String (1)

The relationship between String, Array and Character is a very important subject in any Firmware interview. Here are some common questions and example solution. 

Copy string A to string B (consider A and B might overlap)

char* copy(char *d, char *s)
{
    int n = strlen(s);
    char *ret = d;
    if ((d - s) >= 0) {
        d += n;
        s += n;
        for ( ; n >= 0; n--)
            *d-- = *s--;
    }
    else
        while (*d++ = *s++);
 
    return ret;
}

Reverse a sentense: Ex."I am a real boy!" to "boy! real a am I"
void reverse_string(char *str)
{
    if (!str)
        return;
 
    int n = strlen(str);
    char *c, *tmp;
 
    c = str;
    reverse(str, str + n - 1);
 
    while (1) {
        for (; *c == ' ' && *c != '\0'; c++);
        if (*c == '\0')
            return;
        tmp = c;
 
        for (; *c != ' ' && *c != '\0'; c++);
        reverse(tmp, c - 1);
        if (*c == '\0')
            return;
    }
}
 
void reverse(char *c1, char *c2)
{
    while (c2 > c1) {    // It is simply to reverse a string        char tmp = *c1;      // ex. 'A string' -> 'gnirts A'
        *c1++ = *c2;
        *c2-- = tmp;
    }
}

Remove Duplicate Characters in a string: Ex."hiiii!Iammmbackkkk" to "hi!Iamback"
void remove_dup(char *str)
{
    char pre_c = str[0] + 1;// just to let pre_c be different  
                            // with str[0]. The same as *str + 1;
    char *c1, *c2;
 
    for (c1 = c2 = str; *c2 != '\0'; ) {
        if (*c2 == pre_c) {
            c2++;       // use c2 to compare contiguous chars         }               // If the same, skip it by adding 1
        else {
            pre_c = *c2;   // If not the same, 
            *c1++ = *c2++; // put the char into string c1.        }
    }
    *c1 = '\0';            // the end of the string
}