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\section*{Coursework 9 (Scala)}

This coursework is worth 10\%. It is about a small programming

language called brainf***. The first part is due on 13 December at

11pm; the second, more advanced part, is due on 20 December at

11pm.\bigskip

\IMPORTANT{}

\noindent

Also note that the running time of each part will be restricted to a

maximum of 30 seconds on my laptop.

\DISCLAIMER{}

\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. This allows 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{CW10a} and \texttt{CW10b}, respectively. For example

\begin{lstlisting}[language={},xleftmargin=1mm,numbers=none,basicstyle=\ttfamily\small]

$ scala -cp bf.jar

scala> import CW10a._  

scala> run(load_bff("sierpinski.bf"))

                               *

                              * *

                             *   *

                            * * * *

                           *       *

                          * *     * *

                         *   *   *   *

                        * * * * * * * *

                       *               *

                      * *             * *

                     *   *           *   *

                    * * * *         * * * *

                   *       *       *       *

                  * *     * *     * *     * *

                 *   *   *   *   *   *   *   *

                * * * * * * * * * * * * * * * *

               *                               *

              * *                             * *

             *   *                           *   *

            * * * *                         * * * *

           *       *                       *       *

          * *     * *                     * *     * *

         *   *   *   *                   *   *   *   *

        * * * * * * * *                 * * * * * * * *

       *               *               *               *

      * *             * *             * *             * *

     *   *           *   *           *   *           *   *

    * * * *         * * * *         * * * *         * * * *

   *       *       *       *       *       *       *       *

  * *     * *     * *     * *     * *     * *     * *     * *

 *   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

\end{lstlisting}%$

\subsection*{Part 1 (6 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***}. You

are asked in this part to implement an interpreter for

this language.

Urban M\"uller developed 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 we know 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.\bigskip

\noindent

As mentioned above, brainf*** has 8 single-character commands, namely

\texttt{'>'}, \texttt{'<'}, \texttt{'+'}, \texttt{'-'}, \texttt{'.'},

\texttt{','}, \texttt{'['} and \texttt{']'}. Every other character is

considered a comment.  Brainf*** operates on memory cells containing

integers. For this it uses a single memory pointer that points at each

stage to one memory cell. 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 commands for input/output

are \texttt{','} and \texttt{'.'}. 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. Input works the other way, taking some

user input and storing it in the cell to which the memory pointer

points to. 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 ;o) 

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

\begin{itemize}

\item[(1)]  Write a function that takes a file name 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[1 Mark]

\item[(2)] Brainf*** 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 function, \texttt{sread}, that takes a memory (a \texttt{Map}) and

  a memory pointer (an \texttt{Int}) as argument, 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 argument 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 value is stored at the given memory

  pointer.\hfill[1 Mark]

\item[(3)] Write two functions, \texttt{jumpRight} and

  \texttt{jumpLeft} that are needed to implement the loop constructs

  of brainf***. They take a program (a \texttt{String}) and a program

  counter (an \texttt{Int}) as argument 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 brainf*** 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

  brainf*** 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** 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}\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   

      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}

  \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 2 (4 Marks)}

I am sure you agree while it is fun to look 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

  that given on the pc-position $pc$ is a '\texttt{[}' or a '\texttt{]}',

  then 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}} While it is difficult for compilers to comprehend what

  is intended with whole programs, 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. 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 incrementally 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{[-]} and 

  by using the Scala method \pcode{.replaceAll} you can replace it with the 

  string \pcode{"0"} standing for the new bf-command.\hfill{[1 Mark]}

\item[(7)] Finally, real compilers try to take advantage of 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 of 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. Similarly 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}  

  Similarly for \pcode{-}, \pcode{<} and \pcode{>}. If there are more

  than 26 \pcode{+}'s in a row, then more than on ``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). 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 process 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 so much 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 for example testing

  \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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