strtol wzor

#include <assert.h> /* This library contains assert function*/
#include <ctype.h> /* This library contains functions: isspace, isdigit, isalpha*/
#include <errno.h>  /* variable errno is defined in this library */
#include <limits.h> /*LONG_MIN, LONG_MAX */


long strtol(char* nptr, char **endptr , int base)
{/**
\brief This is my strtol function. It converts strings to long integers
*@param nptr This string points to a character string for function to convert
*@param endptr This is resulting parameter that normally indicates NULL, but if strtol is unable to convert something endptr points to the first unconverted character
*@param base This is the base of the string; it's between 2 and 36
*/
char *string;  /*points at the character being currently converted*/
long number = 0; /*the number returned by this function*/
int sign; /*sign of the number (1-negative, 0-positive)*/
int digit; /* current digit added to the number */
int isoutside = 0; /* variable useful in conditions of number being outside the limit */

assert(nptr); /* quits if string is empty (if the value of nptr is NULL)*/
string = nptr; /* string points to the same place as nptr*/

while(isspace(*string)) /* checks if there are white signes (spaces, new lines etc.) at the beginning of the stringand ommits them */
{
    string++;
}
/* checks the sign of the number */
if(*string=='-')
{
    sign = 1; /* negative number*/
    string++;
}
else
{
    if(*string=='+')
    {
        sign = 0; /*positive number*/
        string++;
    }
    else sign = 0; /*if there's no '+' or'-' sign we assume that the number is positive*/
}
/* Checks the bases and range of digits/letters for bases 8 and 16*/
if((base==8)&&(*string=='0'))
    string++;
if((base==16)&&(*string=='0'))
{
    string++;
    if((*string=='x')||(*string=='X'))
        string++;
    else string--;
}
if((*string=='0')&&(base==0))
{ /*if the base=0 and the first nonwhite character is 0 then the base is 8*/
    base = 8;
    string++;
    if((*string=='x')||(*string=='X'))/*if the base is 0 and the first two nonwhite characters are 0x or 0X then the base is 16*/
    {
        base = 16;
        string++;
    }

}
else if(base==0) /*if the base is 0 and the beginning of string is not 0, 0x or 0X then the base is 10*/
    base = 10;
if(base <2 || base >36)
{
    errno=EINVAL;/*if the base is outside the interval [2;36] then the string will not be converted and will return error-invalid argument*/
    return 0;
}
if((base==8)&&((*string<'0')||(*string>'7')))
/*checks if the values of the string are correct for the base 8: digits 0-7*/
{
    *endptr = string;
    return 0;
}
if((base==16)&&((*string<'0')||((*string>'9')&&(*string<'A'))||((*string>'F')&&(*string<'a'))||(*string>'f')))
/*checks if the values of the string are correct for the base 16: digits 0-9, letters A-F, a-f*/
{
    string--;
    *endptr = string;
    return 0;
}

while(*string!='\0') /* This loop terminates when string points to the end of the file*/
{
    if (isdigit(*string))/* if string does not point to a digit isdigit returns 0, otherwise it returns this digit*/
    {
        digit = *string - '0'; /*converting ASCII code to corresponding digit*/
    }
    else
    {
        if(isalpha(*string)) /* isalpha returns 0 if string does not point to a letter, otherwise it returns that letter */
        {
            if(islower(*string)) /* checks if the string points to the lower case letter */
            {
                digit = (*string - 'a') + 10; /* converts lower case letters in ASCII code to corresponding digits in numerical systems */
            }
            else
                digit = (*string - 'A') + 10; /* converts upper case letters in ASCII code to corresponding digits in numerical systems */
        }
        else /* loop is terminated if string does not point to digits or letters */
        {
            break;
        }
    }
    switch(sign)
    {
    case 0:
    {
        if (isoutside < 0 || number > ((LONG_MAX-digit)/base) /*|| (number == limit && (int) digit > limit_mod)*/)
        {/* checks if the number is inside the limits for long int, if not isoutside=-1 and the string proceedes to the end of the file */
            isoutside =-1;
            string++;
        }
        else /* if the number is inside the limits then isoutside=1 and current digit is attached to the number */
        {
            isoutside = 1;
            number = (number*base)+digit;
            string++;
        }
        break;

    }
    case 1:
    {
        if (isoutside < 0 || number < ((LONG_MIN+digit)/base))
        {/* checks if the number is inside the limits for long int, if not isoutside=-1 and the string proceedes to the end of the file */
            isoutside =-1;
            string++;
        }
        else /* if the number is inside the limits then isoutside=1 and current digit is attached to the number */
        {
            isoutside = 1;
            number = (number*base)-digit;
            string++;
        }

        break;
    }
    }
}
if (isoutside<0)/* if isoutside=-1 then the number is outside limits so function will return LONG_MIN or LONG_MAX depending on sign */
        {
            number = sign ? LONG_MIN : LONG_MAX;
            errno = ERANGE; /* error-value outside range */
        }

if (endptr != '\0') /* if endpointer is something other than NULL then for isoutside=1 (number inside limits) it points to string (current character) and for isoutside=-1 (number outside limits) it points to nptr (the whole string) */
    *endptr = isoutside ? string : (char *) nptr;

return number; /* function returns the converted number */
}