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\begin{document}

\section*{Main Part 5 (Scala, 9 Marks)}

\mbox{}\hfill\textit{``If there's one feature that makes Scala, `Scala',}\\
\mbox{}\hfill\textit{ I would pick implicits.''}\smallskip\\
\mbox{}\hfill\textit{ --- Martin Odersky (creator of the Scala language)}\bigskip\bigskip


\noindent
This part is about a small (esoteric) programming language called
brainf***. We will implement an interpreter and compiler for
this language.\bigskip

%\IMPORTANT{This part is worth 10\% and you need to submit it on \cwTEN{} at 5pm.
%Any 1\% you achieve counts as your ``weekly engagement''.}
\IMPORTANTNONE{}

\noindent
Also note that the running time of each part will be restricted to a
maximum of 30 seconds on my laptop.

\DISCLAIMER{}
\newpage

\subsection*{Reference Implementation}

As usual, this Scala assignment comes with a reference implementation in
form of two \texttt{jar}-files. You can download them from KEATS. They
allow you to run any test cases on your own computer. For example you
can call Scala on the command line with the option \texttt{-cp bf.jar}
and then query any function from the \texttt{bf.scala} template file.
You have to prefix the calls with \texttt{M5a} and \texttt{M5b},
respectively. For example


\begin{lstlisting}[language={},xleftmargin=1mm,numbers=none,basicstyle=\ttfamily\small]
$ scala -cp bf.jar
scala> import M5a._  
scala> run(load_bff("sierpinski.bf")) ; ()
                               *
                              * *
                             *   *
                            * * * *
                           *       *
                          * *     * *
                         *   *   *   *
                        * * * * * * * *
                       *               *
                      * *             * *
                     *   *           *   *
                    * * * *         * * * *
                   *       *       *       *
                  * *     * *     * *     * *
                 *   *   *   *   *   *   *   *
                * * * * * * * * * * * * * * * *
               *                               *
              * *                             * *
             *   *                           *   *
            * * * *                         * * * *
           *       *                       *       *
          * *     * *                     * *     * *
         *   *   *   *                   *   *   *   *
        * * * * * * * *                 * * * * * * * *
       *               *               *               *
      * *             * *             * *             * *
     *   *           *   *           *   *           *   *
    * * * *         * * * *         * * * *         * * * *
   *       *       *       *       *       *       *       *
  * *     * *     * *     * *     * *     * *     * *     * *
 *   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
\end{lstlisting}%$

\newpage

\subsection*{Part A (5 Marks)}

Coming from Java or C++, you might think Scala is a rather esoteric
programming language.  But remember, some serious companies have built
their business on
Scala.\footnote{\url{https://en.wikipedia.org/wiki/Scala_(programming_language)\#Companies}}
I claim functional programming is not a fad.  And there are far, far
more esoteric languages out there. One is called \emph{brainf***}. 
\here{https://esolangs.org/wiki/Brainfuck}
You
are asked in this part to implement an interpreter for this language.

Urban M\"uller developed the original version of brainf*** in 1993.  A close
relative of this language was already introduced in 1964 by Corado
B\"ohm, an Italian computer pioneer. The main feature of brainf*** is
its minimalistic set of instructions---just 8 instructions in total
and all of which are single characters. Despite the minimalism, this
language has been shown to be Turing complete\ldots{}if this doesn't
ring any bell with you: it roughly means that every(!) algorithm can,
in principle, be implemented in brainf***. It just takes a lot of
determination and quite a lot of memory resources.

Some relatively sophisticated sample programs in brainf*** are given
in the file \texttt{bf.scala}, including a brainf*** program for the
Sierpinski triangle and the Mandelbrot set.  There seems to be even a
dedicated Windows IDE for bf programs, though I am not sure whether
this is just an elaborate April fools' joke---judge yourself:

\begin{center}
\url{https://www.microsoft.com/en-us/p/brainf-ck/9nblgggzhvq5}
\end{center}  \bigskip


\noindent
As mentioned above, the original brainf*** has 8 single-character
commands. Our version of bf will contain the commands \texttt{'>'},
\texttt{'<'}, \texttt{'+'}, \texttt{'-'}, \texttt{'.'}, \texttt{'['}
and \texttt{']'}.  Every other character
is considered a comment.

