# Counting numbers performantly
## :couters module
* counter module does addition and subtraction on an array of numbers performantly
* used in Elixir's [Logger.Backends](https://github.com/elixir-lang/elixir/blob/b20ad8a1514008595bebe95ed9f8dfc67380a779/lib/logger/lib/logger/backends/internal.ex)
* See https://www.erlang.org/doc/man/counters.html
* [Elixir 高性能な數の數え方（counters）](https://qiita.com/mnishiguchi/items/b249ce780a064cc52d65)
## Determine counter size
```elixir
# how many numbers to manage
counter_size = 2
# one-based numbering
autorace_index = 1
toukon_index = 2
```
## Create a counter instance
* options
* `:atomics` (default)
* `write_concurrency`
```elixir
my_counter = :counters.new(counter_size, [:write_concurrency])
get_state = fn ->
%{
autorace: :counters.get(my_counter, autorace_index),
toukon: :counters.get(my_counter, toukon_index)
}
end
get_state.()
```
## Add number to counter
```elixir
:counters.add(my_counter, autorace_index, 1)
:counters.add(my_counter, toukon_index, 123)
get_state.()
```
## Subtract number from counter
```elixir
:counters.sub(my_counter, autorace_index, 124)
:counters.sub(my_counter, toukon_index, 24)
get_state.()
```
## Overwrite counter
```elixir
:counters.put(my_counter, autorace_index, 0)
:counters.put(my_counter, toukon_index, 0)
get_state.()
```
## Count large mounts of numbers
```elixir
do_count = fn _ ->
case Enum.random([:toukon, :autorace]) do
:toukon -> :counters.add(my_counter, toukon_index, 1)
:autorace -> :counters.add(my_counter, autorace_index, 1)
end
end
1..100_000
|> Task.async_stream(&do_count.(&1), max_concurrency: 500)
|> Stream.run()
get_state.()
```
## Links
* https://elixirpatterns.dev/
