support lower case with new modulo

src/crypt.c

@@ -9,17 +9,50 @@
 */
 
 #include "crypt.h"
+#include <stdbool.h>
+
+
+// Which of the two below functions is faster (find out!)
+inline int mod(int a, int b)
+{
+	return (a % b + b) % b;
+}
+
+
+inline int fast_mod(int a, int b)
+{
+	const int result = a % b;
+	return result >= 0 ? result : result + b;
+}
+
+
+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 % 26;
+		if (str[i] != ' ')
+		{
+			bool upper = isupper(str[i]);
+			str[i] += key - (upper ? 65 : 97);
+			str[i] %= 26;
+			str[i] += (upper ? 65 : 97);
+		}
 }
 
 
 void caesar_decrypt(char* str, int size, int key)
 {
 	for (int i = 0; i < size && str[i] != '\n'; i++)
-		if (str[i] != ' ') str[i] -= key % 26;
+		if (str[i] != ' ')
+		{
+			bool upper = isupper(str[i]);
+			str[i] = str[i] - key - (upper ? 65 : 97);
+			str[i] = mod(str[i], 26);
+			str[i] += (upper ? 65 : 97);
+		}
 }