add markdown editor

lib/markdown_to_html.rb

@@ -1,13 +1,115 @@
 # frozen_string_literal: true
 
 module MarkdownToHtml
-  LINK_REGEXP = %r{\[([^\]]+)\]\((https?://[^)]+)\)}
+  TAGS = {
+    '' => %w[<em> </em>],
+    '*' => %w[<strong> </strong>],
+    '~' => %w[<s> </s>]
+  }.freeze
+
+  INLINE_TOKENIZER = /(\[)|(\]\(([^)]+?)\))|(?:`([^`].*?)`)|(\*\*\*|\*\*|\*|~~)/
+
+  ALLOWED_TAGS = %w[p br strong b em i u a].freeze
+  ALLOWED_ATTRIBUTES = %w[href].freeze
 
   module_function
 
-  def call(text)
-    text.gsub(LINK_REGEXP) do
-      ApplicationController.helpers.link_to(Regexp.last_match(1), Regexp.last_match(2))
-    end
+  def call(markdown)
+    return '' if markdown.blank?
+
+    text = auto_link_urls(markdown)
+    html = render_markdown(text)
+
+    ActionController::Base.helpers.sanitize(html, tags: ALLOWED_TAGS, attributes: ALLOWED_ATTRIBUTES)
   end
+
+  def auto_link_urls(text)
+    link_parts = text.split(%r{((?:https?://|www\.)[^\s)]+)})
+
+    link_parts.map.with_index do |part, index|
+      if part.match?(%r{\A(?:https?://|www\.)}) &&
+         !link_parts[index - 1]&.match?(/\]\(\s*\z/)
+        trail = part.match(/([.,;:!?]+)\z/)[1] if part.match?(/[.,;:!?]+\z/)
+        clean = trail ? part.chomp(trail) : part
+        url = clean.start_with?('www.') ? "http://#{clean}" : clean
+        "[#{clean}](#{url})#{trail}"
+      else
+        part
+      end
+    end.join
+  end
+
+  def render_markdown(text)
+    text = text.gsub(/\+\+([^+]+)\+\+/, '<u>\1</u>')
+
+    paragraphs = text.split(/\n{2,}/)
+
+    html = paragraphs.filter_map do |para|
+      content = para.strip
+
+      next if content.empty?
+
+      next '<p><br></p>' if ['&nbsp;', '&amp;nbsp;'].include?(content)
+
+      content = content.gsub(/ *\n/, '<br>')
+
+      "<p>#{parse_inline(content)}</p>"
+    end.join
+
+    html.presence || '<p></p>'
+  end
+
+  # rubocop:disable Metrics
+  def parse_inline(text)
+    context = []
+    out = ''
+    last = 0
+
+    tag = lambda do |t|
+      desc = TAGS[t[1] || '']
+      return t unless desc
+      return desc[0] unless desc[1]
+
+      is_end = context.last == t
+      is_end ? context.pop : context.push(t)
+      desc[is_end ? 1 : 0]
+    end
+
+    flush = lambda do
+      str = ''
+      str += tag.call(context.last) while context.any?
+      str
+    end
+
+    while last <= text.length && (m = INLINE_TOKENIZER.match(text, last))
+      prev = text[last...m.begin(0)]
+      last = m.end(0)
+      chunk = m[0]
+
+      if m[4]
+        chunk = "<code>#{ERB::Util.html_escape(m[4])}</code>"
+      elsif m[2]
+        out = out.sub(/\A(.*)<a>/m, "\\1<a href=\"#{ERB::Util.html_escape(m[3])}\">")
+        out = out.gsub('<a>', '[')
+        chunk = "#{flush.call}</a>"
+      elsif m[1]
+        chunk = '<a>'
+      elsif m[5]
+        chunk = if m[5] == '***'
+                  if context.include?('*') && context.include?('**')
+                    tag.call('*') + tag.call('**')
+                  else
+                    tag.call('**') + tag.call('*')
+                  end
+                else
+                  tag.call(m[5])
+                end
+      end
+
+      out += prev.to_s + chunk
+    end
+
+    (out + text[last..].to_s + flush.call).gsub('<a>', '[')
+  end
+  # rubocop:enable Metrics
 end