support lower case with new modulo

src/crypt.c

@@ -9,6 +9,8 @@
 */
 
 #include "crypt.h"
+#include <stdbool.h>
+
 
 // Which of the two below functions is faster (find out!)
 inline int mod(int a, int b)
@@ -23,15 +25,22 @@ inline int fast_mod(int a, int b)
 	return result >= 0 ? result : result + b;
 }
 
-// Below two functions do NOT work for lower case letters (fix this!)
+
+inline bool isupper(char c)
+{
+	return c >= 'A' && c <= 'Z';
+}
+
+
 void caesar_encrypt(char* str, int size, int key)
 {
 	for (int i = 0; i < size && str[i] != '\n'; i++)
 		if (str[i] != ' ')
 		{
-			str[i] += key - 65;
+			bool upper = isupper(str[i]);
+			str[i] += key - (upper ? 65 : 97);
 			str[i] %= 26;
-			str[i] += 65;
+			str[i] += (upper ? 65 : 97);
 		}
 }
 
@@ -41,8 +50,9 @@ void caesar_decrypt(char* str, int size, int key)
 	for (int i = 0; i < size && str[i] != '\n'; i++)
 		if (str[i] != ' ')
 		{
-			str[i] = str[i] - key - 65;
+			bool upper = isupper(str[i]);
+			str[i] = str[i] - key - (upper ? 65 : 97);
 			str[i] = mod(str[i], 26);
-			str[i] += 65;
+			str[i] += (upper ? 65 : 97);
 		}
 }