/* BabelCalc.java
 * Inspired by Jorge Luis Borges' short story, "The Library of Babel".
 * Calculates how many books there are in the Library of Babel
 * that differ from a given book by n characters.
 * Coded by Adam Lee (ebonmusings@gmail.com)
 * March 2006
 * See <http://www.daylightatheism.org/2006/03/how-big-is-the-library-of-babel.html>.
 * This code is released under a Creative Commons license:
 * http://creativecommons.org/licenses/by-nc-sa/2.5/
 */

import java.lang.Integer;
import java.math.BigInteger;

public class BabelCalc {

    /* Twiddle these knobs to set the parameters of the simulation. The values
       given below are as Borges originally defined them.
       NUM_CHARACTERS includes all possible punctuation marks (the original story
       stipulated a 22-letter alphabet, the comma, the period and the space). */

    public static final int NUM_PAGES = 410; /* number of pages per book */
    public static final int NUM_LINES = 40;  /* number of lines per page */
    public static final int NUM_POSITIONS = 80; /* number of positions per line */
    public static final int BOOK_LENGTH = NUM_PAGES * NUM_LINES * NUM_POSITIONS;
    public static final int NUM_CHARACTERS = 25; /* number of possible symbols for any position */
    public static final int BOOK_VOLUME = 2400; /* in cubic centimeters, see below */
    
    /* The observable universe is approximately 10^87 cubic centimeters in volume. */
    public static final int UNIVERSE_VOLUME_MAGNITUDE = 87;

    public static void main(String[] args) {
    	int n;
        if(args == null || args.length == 0) { /* ignore all arguments beyond the first */
	    System.out.println("Please enter a value for n.");
	    return;
        }
        try {
    	    n = Integer.parseInt(args[0]);
    	} catch(NumberFormatException excep) {
    	    System.out.println("That is not a valid number.");
    	    return;
    	}

	/* The reasoning behind this is as follows:
	 * The Library of Babel has BOOK_LENGTH * (NUM_CHARACTERS - 1) books that differ from
	 * a given book by at most 1 character - i.e., we can replace any one character at any
	 * position in the book with a different character.
	 * The number of 2-character variant books is this number, multiplied by
	 * (BOOK_LENGTH - 1) * (NUM_CHARACTERS - 1) - i.e., for each possible 1-character change,
	 * we can then replace any character at some other position in the book
	 * with a different character.
	 * The number of 3-character variant books is the number of 2-character variant books
	 * times (BOOK_LENGTH - 2) * (NUM_CHARACTERS - 1), and so on.
	 */
	BigInteger result = new BigInteger("1");
	for(int i = 0; i < n; i++) {
	    result = result.multiply(new BigInteger(""+((NUM_CHARACTERS - 1) * (BOOK_LENGTH - i))+""));
	}
	
	/* Insert commas correctly. */
	int power = result.toString().length();
	String bignum = commaize(result.toString());
	
	System.out.println("The Library of Babel contains "+bignum+" books that differ "+
	    "from a given book by exactly "+n+" characters. (Order of magnitude = 10^"+power+").");
	    
	/* Assumptions for volume calculation:
	   A sheet of paper is 0.1mm thick.
	   Each sheet is 8.5 x 11 inches, which works out to approximately 21.6 by 28 cm.
	   Given these facts, a 410-page book occupies a volume of approximately 2400cc. */
	
	result = result.multiply(new BigInteger(""+BOOK_VOLUME+""));
	power = result.toString().length();
	bignum = commaize(result.toString());
	
	System.out.println("It would require "+bignum+" cc to store "+
	    "all these books. (Order of magnitude = 10^"+power+").");
	if(power == UNIVERSE_VOLUME_MAGNITUDE) {
	    System.out.println("This many books would approximately fill the observable universe.");
	}
	else if(power > UNIVERSE_VOLUME_MAGNITUDE) {
	    System.out.println("This many books would more than fill the observable universe.");
	}
    }
    
    public static String commaize(String tmp) {
	StringBuffer bignum = new StringBuffer();
	int chars_copied = 0;
	for(int i = tmp.length()-1; i >= 0; i--) {
	    bignum.append(tmp.charAt(i));
	    chars_copied++;
	    if(chars_copied % 3 == 0 && i > 0) bignum.append(',');
	}
	/* Reverse in place. StringBuffer.reverse() doesn't do what you'd expect. */
	for(int i = 0; i < bignum.length()/2; i++) {
	    char c = bignum.charAt(i);
	    bignum.setCharAt(i, bignum.charAt(bignum.length() - (i+1)));
	    bignum.setCharAt(bignum.length() - (i+1), c);
	}
	return bignum.toString();
    }

} /* EOF */