Positioned byte source.

pread/3 reads at most length bytes. That argument is the caller's cap. There is no process-wide size limit. A read that starts at or past the end returns {:ok, ""}. A read that extends past the end returns the bytes that remain. Implementations return a tagged error instead of raising.

F3.IO.Memory is the small implementation. A custom source has the same three callbacks:

defmodule ByteSource do
  @behaviour F3.IO.Source

  defstruct [:data, closed: false]

  @impl true
  def size(%{closed: true}), do: {:error, F3.Error.closed(:size)}
  def size(%{data: data}), do: {:ok, byte_size(data)}

  @impl true
  def pread(%{closed: true}, _offset, _length), do: {:error, F3.Error.closed(:pread)}

  def pread(%{data: data}, offset, length) do
    {:ok, F3.IO.Memory.slice(data, byte_size(data), offset, length)}
  end

  @impl true
  def close(state), do: {:ok, %{state | data: <<>>, closed: true}}
end