brainfuck.lua

local function pointerAdd(state)
  state.pointer = state.pointer + 1
end

local function pointerSub(state)
  state.pointer = state.pointer - 1
end

local function dataAdd(state)
  state.data[state.pointer] = (state.data[state.pointer] or 0) + 1
end

local function dataSub(state)
  state.data[state.pointer] = (state.data[state.pointer] or 0) - 1
end

local function dataOut(state)
  print(string.char(state.data[state.pointer]))
end

local function dataIn(state)
  state.data[state.pointer] = io.read()
end

---@param program string
---@return table
local function analyze(program)
  local tree = {
    pointer = 1,
    result = {}
  }

  for index = 1, #program do
    if (index >= tree.pointer) then
      local char = program:sub(index, index)
      tree.pointer = tree.pointer + 1

      if char == "[" then
        local subtree = analyze(program:sub(index + 1))
        table.insert(tree.result, subtree.result)
        tree.pointer = tree.pointer + subtree.pointer - 1
      elseif char == "]" then
        break
      elseif char == ">" then table.insert(tree.result, pointerAdd)
      elseif char == "<" then table.insert(tree.result, pointerSub)
      elseif char == "+" then table.insert(tree.result, dataAdd)
      elseif char == "-" then table.insert(tree.result, dataSub)
      elseif char == "." then table.insert(tree.result, dataOut)
      elseif char == "," then table.insert(tree.result, dataIn)
      end
    end
  end

  return tree
end

---@param state table
---@param tree table | function
local function execute(state, tree)
  if #tree ~= nil then
    for i = 1, #tree do
      local nodeType = type(tree[i])
      if nodeType == "table" then
        while (state.data[state.pointer] or 0) ~= 0 do
          execute(state, tree[i])
        end
      elseif nodeType == "function" then
        tree[i](state)
      end
    end
  end
end

function Main()
  local INPUT = [[
++       Cell c0 = 2
> +++++  Cell c1 = 5

[        Start your loops with your cell pointer on the loop counter (c1 in our case)
< +      Add 1 to c0
> -      Subtract 1 from c1
]        End your loops with the cell pointer on the loop counter

At this point our program has added 5 to 2 leaving 7 in c0 and 0 in c1
but we cannot output this value to the terminal since it is not ASCII encoded

To display the ASCII character "7" we must add 48 to the value 7
We use a loop to compute 48 = 6 * 8

++++ ++++  c1 = 8 and this will be our loop counter again
[
< +++ +++  Add 6 to c0
> -        Subtract 1 from c1
]
< .        Print out c0 which has the value 55 which translates to "7"!
  ]]

  local program = INPUT:gsub("[^><+-.,%[%]]", "")
  local tree = analyze(program)
  local state = {
    pointer = 0,
    data = {}
  }
  execute(state, tree.result)
end

Main()

brainfuckfile.lua

local function pointerAdd(state)
  state.pointer = state.pointer + 1
end

local function pointerSub(state)
  state.pointer = state.pointer - 1
end

local function dataAdd(state)
  state.data[state.pointer] = (state.data[state.pointer] or 0) + 1
end

local function dataSub(state)
  state.data[state.pointer] = (state.data[state.pointer] or 0) - 1
end

local function dataOut(state)
  print(string.char(state.data[state.pointer]))
end

local function dataIn(state)
  state.data[state.pointer] = io.read()
end

---@param program string
---@return table
local function analyze(program)
  local tree = {
    pointer = 1,
    result = {}
  }

  for index = 1, #program do
    if (index >= tree.pointer) then
      local char = program:sub(index, index)
      tree.pointer = tree.pointer + 1

      if char == "[" then
        local subtree = analyze(program:sub(index + 1))
        table.insert(tree.result, subtree.result)
        tree.pointer = tree.pointer + subtree.pointer - 1
      elseif char == "]" then
        break
      elseif char == ">" then table.insert(tree.result, pointerAdd)
      elseif char == "<" then table.insert(tree.result, pointerSub)
      elseif char == "+" then table.insert(tree.result, dataAdd)
      elseif char == "-" then table.insert(tree.result, dataSub)
      elseif char == "." then table.insert(tree.result, dataOut)
      elseif char == "," then table.insert(tree.result, dataIn)
      end
    end
  end

  return tree
end

---@param state table
---@param tree table | function
local function execute(state, tree)
  if #tree ~= nil then
    for i = 1, #tree do
      local nodeType = type(tree[i])
      if nodeType == "table" then
        while (state.data[state.pointer] or 0) ~= 0 do
          execute(state, tree[i])
        end
      elseif nodeType == "function" then
        tree[i](state)
      end
    end
  end
end

function Main()
  local program = io.input("brain.fuck"):read("*a"):gsub("[^><+-.,%[%]]", "")
  local tree = analyze(program)
  local state = {
    pointer = 0,
    data = {}
  }
  execute(state, tree.result)
end

Main()