feat: add caesar cipher algorithm

cipher/caesar_cipher.c

@@ -0,0 +1,94 @@
+/**
+ * @file
+ * @brief A [caesar cipher](https://en.wikipedia.org/wiki/Caesar_cipher) is a 
+ * substitution cipher that replaces each letter in a text by another located 
+ * a given number of positions along the alphabet.
+ * @author [Andrey Gonçalves Barreto Isidoro](https://github.com/agbi623)
+ */
+
+#include <string.h>  /// for strcmp
+#include <stdio.h>   /// for Input/Output
+#include <ctype.h>   /// for isalpha, isupper, islower and tolower
+#include <assert.h>  /// for assert
+
+/**
+ * @brief calculates the encrypted char for the encryption
+ * @param c the character to be used
+ * @param key the key to be used
+ * @returns the encrypted char
+ */
+int calculate_encrypted_char(char c, int key)
+{
+	int new_c;
+
+	new_c = ((tolower(c) - 'a') + key) % 26;
+
+	// because the % operator sucks
+	if (new_c < 0) new_c += 26;
+
+	return isupper(c) ? new_c + 'A' : new_c + 'a';
+}
+
+/**
+ * @brief encrypts the passed text by calling and assigning the return value 
+ * of calculate_encrypted_char for each alphabetical character in the passed 
+ * text string
+ * @param text the text to be encrypted
+ * @param key used for shifting a letter - shifts left when negative, right 
+ * when positive
+ * @returns void
+ */ 
+void caesar_cipher(char *text, int key)
+{
+	for (int i = 0; text[i] != '\0'; i++)
+	{
+		if (isalpha(text[i]))
+		{
+			text[i] = calculate_encrypted_char(text[i], key);
+		}
+	}
+}
+
+/**
+ * @brief tests the caesar_cipher function by passing some strings to it
+ * @returns void
+ */
+void test(void)
+{
+	char first[] = "ILOVETV";
+	char second[] = "ILOVETV";
+	char third[] = "The smoke... was blue.";
+	char fourth[] = "They think they are the fuckin big shot, like the world revolves around them. It\'s funny at times though x\'D";
+	char fifth[] = "My compass was swallowed by the sea...";
+
+	caesar_cipher(first, 15);
+	assert(strcmp(first, "XADKTIK") == 0);
+	printf("First test OK\n");
+
+	caesar_cipher(second, -15);
+	assert(strcmp(second, "TWZGPEG") == 0);
+	printf("Second test OK\n");
+
+	caesar_cipher(third, 35);
+	assert(strcmp(third, "Cqn bvxtn... fjb kudn.") == 0);
+	printf("Third test OK\n");
+
+	caesar_cipher(fourth, -81);
+	assert(strcmp(fourth, "Qebv qefkh qebv xob qeb crzhfk yfd pelq, ifhb qeb tloia obslisbp xolrka qebj. Fq\'p crkkv xq qfjbp qelrde u\'A") == 0);
+	printf("Fourth test OK\n");
+
+	caesar_cipher(fifth, 0);
+	assert(strcmp(fifth, "My compass was swallowed by the sea...") == 0);
+	printf("Fifth test OK\n");
+}
+
+/**
+ * @brief main function; calls the test function
+ * @returns 0 if all tests pass
+ */
+int main(void)
+{
+	test();
+
+	return 0;
+}