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+++ b/cws/main_cw03.tex Mon Nov 02 02:31:44 2020 +0000
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+% !TEX program = xelatex
+\documentclass{article}
+\usepackage{chessboard}
+\usepackage[LSBC4,T1]{fontenc}
+\let\clipbox\relax
+\usepackage{../style}
+\usepackage{../langs}
+\usepackage{disclaimer}
+
+\begin{document}
+
+\setchessboard{smallboard,
+ zero,
+ showmover=false,
+ boardfontencoding=LSBC4,
+ hlabelformat=\arabic{ranklabel},
+ vlabelformat=\arabic{filelabel}}
+
+\mbox{}\\[-18mm]\mbox{}
+
+\section*{Part 8 (Scala)}
+
+\mbox{}\hfill\textit{``The problem with object-oriented languages is they’ve got all this implicit,}\\
+\mbox{}\hfill\textit{environment that they carry around with them. You wanted a banana but}\\
+\mbox{}\hfill\textit{what you got was a gorilla holding the banana and the entire jungle.''}\smallskip\\
+\mbox{}\hfill\textit{ --- Joe Armstrong (creator of the Erlang programming language)}\medskip\bigskip
+
+\noindent
+This part is about searching and backtracking. You are asked to
+implement Scala programs that solve various versions of the
+\textit{Knight's Tour Problem} on a chessboard. The preliminary part (4\%) is
+due on \cwEIGHT{} at 4pm; the core part is due on \cwEIGHTa{} at 4pm.
+Note the core, more advanced, part might include material you have not
+yet seen in the first three lectures. \bigskip
+
+\IMPORTANT{}
+Also note that the running time of each part will be restricted to a
+maximum of 30 seconds on my laptop: If you calculate a result once,
+try to avoid to calculate the result again. Feel free to copy any code
+you need from files \texttt{knight1.scala}, \texttt{knight2.scala} and
+\texttt{knight3.scala}.
+
+\DISCLAIMER{}
+
+\subsection*{Background}
+
+The \textit{Knight's Tour Problem} is about finding a tour such that
+the knight visits every field on an $n\times n$ chessboard once. For
+example on a $5\times 5$ chessboard, a knight's tour is:
+
+\chessboard[maxfield=d4,
+ pgfstyle= {[base,at={\pgfpoint{0pt}{-0.5ex}}]text},
+ text = \small 24, markfield=Z4,
+ text = \small 11, markfield=a4,
+ text = \small 6, markfield=b4,
+ text = \small 17, markfield=c4,
+ text = \small 0, markfield=d4,
+ text = \small 19, markfield=Z3,
+ text = \small 16, markfield=a3,
+ text = \small 23, markfield=b3,
+ text = \small 12, markfield=c3,
+ text = \small 7, markfield=d3,
+ text = \small 10, markfield=Z2,
+ text = \small 5, markfield=a2,
+ text = \small 18, markfield=b2,
+ text = \small 1, markfield=c2,
+ text = \small 22, markfield=d2,
+ text = \small 15, markfield=Z1,
+ text = \small 20, markfield=a1,
+ text = \small 3, markfield=b1,
+ text = \small 8, markfield=c1,
+ text = \small 13, markfield=d1,
+ text = \small 4, markfield=Z0,
+ text = \small 9, markfield=a0,
+ text = \small 14, markfield=b0,
+ text = \small 21, markfield=c0,
+ text = \small 2, markfield=d0
+ ]
+
+\noindent
+This tour starts in the right-upper corner, then moves to field
+$(3,2)$, then $(4,0)$ and so on. There are no knight's tours on
+$2\times 2$, $3\times 3$ and $4\times 4$ chessboards, but for every
+bigger board there is.
