#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/syscall.h>
 
void print_labels(int i,char* argv[i],int error){
//------------------------------------------------------------------------
// FUNCTION: print_labels(int i,char* argv[i],int error)
// output the labels when we have more than 1 file
// PARAMETERS:
// int i - the current argument number
// char* argv[i] - the filename
// int error - if an error occured up to now
//------------------------------------------------------------------------
    char msg[1024] = "==> ";
 
    if(i >= 2 && error == 0) write(STDOUT_FILENO, "\n", 1); // adds new line
    if(*argv[i] == '-') argv[i] = "standart input"; // the  output when we have - should be ==> stand. input <==
 
    int compare = strcmp(argv[i],"tester.txt"); // argv[i] ?= tester.txt
    if(compare != 0){
        strcat(strcat(msg,argv[i])," <==\n"); // concatenate msg and argv[i] and the result to last string
        write(STDOUT_FILENO, msg, strlen(msg)); // output msg
        argv[i] = "tester.txt";
    }
}
 
void close_error(int i,char* argv[i]){
//------------------------------------------------------------------------
// FUNCTION: close_error(int i,char* argv[i])
// output message on close error
// PARAMETERS:
// int i - the current argument number
// char* argv[i] - the filename
//------------------------------------------------------------------------
    char msg[1024] = "tail: error reading '";
    strcat(msg,argv[i]); // concatenate msg and argv[i]
    write(2, msg, strlen(msg)); //output msg
    perror("'"); // print perror
}
 
void read_error(int i,char* argv[i]){
//------------------------------------------------------------------------
// FUNCTION: read_error(int i,char* argv[i])
// output message on read error, which is the same as close error
// PARAMETERS:
// int i - the current argument number
// char* argv[i] - the filename
//------------------------------------------------------------------------
    close_error(i,argv);
}
 
void open_error(int i,char* argv[i]){
    //------------------------------------------------------------------------
    // FUNCTION: open_error(int i,char* argv[i])
    // output message on file open error
    // PARAMETERS:
    // int i - the current argument number
    // char* argv[i] - the filename
    //------------------------------------------------------------------------
    char msg[1024] = "tail: cannot open '";
    strcat(strcat(msg,argv[i]),"' for ");// concatenate msg and argv[i]
    write(2, msg, strlen(msg)); // output msg
    perror("reading"); // print perror
}
 
 
void standart_read(int fd){
    //------------------------------------------------------------------------
    // FUNCTION: standart_read(int fd)
    // read from the terminal and write the input to a file
    // PARAMETERS:
    // int fd - the file descriptor
    //------------------------------------------------------------------------
    char data[10];
    while(read(0,data,1) != 0){ // 0 indicated EOF
        for(size_t i = 0,len =1; i< len;i++){
            size_t wres = write(fd,data+i,1);
            if(wres < 0) break; // output
            else if(wres == 0) write(fd,data+i,1);
 
        }
    }
}
 
int file_output(int count,char *buff, int fd){
    //------------------------------------------------------------------------
    // FUNCTION: file_output(int count,char *buff, int fd)
    // read from the terminal and write the input to a file
    // PARAMETERS:
    // int fd - the file descriptor
    // int count - number of bytes to be written
    // char* buff -  the buffer for read and write
    //------------------------------------------------------------------------
    count *= -1; // get abs value of bytes
    while(count--){
        ssize_t wres = write(STDOUT_FILENO,buff,read(fd, buff,1)); // output file content
	if(wres == 0){
		do{
		wres = write(STDOUT_FILENO,buff,1);
		}while(wres==0);
	}
        if(wres < 0) {
            perror("tail: error writing 'standard output'"); // output error
            return -1;
        }
    }
    return 0;
}
 
int go_to_tenth_line(int fd,int i, char* argv[]){
    //------------------------------------------------------------------------
    // FUNCTION: go_to_tenth_line(int fd,int i, char* argv[]){
    // move with lseek to the 10th \n from down to up
    // PARAMETERS:
    // int fd - the file descriptor
    // int i - the current argument number
    // char* argv[i] - the filename
    //------------------------------------------------------------------------
    int count,line_count = 10;
    ssize_t rres;
    char buff[1];
    off_t offset;
 
    // go to 10th line from downward          
    for(count = -1;line_count != 0;count--){
        offset = lseek(fd, count, SEEK_END); // go to current byte
        if(offset == -1) break;  // error occured
        if(count != 0) rres = read(fd,buff,1); // read one byte
        if(rres < 0){
            read_error(i,argv); // output read error
            return 1; // error
        }
        if(*buff =='\n') if(count != -1) line_count--; // if found new line char, update how much lines are left
    }
   
    if (*buff != '\n') write(STDOUT_FILENO, buff, 1); // add last char if file is less than 10 lines
    return count;
}
 
int standart_input(int i, char* argv[i]){
    //------------------------------------------------------------------------
    // FUNCTION: standart_input(int i, char* argv[i])
    // get terminal input and output it to a file, then print the last 10 lines
    // PARAMETERS:
    // int i - the current argument number
    // char* argv[i] - the filename
    //------------------------------------------------------------------------
    int fd = open("tester.txt",O_CREAT | O_RDWR , 0664 );  // open file to which the standart input was outputed to
    standart_read(fd); //read from the terminal and write the input to a file
 
    char buff[1];
    int count = go_to_tenth_line(fd,i,argv); // move with lseek to the 10th \n from down to up
 
    if(file_output(count,buff,fd) < 0 || count > 0) return -1; // print last 10 lines;error occured
    if(close(fd) <0) close_error(i,argv);   // close file; error occured
 
    return 0;
}
 
 
int main(int argc,char* argv[]){
    int i = 1,error = 0;
    char buff[1];
 
    if(argc == 1 || ( *argv[i] == '-' && argc == 2 )){ // if there is only - or nothing
        standart_input(++i,argv); // takes care of - input
        return 0;
    }
 
   
    while(i <= argc-1){
        while(*argv[i] == '-'){
            print_labels(i,argv,error); // output messages when more than 2 files
            if(standart_input(++i,argv) < 0) return -1;
            if(i == argc) return 0;
        }
 
        int fd = open(argv[i],O_RDONLY);  // file is opened
 
        while(fd < 0){ // while file can't be opened, go for the next
            open_error(i,argv); // output error mssg
            error = 1;
            if(i++ == argc-1) return -1;
            fd = open(argv[i],O_RDONLY);
        }
 
        if(argc > 2) print_labels(i,argv,error); // output messages when more than 2 files
 
        int count = go_to_tenth_line(fd,i,argv);
        if(count > 0) return -1; // error occured
        if(file_output(count,buff,fd)< 0) return -1; // error
        if(close(fd) <0) close_error(i,argv);       // close file
        i++;
    }
 
    return 0;
}