Implement FiltersPresenter#reset_url

This returns the URL used for the "Clear all" link on both the filter
summary and filter panel in the new "all content" finder UI.

- Add real implementation for `FiltersPresenter#reset_url`, merging the
  query parameters for every applied facet and then removing them from
  the current URL
- Refactor duplication into a `#applied_filters` private method

app/presenters/filters_presenter.rb

@@ -5,11 +5,11 @@ def initialize(facets, finder_url_builder)
   end
 
   def any_filters?
-    facets.any?(&:has_filters?)
+    applied_filters.any?
   end
 
   def summary_items
-    facets.flat_map(&:applied_filters).map do |filter|
+    applied_filters.map do |filter|
       {
         label: filter[:name],
         value: filter[:label],
@@ -20,10 +20,26 @@ def summary_items
   end
 
   def reset_url
-    "#"
+    return nil unless any_filters?
+
+    all_query_params = applied_filters.map { _1[:query_params] }
+    merged_facet_params = all_query_params.reduce do |acc, query_params|
+      acc.deep_merge(query_params) do |_key, old_val, new_val|
+        if old_val.is_a?(Array)
+          old_val + new_val
+        else
+          new_val
+        end
+      end
+    end
+    finder_url_builder.url_except(merged_facet_params)
   end
 
 private
 
   attr_reader :facets, :finder_url_builder
+
+  def applied_filters
+    @applied_filters ||= facets.flat_map(&:applied_filters)
+  end
 end

spec/presenters/filters_presenter_spec.rb

@@ -7,9 +7,27 @@
   let(:finder_url_builder) { instance_double(UrlBuilder) }
 
   let(:facet_without_applied_filters) { double("Facet", has_filters?: false, applied_filters: []) }
-  let(:facet_with_applied_filters) { double("Facet", has_filters?: true, applied_filters:) }
-
-  let(:applied_filters) { [{ name: "name", label: "label", query_params: { key: %w[value] } }] }
+  let(:facet_with_applied_filters) do
+    double(
+      "Facet",
+      has_filters?: true,
+      applied_filters: [{ name: "name", label: "label", query_params: { key: %w[value] } }],
+    )
+  end
+  let(:another_facet_with_applied_filters) do
+    double(
+      "Facet",
+      has_filters?: true,
+      applied_filters: [
+        { name: "name1", label: "label1", query_params: { key1: %w[value1] } },
+        { name: "name2",
+          label: "label2",
+          query_params: {
+            key1: %w[anothervalue1], key2: { value2: "subvalue" }
+          } },
+      ],
+    )
+  end
 
   describe "#any_filters" do
     context "when there are no facets" do
@@ -32,8 +50,34 @@
   end
 
   describe "#reset_url" do
-    it "returns a static anchor link" do
-      expect(subject.reset_url).to eq("#")
+    subject(:reset_url) { filters_presenter.reset_url }
+
+    context "when there are no facets" do
+      let(:facets) { [] }
+
+      it { is_expected.to be_nil }
+    end
+
+    context "when there are only facets without applied filters" do
+      let(:facets) { [facet_without_applied_filters, facet_without_applied_filters] }
+
+      it { is_expected.to be_nil }
+    end
+
+    context "when there are facets with applied filters" do
+      let(:facets) { [facet_with_applied_filters, another_facet_with_applied_filters] }
+
+      before do
+        allow(finder_url_builder).to receive(:url_except).with({
+          key: %w[value],
+          key1: %w[value1 anothervalue1],
+          key2: { value2: "subvalue" },
+        }).and_return("/search/foo")
+      end
+
+      it "returns the expected reset URL" do
+        expect(reset_url).to eq("/search/foo")
+      end
     end
   end