ex17.2.lua

local function disk (cx, cy, r)
  return function (x, y)
    return (x - cx)^2 + (y - cy)^2 <= r^2
  end
end
local function rect (left, right, bottom, up)
  return function (x, y)
    return left <= x and x <= right and
      bottom <= y and y <= up
  end
end
local function complement (r)
  return function (x, y)
    return not r(x, y)
  end
end
local function union (r1, r2)
  return function (x, y)
    return r1(x, y) or r2(x, y)
  end
end
local function intersection (r1, r2)
  return function (x, y)
    return r1(x, y) and r2(x, y)
  end
end
local function difference (r1, r2)
  return function (x, y)
    return r1(x, y) and not r2(x, y)
  end
end
local function translate (r, dx, dy)
  return function (x, y)
    return r(x - dx, y - dy)
  end
end
local function plot (r, M, N)
  io.write("P1\n", M, " ", N, "\n")
  -- header
  for i = 1, N do
    -- for each line
    local y = (N - i*2)/N
    for j = 1, M do
      -- for each column
      local x = (j*2 - M)/M
      io.write(r(x, y) and "1" or "0")
    end
    io.write("\n")
  end
end

local M = {
  disk = disk,
  rect = rect,
  complement = complement,
  union = union,
  intersection = intersection,
  difference = difference,
  translate = translate,
  plot = plot
}

return M