Perform many removal, inserting operations on a table, fast!

batch.lua

local batch = {}

local holes_err = "bad argument #%s to function '%s' (this list doesn't support holes)"

local refill = function (t, ...)
  local newn = select('#', ...)
  local length = math.max(newn, (t.n or #t))

  for i = 1, length do
    t[i] = select(i, ...)

    if (not t.n) and (t[i] == nil) and (i < newn) then
      error(holes_err:format(i + 1, 'batch.refill'), 3)
    end
  end

  if t.n then
    t.n = newn
  end
end

local push = function (t, ...)
  local length = t.n or #t
  local pushed = select('#', ...)

  for i=1, pushed do
    local value = select(i, ...)

    if not t.n and value == nil then
      return i
    end

    t[length + i] = value
  end

  if t.n then
    t.n = length + pushed
  end
end

local splice = function (t, position, removed, ...)
  local inserted = select('#', ...)
  local length = (t.n or #t)
  removed = math.min(removed, length - position + 1)

  -- Hack: Inverting the order of the loop fixes issues with weird removing/inserting
  local a, b, c = position, length + inserted, 1
  if inserted > removed then
    a, b, c = b, a, -c
  end

  for i = a, b, c do
    local j = i - inserted

    if j < position then
      local value = select(i + 1 - position, ...)

      if not t.n and value == nil then
        return i + 1 - position
      end

      t[i] = value
    else
      local k = j + removed
      if k > length then
        t[i] = nil
      else
        t[i] = t[k]
      end
    end
  end

  if t.n then
    t.n = length
  end
end

-- a = {1, 2, 3, 4, 5}
-- b = {}
-- >>> batch.filter(a, b, function(i, t) return i%2 == 0 end)
-- b = {1, 3, 5}
-- Any other argument passed to batch.filter gets passed to the function after i, t
batch.filter = function (a, b, callback, ...)
  local alength = a.n or #a
  local blength = b.n or #b
  local left = alength

  local j = 1
  for i = 1, alength do
    if not callback(i, a, ...) then
      left = left - 1
    else
      b[j] = a[i]
      j = j + 1
    end
  end

  for i = left + 1, blength do
    b[i] = nil
  end

  if b.n then
    b.n = left
  end

  return b
end

-- t = {1, 2, 3, 4, 5}
-- >>> batch.refill(t, 6, 7, 8, 9, 10, 11)
-- t = {6, 7, 8, 9, 10, 11}
batch.refill = function (...)
  return refill(...)
end

-- t = {1, 2, 3, 4, 5}
-- >>> batch.clear(t)
-- t = {}
batch.clear = function (t)
  return refill(t)
end

-- t = {1, 2, 3, 4, 5, 6, 7}
-- >>> batch.splice(t, 3, 2, 8, 9, 10)
-- t = {1, 2, 8, 9, 10, 5, 6, 7}
batch.splice = function (t, ...)
  local index = splice(t, ...)

  if index then
    error(holes_err:format(index + 3, 'batch.splice'), 2)
  end

  return t
end

-- t = {1, 2, 3, 4, 5}
-- >>> batch.insert(t, 3, 6, 7, 8)
-- t = {1, 2, 6, 7, 8, 3, 4, 5}
batch.insert = function (t, position, ...)
  local index
  if not position or position > (t.n or #t) then
    index = push(t, ...)
  else
    index = splice(t, position, 0, ...)
  end

  if index then
    error(holes_err:format(index + 2, 'batch.insert'), 2)
  end

  return t
end

-- t = {1, 2, 3, 4, 5}
-- >>> batch.push(t, 6, 7, 8)
-- t = {1, 2, 3, 4, 5, 6, 7, 8}
batch.push = function (t, ...)
  local index = push(t, ...)

  if index then
    error(holes_err:format(index + 1, 'batch.push'), 2)
  end

  return t
end

-- t = {1, 2, 3, 4, 5, 6, 7}
-- >>> batch.remove(t, 3, 2)
-- t = {1, 2, 5, 6, 7}
batch.remove = function (t, position, n)
  splice(t, position, n)

  return t
end

-- t = {1, 2, nil, 4, 5}
-- >>> batch.maxn(t) == 5
batch.maxn = function (t)
  local n = math.max(t.n, #t)

  for k, _ in next, t do
    if type(k) == 'number' and k > n then
      n = k
    end
  end

  return n
end

-- a = {1, 2, 3, 4, 5}
-- b = {}
-- >>> batch.copy(a, b)
-- b = {1, 2, 3, 4, 5}
batch.copy = function (a, b)
  local alength = a.n or #a
  local blength = b.n or #b

  for i=1, math.max(alength, blength) do
    if i <= alength then
      b[i] = a[i]
    else
      b[i] = nil
    end
  end

  return b
end

return batch

RULE: Batch should never create tables by itself, not even when parameters are not supplied.