Define Xgit.Core.ObjectId for describing SHA-1 object IDs. (#13)

* Define Xgit.Core.ObjectId for describing SHA-1 object IDs.

* A few minor tweaks.

lib/xgit/core/object_id.ex

@@ -0,0 +1,28 @@
+defmodule Xgit.Core.ObjectId do
+  @moduledoc ~S"""
+  An object ID is a string that identifies an object within a repository.
+
+  This string must match the format for a SHA-1 hash (i.e. 40 characters
+  of lowercase hex).
+  """
+
+  @typedoc "A string containing 40 bytes of lowercase hex digits."
+  @type t :: String.t()
+
+  @doc ~S"""
+  Get the special all-null object ID, often used to stand-in for no object.
+  """
+  @spec zero :: t
+  def zero, do: "0000000000000000000000000000000000000000"
+
+  @doc ~S"""
+  Returns `true` if the value is a valid object ID.
+
+  (In other words, is it a string containing 40 characters of lowercase hex?)
+  """
+  @spec valid?(id :: term) :: boolean
+  def valid?(id)
+
+  def valid?(s) when is_binary(s), do: String.length(s) == 40 && String.match?(s, ~r/^[0-9a-f]+$/)
+  def valid?(_), do: false
+end

test/xgit/core/object_id_test.exs

@@ -0,0 +1,23 @@
+defmodule Xgit.Core.ObjectIdTest do
+  use ExUnit.Case, async: true
+
+  alias Xgit.Core.ObjectId
+
+  test "zero/0" do
+    zero = ObjectId.zero()
+    assert is_binary(zero)
+    assert String.length(zero) == 40
+    assert ObjectId.valid?(zero)
+    assert String.match?(zero, ~r/^0+$/)
+  end
+
+  test "valid?/1" do
+    assert ObjectId.valid?("1234567890abcdef12341234567890abcdef1234")
+    refute ObjectId.valid?("1234567890abcdef1231234567890abcdef1234")
+    refute ObjectId.valid?("1234567890abcdef123451234567890abcdef1234")
+    refute ObjectId.valid?("1234567890abCdef12341234567890abcdef1234")
+    refute ObjectId.valid?("1234567890abXdef12341234567890abcdef1234")
+
+    refute ObjectId.valid?(nil)
+  end
+end