I'm using ets via elixir as a simple in-memory persistence layer to store and retrieve keys and also for the occasional foldl which involves reducing many duplicate keys that have different values. I am using the bag option.
Is there a simple, perhaps O(1) way to retrieve a list of just the current keys without having to do a more involved table traversal or match or foldl?
Erlang or Elixir syntax responses welcome.
:ets.new(:cache, [:bag, :named_table, :protected])
I have a static map of atom keys indexed by integer I am using to assist with insertions. But not all the keys are used..
chunk_key_map = %{2 => :chunk_key_2, ..... 28 => :chunk_key_28}
If there's no quick way, I am aware I can do an ets:lookup trying each of my static atom key values and testing for != [] and generating my own list, but wanted to see if ets supports such a feature.
Thanks
So I didn't find the ets technique, but rather implemented the key list retrieve code in constant time in elixir as my key map is static.
list = Enum.reduce(2..28, [], fn head, acc ->
case :ets.lookup(:cache, Map.get(chunk_key_map, head)) do
[] -> acc
_ -> [acc, head]
end
end)
List.flatten(list)
UPDATE: Based on the replies, I took Hamidreza's ets
traversal logic and wrapped it into an Elixir Stream
using Stream.resource/3
.
defp get_ets_keys_lazy(table_name) when is_atom(table_name) do
eot = :"$end_of_table"
Stream.resource(
fn -> [] end,
fn acc ->
case acc do
[] ->
case :ets.first(table_name) do
^eot -> {:halt, acc}
first_key -> {[first_key], first_key}
end
acc ->
case :ets.next(table_name, acc) do
^eot -> {:halt, acc}
next_key -> {[next_key], next_key}
end
end
end,
fn _acc -> :ok end
)
end
Then I ran the stream through a pipeline
get_ets_keys_lazy(table_name)
|> Stream.map(lambda1)
|> Stream.each(lambda2)
|> Stream.run