artist versions: refactor diffs.

app/logical/set_diff.rb

@@ -0,0 +1,35 @@
+class SetDiff
+  attr_reader :added, :removed, :obsolete_added, :obsolete_removed, :changed, :unchanged
+
+  def initialize(new, old, latest)
+    new, old, latest = new.to_a, old.to_a, latest.to_a
+
+    @added, @removed, @changed = changes(new, old)
+    @unchanged = new & old
+    @obsolete_added = added - latest
+    @obsolete_removed = removed & latest
+  end
+
+  def changes(new, old)
+    added = new - old
+    removed = old - new
+    changed = []
+
+    removed.each do |removal|
+      if addition = find_similar(removal, added)
+        changed << [removal, addition]
+        added -= [addition]
+        removed -= [removal]
+      end
+    end
+
+    [added, removed, changed]
+  end
+
+  def find_similar(string, candidates, max_dissimilarity: 0.75)
+    distance = ->(other) { DidYouMean::Levenshtein.distance(string, other) }
+    max_distance = string.size * max_dissimilarity
+
+    candidates.select { |candidate| distance[candidate] <= max_distance }.sort_by(&distance).first
+  end
+end