"sandbox" (1.3 pr2)

local myenv = getfenv()
local turtle = turtle
local env = {
    turtle = {
        select = turtle.select,
        drop = turtle.drop
    }
}

setmetatable(env, {__index = myenv, __newindex = myenv})

local undolist = {}

-- initially: +X is forward, +Y is left, +Z is up

local dx, dy = 1, 0
local x, y, z = 0, 0, 0

function env.turtle.gps()
    return x, y, z
end

function env.turtle.face(ndx, ndy)
    if ndx == -dx and ndy == -dy then
        env.turtle.turnLeft()
        env.turtle.turnLeft()
    elseif ndy == dx and ndx == -ndy then
        env.turtle.turnLeft()
    elseif ndy == -dx and ndx == ndy then
        env.turtle.turnRight()
    end
end

local gpshook = {
    [turtle.turnLeft] = function()
        dy, dx = dx, -dy
    end,
    [turtle.turnRight] = function()
        dy, dx = -dx, dy
    end,
    [turtle.forward] = function()
        x = x + dx
        y = y + dy
    end,
    [turtle.back] = function()
        x = x - dx
        y = y - dy
    end,
    [turtle.up] = function()
        z = z + 1
    end,
    [turtle.down] = function()
        z = z - 1
    end,
}

local function wrap(fn, rev)
    return function(...)
        table.insert(undolist, rev)
        local a = {fn(...)}
        if #a == 0 or a[1] then
            if gpshook[fn] then gpshook[fn]() end
        else
            table.remove(undolist,#undolist)
        end
        return unpack(a)
    end
end

local function pair(a, b)
    env.turtle[a] = wrap(turtle[a], turtle[b])
    env.turtle[b] = wrap(turtle[b], turtle[a])
end

pair("forward", "back")
pair("up", "down")
pair("turnLeft", "turnRight")
pair("dig", "place")
pair("digUp", "placeUp")
pair("digDown", "placeDown")

local args = {...}

local noundo = (args[1] == "noundo")
if noundo then table.remove(args, 1) end

local prog = shell.resolveProgram(table.remove(args, 1))
print("Beginning sandboxed execution")
local ok, result = pcall(os.run, env, prog, unpack(args))
print("Ended sandboxed execution")

if not ok then print("Error: ",result) end

if noundo then
    print("Not undoing ",#undolist," actions")
else
    print("Undoing ",#undolist," actions")

    for k = #undolist,1,-1 do
        undolist[k]()
    end

    print("Undo complete")
end