Source code in the seed7 programming language

$ include "seed7_05.s7i";
  include "draw.s7i";
  include "keybd.s7i";

const integer: SIZE is 300;
const integer: SCALE is 1;

const proc: genBrownianTree (in integer: fieldSize, in integer: numParticles) is func
  local
    var array array integer: world is 0 times 0 times 0;
    var integer: px is 0;
    var integer: py is 0;
    var integer: dx is 0;
    var integer: dy is 0;
    var integer: i is 0;
    var boolean: bumped is FALSE;
  begin
    world := fieldSize times fieldSize times 0;
    world[rand(1, fieldSize)][rand(1, fieldSize)] := 1;  # Set the seed
    for i range 1 to numParticles do
      # Set particle's initial position
      px := rand(1, fieldSize);
      py := rand(1, fieldSize);
      bumped := FALSE;
      repeat
        # Randomly choose a direction
        dx := rand(-1, 1);
        dy := rand(-1, 1);
        if dx + px < 1 or dx + px > fieldSize or dy + py < 1 or dy + py > fieldSize then
          # Plop the particle into some other random location
          px := rand(1, fieldSize);
          py := rand(1, fieldSize);
        elsif world[py + dy][px + dx] <> 0 then
          # Bumped into something
          world[py][px] := 1;
          rect(SCALE * pred(px), SCALE * pred(py), SCALE, SCALE, white);
          DRAW_FLUSH;
          bumped := TRUE;
        else
          py +:= dy;
          px +:= dx;
        end if;
      until bumped;
    end for;
  end func;
 
const proc: main is func
  begin
    screen(SIZE * SCALE, SIZE * SCALE);
    KEYBOARD := GRAPH_KEYBOARD;
    genBrownianTree(SIZE, 20000);
    readln(KEYBOARD);
  end func;