Our interpreter for bf operates on memory cells containing
integers. For this it uses a single memory pointer, called
\texttt{mp}, that points at each stage to one memory cell.

\begin{center}
\begin{tikzpicture}
  \draw [line width=1mm, rounded corners] (0,0) rectangle (5, 0.5);
  \draw (0.5, 0) -- (0.5, 0.5);
  \draw (1.0, 0) -- (1.0, 0.5);

  \draw (2.5, 0) -- (2.5, 0.5);
  \draw (2.0, 0) -- (2.0, 0.5);

  \draw (4.5, 0) -- (4.5, 0.5);
  \draw (4.0, 0) -- (4.0, 0.5);

  \draw (1.5,0.25) node {$\cdots$};
  \draw (3.0,0.25) node {$\cdots$};

  \draw [->, thick] (2.25, -0.5) -- (2.25, -0.15);
  \draw (2.25,-0.8) node {\texttt{mp}};

  \draw (0.7,0.7) node {\sf\footnotesize memory};
\end{tikzpicture}    
\end{center}  

\noindent
This pointer can be moved forward by one memory cell by using the
command \texttt{'>'}, and backward by using \texttt{'<'}. The commands
\texttt{'+'} and \texttt{'-'} increase, respectively decrease, by 1
the content of the memory cell to which the memory pointer currently
points to. The command for output in bf is \texttt{'.'} whereby output works
by reading the content of the memory cell to which the memory pointer
points to and printing it out as an ASCII character.\footnote{In the
  original version of bf, there is also a command for input, but we
  omit it here. All our programs will be ``autonomous''.}  The
commands \texttt{'['} and \texttt{']'} are looping
constructs. Everything in between \texttt{'['} and \texttt{']'} is
repeated until a counter (memory cell) reaches zero.  A typical
program in brainf*** looks as follows:

\begin{center}
\begin{verbatim}
   ++++++++[>++++[>++>+++>+++>+<<<<-]>+>+>->>+[<]<-]>>.>---.++
   +++++..+++.>>.<-.<.+++.------.--------.>>+.>++.
\end{verbatim}
\end{center}  

\noindent
This one prints out Hello World\ldots{}obviously \texttt{;o)} 


\subsubsection*{Tasks (file bf.scala)}

\begin{itemize}
\item[(1)]  Write a function that takes a filename (a string) as an argument
  and requests the corresponding file from disk. It returns the
  content of the file as a string. If the file does not exists,
  the function should return the empty string.
  \mbox{}\hfill[0.5 Marks]
  
\item[(2)] Brainf**k memory is represented by a \texttt{Map} from
  integers to integers. The empty memory is represented by
  \texttt{Map()}, that is nothing is stored in the
  memory; \texttt{Map(0 -> 1, 2 -> 3)} stores \texttt{1} at
  memory location \texttt{0}, and at \texttt{2} it stores \texttt{3}. The
  convention is that if we query the memory at a location that is
  \emph{not} defined in the \texttt{Map}, we return \texttt{0}. Write
  a `safe-read' function, \texttt{sread}, that takes a memory (a \texttt{Map}) and
  a memory pointer (an \texttt{Int}) as arguments, and `safely' reads the
  corresponding memory location. If the \texttt{Map} is not defined at
  the memory pointer, \texttt{sread} returns \texttt{0}.

  Write another function \texttt{write}, which takes a memory, a
  memory pointer and an integer value as arguments and updates the
  \texttt{Map} with the value at the given memory location. As usual,
  the \texttt{Map} is not updated `in-place' but a new map is created
  with the same data, except the new value is stored at the given memory
  pointer.\hfill[0.5 Marks]

