Brainfuck Interpreter

--  An interpreter to run the Brainfuck language in lua
--  The size of the memory array is in this example 1024 (1 - 1025)

local _args = {...}

local _nSize = 1024
local _nMaxVal = 255

local _nVars = {}
local _nPtr = 1

local _nChar = 1
local _sScript
local _nEntries = {}

local _usage = [[
Brainfuck - Usage

  'bfq <filename>'
  'bfq :<usage|info|help|test|learn>'
]]
local _info = [[
Brainfuck - Info

  Brainfuck is an esoteric programming language
  created in 1993 by Urban Mueller, and notable
  for its extreme minimalism.

  The language consists of only eight simple
  commands and an instruction pointer.

  While it is fully Turing-complete, it is not
  intended for practical use, but to challenge
  and amuse programmers.

  https://en.wikipedia.org/wiki/Brainfuck
  8th of December 2016
  ]]
 local _help = [[
Brainfuck - Help

  :usage - How to use this program
  :info  - Information about Brainfuck
  :help  - This message
  :test  - Runs a small program
  :learn - Explains how to use Brainfuck
]]
local _test = "Brainfuck - Test\n\n  Program output:"
 local _learn = [[
Brainfuck - Learn

  There are eight commands:
    < - Decrease the pointer
    > - Increase the pointer
    + - Increase the current cell value
    - - Decrease the current cell value
    [ - Declare a loop
    ] - Declare the end of a lopp
    . - Display the current cell value in ASCII
    , - Read the user's input into the cell

  A loop gets executed when the current cell has
  a value greater than zero. Cells overflow and
  underflow. The pointer does so too.
  Cell range: 0-255
  Pointer range: 1-1025
]]
 local _notFound = [[
Brainfuck - Error

  Command/File not found
  Try ':help' instead
 ]]

--  Several cleaning/ready functions (mostly for aestetic reasons)
local function _printf(_sInput)
    term.write(_sInput)
end

local function _readyAll()
    for i = 1,_nSize do
        _nVars[i] = 0
    end
    _nPtr = 1
end

--  The six functions for memory manipulation/output
--  < > - + ,
local function _decPtr()
    _nPtr = _nPtr-1
    if _nPtr < 1 then
        _nPtr = _nSize
    end
end

local function _nncPtr()
    _nPtr = _nPtr+1
    if _nPtr > _nSize then
        _nPtr = 1
    end
end

local function _decVal()
    _nVars[_nPtr] = _nVars[_nPtr] - 1
    if _nVars[_nPtr] < 0 then
        _nVars[_nPtr] = _nMaxVal
    end
end

local function _nncVal()
    _nVars[_nPtr] = _nVars[_nPtr] + 1
    if _nVars[_nPtr] > _nMaxVal then
        _nVars[_nPtr] = 0
    end
end

local function _getChar()
    local _sInput = read()
    if string.len(_sInput)  < 1 then
        _nVars[_nPtr] = 0
    else
        _nVars[_nPtr] = (string.byte(string.sub(_sInput,1,1)) > _nMaxVal and 0)or string.byte(string.sub(_sInput,1,1))
    end
end

local function _putChar()
    _printf(string.char(_nVars[_nPtr]))
end

--  The remaining functions for the loop
--  [ ]
local function _jmpLoop()
    local _nCount = 0
    while _nChar < string.len(_sScript) do
        _sChar = string.sub(_sScript,_nChar,_nChar)
        if _sChar == "[" then
            _nCount = _nCount + 1
        elseif _sChar == "]" then
            _nCount = _nCount -1
        else

        end
        _nChar = _nChar +1
    end
end

--  The interpreter
local function _doString(_sInput)
    _sScript = _sInput
    _readyAll()
    repeat
        c = string.sub(_sScript,_nChar,_nChar)
        if c == nil then
        elseif c=="<" then
            _decPtr()
        elseif c==">" then
            _nncPtr()
        elseif c=="-" then
            _decVal()
        elseif c=="+" then
            _nncVal()
        elseif c=="." then
            _putChar()
        elseif c=="," then
            _getChar()
        elseif c=="[" then
            if _nVars[_nPtr] > 0 then
                _nEntries[#_nEntries + 1] = _nChar
            else
                _jmpLoop()
            end
        elseif c=="]" then
            if _nVars[_nPtr] > 0 then
                _nChar = _nEntries[#_nEntries]
            else
            end
        end

        _nChar = _nChar+1
    until _nChar > string.len(_sScript)
    print("")
end

--  The frontend
local function _commands()
    if _args[1] == ":usage" then
        print(_usage)
    elseif _args[1] == ":info" then
        print(_info)
    elseif _args[1] == ":help" then
        print(_help)
    elseif _args[1] == ":test" then
        print(_test)
        _doString("+++++[->+++++<]>+++++++..<++++++[->++++++<]>+++++.<+++++[->+++++<]>++++.<++++++++[->--------<]>------.<+++++++++[->+++++++++<]>++++++++.<+++[->---<]>-.<>++++++.<+++++++++[->---------<]>----.<+++++++++[->+++++++++<]>++.<+++[->---<]>-----.<>.<++++++++[->--------<]>-----.<+++++++++[->+++++++++<]>+++.<+++[->---<]>---.<>+.<+++[->+++<]>+.<+++++++++[->---------<]>--.<++++++++[->++++++++<]>+++++++++++++.<>--------.<+++[->+++<]>+++++.<>.<++++[->----<]>--.<>++++++.<>--.<+++++++[->-------<]>--------.<+++[->---<]>---.<+++++++++[->+++++++++<]>+++.<+++[->---<]>---.<>---.<++++++++[->--------<]>-----.<++++++++[->++++++++<]>+++++++++.<>+++++.<>++++++.<+++[->---<]>------.<+++[->+++<]>++++.<>--.<>++.<+++[->---<]>----.<+++[->+++<]>++++++.<+++[->---<]>------.<+++[->+++<]>++++.<+++++++++[->---------<]>-.<+++++++++[->+++++++++<]>++++++.<>--------.<>+++.<>-------.<>++++++++.<+++++++++[->---------<]>-.<++++[->----<]>-------.<")
    elseif _args[1] == ":learn" then
        print(_learn)
    else
        print(_notFound)
    end
end

local function _run()
    if fs.exists(_args[1]) then
        _fFile = fs.open(_args[1],"r")
        _sFile = _fFile.readAll()
        _fFile.close()
        _doString(_sFile)
    else
        print(_notFound)
    end
end

local function _main()
    shell.run("clear")
    if #_args < 1 then
        print(_usage)
        return
    end
    if string.sub(_args[1],1,1) == ":" then
        _commands()
    else
        _run()
    end
end

_main()