
+
+    return number;
+}
+
+void find_issuer(long long card_number)
+{
+    int id = get_id(card_number);
+
+    // Number beginning with 34 or 37
+    if (id / 100 == 34 || id / 100 == 37)
+    {
+        printf("AMEX\n");
+        return;
+    }
+
+    // Number beginning with 4
+    if (id / 1000 == 4)
+    {
+        printf("VISA\n");
+        return;
+    }
+
+    // Number beginning with 51-55 or 2221-2720
+    if ((id / 100 >= 51 && id / 100 <= 55) || (id >= 2221 && id <= 2720))
+    {
+        printf("MASTERCARD\n");
+        return;
+    }
+    else
+    {
+        printf("INVALID\n");
+        return;
+    }
+}
+
+int validate(long long card_number)
+{
+    // Add together individual digits while multiplying every other digit from one after least significant before addition
+    // If multiplication product is geater than 9 then add individual digits together. 3456 => 6 + (1 + 0) + 4 + 6
+    int sum = 0, multiply = 0;
+
+    while (card_number > 0)
+    {
+        if (multiply)
+        {
+            if (card_number % 10 * 2 > 9) // Is multiplication product greater than 9?
+            {
+                int n = card_number % 10 * 2;
+                while (n > 0) // If so, add individual digits to the sum
+                {
+                    sum += n % 10;
+                    n /= 10;
+                }
+            }
+            else
+            {
+                sum += card_number % 10 * 2;
+            }
+            card_number /= 10;
+            multiply = 0;
+        }
+        else
+        {
+            sum += card_number % 10;
+            card_number /= 10;
+            multiply = 1;
+        }
+    }
+
+    if (sum % 10 != 0)
+    {
+        printf("INVALID\n");
+        return 0; // Invalid number
+    }
+    else
+    {
+        return 1; // Valid number
+    }
+}
+
+int get_id(long long card_number)
+{
+    int count = 0, id;
+    long long n, trunc = 1;
+
+    n = card_number;
+
+    while (n > 0)
+    {
+        n /= 10;
+        count++;
+    }
+
+    if (count < 13 || count > 16)
+    {
+        return 0;
+    }
+
+    for (int i = 0; i < count - 4; i++) // Get four most significant digits
+    {
+        trunc *= 10;
+    }
+    id = card_number / trunc;
+    return id;
+}