commit 3cde7259a8956cde2fcf80a6000b45b2000e314e
Author: bot50 <bot@cs50.harvard.edu>
Date:   Sun Feb 25 09:57:34 2024 +0000

    kukemuna-cs50/problems/2024/x/substitution@20240225T095734.427910511Z

diff --git a/substitution.c b/substitution.c
new file mode 100644
index 0000000..4732453
--- /dev/null
+++ b/substitution.c
@@ -0,0 +1,78 @@
+#include <cs50.h>
+#include <ctype.h>
+#include <stdio.h>
+#include <string.h>
+
+int validate_key(string key); // NQXPOMAFTRHLZGECYJIUWSKDVB
+void substitute(string plaintext, string key);
+
+int main(int argc, string argv[])
+{
+    // Print usage and exit if argc is not 2 or key is not integer
+    if (argc != 2 || validate_key(argv[1]) > 0)
+    {
+        printf("Usage: ./substitution key\n");
+        return 1;
+    }
+    else
+    {
+        string key = argv[1];
+        string plaintext = get_string("plaintext:  ");
+        substitute(plaintext, key);
+    }
+    return 0;
+}
+
+// Convert a string of numbers to integer
+int validate_key(string key)
+{
+    // Sum of consecutive numbers: (b + a) x (b - a + 1) / 2
+    // If key length is 26 then iterate each string element
+    if (strlen(key) == 26)
+    {
+        int sum = 0;
+
+        for (char c = 'A'; c <= 'Z'; c++)
+        {
+            for (int i = 0; i < 26; i++)
+            {
+                if (toupper(key[i]) == c)
+                {
+                    sum++;
+                    break;
+                }
+            }
+        }
+
+        if (sum == 26) // Sum of 26 unique uppercase characters, but also MMCcEFGHIJKLMNOPQRqTUVWXeZ
+        {
+            return 0;
+        }
+    }
+    return 1;
+}
+
+void substitute(string plaintext, string key)
+{
+    printf("ciphertext: ");
+
+    for (int i = 0; i < strlen(plaintext); i++)
+    {
+        if (isalpha(plaintext[i]))
+        {
+            if (isupper(plaintext[i]))
+            {
+                printf("%c", toupper(key[plaintext[i] - 65]));
+            }
+            else
+            {
+                printf("%c", tolower(key[plaintext[i] - 97]));
+            }
+        }
+        else
+        {
+            printf("%c", plaintext[i]);
+        }
+    }
+    printf("\n");
+}