# `F3.Footer`

Decodes an F3 footer into `F3.Metadata`.

The footer FlatBuffers buffer is the last `footer_size` bytes of the
metadata span. Column metadata is a separate FlatBuffers root in the
bytes before that footer. Both reads stay inside the span already
accepted by `F3.Postscript`. Wasm section payloads are not read.

# `decode`

```elixir
@spec decode(binary(), F3.Postscript.t()) ::
  {:ok, F3.Metadata.t()} | {:error, F3.Error.t()}
```

Decodes a metadata span that was already bounds-checked.

`span` is the metadata bytes. `postscript.footer_size` selects the footer
at the end of that span. A span shorter than `footer_size` raises
`ArgumentError`. A span that is not a FlatBuffers table returns
`:corrupt_metadata`.

## Examples

    iex> post = %F3.Postscript{
    ...>   metadata_size: 1,
    ...>   footer_size: 1,
    ...>   footer_compression: :uncompressed,
    ...>   checksum_type: :xxhash,
    ...>   data_checksum: 0,
    ...>   schema_checksum: 0,
    ...>   major_version: 0,
    ...>   minor_version: 1,
    ...>   metadata_offset: 0,
    ...>   footer_offset: 0,
    ...>   file_size: 1
    ...> }
    iex> {:error, error} = F3.Footer.decode(<<0>>, post)
    iex> error.category
    :corrupt_metadata

# `load`

```elixir
@spec load(F3.File.t(), non_neg_integer()) ::
  {:ok, F3.Metadata.t()} | {:error, F3.Error.t()}
```

Reads the metadata span and returns the decoded file metadata.

`max` is the metadata cap in bytes. A short file is `:truncated`. A file
that is not an `F3.File`, or a negative `max`, raises `FunctionClauseError`.

## Examples

    iex> {:ok, file} = F3.open("test/fixtures/small.f3")
    iex> {:ok, meta} = F3.Footer.load(file, F3.Postscript.max_metadata_bytes())
    iex> Enum.map(meta.schema.fields, & &1.name)
    ["trade_id", "symbol", "price"]

    iex> {:ok, short} = F3.open({:memory, "no"})
    iex> {:error, error} = F3.Footer.load(short, 32)
    iex> error.category
    :truncated

---

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