1 files changed, 83 insertions, 0 deletions
diff --git a/lib/gitlab/relative_positioning/range.rb b/lib/gitlab/relative_positioning/range.rb
new file mode 100644
index 00000000000..174d5ef4b35
--- /dev/null
+++ b/lib/gitlab/relative_positioning/range.rb
@@ -0,0 +1,83 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module RelativePositioning
+    IllegalRange = Class.new(ArgumentError)
+
+    class Range
+      attr_reader :lhs, :rhs
+
+      def open_on_left?
+        lhs.nil?
+      end
+
+      def open_on_right?
+        rhs.nil?
+      end
+
+      def cover?(item_context)
+        return false unless item_context
+        return false unless item_context.positioned?
+        return true if item_context.object == lhs&.object
+        return true if item_context.object == rhs&.object
+
+        pos = item_context.relative_position
+
+        return lhs.relative_position < pos if open_on_right?
+        return pos < rhs.relative_position if open_on_left?
+
+        lhs.relative_position < pos && pos < rhs.relative_position
+      end
+
+      def ==(other)
+        other.is_a?(RelativePositioning::Range) && lhs == other.lhs && rhs == other.rhs
+      end
+    end
+
+    def self.range(lhs, rhs)
+      if lhs && rhs
+        ClosedRange.new(lhs, rhs)
+      elsif lhs
+        StartingFrom.new(lhs)
+      elsif rhs
+        EndingAt.new(rhs)
+      else
+        raise IllegalRange, 'One of rhs or lhs must be provided' unless lhs && rhs
+      end
+    end
+
+    class ClosedRange < RelativePositioning::Range
+      def initialize(lhs, rhs)
+        @lhs, @rhs = lhs, rhs
+        raise IllegalRange, 'Either lhs or rhs is missing' unless lhs && rhs
+        raise IllegalRange, 'lhs and rhs cannot be the same object' if lhs == rhs
+      end
+    end
+
+    class StartingFrom < RelativePositioning::Range
+      include Gitlab::Utils::StrongMemoize
+
+      def initialize(lhs)
+        @lhs = lhs
+        raise IllegalRange, 'lhs is required' unless lhs
+      end
+
+      def rhs
+        strong_memoize(:rhs) { lhs.rhs_neighbour }
+      end
+    end
+
+    class EndingAt < RelativePositioning::Range
+      include Gitlab::Utils::StrongMemoize
+
+      def initialize(rhs)
+        @rhs = rhs
+        raise IllegalRange, 'rhs is required' unless rhs
+      end
+
+      def lhs
+        strong_memoize(:lhs) { rhs.lhs_neighbour }
+      end
+    end
+  end
+end