127 lines
4.4 KiB
TeX
Executable File
127 lines
4.4 KiB
TeX
Executable File
\section{Befunge}
|
|
|
|
\definition{}
|
|
\textit{Befunge} is another esoteric programming langauge, designed to be very difficult to compile. \par
|
|
It consists of a \say{field} of instructions, a two-dimensional program counter, and a stack of values. \par
|
|
|
|
\vspace{2mm}
|
|
|
|
The program counter starts in the top-left corner of the field, moving right. \par
|
|
It executes any instruction it encounters.
|
|
The instructions \texttt{>}, \texttt{<}, \texttt{\^}, and \texttt{v} can be used to direct the program counter,
|
|
and \texttt{\_} and \texttt{|} are used for control flow.
|
|
|
|
|
|
|
|
\vfill
|
|
|
|
An instruction reference is below:
|
|
\begin{itemize}[itemsep=2mm]
|
|
\item \texttt{+} Addition: Pop two values $a$ and $b$, then push the result of $a+b$
|
|
\item \texttt{-} Subtraction: Pop two values $a$ and $b$, then push the result of $b-a$
|
|
\item \texttt{*} Multiplication: Pop two values $a$ and $b$, then push the result of $a \times b$
|
|
\item \texttt{/} Integer division: Pop two values $a$ and $b$, then push the result of $b \div a$, rounded down.
|
|
\item \texttt{\%} Modulo: Pop two values $a$ and $b$, then push the remainder of the integer division of $b \div a$.
|
|
\item \texttt{!} Logical NOT: Pop a value. If the value is zero, push 1; otherwise, push zero.
|
|
\item \texttt{ \`} Greater than: Pop two values $a$ and $b$, then push 1 if $b>a$, otherwise zero.
|
|
\item \texttt{>} Program counter direction right
|
|
\item \texttt{<} Program counter direction left
|
|
\item \texttt{\textasciicircum} Program counter direction up
|
|
\item \texttt{v} Program counter direction down
|
|
\item \texttt{?} Random program counter direction
|
|
\item \texttt{\_} Horizontal \texttt{if}: pop a value; set direction to right if value=0, set to left otherwise
|
|
\item \texttt{|} Vertical \texttt{if}: pop a value; set direction to down if value=0, set to up otherwise
|
|
\item \texttt{"} Toggle string mode (push each character's ASCII value all the way up to the next ")
|
|
\item \texttt{:} Duplicate top stack value
|
|
\item \texttt{\textbackslash} Swap top stack values
|
|
\item \texttt{\$} Pop top of stack and discard
|
|
\item \texttt{.} Pop top of stack and output as integer
|
|
\item \texttt{,} Pop top of stack and output as ASCII character
|
|
\item \texttt{\#} Bridge: jump over next command in the current direction of the current PC
|
|
\item \texttt{g} A "get" call (a way to retrieve data in storage). Pop two values y and x, then push the ASCII value of the character at that position in the program. If (x,y) is out of bounds, push 0
|
|
\item \texttt{p} A "put" call (a way to store a value for later use). Pop three values y, x and v, then change the character at the position (x,y) in the program to the character with ASCII value v
|
|
\item \texttt{\&} Get integer from user and push it
|
|
\item \texttt{\~{}} Get character from user and push it
|
|
\item \texttt{@} End program
|
|
\item \texttt{0}-\texttt{9} Push corresponding number onto the stack
|
|
\end{itemize}
|
|
|
|
\vspace{2mm}
|
|
|
|
Note that the \texttt{p} instruction allows us to write self-modifying code.
|
|
|
|
\vfill
|
|
\pagebreak
|
|
|
|
\problem{}
|
|
Write a program that prints \say{\texttt{Hello World}}.
|
|
|
|
\problem{}
|
|
Write a program that prints the alphabet
|
|
|
|
\problem{}
|
|
Write a program that generates a random sequence of numbers (\texttt{0}-\texttt{9}).
|
|
|
|
\vfill
|
|
|
|
|
|
|
|
\problem{}
|
|
Write a program that does not contain the string \say{\texttt{Hello World}}, \par
|
|
but writes that string somewhere inside its source.
|
|
|
|
\problem{}
|
|
Replace the \texttt{x}s in the following program
|
|
so that the loop runs forever. \par
|
|
Do not use any control-flow instructions
|
|
(\texttt{>}, \texttt{<}, \texttt{\textasciicircum{}}, \texttt{v}, \texttt{\_}, \texttt{|}, \texttt{\#}, or \texttt{?}) \par
|
|
\hint{
|
|
Start by replacing all the \texttt{x}s with spaces. \par
|
|
You may not need all the \texttt{x}s, feel free to use a smaller rectangle.
|
|
}
|
|
|
|
\vspace{2mm}
|
|
|
|
\begin{center}
|
|
\texttt{>xxxxxxxxv \\
|
|
x ~ ~ ~ ~x \\
|
|
x ~ ~ ~ ~x \\
|
|
x ~ ~ ~ ~x \\
|
|
x ~ ~ ~ ~x \\
|
|
x ~ ~ ~ ~x \\
|
|
x ~ ~ ~ ~x \\
|
|
x ~ ~ ~ ~x \\
|
|
\textasciicircum{}xxxxxxxx@
|
|
}
|
|
\end{center}
|
|
|
|
\begin{solution}
|
|
The intended solution is self-modifying code:
|
|
|
|
\begin{center}
|
|
\texttt{>69*6+97pv\\
|
|
p ~ ~ ~ ~8\\
|
|
7 ~ ~ ~ ~8\\
|
|
9 ~ ~ ~ ~*\\
|
|
* ~ ~ ~ ~0\\
|
|
8 ~ ~ ~ ~0\\
|
|
8 ~ ~ ~ ~p\\
|
|
\textasciicircum{}p00-2*88@
|
|
}
|
|
\end{center}
|
|
|
|
\end{solution}
|
|
|
|
|
|
\vfill
|
|
|
|
|
|
\problem{Bonus}
|
|
Write a quine. (i.e, write a program that outputs itself)
|
|
|
|
\begin{solution}
|
|
\texttt{01->1\# +\# :\# 0\# g\# ,\# :\# 5\# 8\# *\# 4\# +\# -\# \_@}
|
|
\end{solution}
|
|
|
|
\vfill
|
|
\pagebreak |