+
+A knight's tour is called \emph{closed}, if the last step in the tour
+is within a knight's move to the beginning of the tour. So the above
+knight's tour is \underline{not} closed because the last
+step on field $(0, 4)$ is not within the reach of the first step on
+$(4, 4)$. It turns out there is no closed knight's tour on a $5\times
+5$ board. But there are on a $6\times 6$ board and on bigger ones, for
+example
+
+\chessboard[maxfield=e5,
+ pgfstyle={[base,at={\pgfpoint{0pt}{-0.5ex}}]text},
+ text = \small 10, markfield=Z5,
+ text = \small 5, markfield=a5,
+ text = \small 18, markfield=b5,
+ text = \small 25, markfield=c5,
+ text = \small 16, markfield=d5,
+ text = \small 7, markfield=e5,
+ text = \small 31, markfield=Z4,
+ text = \small 26, markfield=a4,
+ text = \small 9, markfield=b4,
+ text = \small 6, markfield=c4,
+ text = \small 19, markfield=d4,
+ text = \small 24, markfield=e4,
+ % 4 11 30 17 8 15
+ text = \small 4, markfield=Z3,
+ text = \small 11, markfield=a3,
+ text = \small 30, markfield=b3,
+ text = \small 17, markfield=c3,
+ text = \small 8, markfield=d3,
+ text = \small 15, markfield=e3,
+ %29 32 27 0 23 20
+ text = \small 29, markfield=Z2,
+ text = \small 32, markfield=a2,
+ text = \small 27, markfield=b2,
+ text = \small 0, markfield=c2,
+ text = \small 23, markfield=d2,
+ text = \small 20, markfield=e2,
+ %12 3 34 21 14 1
+ text = \small 12, markfield=Z1,
+ text = \small 3, markfield=a1,
+ text = \small 34, markfield=b1,
+ text = \small 21, markfield=c1,
+ text = \small 14, markfield=d1,
+ text = \small 1, markfield=e1,
+ %33 28 13 2 35 22
+ text = \small 33, markfield=Z0,
+ text = \small 28, markfield=a0,
+ text = \small 13, markfield=b0,
+ text = \small 2, markfield=c0,
+ text = \small 35, markfield=d0,
+ text = \small 22, markfield=e0,
+ vlabel=false,
+ hlabel=false
+ ]
+
+
+\noindent
+where the 35th move can join up again with the 0th move.
+
+If you cannot remember how a knight moves in chess, or never played
+chess, below are all potential moves indicated for two knights, one on
+field $(2, 2)$ (blue moves) and another on $(7, 7)$ (red moves):
+
+{\chessboard[maxfield=g7,
+ color=blue!50,
+ linewidth=0.2em,
+ shortenstart=0.5ex,
+ shortenend=0.5ex,
+ markstyle=cross,
+ markfields={a4, c4, Z3, d3, Z1, d1, a0, c0},
+ color=red!50,
+ markfields={f5, e6},
+ setpieces={Ng7, Nb2},
+ boardfontsize=12pt,labelfontsize=9pt]}
+
+\subsection*{Reference Implementation}
+
+This Scala part comes with three reference implementations in form of
+\texttt{jar}-files. 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 knight1.jar} and then query any function from the
+\texttt{knight1.scala} template file. As usual you have to
+prefix the calls with \texttt{CW8a}, \texttt{CW8b} and \texttt{CW8c}.
+Since some of the calls are time sensitive, I included some timing
+information. For example
+
+\begin{lstlisting}[language={},numbers=none,basicstyle=\ttfamily\small]
+$ scala -cp knight1.jar
+scala> CW8a.enum_tours(5, List((0, 0))).length
+Time needed: 1.722 secs.
+res0: Int = 304
+
+scala> CW8a.print_board(8, CW8a.first_tour(8, List((0, 0))).get)
+Time needed: 15.411 secs.
+
+ 51 46 55 44 53 4 21 12
+ 56 43 52 3 22 13 24 5
+ 47 50 45 54 25 20 11 14
+ 42 57 2 49 40 23 6 19
+ 35 48 41 26 61 10 15 28
+ 58 1 36 39 32 27 18 7
+ 37 34 31 60 9 62 29 16
+ 0 59 38 33 30 17 8 63
+\end{lstlisting}%$
+
+
+\subsection*{Hints}
+
+\noindent
+\textbf{Preliminary Part} useful list functions: \texttt{.contains(..)} checks
+whether an element is in a list, \texttt{.flatten} turns a list of
+lists into just a list, \texttt{\_::\_} puts an element on the head of
+the list, \texttt{.head} gives you the first element of a list (make
+sure the list is not \texttt{Nil}); a useful option function:
+\texttt{.isDefined} returns true, if an option is \texttt{Some(..)};
+anonymous functions can be constructed using \texttt{(x:Int) => ...},
+this function takes an \texttt{Int} as an argument.\medskip
+
+
+\noindent
+\textbf{Core Part} a useful list function: \texttt{.sortBy} sorts a list
+according to a component given by the function; a function can be
+tested to be tail-recursive by annotation \texttt{@tailrec}, which is
+made available by importing \texttt{scala.annotation.tailrec}.\medskip
+
+
+
+
+\subsection*{Preliminary Part (4 Marks)}
+
+You are asked to implement the knight's tour problem such that the
+dimension of the board can be changed. Therefore most functions will
+take the dimension of the board as an argument. The fun with this
+problem is that even for small chessboard dimensions it has already an
+incredibly large search space---finding a tour is like finding a
+needle in a haystack. In the first task we want to see how far we get
+with exhaustively exploring the complete search space for small
+chessboards.\medskip
+
+\noindent
+Let us first fix the basic datastructures for the implementation. The
+board dimension is an integer.