\item[(3)] Write two functions, \texttt{jumpRight} and
  \texttt{jumpLeft}, that are needed to implement the loop constructs
  in brainf**k. They take a program (a \texttt{String}) and a program
  counter (an \texttt{Int}) as arguments and move right (respectively
  left) in the string in order to find the \textbf{matching}
  opening/closing bracket. For example, given the following program
  with the program counter indicated by an arrow:

  \begin{center}
  \texttt{--[\barbelow{.}.+>--].>.++}
  \end{center}

  then the matching closing bracket is in 9th position (counting from 0) and
  \texttt{jumpRight} is supposed to return the position just after this
  
  \begin{center}
  \texttt{--[..+>--]\barbelow{.}>.++}
  \end{center}

  meaning it jumps to after the loop. Similarly, if you are in 8th position,
  then \texttt{jumpLeft} is supposed to jump to just after the opening
  bracket (that is jumping to the beginning of the loop):

  \begin{center}
    \texttt{--[..+>-\barbelow{-}].>.++}
    \qquad$\stackrel{\texttt{jumpLeft}}{\longrightarrow}$\qquad
    \texttt{--[\barbelow{.}.+>--].>.++}
  \end{center}

  Unfortunately we have to take into account that there might be
  other opening and closing brackets on the `way' to find the
  matching bracket. For example in the brain*ck program

  \begin{center}
  \texttt{--[\barbelow{.}.[+>]--].>.++}
  \end{center}

  we do not want to return the index for the \texttt{'-'} in the 9th
  position, but the program counter for \texttt{'.'} in 12th
  position. The easiest to find out whether a bracket is matched is by
  using levels (which are the third argument in \texttt{jumpLeft} and
  \texttt{jumpLeft}). In case of \texttt{jumpRight} you increase the
  level by one whenever you find an opening bracket and decrease by
  one for a closing bracket. Then in \texttt{jumpRight} you are looking
  for the closing bracket on level \texttt{0}. For \texttt{jumpLeft} you
  do the opposite. In this way you can find \textbf{matching} brackets
  in strings such as

  \begin{center}
  \texttt{--[\barbelow{.}.[[-]+>[.]]--].>.++}
  \end{center}

  for which \texttt{jumpRight} should produce the position:

  \begin{center}
  \texttt{--[..[[-]+>[.]]--]\barbelow{.}>.++}
  \end{center}

  It is also possible that the position returned by \texttt{jumpRight} or
  \texttt{jumpLeft} is outside the string in cases where there are
  no matching brackets. For example

  \begin{center}
  \texttt{--[\barbelow{.}.[[-]+>[.]]--.>.++}
  \qquad$\stackrel{\texttt{jumpRight}}{\longrightarrow}$\qquad
  \texttt{--[..[[-]+>[.]]-->.++\barbelow{\;\phantom{+}}}
  \end{center}
  \hfill[2 Marks]


\item[(4)] Write a recursive function \texttt{compute} that runs a
  brain*u*k program. It takes a program, a program counter, a memory
  pointer and a memory as arguments. If the program counter is outside
  the program string, the execution stops and \texttt{compute} returns the
  memory. If the program counter is inside the string, it reads the
  corresponding character and updates the program counter \texttt{pc},
  memory pointer \texttt{mp} and memory \texttt{mem} according to the
  rules shown in Figure~\ref{comms}. It then calls recursively
  \texttt{compute} with the updated data. The most convenient way to
  implement the brainf**k rules in Scala is to use pattern-matching
  and to calculate a triple consisting of the updated \texttt{pc},
  \texttt{mp} and \texttt{mem}.

  Write another function \texttt{run} that calls \texttt{compute} with a
  given brainfu*k program and memory, and the program counter and memory pointer
  set to~$0$. Like \texttt{compute}, it returns the memory after the execution
  of the program finishes. You can test your brainf**k interpreter with the
  Sierpinski triangle or the Hello world programs (they seem to be particularly
  useful for debugging purposes), or have a look at

  \begin{center}
  \url{https://esolangs.org/wiki/Brainfuck}
  \end{center}

  \noindent for more bf-programs and the test cases given in \texttt{bf.scala}.\\
  \mbox{}\hfill[2 Marks]
  
