From a1cbbd24b603c53c29cafaf9a23e59c1f2ae00b6 Mon Sep 17 00:00:00 2001
From: Marcel Otto <marcelotto@gmx.de>
Date: Wed, 20 Nov 2019 01:45:34 +0100
Subject: [PATCH] Add RDF.Diff.apply/2

---
 lib/rdf/diff.ex         | 23 ++++++++++-
 test/unit/diff_test.exs | 85 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 107 insertions(+), 1 deletion(-)

diff --git a/lib/rdf/diff.ex b/lib/rdf/diff.ex
index d8224ef..5aa6d11 100644
--- a/lib/rdf/diff.ex
+++ b/lib/rdf/diff.ex
@@ -30,7 +30,7 @@ defmodule RDF.Diff do
   defp coerce_graph(data), do: Graph.new(data)
 
   @doc """
-  Computes the diff between two `RDF.Graph`s or `RDF.Description`s.
+  Computes a diff between two `RDF.Graph`s or `RDF.Description`s.
 
   The first argument represents the original and the second argument the new version
   of the RDF data to be compared. Any combination of `RDF.Graph`s or
@@ -149,4 +149,25 @@ defmodule RDF.Diff do
       deletions: Graph.add(diff1.deletions, diff2.deletions)
     )
   end
+
+  @doc """
+  Applies a diff to a `RDF.Graph` or `RDF.Description` by deleting the `deletions` and adding the `additions` of the `diff`.
+
+  Deletions of statements which are not present in the given graph or description
+  are simply ignored.
+
+  The result of an application is always a `RDF.Graph`, even if a `RDF.Description`
+  is given and the additions from the diff are all about the subject of this description.
+  """
+  def apply(diff, rdf_data)
+
+  def apply(%__MODULE__{} = diff, %Graph{} = graph) do
+    graph
+    |> Graph.delete(diff.deletions)
+    |> Graph.add(diff.additions)
+  end
+
+  def apply(%__MODULE__{} = diff, %Description{} = description) do
+    __MODULE__.apply(diff, Graph.new(description))
+  end
 end
diff --git a/test/unit/diff_test.exs b/test/unit/diff_test.exs
index 3e9364e..2336383 100644
--- a/test/unit/diff_test.exs
+++ b/test/unit/diff_test.exs
@@ -197,4 +197,89 @@ defmodule RDF.DiffTest do
                ])
              )
   end
+
+  describe "apply/2" do
+    test "on a graph" do
+      assert Diff.new(
+                 additions: Graph.new([
+                   EX.S1
+                   |> EX.p(EX.O3)
+                   |> EX.p3(EX.O),
+                   EX.S3
+                   |> EX.p(EX.O)
+                 ]),
+                 deletions: Graph.new([
+                   EX.S1
+                   |> EX.p(EX.O1)
+                   |> EX.p2(EX.O),
+                   EX.S2
+                   |> EX.p(EX.O)
+                 ]))
+             |> Diff.apply(Graph.new([
+                              EX.S1
+                              |> EX.p(EX.O1, EX.O2)
+                              |> EX.p2(EX.O),
+                              EX.S2
+                              |> EX.p(EX.O)
+                            ])) ==
+               Graph.new([
+                 EX.S1
+                 |> EX.p(EX.O2, EX.O3)
+                 |> EX.p3(EX.O),
+                 EX.S3
+                 |> EX.p(EX.O)
+               ])
+    end
+
+    test "on a description" do
+      assert Diff.new(
+               additions: Graph.new([
+                 EX.S1
+                 |> EX.p(EX.O3)
+                 |> EX.p3(EX.O),
+                 EX.S3
+                 |> EX.p(EX.O)
+               ]),
+               deletions: Graph.new([
+                 EX.S1
+                 |> EX.p(EX.O1)
+                 |> EX.p2(EX.O),
+               ]))
+             |> Diff.apply(
+                  EX.S1
+                  |> EX.p(EX.O1, EX.O2)
+                  |> EX.p2(EX.O)
+                ) ==
+               Graph.new([
+                 EX.S1
+                 |> EX.p(EX.O2, EX.O3)
+                 |> EX.p3(EX.O),
+                 EX.S3
+                 |> EX.p(EX.O)
+               ])
+    end
+
+    test "when the statements to be deleted are not present" do
+      assert Diff.new(
+               additions: Graph.new(
+                 EX.S1
+                 |> EX.p(EX.O4)
+               ),
+               deletions: Graph.new([
+                 EX.S1
+                 |> EX.p(EX.O2, EX.O3)
+                 |> EX.p2(EX.O),
+                 EX.S2
+                 |> EX.p(EX.O)
+               ]))
+             |> Diff.apply(Graph.new(
+               EX.S1
+               |> EX.p(EX.O1, EX.O2)
+             )) ==
+               Graph.new(
+                 EX.S1
+                 |> EX.p(EX.O1, EX.O4)
+               )
+    end
+  end
 end