+A \emph{position} (or field) on the chessboard is
+a pair of integers, like $(0, 0)$. A \emph{path} is a list of
+positions. The first (or 0th move) in a path is the last element in
+this list; and the last move in the path is the first element. For
+example the path for the $5\times 5$ chessboard above is represented
+by
+
+\[
+\texttt{List($\underbrace{\texttt{(0, 4)}}_{24}$,
+ $\underbrace{\texttt{(2, 3)}}_{23}$, ...,
+ $\underbrace{\texttt{(3, 2)}}_1$, $\underbrace{\texttt{(4, 4)}}_0$)}
+\]
+
+\noindent
+Suppose the dimension of a chessboard is $n$, then a path is a
+\emph{tour} if the length of the path is $n \times n$, each element
+occurs only once in the path, and each move follows the rules of how a
+knight moves (see above for the rules).
+
+
+\subsubsection*{Tasks (file knight1.scala)}
+
+\begin{itemize}
+\item[(1)] Implement an \texttt{is\_legal} function that takes a
+ dimension, a path and a position as arguments and tests whether the
+ position is inside the board and not yet element in the
+ path. \hfill[1 Mark]
+
+\item[(2)] Implement a \texttt{legal\_moves} function that calculates for a
+ position all legal onward moves. If the onward moves are
+ placed on a circle, you should produce them starting from
+ ``12-o'clock'' following in clockwise order. For example on an
+ $8\times 8$ board for a knight at position $(2, 2)$ and otherwise
+ empty board, the legal-moves function should produce the onward
+ positions in this order:
+
+ \begin{center}
+ \texttt{List((3,4), (4,3), (4,1), (3,0), (1,0), (0,1), (0,3), (1,4))}
+ \end{center}
+
+ If the board is not empty, then maybe some of the moves need to be
+ filtered out from this list. For a knight on field $(7, 7)$ and an
+ empty board, the legal moves are
+
+ \begin{center}
+ \texttt{List((6,5), (5,6))}
+ \end{center}
+ \mbox{}\hfill[1 Mark]
+
+\item[(3)] Implement two recursive functions (\texttt{count\_tours} and
+ \texttt{enum\_tours}). They each take a dimension and a path as
+ arguments. They exhaustively search for tours starting
+ from the given path. The first function counts all possible
+ tours (there can be none for certain board sizes) and the second
+ collects all tours in a list of paths. These functions will be
+ called with a path containing a single position---the starting field.
+ They are expected to extend this path so as to find all tours starting
+ from the given position.\\
+ \mbox{}\hfill[2 Marks]
+\end{itemize}
+
+\noindent \textbf{Test data:} For the marking, the functions in (3)
+will be called with board sizes up to $5 \times 5$. If you search
+for tours on a $5 \times 5$ board starting only from field $(0, 0)$,
+there are 304 of tours. If you try out every field of a $5 \times
+5$-board as a starting field and add up all tours, you obtain
+1728. A $6\times 6$ board is already too large to be searched
+exhaustively.\footnote{For your interest, the number of tours on
+ $6\times 6$, $7\times 7$ and $8\times 8$ are 6637920, 165575218320,
+ 19591828170979904, respectively.}\smallskip
+
+
+\subsection*{Core Part (6 Marks)}
+
+
+\subsubsection*{Tasks (file knight1.scala cont.)}
+
+\begin{itemize}
+\item[(4)] Implement a \texttt{first}-function. This function takes a list of
+ positions and a function $f$ as arguments; $f$ is the name we give to
+ this argument). The function $f$ takes a position as argument and
+ produces an optional path. So $f$'s type is \texttt{Pos =>
+ Option[Path]}. The idea behind the \texttt{first}-function is as follows:
+
+ \[
+ \begin{array}{lcl}
+ \textit{first}(\texttt{Nil}, f) & \dn & \texttt{None}\\
+ \textit{first}(x\!::\!xs, f) & \dn & \begin{cases}
+ f(x) & \textit{if}\;f(x) \not=\texttt{None}\\
+ \textit{first}(xs, f) & \textit{otherwise}\\
+ \end{cases}
+ \end{array}
+ \]
+
+ \noindent That is, we want to find the first position where the
+ result of $f$ is not \texttt{None}, if there is one. Note that
+ `inside' \texttt{first}, you do not (need to) know anything about
+ the argument $f$ except its type, namely \texttt{Pos =>
+ Option[Path]}. If you want to find out what the result of $f$ is
+ on a particular argument, say $x$, you can just write $f(x)$.
