main.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef char *sds;

typedef struct String {
  int len;
  int free;
  char data[];
} String;

#define GET_STRING_LEN(data_ptr) (*((int *)((sds)(data_ptr) - sizeof(int) * 2)))
#define GET_STRING_FREE(data_ptr) (*((int *)((sds)(data_ptr) - sizeof(int))))
#define SET_STRING_LEN(data_ptr, new_len)                                      \
  do {                                                                         \
    int *len_ptr = (int *)((char *)(data_ptr) - sizeof(int) - sizeof(int));    \
    *len_ptr = (new_len);                                                      \
  } while (0)
#define SET_STRING_FREE(data_ptr, new_free)                                    \
  do {                                                                         \
    int *free_ptr = (int *)((char *)(data_ptr) - sizeof(int));                 \
    *free_ptr = (new_free);                                                    \
  } while (0)
#define GET_STRING_PTR(data_ptr) ((String *)((sds)(data_ptr) - sizeof(int) * 2))
#define FREE_STRING(data_ptr)                                                  \
  do {                                                                         \
    String *str_ptr = (String *)((sds)(data_ptr) - sizeof(int) * 2);           \
    free(str_ptr);                                                             \
  } while (0)

sds new_string(int length) {
  String *str = (String *)malloc(sizeof(String) + length + 1);
  str->len = length;
  str->free = 0;
  return str->data;
}

sds init_string(const char *s) {
  sds string = new_string(strlen(s));
  snprintf(string, strlen(s) + 1, "%s", s);
  return string;
}

sds append_string(sds a, const char *b) {
  int length = strlen(b);
  String *str = GET_STRING_PTR(a);
  if (str->free < length) {
    str->free += str->len + str->free + length;
    str = realloc(str, sizeof(String) + str->len + str->free + length + 1);
  } else {
    str->free -= length;
  }
  strcpy(str->data + str->len, b);
  str->len += length;
  return str->data;
}

int main() {
  sds string = init_string("Hello");
  printf("[0] SDS: %s, Length: %d, Free: %d, strlen: %lu\n", string,
         GET_STRING_LEN(string), GET_STRING_FREE(string), strlen(string));
  string = append_string(string, " world");
  printf("[1] SDS: %s, Length: %d, Free: %d, strlen: %lu\n", string,
         GET_STRING_LEN(string), GET_STRING_FREE(string), strlen(string));
  string = append_string(string, "!");
  printf("[2] SDS: %s, Length: %d, Free: %d, strlen: %lu\n", string,
         GET_STRING_LEN(string), GET_STRING_FREE(string), strlen(string));
  string = append_string(string, " Hello world!");
  printf("[3] SDS: %s, Length: %d, Free: %d, strlen: %lu\n", string,
         GET_STRING_LEN(string), GET_STRING_FREE(string), strlen(string));
  FREE_STRING(string);
  return 0;
}