defmodule HashFS do

  @moduledoc """
  HashFS is a small library providing a content-addressable file system. This
  is the documentation generated from the source.
  """

  @readahead 10_000_000
  @buffer_size 0xfff

  defmacrop try!(clause) do
    quote do
      case unquote(clause) do
        {:error, err} -> raise err
        {:ok, val} -> val
      end
    end
  end
  
  defstruct [:path]

  def new(path) do
    File.mkdir_p(Path.join(path, "cache"))
    File.mkdir_p(Path.join(path, "data"))
    %HashFS{ path: path }
  end

  @doc """
  Get a new `File` with the contents corresponding to the calculated hash.
  """
  def retrieve(fs, hash) do
    hex = Base.encode16(hash, case: :lower)
    path = Path.join([fs.path, "data", binary_part(hex, 0, 2), binary_part(hex, 2, byte_size(hex)-2)])
    File.open path
  end

  @doc """
  Same as `retrieve/2` but raises an error when something went wrong.
  """
  def retrieve!(fs, path) do
    try!(retrieve(fs, path))
  end

  """
  Store a file. The file's content is given by `input`, which is a `Stream`, and a `length` in bytes.
  """
  def store(fs, input, length) do
    sha = :crypto.hash_init(:sha)
    sha = :crypto.hash_update(sha, "blob " <> to_string(length) <> "\0")
		out_path = Path.join([fs.path, "cache", UUID.uuid4])
    out = File.open!(out_path, [:write, :exclusive, :delayed_write])
    hash = :crypto.hash_final(Enum.reduce(input, sha, fn(chunk, acc) -> 
      IO.binwrite(out, chunk)
      :crypto.hash_update(acc, chunk)
    end))
    hex = Base.encode16(hash, case: :lower)
    dir_path = Path.join([fs.path, "data", binary_part(hex, 0, 2)])
    File.mkdir dir_path
		File.close out
    File.rename out_path, Path.join(dir_path, binary_part(hex, 2, byte_size(hex)-2))
    {:ok,hash}
  end

  @doc """
  Import a file into the repository by giving a reference to its location.
  """
  def store(fs, path) do
    info = try!(File.stat(path))
    file = File.stream!(path, [read_ahead: @readahead], @buffer_size)
    store(fs, file, info.size)
  end

  @doc """
  The same as `store/3`, except that this raises an error when something went wrong.
  """
  def store!(fs, input, length) do
    try!(store(fs, input, length))
  end

  @doc """
  Same as `store/2`, but raises an error when something went wrong.
  """
  def store!(fs, path) do
    try!(store(fs, path))
  end

end