+ There is one additional point however you should
+ take into account when implementing \texttt{first}: you will need to
+ calculate what the result of $f(x)$ is; your code should do this
+ only \textbf{once} and for as \textbf{few} elements in the list as
+ possible! Do not calculate $f(x)$ for all elements and then see which
+ is the first \texttt{Some}.\\\mbox{}\hfill[1 Mark]
+
+\item[(5)] Implement a \texttt{first\_tour} function that uses the
+ \texttt{first}-function from (4), and searches recursively for single tour.
+ As there might not be such a tour at all, the \texttt{first\_tour} function
+ needs to return a value of type
+ \texttt{Option[Path]}.\\\mbox{}\hfill[1 Mark]
+\end{itemize}
+
+\noindent
+\textbf{Testing:} The \texttt{first\_tour} function will be called with board
+sizes of up to $8 \times 8$.
+\bigskip
+
+%%\newpage
+
+\noindent
+As you should have seen in the earlier parts, a naive search for tours beyond
+$8 \times 8$ boards and also searching for closed tours even on small
+boards takes too much time. There is a heuristics, called \emph{Warnsdorf's
+Rule} that can speed up finding a tour. This heuristics states that a
+knight is moved so that it always proceeds to the field from which the
+knight will have the \underline{fewest} onward moves. For example for
+a knight on field $(1, 3)$, the field $(0, 1)$ has the fewest possible
+onward moves, namely 2.
+
+\chessboard[maxfield=g7,
+ pgfstyle= {[base,at={\pgfpoint{0pt}{-0.5ex}}]text},
+ text = \small 3, markfield=Z5,
+ text = \small 7, markfield=b5,
+ text = \small 7, markfield=c4,
+ text = \small 7, markfield=c2,
+ text = \small 5, markfield=b1,
+ text = \small 2, markfield=Z1,
+ setpieces={Na3}]
+
+\noindent
+Warnsdorf's Rule states that the moves on the board above should be
+tried in the order
+
+\[
+(0, 1), (0, 5), (2, 1), (2, 5), (3, 4), (3, 2)
+\]
+
+\noindent
+Whenever there are ties, the corresponding onward moves can be in any
+order. When calculating the number of onward moves for each field, we
+do not count moves that revisit any field already visited.
+
+\subsubsection*{Tasks (file knight2.scala)}
+
+\begin{itemize}
+\item[(6)] Write a function \texttt{ordered\_moves} that calculates a list of
+ onward moves like in (2) but orders them according to
+ Warnsdorf’s Rule. That means moves with the fewest legal onward moves
+ should come first (in order to be tried out first). \hfill[1 Mark]
+
+\item[(7)] Implement a \texttt{first\_closed\_tour\_heuristics}
+ function that searches for a single
+ \textbf{closed} tour on a $6\times 6$ board. It should try out
+ onward moves according to
+ the \texttt{ordered\_moves} function from (6). It is more likely to find
+ a solution when started in the middle of the board (that is
+ position $(dimension / 2, dimension / 2)$). \hfill[1 Mark]
+
+\item[(8)] Implement a \texttt{first\_tour\_heuristics} function
+ for boards up to
+ $30\times 30$. It is the same function as in (7) but searches for
+ tours (not just closed tours). It might be called with any field on the
+ board as starting field.\\
+ %You have to be careful to write a
+ %tail-recursive function of the \texttt{first\_tour\_heuristics} function
+ %otherwise you will get problems with stack-overflows.\\
+ \mbox{}\hfill[1 Mark]
+\end{itemize}
+
+\subsubsection*{Task (file knight3.scala)}
+\begin{itemize}
+\item[(9)] Implement a function \texttt{tour\_on\_mega\_board} which is
+ the same function as in (8), \textbf{but} should be able to
+ deal with boards up to
+ $70\times 70$ \textbf{within 30 seconds} (on my laptop). This will be tested
+ by starting from field $(0, 0)$. You have to be careful to
+ write a tail-recursive function otherwise you will get problems
+ with stack-overflows. Please observe the requirements about
+ the submissions: no tricks involving \textbf{.par}.\medskip
+
+ The timelimit of 30 seconds is with respect to the laptop on which the
+ marking will happen. You can roughly estimate how well your
+ implementation performs by running \texttt{knight3.jar} on your
+ computer. For example the reference implementation shows
+ on my laptop:
+
+ \begin{lstlisting}[language={},numbers=none,basicstyle=\ttfamily\small]
+$ scala -cp knight3.jar
+
+scala> CW8c.tour_on_mega_board(70, List((0, 0)))
+Time needed: 9.484 secs.
+...<<long_list>>...
+\end{lstlisting}%$
+
+ \mbox{}\hfill[1 Mark]
+\end{itemize}
+\bigskip
+
+
+
+
+\end{document}
+
+%%% Local Variables:
+%%% mode: latex
+%%% TeX-master: t
+%%% End: