-- Removes blocks which are not placeable
local function removeNonBlocks()
	-- Travel to the ground to test if it can place each item
	local traveled = 0
	while not turtle.detectDown() do turtle.down() traveled = traveled + 1 end
	local blocks = {}

	-- Try placing each item and look at the error message produced
	-- If the message contains 'item' then it is an item
	for i=1, 16 do
		blocks[i] = turtle.getItemDetail(i)
		if blocks[i] then
			turtle.select(i)
			local _, reason = turtle.placeDown()
			if reason:match("item") then
				print(("Found non-block in slot '%s'"):format(i))
				blocks[i] = true
			end
		end
	end

	-- Travel back up to where the turtle started
	for _=1, traveled do turtle.up() end
	-- Drop all the items marked as non-blocks
	for i, details in pairs(blocks) do
		if type(details) ~= "table" then
			turtle.select(i)
			turtle.drop()
			blocks[i] = nil
		end
	end
	return blocks
end

local function followPath(walk_path)
	local command_lookup = {
		U = turtle.up, D = turtle.down,
		F = turtle.forward, B = turtle.back,
		L = turtle.turnLeft, R = turtle.turnRight
	}
	local count, op = 0, ""
	local pattern_iterator = walk_path:gmatch("(%d+)([UDFBLR])")
	return function()
		while true do
			if count == 0 then
				count, op = pattern_iterator()
				if not (count and op) then return end
			end

			command_lookup[op]()
			count = count - 1
			if op ~= "L" and op ~= "R" then
				return count, op
			end
		end
	end
end

local function getTravelCost(walk_path)
	local distance = 0
	for count, op in walk_path:gmatch("(%d+)([UDFBLR])") do
		if op ~= "L" and op ~= "R" then
			distance = distance + count
		end
	end
	return distance
end

-- Main
math.randomseed(os.epoch())
local build_path = "6F 1R 20F 1R 14F 1R 20F 1R 5F"

local travel_cost = getTravelCost(build_path)
if travel_cost > turtle.getFuelLevel() then
	print(("- Insufficient fuel level %s, needed %s"):format(turtle.getFuelLevel(), travel_cost))
	return
end
print(("- Sufficient fuel level %s, needed %s"):format(turtle.getFuelLevel(), travel_cost))

print("- Removing items which are not blocks")
local blocks_lookup = {}
local total_blocks = 0
for i, details in pairs(removeNonBlocks()) do
	local name = ("%s/%s"):format(details.name, details.damage or 0)
	blocks_lookup[name] = blocks_lookup[name] or {}
	blocks_lookup[name][i] = details.count
	blocks_lookup[name].total = (blocks_lookup[name].total or 0) + details.count
	total_blocks = total_blocks + details.count
end
turtle.up()

if travel_cost > total_blocks then
	print(("- Insufficient block count %s, needed %s"):format(total_blocks, travel_cost))
	return
end
print(("- Sufficient block count %s, needed %s"):format(total_blocks, travel_cost))

print(("- Following path and building: '%s'"):format(build_path))
for _ in followPath(build_path) do
	local target_name, target_score

	for name, details in pairs(blocks_lookup) do
		local score = details.total * math.random()
		if not target_score or score > target_score then
			target_score = score
			target_name = name
		end
	end

	local target_block = blocks_lookup[target_name]
	for i=1, 16 do
		if target_block[i] and target_block[i] > 0 then
			turtle.select(i)
			target_block[i] = target_block[i] - 1
			target_block.total = target_block.total - 1
			break
		end
	end
	turtle.placeDown()
end

print("- Finished")