string库函数的实现

练手代码。已测试。

包含 strlen, rev, strcat, strcmp, strstr, itoa

#include <stdio.h>
#include <assert.h>
#define SWAP(a,b) (a)^=(b);(b)^=(a);(a)^=(b)
int mystrlen(const char* src){
    assert( src!=NULL);
    char* ptr = src;
    while(*ptr){
         ptr++;
    }
    return ptr- src;
}
char * mystrcat(char* src, const char* dest){
    assert( src!=NULL && dest != NULL);
        char* ret = src;
          while(*src) src++;
        while(*src++ = *dest++);
        return ret;
}
char *myrev(char* str){
        assert(str!=NULL);
        char* ret, *start ;
        ret = start = str;
    while(*str) str++;
        str--;

        while(start<str){
          SWAP(*start, *str);
          start++,str--;
        }
        return ret;
}
int mystrcmp(const char* src, const char* dest){
    assert( src!=NULL && dest != NULL);
    while(*src && *dest && *src == *dest) src++, dest++;
        return *src - *dest;
}
char *myitoa(int num, char* buf){
        assert(buf!=NULL);
        char * ret = buf;
        int isNegtive = 0;
        if(num<0){
             num*=-1;
             isNegtive = 1;
        }
        do{
           *buf++ = num%10 + '0';
        }while(num/=10);
        // negtive
        if(isNegtive) *buf++ = '-';
        *buf = '\0';
        return myrev(ret);
}
char* mystrstr(const char* src, const char* dest){
      assert( src!=NULL && dest != NULL);
      if(!*dest) return src;
      while(*src){
           int i = 0;
           for(;*(src+i)==*(dest+i);i++){
                if(!*(dest+i+1))
                      return src;
           }
           src++;
      }
      return NULL;

}
int main(){
    char test[10] = "abcd";
    printf("strlen: %d\n", mystrlen(test));
    printf("strcat %s\n", mystrcat(test,"xyz"));
    printf("myrev %s\n", myrev(test));
    printf("mystrcmp %d\n",mystrcmp("test",test));
    printf("mystrcmp %d\n",mystrcmp(test,"zyxdcba"));
    printf("mystrstr %s\n",mystrstr(test, "bc"));
    printf("myitoa %s \n", myitoa(-124, test));
}