  \begin{figure}[p]
  \begin{center}
    \begin{tabular}{|@{\hspace{0.5mm}}p{0.8cm}|l|}
      \hline
      \hfill\texttt{'>'} & \begin{tabular}[t]{@{}l@{\hspace{2mm}}l@{}}
                       $\bullet$ & $\texttt{pc} + 1$\\
                       $\bullet$ & $\texttt{mp} + 1$\\
                       $\bullet$ & \texttt{mem} unchanged
                     \end{tabular}\\\hline   
      \hfill\texttt{'<'} & \begin{tabular}[t]{@{}l@{\hspace{2mm}}l@{}}
                       $\bullet$ & $\texttt{pc} + 1$\\
                       $\bullet$ & $\texttt{mp} - 1$\\
                       $\bullet$ & \texttt{mem} unchanged
                     \end{tabular}\\\hline   
      \hfill\texttt{'+'} & \begin{tabular}[t]{@{}l@{\hspace{2mm}}l@{}}
                       $\bullet$ & $\texttt{pc} + 1$\\
                       $\bullet$ & $\texttt{mp}$ unchanged\\
                       $\bullet$ & \texttt{mem} updated with \texttt{mp -> mem(mp) + 1}\\
                     \end{tabular}\\\hline   
      \hfill\texttt{'-'} & \begin{tabular}[t]{@{}l@{\hspace{2mm}}l@{}}
                       $\bullet$ & $\texttt{pc} + 1$\\
                       $\bullet$ & $\texttt{mp}$ unchanged\\
                       $\bullet$ & \texttt{mem} updated with \texttt{mp -> mem(mp) - 1}\\
                     \end{tabular}\\\hline   
      \hfill\texttt{'.'} & \begin{tabular}[t]{@{}l@{\hspace{2mm}}l@{}}
                       $\bullet$ & $\texttt{pc} + 1$\\
                       $\bullet$ & $\texttt{mp}$ and \texttt{mem} unchanged\\
                       $\bullet$ & print out \,\texttt{mem(mp)} as a character\\
                     \end{tabular}\\\hline   
      %\hfill\texttt{','} & \begin{tabular}[t]{@{}l@{\hspace{2mm}}l@{}}
      %                 $\bullet$ & $\texttt{pc} + 1$\\
      %                 $\bullet$ & $\texttt{mp}$ unchanged\\
      %                 $\bullet$ & \texttt{mem} updated with \texttt{mp -> \textrm{input}}\\
      %                 \multicolumn{2}{@{}l}{the input is given by \texttt{Console.in.read().toByte}}
      %               \end{tabular}\\\hline   
      \hfill\texttt{'['} & \begin{tabular}[t]{@{}l@{\hspace{2mm}}l@{}}
                       \multicolumn{2}{@{}l}{if \texttt{mem(mp) == 0} then}\\
                       $\bullet$ & $\texttt{pc = jumpRight(prog, pc + 1, 0)}$\\
                       $\bullet$ & $\texttt{mp}$ and \texttt{mem} unchanged\medskip\\
                       \multicolumn{2}{@{}l}{otherwise if \texttt{mem(mp) != 0} then}\\
                       $\bullet$ & $\texttt{pc} + 1$\\
                       $\bullet$ & $\texttt{mp}$ and \texttt{mem} unchanged\\
                     \end{tabular}
                     \\\hline   
      \hfill\texttt{']'} & \begin{tabular}[t]{@{}l@{\hspace{2mm}}l@{}}
                       \multicolumn{2}{@{}l}{if \texttt{mem(mp) != 0} then}\\
                       $\bullet$ & $\texttt{pc = jumpLeft(prog, pc - 1, 0)}$\\
                       $\bullet$ & $\texttt{mp}$ and \texttt{mem} unchanged\medskip\\
                       \multicolumn{2}{@{}l}{otherwise if \texttt{mem(mp) == 0} then}\\
                       $\bullet$ & $\texttt{pc} + 1$\\
                       $\bullet$ & $\texttt{mp}$ and \texttt{mem} unchanged\\
                           \end{tabular}\\\hline
      %\hfill\texttt{'*'} & \begin{tabular}[t]{@{}l@{\hspace{2mm}}l@{}}
      %                 $\bullet$ & $\texttt{pc} + 1$\\
      %                 $\bullet$ & $\texttt{mp}$ unchanged\\
      %                 $\bullet$ & \texttt{mem} updated with \texttt{mp -> mem(mp) * mem(mp - 1)}\\
      %                       \multicolumn{2}{@{}l}{this multiplies the content of the memory cells at
      %                       \texttt{mp} and \texttt{mp - 1}}\\
      %                       \multicolumn{2}{@{}l}{and stores the result at \texttt{mp}}
      %                     \end{tabular}\\\hline
      %\hfill\texttt{'@'} & \begin{tabular}[t]{@{}l@{\hspace{2mm}}l@{}}
      %                 $\bullet$ & $\texttt{pc} + 1$\\
      %                 $\bullet$ & $\texttt{mp}$ unchanged\\
      %                       $\bullet$ & \texttt{mem} updated with
      %                                   \texttt{mem(mp) -> mem(mp - 1)}\\
      %                       \multicolumn{2}{@{}l}{this updates the memory cell having the index stored at \texttt{mem(mp)},}\\
      %                       \multicolumn{2}{@{}l}{with the value stored at \texttt{mem(mp - 1)},}
      %                     \end{tabular}\\\hline
      %\hfill\texttt{'\#'} & \begin{tabular}[t]{@{}l@{\hspace{2mm}}l@{}}
      %                 $\bullet$ & $\texttt{pc} + 1$\\
      %                 $\bullet$ & $\texttt{mp}$ and \texttt{mem} unchanged\\
      %                 $\bullet$ & print out \,\texttt{mem(mp)} as a number\\
      %               \end{tabular}\\\hline  
      any other char & \begin{tabular}[t]{@{}l@{\hspace{2mm}}l@{}}
      %                   $\bullet$ & $\texttt{pc} + 1$\\
                         $\bullet$ & \texttt{mp} and \texttt{mem} unchanged
                       \end{tabular}\\
      \hline                 
    \end{tabular}
    \\\mbox{}\\[-10mm]\mbox{}
  \end{center}
  \caption{The rules for how commands in the brainf*** language update the
    program counter \texttt{pc},
    the memory pointer \texttt{mp} and the memory \texttt{mem}.\label{comms}}
  \end{figure}
\end{itemize}\bigskip  

