#include <stdio.h>
#include <assert.h>
#include <string.h>
char* my_strncpy(char* dest, const char* src, int n)
{
  assert(dest && src);
  char* ret = dest;
  // 这里求src的长度是为了解决拷贝个数大于源字符串的长度 + 1的情况
  // 每拷贝一个lenSrc--，当 lenSrc == 0，而 n != 0,此时应该在后面拷贝\0直至n == 0；
  // 因为字符串后面还有一个\0,要把他算在内，所以lenSrc要等于长度 + 1；
  int lenSrc = (int)strlen(src) + 1;
  while (n--)
  {
    if (!lenSrc) // 当lenSrc为0时，!lenSrc为真，后面拷贝\0直至 n == 0；
    {
      *dest++ = '\0';
    }
    else
    {
      *dest++ = *src++;
      lenSrc--;  // 每拷贝一次源字符串的内容，lenSrc--；
    }
  }
  return ret;
}
int main()
{
  char arr1[20] = "xxxxxxxxxxxxxxxx";
  char arr2[] = "abcdef";
  printf("%s\n", my_strncpy(arr1, arr2, 5));
  return 0;
}

#include <stdio.h>
#include <string.h>
int main()
{
  char a1[20] = "xxxxxxxxxxx";
  char a2[] = "abcdef";
  printf("%s\n", strncat(a1, a2, 4));
  return 0;
}

#include <stdio.h>
#include <string.h>
int main()
{
  char a1[20] = "xxx\0xxxxxxx";
  char a2[] = "abcdef";
  printf("%s\n", strncat(a1, a2, 5));
  return 0;
}

#include <stdio.h>
#include <string.h>
int main()
{
  char a1[] = "abcdef";
  char a2[] = "abcdq";
  printf("%d\n", strncmp(a1, a2, 5));
  return 0;
}

#include <stdio.h>
#include <string.h>
int main()
{
  char arr1[] = "abcdef";
  char arr2[] = "bcde";
  char arr3[] = "bcb";
  printf("%s\n", strstr(arr1, arr2));
  printf("%s\n", strstr(arr1, arr3));
  return 0;
}

#include <stdio.h>
#include <string.h>
#include <assert.h>
char* my_strstr(const char* str1, const char* str2)
{
  assert(str1 && str2);
  if (*str2 == '\0')
    return (char*)str1;
  const char* s1 = NULL;
  const char* s2 = NULL;
  const char* cur = str1;
  while (*cur)
  {
    s1 = cur;
    s2 = str2;
    while (!(*s1 - *s2) && *s1 && *s2)
    {
      s1++;
      s2++;
    }
    if (!*s2)  // 如果上面的循环停止是因为*s2 == \0,那么匹配成功
      return (char*)cur;
    if ((int)strlen(cur) < (int)strlen(str2))
      return NULL;
    cur++;
  }
  return NULL;
}
int main()
{
  char arr1[] = "abbbce";
  char arr2[] = "bbc";
  printf("%s\n", my_strstr(arr1, arr2));
  return 0;
}

#include <stdio.h>
#include <string.h>
int main()
{
  printf("%s\n", strerror(0));
  printf("%s\n", strerror(1));
  printf("%s\n", strerror(2));
  printf("%s\n", strerror(3));
  printf("%s\n", strerror(4));
  printf("%s\n", strerror(5));
  return 0;
}