ComputerCraft "go" utility alternative v1

--   Block actions combinators v1 (go)
--
-- f b u d l r   -- Move forward, backward, up, down, turn left, turn right
-- F B U D       -- same moves, but move until block detected
-- m M   --  mine block, mine blocks while block present (usefull for gravel mining)
-- [commands lastcommand]   --  loop while lastcommand returns success
-- commandNUMBER   -- loop command NUMBER times
-- [commands]NUMBER -- loop commands list NUMBER times
-- >DIGIT   --   place block from DIGIT slot. When DIGIT == 0, place first available
-- ^DIGIT   --   place block up
-- _DIGIT   --   place block down

local tArgs = { ... }
if #tArgs ~= 1 then
  print("Usage: go <list of commands>")
  return
end

-- CAPS-letter commands evaluator, move-only
-- Remove code duplication by encapsulating common pattern in function
local function mdMove(command, detector)
  while detector() == false do
    command()
  end
end

-- CAPS-letter commands evaluator, dig-only
-- Remove code duplication by encapsulating common pattern in function
local function mdDig(command, detector)
  while detector() do
    command()
    sleep(0.4)
  end
end

-- Evaluate prefix commands
local function runPrefixCmd(ch, digCmd, detectCmd, placeCmd)
  local chCode = string.byte(ch, 1) - 48
  if false then
  elseif ch == 'm' then return true, digCmd()
  elseif ch == 'M' then mdDig(digCmd, detectCmd); return true, false -- todo: better result return
  elseif ch == '0' then return true, placeCmd()
  elseif chCode > 0 and chCode < 10 then
    turtle.select(chCode)
    res = placeCmd()
    return true, res
  end
  return false, false
end

-- Lua specific array length
local function lngth(lst)
  if lst == nil then return 0 else return #lst end
end

local function parseNumber(st)
  _, _, num, rest = string.find(st, "^(%d+)(.*)$")
  return num, lngth(st) - lngth(rest)
end

local function parseLoop(st)
  _, _, loop, rest = string.find(st, "^(%b[])(.*)$")
  if loop ~= nil and loop ~= '' then
    return loop:sub(2,-2), lngth(st) - lngth(rest)
  elseif st:sub(1,1) == '[' then return nil, 1
  else return nil, 0
  end
end

function go(command)
  local i = 1
  local prevCommand = ''
  local result = false

  while i <= #command do
    local ch = command:sub(i, i)
    local nextCh = i == #command and '' or command:sub(i+1, i+1)

    local number, nLen = parseNumber(command:sub(i))
    local loop, lLen   = parseLoop(command:sub(i))

    -- if we found number, repeat last command number times
    if number ~= nil then
      number = tonumber(number)
      while number > 1 do
        go(prevCommand)
        number = number - 1
      end
      i = i + nLen - 1
    -- if we found loop, parse possible number and evaluate expression
    elseif loop ~= nil then
      local number, nLen = parseNumber(command:sub(i + lLen))
      if number ~= nil then
        number = tonumber(number)
        while number > 0 do
          go(loop)
          number = number - 1
        end
        i = i + nLen
      else
        while go(loop) do end
      end
      i = i + lLen - 1

    elseif ch == 'f' then result = turtle.forward()
    elseif ch == 'b' then result = turtle.back()
    elseif ch == 'u' then result = turtle.up()
    elseif ch == 'd' then result = turtle.down()

    elseif ch == 'F' then mdMove(turtle.forward, turtle.detect)
    elseif ch == 'B' then go("aFa") -- this is not primitive
    elseif ch == 'U' then mdMove(turtle.up, turtle.detectUp)
    elseif ch == 'D' then mdMove(turtle.down, turtle.detectDown)

    elseif ch == 'l' then turtle.turnLeft()
    elseif ch == 'r' then turtle.turnRight()
    elseif ch == 'a' then go("ll")

    elseif ch == 'm' then result = turtle.dig()
    elseif ch == 'M' then mdDig(turtle.dig, turtle.detect)

    elseif ch == '>' then
      local result1,result2 = runPrefixCmd(nextCh, turtle.dig, turtle.detect, turtle.place)
      if result1 then
        i = i + 1 -- todo: better prefix parsing
      end
      result = result2
    elseif ch == '^' then
      local result1,result2 = runPrefixCmd(nextCh, turtle.digUp, turtle.detectUp, turtle.placeUp)
      if result1 then
        i = i + 1
      end
      result = result2
    elseif ch == '_' then
      local result1,result2 = runPrefixCmd(nextCh, turtle.digDown, turtle.detectDown, turtle.placeDown)
      if result1 then
        i = i + 1
      end
      result = result2
    end

    prevCommand = ch
    i = i + 1
  end
  return result
end

go(tArgs[1])