
diff -r 554278cd4b70 -r 3c81352ec565 cws/resit.tex
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/cws/resit.tex	Sat Oct 08 00:30:51 2022 +0100
@@ -0,0 +1,273 @@
+
+% !TEX program = xelatex
+\documentclass{article}
+\usepackage{../styles/style}
+\usepackage{disclaimer}
+\usepackage{../styles/langs}
+\usepackage{graphicx}
+
+\begin{document}
+
+
+%% should ask to lower case the words.
+
+\section*{Resit: Evil Wordle Game (Scala, 6 Marks)}
+
+
+\noindent
+You are asked to implement a Scala program for making the popular Wordle game as difficult
+as possible. The deadline for your submission is on 4th August at 16:00. There will be no
+automated tests for the resit, but there are plenty of testcases in the template and the
+task description. \bigskip
+
+\IMPORTANTNONE{}
+
+\noindent
+Also note that the running time of each part will be restricted to a
+maximum of 30 seconds on my laptop.
+
+\DISCLAIMER{}
+
+\subsection*{Hints}
+
+\noindent
+Useful data functions: \texttt{Source.fromURL},
+\texttt{Source.fromFile} for obtaining a webpage and reading a file,
+\texttt{.getOrElse(..,..)} allows to query a Map, but also gives a
+default value if the Map is not defined, a Map can be `updated' by
+using \texttt{+}, \texttt{.contains} and \texttt{.filter} can test whether
+an element is included in a list, and respectively filter out elements in a list,
+\texttt{.sortBy(\_.\_2)} sorts a list of pairs according to the second
+elements in the pairs---the sorting is done from smallest to highest,
+\texttt{.groupBy} orders lists according to same elements
+.
+
+
+\newpage
+
+
+\subsection*{Resit (6 Marks, file wordle.scala)}
+
+You probably know the game of Wordle\footnote{\url{https://en.wikipedia.org/wiki/Wordle}}
+where you are supposed to guess a five-letter word. The feedback for guesses can help
+with the next guess (green letters are correct, orange letters are present, but in the
+wrong place). For example:
+
+\begin{center}
+\includegraphics[scale=0.2]{../pics/w.jpeg}
+\end{center}  
+
+\noindent
+The idea of the program to be implemented here is to make the Wordle game as evil as possible
+by finding words that are the most difficult to guess. A word list of five-letter words is available
+from 
+
+\begin{center}
+\begin{tabular}{ll}  
+  \url{https://nms.kcl.ac.uk/christian.urban/wordle.txt} & (78 KByte)\\
+\end{tabular}
+\end{center}
+
+\noindent
+In your program you need to download this list and implement some
+functions that in the end select the most difficult words (given an
+input from the user).  If bandwidth is an issue for you, download the
+file locally, but in the submitted version use \texttt{Source.fromURL}
+instead of \texttt{Source.fromFile}.
+
+\subsection*{Tasks}
+
+\begin{itemize}
+\item[(1)] Implement the function \pcode{get_wordle_list} which takes an
+  URL-string as argument and requests the corresponding file. The function should
+  return the word list appropriately broken up into lines.
+  The result should be a list of strings (the lines in the file). In case
+  the url does not produce a file, return the empty list.\\
+  \mbox{}\hfill [0.25 Marks]
+
+\item[(2)] Implement a polymorphic function \pcode{removeN}, which removes $n$ occurences of an
+  element from a list (if this element is less than $n$ times pressent, then remove all occurences).
+  For example
+
+\begin{lstlisting}[numbers=none]
+removeN(List(1,2,3,2,1), 3, 2)  => List(1, 2, 2, 1)
+removeN(List(1,2,3,2,1), 2, 1)  => List(1, 3, 2, 1)
+removeN(List(1,2,3,2,1), 2, 2)  => List(1, 3, 1)
+removeN(List(1,2,3,2,1), 1, 1)  => List(2, 3, 2, 1)
+removeN(List(1,2,3,2,1), 1, 3)  => List(2, 3, 2)
+removeN(List(1,2,3,2,1), 0, 2)  => List(1, 2, 3, 2, 1)
+\end{lstlisting}
+
+Make sure you only remove at most $n$ occurrences of the element from the list.
