Place relative_positioning specs into the lib tree

This divides up the relative_positioning class specs into the
appropriate places in the lib tree

spec/lib/gitlab/relative_positioning/item_context_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::RelativePositioning::ItemContext do
+  let_it_be(:default_user) { create_default(:user) }
+  let_it_be(:project, reload: true) { create(:project) }
+
+  def create_issue(pos)
+    create(:issue, project: project, relative_position: pos)
+  end
+
+  # Increase the range size to convice yourself that this covers ALL arrangements
+  range = (101..108)
+  indices = (0..).take(range.size)
+
+  let(:start) { ((range.first + range.last) / 2.0).floor }
+  let(:subjects) { issues.map { |i| described_class.new(i.reset, range) } }
+
+  context 'there are gaps at the start and end' do
+    let_it_be(:issues) { (range.first.succ..range.last.pred).map { |pos| create_issue(pos) } }
+
+    it 'is always possible to find a gap' do
+      expect(subjects)
+        .to all(have_attributes(find_next_gap_before: be_present, find_next_gap_after: be_present))
+    end
+
+    where(:index) { indices.reverse.drop(2) }
+
+    with_them do
+      subject { subjects[index] }
+
+      it 'is possible to shift_right, which will consume the gap at the end' do
+        subject.shift_right
+
+        expect(subject.find_next_gap_after).not_to be_present
+      end
+
+      it 'is possible to shift_left, which will consume the gap at the start' do
+        subject.shift_left
+
+        expect(subject.find_next_gap_before).not_to be_present
+      end
+    end
+  end
+
+  context 'there is a gap of multiple spaces' do
+    let_it_be(:issues) { [range.first, range.last].map { |pos| create_issue(pos) } }
+
+    it 'is possible to find the gap from the right' do
+      gap = Gitlab::RelativePositioning::Gap.new(range.last, range.first)
+
+      expect(subjects.last).to have_attributes(
+        find_next_gap_before: eq(gap),
+        find_next_gap_after: be_nil
+      )
+    end
+
+    it 'is possible to find the gap from the left' do
+      gap = Gitlab::RelativePositioning::Gap.new(range.first, range.last)
+
+      expect(subjects.first).to have_attributes(
+        find_next_gap_before: be_nil,
+        find_next_gap_after: eq(gap)
+      )
+    end
+  end
+
+  context 'there are several free spaces' do
+    let_it_be(:issues) { range.select(&:even?).map { |pos| create_issue(pos) } }
+    let_it_be(:gaps) do
+      range.select(&:odd?).map do |pos|
+        rhs = pos.succ.clamp(range.first, range.last)
+        lhs = pos.pred.clamp(range.first, range.last)
+
+        {
+          before: Gitlab::RelativePositioning::Gap.new(rhs, lhs),
+          after: Gitlab::RelativePositioning::Gap.new(lhs, rhs)
+        }
+      end
+    end
+
+    where(:current_pos) { range.select(&:even?) }
+
+    with_them do
+      let(:subject) { subjects.find { |s| s.relative_position == current_pos } }
+
+      it 'finds the closest gap' do
+        closest_gap_before = gaps
+          .map { |gap| gap[:before] }
+          .select { |gap| gap.start_pos <= subject.relative_position }
+          .max_by { |gap| gap.start_pos }
+        closest_gap_after = gaps
+          .map { |gap| gap[:after] }
+          .select { |gap| gap.start_pos >= subject.relative_position }
+          .min_by { |gap| gap.start_pos }
+
+        expect(subject).to have_attributes(
+          find_next_gap_before: closest_gap_before,
+          find_next_gap_after: closest_gap_after
+        )
+      end
+    end
+  end
+
+  context 'there is at least one free space' do
+    where(:free_space) { range.to_a }
+
+    with_them do
+      let(:issues) { range.reject { |x| x == free_space }.map { |p| create_issue(p) } }
+      let(:gap_rhs) { free_space.succ.clamp(range.first, range.last) }
+      let(:gap_lhs) { free_space.pred.clamp(range.first, range.last) }
+
+      it 'can always find a gap before if there is space to the left' do
+        expected_gap = Gitlab::RelativePositioning::Gap.new(gap_rhs, gap_lhs)
+
+        to_the_right_of_gap = subjects.select { |s| free_space < s.relative_position }
+
+        expect(to_the_right_of_gap)
+          .to all(have_attributes(find_next_gap_before: eq(expected_gap)))
+      end
+
+      it 'can always find a gap after if there is space to the right' do
+        expected_gap = Gitlab::RelativePositioning::Gap.new(gap_lhs, gap_rhs)
+
+        to_the_left_of_gap = subjects.select { |s| s.relative_position < free_space }
+
+        expect(to_the_left_of_gap)
+          .to all(have_attributes(find_next_gap_after: eq(expected_gap)))
+      end
+    end
+  end
+end