#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define DEFAULT_STRING_NUM 10
#define MAXLEN 1000
#define MAXLINES 5000

char *lineptr[MAXLINES];

int getline(char *, int);
char *smartalloc(int n, char *begin);
int readlines(char *linestack[], int tail_num);
void writelines(char *lineptr[], int tail_num, int nlines);
void writespecific(char *lineptr[]);

int main(int argc, char *argv[])
{
    int num, nlines;

    if (argc == 3 && strcmp(*(argv+1),"-n") == 0) {
        num = atoi(*(argv+2));
        if (num < 1) {
            printf("usage: 'tail -n COUNT_LINES' \n");
            return -1;
        }
        else if (num > MAXLINES) {
            printf("n must be in {1..%d}\n",MAXLINES);
            return -1;
        }
    }
    else if (argc == 1)
        num = DEFAULT_STRING_NUM;
    else {
        printf("usage: 'tail -n COUNT_LINES' \n");
        return -1;
    }
    if ((nlines=readlines(lineptr, num)) >= 0) {
        num = (num > nlines) ? nlines : num ; /* if in stdio lines less, than user want (num) */
        writelines(lineptr, num, nlines);
        return 0;
    } else {
        printf("error: input too big to tail\n");
        return -1;
    }
}

/* readlines: */
int readlines(char *lineptr[], int tail_num)
{
    int len;                   /* length string getted from getline */
    int nlines = 0;            /* total count strings ( <= tail_num )*/
    char *p;                   /* pointer to current free position (returned by smartalloc) */
    char *leftpos = NULL;      /* pointer to beginning interest block (use in smartalloc) */
    char *rightpos = NULL;     /* pointer to end interest block (calculate as p+len)  */
    char line[MAXLEN];         /* current string */

    while ((len = getline(line, MAXLEN)) > 0) {
        if ((p = smartalloc(len+1, leftpos)) == NULL) { /* if buffer overfull */
            return -1;
        }
        else {
            /* check is `memory moved`? if next pointer not "next" */
            if (rightpos+1 > p) {
                /* moving interest pointers value */
                for (int i = 0; i < tail_num; i++)
                    lineptr[nlines-i-1] -= rightpos - p + 1;
                    /* (rightpos - p + 1) - moving 'diff' (in pointers) */
            }
            /* copying string */
            line[len] = '\0'; /* delete \n in line */
            strcpy(p, line);
            lineptr[nlines++] = p;
            if (nlines <= tail_num)
                leftpos = lineptr[0]; /* first element */
            else
                leftpos = lineptr[nlines-tail_num]; /* first interest element */
            rightpos = p + len;
        }
    }
    return nlines;
}

void writelines(char *lineptr[], int tail_num, int nlines)
{
    for (int i = 0; i < tail_num; i++)
        printf("%s\n",lineptr[nlines-tail_num+i]);
}