c htoi

/*  * The C Programming Language, second edition, * by Brian Kernighan and Dennis Ritchie * * Exercise 2-3, page 46 * * Write the function htoi(s), which converts a string of hexadecimal * digits (including an optional 0x or 0X) into its equivalent integer * value. The allowable digits are 0 through 9, a through f, and A * through F.  *//* htoi:  convert hexdicimal string s to integer */int htoi(char s[]){    int i;    int c, n;    n = 0;    for (i = 0; (c = s[i]) != '\0'; ++i) {        n *= 16;        if (i == 0 && c == '0') {            /* Drop the 0x of 0X from the start of the string. */            c = s[++i];            if (c != 'x' && c != 'X')                --i;        } else if (c >= '0' && c <= '9')            /* c is a numerical digit. */            n += c - '0';        else if (c >= 'a' && c <= 'f')            /* c is a letter in the range 'a'-'f' */            n += 10 + (c - 'a');        else if (c >= 'A' && c <= 'F')            /* c is a letter in the range 'A'-'F' */            n += 10 + (c - 'A');        else            /* invalid input */            return n;    }    return n;}