Retire vendored cacycle_diff.js for a diff-lcs-based diff view

Revisions#diff now computes an inline or side-by-side word diff
server-side (Page#diff_against, lib/html_word_diff.rb) instead of
shipping raw/escaped content to the browser for a 2008-era vendored
JS differ to mangle. View mode is picked via a `view` param, settable
either on the diff page itself or directly from the revisions#index
sticky bar next to "Diff revisions".

Also fixes a pre-existing bug in RevisionsController#diff's single-
revision branch, which set params[:start]/params[:end] instead of
params[:start_revision]/params[:end_revision].

1 files changed, 52 insertions, 0 deletions

diff --git a/lib/html_word_diff.rb b/lib/html_word_diff.rb
new file mode 100644
index 0000000..1f28cf5
--- /dev/null
+++ b/lib/html_word_diff.rb
@@ -0,0 +1,52 @@
+require 'diff/lcs'
+
+module HtmlWordDiff
+  TOKEN_REGEXP = /<[^>]*>|[[:space:]]+|[[:alnum:]]+|[^[:space:][:alnum:]<>]/
+
+  def self.inline(old_html, new_html)
+    html = +''
+    each_change(old_html, new_html) do |action, old_token, new_token|
+      case action
+      when '='
+        html << new_token
+      when '-'
+        html << "<del>#{old_token}</del>"
+      when '+'
+        html << "<ins>#{new_token}</ins>"
+      when '!'
+        html << "<del>#{old_token}</del><ins>#{new_token}</ins>"
+      end
+    end
+    html
+  end
+
+  def self.side_by_side(old_html, new_html)
+    old_out = +''
+    new_out = +''
+    each_change(old_html, new_html) do |action, old_token, new_token|
+      case action
+      when '='
+        old_out << old_token
+        new_out << new_token
+      when '-'
+        old_out << "<del>#{old_token}</del>"
+      when '+'
+        new_out << "<ins>#{new_token}</ins>"
+      when '!'
+        old_out << "<del>#{old_token}</del>"
+        new_out << "<ins>#{new_token}</ins>"
+      end
+    end
+    [old_out, new_out]
+  end
+
+  def self.each_change(old_html, new_html)
+    Diff::LCS.sdiff(tokenize(old_html), tokenize(new_html)).each do |change|
+      yield change.action, change.old_element, change.new_element
+    end
+  end
+
+  def self.tokenize(html)
+    html.to_s.scan(TOKEN_REGEXP)
+  end
+end