ComputerCraft Turtle Circle program (not mine)

local r = 0
currSlot = 2 --reserve slot #1 for fuel all time!
mfid = 0 --monitor computer's id
osid = os.getComputerID() --turtle's computer id
isSent = false --send all messages just once
fReserve = 10 --fuel can't go below this
print("What radius of circle would you like me to create?")
r = tonumber(io.read())
if r == 0 then
  print("Radius 0 circle? Done!")
end
--send message to monitor computer(if any) and write it out locally
function sendM(message)
  if peripheral.isPresent("right") and peripheral.getType("right") == "modem" then
        rednet.open("right")
        rednet.send(mfid, "Turtle("..osid.."):"..message, false)
        rednet.close("right")
  end
  print(message)
end
--return gps location if it has a modem and can retrive turtles's location
--if not then return x,y based on turtle's startup location
function gpsLoc(fx,fy)
  if peripheral.isPresent("right") and peripheral.getType("right") == "modem" then
        rednet.open("right")
        local x, y, z = gps.locate(5)
        rednet.close("right")
        if x == nil then
          return "x:"..tostring(fx)..",y:"..tostring(fy).."(rel)"
        else
          return "x:"..tostring(x)..",y:"..tostring(y)..",z:"..tostring(z)
        end
  else
        return "x:"..tostring(fx)..",y:"..tostring(fy).."(rel)"
  end
end
function findBlock ()
  if turtle.getItemCount(currSlot) ~= 0 then
        return true
  end

  local tmp = 2
  turtle.select(tmp)

  while (turtle.getItemCount(tmp) == 0 and tmp <= 16) do
        if tmp == 16 then
          if isSent == false then --only print once
                sendM("Out of blocks! Turtle at "..gpsLoc(x,y))
                isSent = true
          end
          sleep(5)
          tmp = 2
          turtle.select(tmp)
        else
          tmp = tmp + 1
          turtle.select(tmp)
        end
  end
  isSent = false
  currSlot = tmp
  tmp = 2

  return true
end
function tryPlaceDown (x,y)
  if turtle.detectDown() == true then
        if turtle.compareDown() == true then
          return true
        else
          sendM("Can't place block("..gpsLoc(x,y)..")!")
          return false
        end
  else
        if turtle.placeDown() == true then
          return true
        else
          sendM("Can't place block("..gpsLoc(x,y)..")!")
          return false
        end
  end
end
round = function (inputN)
  if inputN % 2 ~= 0.5 then
        return math.floor(inputN + 0.5)
  end
  return inputN - 0.5
end
turtle.select(currSlot)
local coords = {}
local coordsCount = 0
local f = 1 - r
local ddF_x = 1
local ddF_y = -2 * r
x = 0
y = r
coords[0 .. "," .. round(0+r)] = true
coords[0 .. "," .. round(0-r)] = true
coords[0+r .. "," .. 0] = true
coords[0-r .. "," .. 0] = true
coordsCount = 4
while (x < y) do
  if (f >= 0) then
        y = y - 1
        ddF_y = ddF_y + 2
        f = f + ddF_y
  end
  x = x + 1
  ddF_x = ddF_x + 2
  f = f + ddF_x
  coords[round(0+x) .. "," .. round(0+y)] = true
  coords[round(0-x) .. "," .. round(0+y)] = true
  coords[round(0+x) .. "," .. round(0-y)] = true
  coords[round(0-x) .. "," .. round(0-y)] = true
  coords[round(0+y) .. "," .. round(0+x)] = true
  coords[round(0-y) .. "," .. round(0+x)] = true
  coords[round(0+y) .. "," .. round(0-x)] = true
  coords[round(0-y) .. "," .. round(0-x)] = true
  coordsCount = coordsCount + 8
end
checkCoords = function (x0,y0)
  if coords[x0 .. "," .. y0] == true then
        return true
  else
        return false
  end
end
--watch fuel level and if smaller than the reserve level refuel from slot #1
function fuelProc()
  local flvl = turtle.getFuelLevel()
  if flvl == "unlimited" then
        return true
  elseif flvl > fReserve then
        return true
  else
        turtle.select(1)
        while turtle.refuel(1) == false do
          if isSent == false then
                sendM("Out of fuel! turtle at "..gpsLoc(x,y))
                print("Please insert fuel to slot #1 to resume!")
                isSent = true
          end
          sleep(5)
        end
        isSent = false
        turtle.select(currSlot)
  end
end
-- Move from the center to the first position it can find.
local x = 0
local y = 0
while checkCoords(x,y) ~= true do
  fuelProc()
  turtle.forward()
  x = x + 1
end
-- Place a block in the first position.
--findBlock()
--tryPlaceDown() <-block will be placed last so won't be any route blocking at multi lvl structures
-- Keep track of which way the turtle is facing.
facingX = 1
facingY = 0
-- Look clockwise from each block and find the next block in the space.
spinsDone = 0
while (coordsCount > 0 and spinsDone < 5) do
  fuelProc()
  cX = x + facingX
  cY = y + facingY
  if checkCoords(cX, cY) then
        turtle.forward()
        findBlock()
        tryPlaceDown(x,y)
        x = cX
        y = cY
        coords[x .. "," .. y] = nil
        coordsCount = coordsCount - 1
        spinsDone = 0
  else
        -- Check diagnals
        if facingX == 1 then
                  cY = y + 1
                  newFacingX = 0
                  newFacingY = 1
        elseif facingX == -1 then
                  cY = y - 1
                  newFacingX = 0
                  newFacingY = -1
        elseif facingY == 1 then
                  cX = x - 1
                  newFacingX = -1
                  newFacingY = 0
        elseif facingY == -1 then
                  cX = x + 1
                  newFacingX = 1
                  newFacingY = 0
        end

        if checkCoords(cX,cY) then
                  turtle.forward()
                  turtle.turnRight()
                  turtle.forward()
                  turtle.turnLeft()
                  findBlock()
                  tryPlaceDown(x,y)
                  x = cX
                  y = cY
                  coords[x .. "," .. y] = nil
                  coordsCount = coordsCount - 1
                  spinsDone = 0
        else
                  turtle.turnRight()
                  facingX = newFacingX
                  facingY = newFacingY
                  spinsDone = spinsDone + 1
        end
  end
end
--return to starting point
repeat
  fuelProc()
  turtle.forward()
  x = x - 1
until x == 0
turtle.turnRight()
turtle.turnRight()
sendM("Work done!")
x = nil
y = nil
r = nil
coords = nil
coordsCount = nil