#include <iostream>
#include <fstream>

using namespace std;

#define ROWS 23
#define COLUMNS 80

//this version does not print the percentage
//of bytes seen but it is trivial to add

struct undo {
  bool canundo;   //can we undo?
  int seek_last;  //last page action before current action
  int seek_pos;   //last page action 
  int seek_end;   //end of last page
  int line_skip;  //lines to skip since seek_last
  int line_count; //lines counted since seek_last

  undo() {
    canundo=false;
    seek_last=seek_pos=seek_end=line_skip=line_count=0;
  }
};

void do_more_of(istream& is);
int get_user_input(istream& is, ifstream& tty, undo& myundo);

int main( int argc , char *argv[] ) {
  if (argc==1)
    do_more_of(cin);
  else 
    while (0<--argc) {
      ifstream is(*++argv); 
      if (is.good()) {
	do_more_of(is) ;
	is.close();
      }
      else
	cout<<"skipping "<<*argv<<"\n";
    }
  return 0;
}

void do_more_of(istream& is) {
  undo myundo;
  char line[COLUMNS+1]; // buffer to store line of input
  int num_of_lines = ROWS; // # of lines left on screen
  ifstream tty("/dev/tty");
  if (!tty.good()) {
    exit(1);
  }
  while (true) { 
    if (num_of_lines==0) {
      myundo.seek_end=is.tellg(); //end of current page
      num_of_lines = get_user_input(is, tty, myundo);
      if (num_of_lines==-1)
	exit(1); //quit
      if (num_of_lines==-2)
	return; //next file
    }
    else {
      is.getline(line, COLUMNS+1);
      if (is.fail() && !is.eof()) //if line is long, clear error bit
	is.clear();
      cout<<line<<"\n";
      if (is.eof()) {
	return;
      }
      num_of_lines--;
    }
  }
}

int get_user_input(istream& is, ifstream& tty, undo& myundo) {
  char c[2];
  cout<<"\033[7m more? \033[m";
  cout.flush(); //make sure to output above message
  tty.getline(c,2); 
  if (c[0]=='\0') { //line action
    myundo.line_skip=myundo.line_count;
    myundo.line_count++;
    myundo.seek_last=myundo.seek_pos;
    myundo.canundo=true;
    //optimization option for not going back too far
    if (myundo.line_skip==COLUMNS) { 
      myundo.line_skip=0;
      myundo.line_count=1;
      myundo.seek_last=myundo.seek_pos=myundo.seek_end; //end of last page
    }
    return 1;
  }
  if (c[0]==' ') { //space
    myundo.line_skip=myundo.line_count;
    myundo.line_count=0; //reset line count
    myundo.seek_last=myundo.seek_pos;
    myundo.seek_pos=is.tellg(); //update last page action
    myundo.canundo=true;
    return ROWS;
  }
  if (c[0]=='b')  //undo
    if (myundo.canundo) {
      is.seekg(myundo.seek_last); //go back to the last page
      char line[COLUMNS+1]; 
      for (int i=0; i<myundo.line_skip; i++) {//skip enough lines 
	is.getline(line, COLUMNS+1);
	if (is.fail() && !is.eof()) 
	  is.clear();
      }
      myundo.seek_last=is.tellg(); //set last page action
      myundo.seek_pos=myundo.seek_last;
      myundo.line_skip=0;
      myundo.line_count=0;
      myundo.canundo=false; //can't undo anymore
      return ROWS;
    }
  if (c[0]=='q') 
    return -1;
  if (c[0]=='n')  //skip to next file
    return -2;
  return 0;
}