#include <stdio.h>
#include <string.h>
#include <malloc.h>
 
typedef struct tag_string {
  char* c_str;
  size_t length;
  size_t capacity;
} string;
 
string create_string(const char* str, size_t capacity) {
  string ret_val = {0};
  ret_val.length = strlen(str);
  ret_val.capacity = capacity;
  if (ret_val.length > ret_val.capacity) { /* 指定された大きさのバッファでは足りない */
    ret_val.capacity = ret_val.length;
  }
  ret_val.c_str = (char*) malloc(sizeof(char) * ret_val.capacity + sizeof(char)); /* 終端文字を考慮 */
  memset(ret_val.c_str, 0, sizeof(char) * ret_val.capacity + sizeof(char));
  strncpy(ret_val.c_str, str, ret_val.length);
  return ret_val;
}
 
void destroy_string(string* p) {
  if (p->c_str) {
    free(p->c_str);
    p->c_str = NULL;
    p->length = p->capacity = 0;
  }
}
 
/* begin, end */
char* begin_string(string* str) {
  return str->c_str;
}
char* end_string(string* str) {
  return str->c_str + str->length; /* 終端文字を返す */
}
const char* cbegin_string(const string* str) {
  return str->c_str;
}
const char* cend_string(const string* str) {
  return str->c_str + str->length; /* 終端文字を返す */
}
 
/* copy */
void copy_string(string* s1, const string* s2) {
  if (s1->capacity >= s2->length) {
    strncpy(s1->c_str, s2->c_str, s2->length);
    s1->length = s2->length;
    memset(end_string(s1), 0, (s1->capacity - s1->length + 1) * sizeof(char));
    return;
  }
  /* s1のバッファには入りきらない */
  string ret_val = create_string(s2->c_str, s2->length);
  destroy_string(s1);
  *s1 = ret_val;
}
 
/* compare */
_Bool equal_string(const string* s1, const string* s2) {
  return !strcmp(s1->c_str, s2->c_str);
}
 
 
int main(void){
  string str1 = create_string("abcdef", 6);
  string str2 = create_string("abcdefg", 0);
  string str3 = create_string("", 10);
 
  copy_string(&str2, &str1);
  copy_string(&str3, &str1);
 
  printf("string: %s\nlength: %d\ncapacity:%d\n",
    str2.c_str, str2.length, str2.capacity);
  printf("string: %s\nlength: %d\ncapacity:%d\n",
    str3.c_str, str3.length, str3.capacity);
 
  if (equal_string(&str1, &str2)) {
    printf("str1 == str2\n");
  }
 
  destroy_string(&str1);
  destroy_string(&str2);
  destroy_string(&str3);
  return 0;
}
 

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