+This function should work for lists of intergers but also lists of chars, strings etc.\\
+  \mbox{}\hfill [0.25 Marks]
+
+\item[(3)] Implement a function \pcode{score} that calculates the
+  feedback for a word against a secret word using the rules of the
+  Wordle game. The output of \pcode{score} should be a list of 5
+  elements of type \pcode{Tip} representing three outcomes: a letter
+  in the correct position, a letter that is present, but not in the
+  correct position and a letter that is absent. For example given the
+  secret word "chess" the score for the word "caves" is
+
+\begin{lstlisting}[numbers=none]
+List(Correct, Absent, Absent, Present, Correct)
+\end{lstlisting}
+
+  You have to be careful with multiple occurrences of letters. For example
+  the secret "chess" with the guess "swiss" should produce
+
+\begin{lstlisting}[numbers=none]
+List(Absent, Absent, Absent, Correct, Correct)
+\end{lstlisting}
+
+even though the first 's' in "swiss" is present in the secret word, the 's' are already
+`used up' by the two letters that are correct. To implement this you need to
+implement first a function \pcode{pool} which calculates all the letters in
+a secret that are not correct in a word. For example
+
+\begin{lstlisting}[numbers=none]
+  pool("chess", "caves")  => List(h, e, s)
+  pool("chess", "swiss")  => List(c, h, e)
+\end{lstlisting}
+
+  Now the helper function \pcode{aux} can analyse the arguments secret and word recursively letter-by-letter and
+  decide: if the letters are the same, then return \pcode{Correct} for the corresponding position.
+  If they are not the same, but the letter is in the pool, then return \pcode{Present} and also remove
+  this letter from the pool in the next recursive call of \pcode{aux}. Otherwise return \pcode{Absent} for the
+  corresponding position. The function \pcode{score} is a wrapper for the function \pcode{aux}
+  calling \pcode{aux} with the appropriate arguments (recall what is calculated with \pcode{pool}).\mbox{}\hfill [1.5 Marks]
+
+\item[(4)] Implement a function \pcode{eval} that gives an integer value to each of the
+  \pcode{Tip}s such that
+
+  \begin{center}
+  \begin{tabular}{lcl}  
+    \textit{eval (Correct)} & $\dn$ & $10$\\
+    \textit{eval (Present)} & $\dn$ & $1$\\
+    \textit{eval (Absent)} & $\dn$ & $0$
+  \end{tabular}                                   
+  \end{center}  
+
+  The function \pcode{iscore} then takes an output of \pcode{score} and sums
+  up all corresponding values. For example for 
+
+\begin{lstlisting}[numbers=none]
+  iscore("chess", "caves")  => 21
+  iscore("chess", "swiss")  => 20
+\end{lstlisting}
+  \mbox{}\hfill [0.5 Marks]
+
+\item[(5)] The function \pcode{evil} takes a list of secrets (the list from Task 1)
+  and a word as arguments, and calculates the list of words with the lowest
+  score (remember we want to make the Wordle game as difficult as possible---therefore
+  when the user gives us a word, we want to find the secrets that produce the lowest
+  score). For this implement a helper function \pcode{lowest} that goes through
+  the secrets one-by-one and calculates the score. The argument \pcode{current} is
+  the score of the ``currently'' found secrets. When the function \pcode{lowest}
+  is called for the first time then this will be set to the maximum integer value
+  \pcode{Int.MaxValue}. The accumulator will be first empty. If a secret is found
+  with the same score as \pcode{current} then this word is added to the accumulator.
+  If the secret has a lower score, then the accumulator will be discarded and this
+  secret will be the new accumulator. If the secret has a higher score, then it can be
+  ignored. For example \pcode{evil} (the wrapper for \pcode{lowest}) generates
+
+\begin{lstlisting}[numbers=none]
+evil(secrets, "stent").length => 1907
+evil(secrets, "hexes").length => 2966
+evil(secrets, "horse").length => 1203
+evil(secrets, "hoise").length => 971
+evil(secrets, "house").length => 1228
+\end{lstlisting}
+
+where \pcode{secrets} is the list generated under Task 1.
