buffon's needle with luamplib

bneedle.tex

\input luamplib.sty
  \mplibnumbersystem{double}
  \mplibtextextlabel{enable}
\ifdefined\n\else \def\n{5000}\fi
\ifdefined\length\else \def\length{100}\fi
\ifdefined\distance\else \def\distance{100}\fi
\ifnum\length>\distance \let\length\distance\fi
\mplibcode
total := \n; l := \length; d := \distance;
path needle, hline;

beginfig(0);

hline := (-1/2l,0) -- (3/2l,0);
draw hline; draw hline shifted (d*up);

strike := 0;
for i=1 upto total:
  posx := uniformdeviate(l); posy := uniformdeviate(d);
  ang  := uniformdeviate(180);

  needle := (dir ang * -l/2 -- dir ang * l/2) shifted (posx, posy);
  if i <= 50: draw needle withcolor red; fi

  if (ypart point 1 of needle >= d) or (ypart point 0 of needle <= 0):
    strike := strike + 1;
  fi
endfor

pi_a := 2*l*total/(d*strike);
label.rt("$n = " & decimal total & "$", 20*down);
label.rt("$\pi \approx " & decimal pi_a & "$", 30*down);

endfig;
\endmplibcode
\end