Diamond Squared algorithm in #pico8

diamond.p8

-- not actually a cart, but paste it into the pico-8 code section
-- diamond squared implementation
-- by ben jones/@powersaurus
-- based on this wikipedia image https://en.wikipedia.org/wiki/diamond-square_algorithm#/media/file:diamond_square.svg
-- https://en.wikipedia.org/wiki/diamond-square_algorithm
-- it is probably wrong. looks nice though
function _init()
 printh("\n ███████████████")
 printh(" █new board!!!!█")
 printh(" ███████████████")
 grid={}
 height=129
 width=129
 step=height-1
 rand=255
 randomness=1.3
 iter=0
 -- everything starts at same height
 for i=0,height-1 do
  grid[i]={}
  for j=0,width-1 do
   grid[i][j]=64--rnd(128)
  end
 end
end

function _update()
 if btnp(❎) then
  do_step()
  iter+=1
  printh("iteration "..iter)
 end
end

-- iterate on the heightmap
function do_step()
 printh(step)
 if(step<1)step=1
 printh("\n\n>>>")
 for y=-height*2,height*2,step do
  for x=-width*2,width*2,step do
   half_step=flr(step/2)
   vx=(x+half_step)%width
   vy=(y+half_step)%height
   printh("visit "..vx..", "..vy)
   xx=x%width
   yy=y%height
   a=grid[yy][xx]
   b=grid[yy][(xx+step)%width]
   c=grid[(yy+step)%height][xx]
   d=grid[(yy+step)%height][(xx+step)%width]
   grid[vy][vx]=
    flr(
     (a+b+
      c+d)
      /4)
   new_h=grid[vy][vx]
   grid[(vy-half_step)%height][vx]=randomise(new_h)
   grid[vy][(vx+half_step)%width]=randomise(new_h)
   grid[(vy+half_step)%width][vx]=randomise(new_h)
   grid[vy][(vx-half_step)%width]=randomise(new_h)
  end
 end
  step/=2
  rand/=randomness
  step=flr(step)
 printh("---")
end
-- add some randomness to a value
function randomise(v)
 return new_h+flr(rnd(rand))-rand/2
end
-- heightmap, lighter colours for
-- higher points, darker colours for
-- lower points
function hm(v)
 if v>90 then
  return 7
 elseif v>70 then
  return 6
 elseif v>40 then
  return 13
 elseif v>10 then
  return 5
 else
  return 1
 end
end

function cloud(v)
 if v>40 then
  return 7
 else
  return 1
 end
end
function _draw()
 -- draw grid of pixels
 for i=0,0 do
  for j=0,0 do
  for y=0,height-1 do
   for x=0,width-1 do
    pset(
     i*(width-1)+x,
     j*(height-1)+y,
     hm(grid[y][x]))
   end
  end
  end
 end

end