+In all cases above the iscore of the resulting secrets is 0, but this does not need to be the case
+in general.\\
+  \mbox{}\hfill [1.5 Marks]
+
+\item[(6)] The secrets generated in Task 5 are the ones with the lowest score
+  with respect to the word, or the secrets that are furthest ``away'' from the
+  given word. This is already quite evil for a secret word---remember we can choose
+  a secret \emph{after} a user has given a first word. Now we want to make it
+  even more evil by choosing words that have the most obscure letters. For this we
+  calculate the frequency of how many times certain letters occur in our secrets
+  list (see Task 1). The \emph{frequency} of the letter $c$, say, is given by the formula
+
+  \begin{center}
+  $\textit{freq(c)} \dn 1 - \frac{\textit{number of occurrences of c}}{\textit{number of all letters}}$  
+  \end{center}  
+
+  That means that letters that occur fewer times in our secrets have a higher frequency.
+  For example the letter 'y' has the frequency 0.9680234350909651 while the much more
+  often occurring letter 'e' has only 0.897286463151403 (all calculations should be done
+  with Doubles).
+
+  The function \pcode{frequencies} should calculate the frequencies for all lower-case letters
+  by generating a Map from letters (\pcode{Char}) to Doubles (frequencies).\\ 
+  \mbox{}\hfill [1 Mark]
+
+\item[(7)] In this task we want to use the output of \pcode{evil}, rank each string in the
+  generated set and then filter out the strings that are ranked highest (the ones with the most obscure letters).
+  This list of strings often contains only a single word, but in general there might be more (see below).
+  First implement a function \pcode{rank} that takes a frequency map (from 6) and a string as arguments and
+  generates a rank by summing up all frequencies of the letters in the string. For example
+
+\begin{lstlisting}[numbers=none]
+rank(frequencies(secrets), "adobe") => 4.673604687018193
+rank(frequencies(secrets), "gaffe") => 4.745205057045945
+rank(frequencies(secrets), "fuzzy") => 4.898735738513722
+\end{lstlisting}
+
+  Finally, implement a function \pcode{ranked_evil} that selects from the output of \pcode{evil}
+  the string(s) which are highest ranked in evilness.
+
+  
+\begin{lstlisting}[numbers=none]
+ranked_evil(secrets, "abbey") => List(whizz)
+ranked_evil(secrets, "afear") => List(buzzy)
+ranked_evil(secrets, "zincy") => List(jugum)
+ranked_evil(secrets, "zippy") => List(chuff)
+\end{lstlisting}
+
+This means if the user types in "abbey" then the most evil word to choose as secret is "whizzy" (according to
+our calculations). This word has a zero \pcode{iscore} and the most obscure letters.
+
+%
+%\color{red}
+%\section*{Correction with \texttt{ranked\_evil}}
+%
+%The testcases above are actually not the maximum, but the minimum! I will make sure
+%that the task will count as solved when either the minimum (as above) or the maximum (as intended)
+%is used. The correct solutions for the above testcases using the maximum are:
+%\color{black}
+%
+%\begin{lstlisting}[numbers=none]
+%ranked_evil(secrets, "beats") => List(fuzzy)
+%ranked_evil(secrets, "vitae") => List(fuzzy)
+%ranked_evil(secrets, "bento") => List(fuzzy)
+%ranked_evil(secrets, "belts") => List(fuzzy)
+%\end{lstlisting}
+%
+%\noindent \textcolor{red}{Some further testcases for the maximum are:}
+%
+%\begin{lstlisting}[numbers=none]
+%ranked_evil(secrets, "abbey") => List(whizz)
+%ranked_evil(secrets, "afear") => List(buzzy)
+%ranked_evil(secrets, "zincy") => List(jugum)
+%ranked_evil(secrets, "zippy") => List(chuff)
+%\end{lstlisting}
+% 
+%
+
+\mbox{}\hfill [1 Mark]  
+\end{itemize}
+
+\end{document} 
+
+%%% Local Variables: 
+%%% mode: latex
+%%% TeX-master: t
+%%% End: 
+
+
+




