Add inline math filter

lib/qiita/markdown.rb

@@ -36,6 +36,7 @@
 require "qiita/markdown/filters/html_toc"
 require "qiita/markdown/filters/image_link"
 require "qiita/markdown/filters/inline_code_color"
+require "qiita/markdown/filters/inline_math"
 require "qiita/markdown/filters/mention"
 require "qiita/markdown/filters/qiita_marker"
 require "qiita/markdown/filters/simplify"

lib/qiita/markdown/filters/inline_math.rb

@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+module Qiita
+  module Markdown
+    module Filters
+      class InlineMath < HTML::Pipeline::Filter
+        def call
+          doc.search(".//code").each do |code|
+            opening = code.previous
+            closing = code.next
+            replace_with_math_span(code, opening, closing) if inline_math_code?(opening, closing)
+          end
+
+          doc
+        end
+
+        private
+
+        def inline_math_code?(opening, closing)
+          opening.present? && closing.present? && valid_opening?(opening) && valid_closing?(closing)
+        end
+
+        def valid_opening?(opening)
+          opening.text? && opening.content.end_with?("$") && !opening.content.end_with?("$$")
+        end
+
+        def valid_closing?(closing)
+          closing.text? && closing.content.start_with?("$") && !closing.content.start_with?("$$")
+        end
+
+        def replace_with_math_span(code, opening, closing)
+          span = Nokogiri::XML::Node.new("span", doc)
+          span.add_child(Nokogiri::XML::Text.new("$#{code.text}$", doc))
+          code.replace(span)
+          opening.content = opening.content.delete_suffix("$")
+          opening.remove if opening.content.empty?
+          closing.content = closing.content.delete_prefix("$")
+          closing.remove if closing.content.empty?
+        end
+      end
+    end
+  end
+end

lib/qiita/markdown/processor.rb

@@ -24,6 +24,7 @@ def self.default_filters
           Filters::GroupMention,
           Filters::ExternalLink,
           Filters::InlineCodeColor,
+          Filters::InlineMath,
           Filters::FinalSanitizer,
         ]
       end

spec/qiita/markdown/filters/inline_math_spec.rb

@@ -0,0 +1,77 @@
+# frozen_string_literal: true
+
+describe Qiita::Markdown::Filters::InlineMath do
+  subject(:filter) do
+    described_class.new(html)
+  end
+
+  context "with dollar signs" do
+    let(:html) do
+      <<~HTML
+        <div>
+          $<code>A = B</code>$
+        </div>
+      HTML
+    end
+
+    it "replaces <code> to <span> with dollars" do
+      expect(filter.call.to_html).to eq(
+        <<~HTML,
+          <div>
+            <span>$A = B$</span>
+          </div>
+        HTML
+      )
+    end
+  end
+
+  context "with dollar signs with surrounding text" do
+    let(:html) do
+      <<~HTML
+        <div>
+          Some text before$<code>A = B</code>$Some text after
+        </div>
+      HTML
+    end
+
+    it "replaces <code> to <span> with dollars" do
+      expect(filter.call.to_html).to eq(
+        <<~HTML,
+          <div>
+            Some text before<span>$A = B$</span>Some text after
+          </div>
+        HTML
+      )
+    end
+  end
+
+  context "with double dollar signs" do
+    let(:html) do
+      <<~HTML
+        <div>
+          $$
+          <code>A = B</code>
+          $$
+        </div>
+      HTML
+    end
+
+    it "does not replace <code>" do
+      expect(filter.call.to_html).to eq(html)
+    end
+  end
+
+  context "without dollar signs" do
+    let(:html) do
+      <<~HTML
+        <div>
+          <code>A = B</code>
+        </div>
+      HTML
+    end
+
+    it "does not replace <code>" do
+      expect(filter.call.to_html).to eq(html)
+    end
+  end
+end