# `F3.Decoder`

Decodes one CASCADE encoding unit into an owned `F3.Column`.

The default implementation is `F3.Decoder.Native`. Tests replace it by
setting `Application.put_env(:f3, :decoder, module)`. The module must
implement this behaviour. `F3.read/2` uses that module for every unit
and does not call it when the native build is absent.

## Examples

A test double returns one owned column and does not touch a file:

    defmodule SampleDecoder do
      @behaviour F3.Decoder

      @impl true
      def decode(_bytes, :uncompressed, :i64) do
        F3.Column.from_native(:i64, 1, <<1::little-64>>, nil)
      end

      def decode(_bytes, _compression, _type) do
        {:error, F3.Error.unsupported_encoding(:decode, "unsupported unit")}
      end
    end

`decode/3` receives the unit bytes, the compression atom, and the
physical type atom. It returns `{:ok, column}` or `{:error, %F3.Error{}}`.
It should not raise for a unit it refuses.

# `decode`

```elixir
@callback decode(binary(), atom(), atom()) ::
  {:ok, F3.Column.t()} | {:error, F3.Error.t()}
```

Decodes one CASCADE unit.

`bytes` is the unit payload. `compression` is `:uncompressed`, `:zstd`,
or `:lz4`. `type` is the physical kind, such as `:i64`. The return is an
owned column or an error. A refusal does not raise.

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*Consult [api-reference.md](api-reference.md) for complete listing*