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

challenges/caesar-cipher/solutions/c/function/README.md

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+# caesar-cipher/c/function
+
+Created by [@Divlo](https://github.com/Divlo) on 7 October 2021.

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

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+#include "character.h"
+
+#include <stdlib.h>
+#include <string.h>
+
+void character_append(char* string, char character) {
+  size_t length = strlen(string);
+  string[length] = character;
+  string[length + 1] = '\0';
+}

challenges/caesar-cipher/solutions/c/function/character.h

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+#ifndef CHARACTER_H
+#define CHARACTER_H
+
+/**
+ * @brief Append a character to a string, assuming string points to an array
+ * with enough space.
+ *
+ * @param string
+ * @param character
+ */
+void character_append(char* string, char character);
+
+#endif

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

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+#include "input.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "character.h"
+
+char* input() {
+  char character;
+  size_t length = 1;
+  char* string = malloc(length * sizeof(char));
+  *string = '\0';
+  while ((character = getchar()) != '\n' && character != EOF) {
+    length++;
+    string = realloc(string, length * sizeof(char));
+    character_append(string, character);
+  }
+  return string;
+}

challenges/caesar-cipher/solutions/c/function/input.h

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+#ifndef INPUT_H
+#define INPUT_H
+
+/**
+ * @brief Read a line from stdin.
+ *
+ * @return char*
+ */
+char* input();
+
+#endif

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

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+#include <stdio.h>
+#include <stdlib.h>
+
+#include "character.h"
+#include "input.h"
+#include "string.h"
+
+int main() {
+  char* string = input();
+  int shift;
+  scanf("%d", &shift);
+  string = string_caesar_cipher(string, shift);
+  printf("%s\n", string);
+  free(string);
+  return EXIT_SUCCESS;
+}

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

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+#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;
+}

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

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+#ifndef STRING_H
+#define STRING_H
+
+#define ALPHABET "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+
+/**
+ * @brief Shift the alphabet by a given amount.
+ *
+ * @param shift
+ * @return char*
+ */
+char* string_shift_alphabet(int shift);
+
+/**
+ * @brief Encrypts a string using the Caesar cipher.
+ *
+ * @param string
+ * @param shift
+ * @return char*
+ */
+char* string_caesar_cipher(char* string, int shift);
+
+#endif