// functions that can move the file position conveniently
// remember that the last line of a file is usually empty and goes from '\n' to EOF

#include "move_lines.h"
#include <unistd.h>

// function to move back/up
void go_back_x_lines(int fd, int lines) {
  for (int i = 0; i < lines; i++) {
    seek_previous_line(fd);
  }
}

void seek_previous_line(int fd) {
  seek_line_start(fd);
  lseek(fd, -1, SEEK_CUR);
  seek_line_start(fd);
}

// set file position to the beginning of the line
void seek_line_start(int fd) {
  char cur_char = '\0';

  while (cur_char != '\n') {
    int ret_lseek = lseek(fd, -1, SEEK_CUR);
    if (ret_lseek == -1)
      break;
    
    read(fd, &cur_char, 1);
    if (cur_char != '\n')
      lseek(fd, -1, SEEK_CUR);
  }
}

// function to move forward/down
void go_forward_x_lines(int fd, int lines) {
  for (int i = 0; i < lines; i++) {
    seek_next_line(fd);
  }
}

void seek_next_line(int fd) {
  seek_line_end(fd);
  lseek(fd, 1, SEEK_CUR);
}

// set file position to the end of the line
void seek_line_end(int fd) {
  char cur_char = '\0';

  while (cur_char != '\n') {
    // return code 0 of read indicates EOF
    if (read(fd, &cur_char, 1) == 0)
      break;
  }
  
  lseek(fd, -1, SEEK_CUR);
}