%   Animals   (in TeX, no less!!!)
%
%   This is the program that uses a binary tree of questions to
%   guess the type of animal of which the user is thinking.
%
%   Andrew Marc Greene, March-April 1988
%
%%% modified May 1991, Michael Downes:
%%%   ---removed path names to leave bare file names
%%%   ---added animals2.tex so that you can repeat without
%%%      restarting TeX
%%%   ---added code so that lowercase and uppercase user
%%%      responses will both be recognized.
%
%   Moral support (i.e., ``You can't do that!  Show us!'')
%   provided by the Student Information Processing Board
%   of MIT.
%
% Instructions on running this program:
%
%     tex animals
%
% Think of an animal.  The program will try to guess your animal.
%
% You will be asked a whole bunch of yes/no questions.  This is a
% spartan implementation, so answer with a capital Y or N.  When
% the program finishes going through its tree, it will either have
% guessed your animal or it will ask you to enter a question that
% it can ask to differentiate between your animal and its guess.
% It will then ask you which one is `yes.'

% Here's where I declare all my variables, etc.
%
% ``curcode'' is the current index into the data file.
% ``temp'' is a temporary holding variable.
% ``lc'' is a loop counter
% ``ifamg'' is a general-purpose flag.  amg are my initials.
% ``ifreploop'' controls loop repetitions.
% ``ifmainlooprep'' controls repetitions of the main loop.
% ``inp'' is the input file.
% ``outp'' is the output file.
% ``amgY'' and ``amgN'' are character constants.  Why I did it this way I
%                        don't remember.
%
\newcount\curcode\curcode=1\newcount\temp\temp=0\newcount\lc
\newif\ifamg\newif\ifyn\newif\ifreploop\newif\ifmainlrep
\newread\inp\newwrite\outp\def\foo{}
\def\amgY{Y}\def\amgN{N}

% The data file consists of records stored in the following format:
%
% Record Number <newline>
% Question <newline>
% If-Yes-Goto-Record Number <newline>
% If-No-Goto-Record Number <newline>
%
% The following routine scans the data file until it reaches the
% record requested in \curcode
%
\def\Scan{%
{\loop
 \global\read\inp to \foo
 \ifnum\foo=\curcode\amgfalse\else\amgtrue\fi
 \ifamg\read\inp to \foo\read\inp to \foo\read\inp to \foo
\repeat}}

% The following routine displays the question and waits for a Y or N
% answer
%
\def\Query{%
{\read\inp to \foo
\immediate\write16{}%
\message{\foo}%
\GetYN
\ifyn
  \read\inp to \foo\global\curcode=\foo\read\inp to \foo
\else
  \read\inp to \foo\read\inp to \foo\global\curcode=\foo
\fi
}}

% The following routines deal with the user's input.
% \vread (verbatim read) ignores <newline>s and makes <space>s normal
% \GetYN gets input and repeats until it gets a Y or N response.
%
\def\vread#1{{\catcode`\^^M=9\catcode`\ =12\global\read-1 to #1}}
\def\first#1#2\end{#1}
%
\def\GetYN{%
{\loop
\vread{\bar}%
\edef\bar{\uppercase{\def\noexpand\bar{%
  \expandafter\first\bar\empty\end}}}\bar
\reploopfalse
\if Y\bar\global\yntrue\else
 \if N\bar\global\ynfalse\else\replooptrue\fi\fi
\ifreploop
\immediate\write16{Hey, you!  Answer Y or N, please.}%
\message{Please enter Y or N --> }%
\repeat
}}

% The following routine is called if the ``Goto-Record'' is -1,
% meaning that the program didn't guess correctly and is clueless.
% It gets the new animal and the differentiating question, and
% modifies the data file.  Actually, it makes a modified copy of
% the file, then copies the temporary new one over the old outdated
% one.
%
\def\NewAnimal{
\immediate\write16{Well, I'm stumped.  What animal did you have in mind?}
\vread{\usersanimal}
\immediate\write16{OK.  What question would let me tell the difference?}
\vread{\userquery}
\immediate\write16{Is the answer to that question Yes or No if I ask about}
\message{\usersanimal? }
\GetYN
\curcode=-1
\Scan
\read\inp to \lastcode\lc=\lastcode
\closein\inp
%
% Open up the files.  These names are system-dependent.   *FLAG*
%
\openin\inp=animal.dat
\ifeof\inp \errmessage{Unable to open animal.dat!}\fi
\immediate\openout\outp=newaniml.dat
%
% Read through the inp file, copying all records that don't need to
% be changed, outputting modified versions of the changed ones (and
% discarding the old), and appending the new records.
%
{\loop
 \read\inp to \foo
 \amgtrue
 \ifnum\foo=\temp\amgfalse\fi
 \ifnum\foo=-1 \amgfalse\fi
 \ifamg\immediate\write\outp{\foo}%
  \read\inp to \foo\immediate\write\outp{\foo}%
  \read\inp to \foo\immediate\write\outp{\foo}%
  \read\inp to \foo\immediate\write\outp{\foo}%
  \amgtrue
 \else\ifnum\foo=\temp
  \immediate\write\outp{\foo}%
  \immediate\write\outp{\userquery}%
  \immediate\write\outp{\number\lc}%
  \global\advance\lc by 1
  \immediate\write\outp{\number\lc}%
  \read\inp to \animal\read\inp to \foo\read\inp to \foo
  \amgtrue
 \else
  \lc=\lastcode
  \ifyn\WriteUsers\WriteAnimal
  \else\WriteAnimal\WriteUsers
  \amgfalse\fi
 \fi\fi
 \ifamg
\repeat}
\closein\inp
\immediate\write\outp{-1}
\immediate\write\outp{\number\lc}
\immediate\closeout\outp
%
% Now copy the temporary file over the original one
%
% These filenames are also system-dependent.           *FLAG*
%
\openin\inp=newaniml.dat
\ifeof\inp \errmessage{Unable to open newaniml.dat!}\fi
\immediate\openout\outp=animal.dat
{\endlinechar=-1
  \loop
  \read\inp to \foo
  \immediate\write\outp{\foo}
  \amgtrue
  \ifeof\inp\amgfalse\fi
 \ifamg\repeat}
\immediate\closeout\outp
}
%
% This routine is called by NewAnimal and writes the record for
% the user's new animal
%
\def\WriteUsers{
\immediate\write\outp{\number\lc}
\immediate\write\outp{Is it \usersanimal?}
\immediate\write\outp{0}
\immediate\write\outp{-1}
\global\advance\lc by 1 }

% This one writes the modified old animal
%
\def\WriteAnimal{
\immediate\write\outp{\number\lc}
\immediate\write\outp{\animal}
\immediate\write\outp{0}
\immediate\write\outp{-1}
\global\advance\lc by 1 }

% Now we get into the main routine.
% It simply repeats the scan-query loop until it gets a 0 (right answer)
% or a -1 (wrong answer, I'm stumped), and calls the appropriate routine.
%
\def\maybecontinue{\input animals2.tex
  \maybecontinue}

\maybecontinue

% Ah, the joys of a job well-done.  We can now exit to the system, knowing
% that the world is a slightly better place for our efforts.
%
% The following line of code, probably the most profound in the entire
% program, sums up this philosophy of life in four characters.  The
% Puritan work ethic is embodied in this amazingly meaning-laden
% command designed by Donald Knuth.

\end