feat(solutions): add caesar-cipher/c/function

challenges/caesar-cipher/solutions/c/function/string.c

@@ -0,0 +1,43 @@
+#include "string.h"
+
+#include <stdbool.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "character.h"
+
+#define ALPHABET_LENGTH 26
+
+char* string_shift_alphabet(int shift) {
+  char* result = malloc(sizeof(char) * (ALPHABET_LENGTH + 1));
+  for (char index = 0; index < ALPHABET_LENGTH; index++) {
+    char letter = 'A' + index + shift;
+    if (letter < 'A') {
+      letter = 'Z' + shift + index + 1;
+    } else if (letter > 'Z') {
+      letter = letter - ALPHABET_LENGTH;
+    }
+    character_append(result, letter);
+  }
+  return result;
+}
+
+char* string_caesar_cipher(char* string, int shift) {
+  size_t string_length = strlen(string);
+  char* result = malloc(sizeof(char) * (string_length + 1));
+  char* shifted_alphabet = string_shift_alphabet(shift);
+  for (size_t index = 0; index < string_length; index++) {
+    char letter = string[index];
+    if (letter != ' ') {
+      for (char index_alphabet = 0; index_alphabet < ALPHABET_LENGTH; index_alphabet++) {
+        char current_letter = 'A' + index_alphabet;
+        if (string[index] == current_letter) {
+          letter = shifted_alphabet[index_alphabet];
+          break;
+        }
+      }
+    }
+    character_append(result, letter);
+  }
+  return result;
+}