%%\newpage

\subsection*{Part B (4 Marks)}

I am sure you agree while it is fun to marvel at bf-programs, like the
Sierpinski triangle or the Mandelbrot program, being interpreted, it
is much more fun to write a compiler for the bf-language.


\subsubsection*{Tasks (file bfc.scala)}

\begin{itemize}
\item[(5)] Compilers, in general, attempt to make programs run
  faster by precomputing as much information as possible
  before running the program. In our case we can precompute the
  addresses where we need to jump at the beginning and end of
  loops. 

  For this write a function \texttt{jtable} that precomputes the ``jump
  table'' for a bf-program. This function takes a bf-program 
  as an argument and returns a \texttt{Map[Int, Int]}. The 
  purpose of this Map is to record the information, in cases
  a pc-position points to a '\texttt{[}' or a '\texttt{]}',
  to which pc-position do we need to jump next?
 
  For example for the program
    
  \begin{center}
    \texttt{+++++[->++++++++++<]>--<+++[->>++++++++++}
    \texttt{<<]>>++<<----------[+>.>.<+<]}
  \end{center}

  we obtain the Map (note the precise numbers might differ depending on white
  spaces etc.~in the bf-program):

  \begin{center}
  \texttt{Map(69 -> 61, 5 -> 20, 60 -> 70, 27 -> 44, 43 -> 28, 19 -> 6)}
  \end{center}  
  
  This Map states that for the '\texttt{[}' on position 5, we need to
  jump to position 20, which is just after the corresponding '\texttt{]}'.
  Similarly, for the '\texttt{]}' on position 19, we need to jump to
  position 6, which is just after the '\texttt{[}' on position 5, and so
  on. The idea is to not calculate this information each time
  we hit a bracket, but just look up this information in the 
  \texttt{jtable}. 

  Then adapt the \texttt{compute} and \texttt{run} functions
  from Part 1 in order to take advantage of the information
  stored in the \texttt{jtable}. This means whenever \texttt{jumpLeft}
  and \texttt{jumpRight} was called previously, you should look
  up the jump address in the \texttt{jtable}. Feel free to reuse
  the function \texttt{jumpLeft} and \texttt{jumpRight} for
  calculating the \texttt{jtable}.\hfill{[1 Mark]}

\item[(6)] Compilers try to eliminate any ``dead'' code that could
  slow down programs and also perform what is often called
  \emph{peephole
    optimisations}.\footnote{\url{https://en.wikipedia.org/wiki/Peephole_optimization}}
  For the latter consider that it is difficult for compilers to
  comprehend what is intended with whole programs, but they are very good
  at finding out what small snippets of code do, and then try to
  generate faster code for such snippets.

