-- Set constants.

DEFAULT_RADIUS = 10
MAX_RADIUS = 20
EMPTY_HARVEST_LIMIT = 15
EMPTY_LEVELS_LIMIT = 2

-- Define functions.

function refuel(distance)

   if distance == nil then
      distance = 1
   end

   if turtle.getFuelLevel() < distance then
      continue = true
   else
      continue = false
   end

   slot = 0

   while continue do

      slot = slot + 1
      turtle.select(slot)

      if turtle.refuel(0) then

         while continue and turtle.getItemCount() > 0 do

            turtle.refuel(1)

            if turtle.getFuelLevel() >= distance then
               continue = false
            end

         end

      end

      if slot == 16 then
         continue = false
      end

   end

end

function burrow(levels)

   returnValue = false

   if levels == nil then
      levels = 1
   end

   for i = 1, levels, 1 do

      if turtle.detectDown() then
         turtle.digDown()
         harvestCount = harvestCount + 1
         returnValue = true
      end

      turtle.down()
      levelTotal = levelTotal + 1

   end

   return returnValue

end

function move()

   turtle.forward()

   if heading == 0 then
      yPos = yPos + 1
   elseif heading == 90 then
      xPos = xPos + 1
   elseif heading == 180 then
      yPos = yPos - 1
   elseif heading == 270 then
      xPos = xPos - 1
   end

end

function turn(direction)

   if direction == nill then
      direction = "RIGHT"
   end

   if direction == "RIGHT" then
      turtle.turnRight()
      heading = heading + 90
   elseif direction == "LEFT" then
      turtle.turnLeft()
      heading = heading - 90
   end

   if heading == -90 then
      heading = 270
   elseif heading == 360 then
      heading = 0
   end

end

-- Get the first parameter value that is passed to the program.  This will be
-- the radius of the expanding square search.  If no paramter is passed, default
-- to a radius of 10.

args = {...}

if #args == 0 then
   args[1] = DEFAULT_RADIUS
end

args[1] = tonumber(args[1])

if args[1] == nil then
   args[1] = DEFAULT_RADIUS
elseif args[1] > MAX_RADIUS then
   args[1] = MAX_RADIUS
end

harvestLeg = {}

for i = 1, 3, 1 do
   harvestLeg[i] = args[1] * 4
end

-- Initialize the variables needed to run the loop and keep track of the
-- harvesting progress.

levelTotal = 0
legTotal = 0
harvestTotal = 0

heading = 0
xPos = 0
yPos = 0
emptyLevels = 0

moveDown = 1

-- Refuel the turtle as needed.

refuel(2)

-- Loop for each level that is harvested.

while moveDown == 1 do

   legCount = 0
   legTarget = 0
   legLength = 0
   lastHarvestLeg = 0

   levelUp = 0
   levelEven = 0
   levelDown = 0
   harvestCount = 0

   -- Examine the last five levels that have been processed, and find the
   -- maximum leg on which something was harvested.

   for i = 1, 3, 1 do
      if harvestLeg[i] > legTarget then
         legTarget = harvestLeg[i]
      end
   end

   legTarget = legTarget + 8

   -- Move the turtle down to the next level and begin recording whether
   -- anything is harvested.

   if burrow(1) then
      levelUp = 1
   end

   if burrow(1) then
      levelEven = 1
   end

   if turtle.detectDown() then
      turtle.digDown()
      harvestCount = harvestCount + 1
   end

   -- Begin looping through the different legs of the search. Note that the
   -- target leg value will be revaluated after each leg, and can expand as
   -- more harvesting takes place.

   while legCount < legTarget do

      legCount = legCount + 1

      -- On every second leg, the length of the leg increases by 1.  Ensure
      -- that there is enough fuel to accomplish the leg.

      if legCount % 2 == 1 then
         legLength = legLength + 1
      end

      refuel(legLength)

      -- Run through the length of the leg, harvesting as needed and keeping
      -- track of the progress.

      for blockIndex = 1, legLength, 1 do

         if turtle.detect() then
            turtle.dig()
            harvestCount = harvestCount + 1
            lastHarvestLeg = legCount
            levelEven = 0
         end

         move()

         if turtle.detectUp() then
            turtle.digUp()
            harvestCount = harvestCount + 1
            lastHarvestLeg = legCount
            levelUp = 1
         end

         if turtle.detectDown() then
            turtle.digDown()
            harvestCount = harvestCount + 1
            lastHarvestLeg = legCount
            levelDown = 1
         end

      end

      -- As new items are harvested within the canopy, reset the target value.

      if legTarget < lastHarvestLeg + 8 then
         legTarget = lastHarvestLeg + 8
      end

      if legTarget > MAX_RADIUS * 4 then
         legTarget = MAX_RADIUS * 4
      end

      -- Turn the turtle 90 degrees to the right and adjust its heading value.

      turn("RIGHT")

   end

   -- The turtle has now completed all of the legs needed for this level.
   -- Update all of the tracking variables and print progress messages.

   legTotal = legTotal + legCount
   harvestTotal = harvestTotal + harvestCount

   print("Level " .. levelTotal .. ": " .. legCount .. " legs, " .. harvestCount .. " harvested")

   -- Ensure that the turtle has enough fuel to return to the starting position.

   refuel(legLength + 10)

   -- Return the turtle to its starting position. Turn it 90 degrees to the
   -- right to backtrack along the last leg.

   turn("RIGHT")

   -- Move the turtle foward until it is even with its starting position.

   if heading == 0 or heading == 180 then
      while yPos ~= 0 do
         move()
      end
   elseif heading == 90 or heading == 270 then
      while xPos ~= 0 do
         move()
      end
   end

   -- Turn the turtle 90 degrees to the left, and move it to the starting
   -- position.

   turn("LEFT")

   if heading == 0 or heading == 180 then
      while yPos ~= 0 do
         move()
      end
   elseif heading == 90 or heading == 270 then
      while xPos ~= 0 do
         move()
      end
   end

   -- Perform a series of right turns until the turtle is facing in its
   -- original direction.

   while heading ~= 0 do
      turn("RIGHT")
   end

   -- Move down one block to sit on top of what will now be the next solid level.

   burrow(1)

   -- Store the last leg which resulted in harvesting in the historical
   -- recording array, deleting the least-recent record.

   harvestLeg[3] = harvestLeg[2]
   harvestLeg[2] = harvestLeg[1]
   harvestLeg[1] = lastHarvestLeg

   -- Determine whether so few items were harvested on this level that it
   -- qualifies as an empty level, keeping track of how many empty levels
   -- have been encountered in a row.

   if harvestCount / (levelUp + levelEven + levelDown) < EMPTY_HARVEST_LIMIT then
      emptyLevels = emptyLevels + 1
   else
      emptyLevels = 0
   end

   if emptyLevels >= EMPTY_LEVELS_LIMIT then
       moveDown = 0
   end if

end

while not turtle.detectDown() do
   turtle.down()
end