  In our case, dead code is everything that is not a bf-command.
  Therefore write a function \texttt{optimise} which deletes such
  dead code from a bf-program. Moreover this function should replace every substring
  of the form \pcode{[-]} by a new command \texttt{0}. 
  The idea is that the loop \pcode{[-]} just resets the
  memory at the current location to 0. It is more efficient
  to do this in a single step, rather than stepwise in a loop as in
  the original bf-programs.

  In the extended \texttt{compute3} and \texttt{run3} functions you should
  implement this command by writing 0 to \pcode{mem(mp)}, that is use
  \pcode{write(mem, mp, 0)} as the rule for the command \texttt{0}.
  The easiest way to modify a string in this way is to use the regular
  expression \pcode{"""[^<>+-.\\[\\]]"""}, which recognises everything that is 
  not a bf-command. Similarly, the
  regular expression \pcode{"""\\[-\\]"""} finds all occurrences of \pcode{[-]}.  By using the Scala method \pcode{.replaceAll} you can replace substrings
  with new strings.\\
  \mbox{}\hfill{[1 Mark]}

\item[(7)] Finally, real compilers try to take advantage of modern
  CPUs which often provide complex operations in hardware that can
  combine many smaller instructions into a single faster instruction.

  In our case we can optimise the several single increments performed at a
  memory cell, for example \pcode{++++}, by a single ``increment by
  4''. For this optimisation we just have to make sure these single
  increments are all next to each other. Similar optimisations should apply
  for the bf-commands \pcode{-}, \pcode{<} and
  \pcode{>}, which can all be replaced by extended versions that take
  the amount of the increment (decrement) into account. We will do
  this by introducing two-character bf-commands. For example

  \begin{center}
    \begin{tabular}{l|l}
      original bf-cmds & replacement\\
      \hline
      \pcode{+} & \pcode{+A}\\
      \pcode{++} & \pcode{+B}\\
      \pcode{+++} & \pcode{+C}\\
      \ldots{} & \ldots{}\\
      \pcode{+++....++} & \pcode{+Z}\\
      \hspace{5mm}(these are 26 \pcode{+}'s)\\
    \end{tabular} 
  \end{center}  


  If there are more
  than 26 \pcode{+}'s in a row, then more than one ``two-character''
  bf-commands need to be generated (the idea is that more than
  26 copies of a single bf-command in a row is a rare occurrence in
  actual bf-programs). Similar replacements apply
  for \pcode{-}, \pcode{<} and \pcode{>}, but
  all other bf-commands should be unaffected by this
  change. 

  For this write a function \texttt{combine} which replaces sequences
  of repeated increment and decrement commands by appropriate
  two-character commands. In the functions \pcode{compute4} and
  \pcode{run4}, the ``combine'' and the optimisation from (6) should
  be performed. Make sure that when a two-character bf-command is
  encountered you need to increase the \pcode{pc}-counter by two in
  order to progress to the next command. For example

  \begin{center}
  \pcode{combine(optimise(load_bff("benchmark.bf")))}  
  \end{center}  

  generates the improved program

  \begin{center}
  \pcode{>A+B[<A+M>A-A]<A[[}\hspace{3mm}\ldots{}
  \end{center}  

  for the original benchmark program

  \begin{center}
    \pcode{>++[<+++++++++++++>-]<[[}\hspace{3mm}\ldots
  \end{center}    

  As you can see, the compiler bets on saving a lot of time on the
  \pcode{+B} and \pcode{+M} steps so that the optimisations is
  worthwhile overall (of course for the \pcode{>A}'s and so on, the compiler incurs a
  penalty). Luckily, after you have performed all
  optimisations in (5) - (7), you can expect that the
  \pcode{benchmark.bf} program runs four to five times faster.
  You can also test whether your compiler produces the correct result
  by  testing for example

  \begin{center}
  \pcode{run(load_bff("sierpinski.bf")) == run4(load_bff("sierpinski.bf"))}
  \end{center}

  which should return true for all the different compiler stages. \\ 
  \mbox{}\hfill{[2 Marks]}
\end{itemize}  

\end{document}


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