Improve highlighting for merge diffs

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/16950

**Problem**

Current diff detection implementation has a limitation for cases with
multiple changes in the same line. It highlights the whole block
instead of marking the exact difference.

**Solution**

Use a different diff detection algorithm to improve the detection

Other changes:

Extract char_diff.rb from ee namespace

ee/lib/gitlab/diff/char_diff.rb → lib/gitlab/diff/char_diff.rb

@@ -18,6 +18,33 @@ module Gitlab
         @changes
       end
 
+      def changed_ranges(offset: 0)
+        old_diffs = []
+        new_diffs = []
+        new_pointer = old_pointer = offset
+
+        generate_diff.each do |(action, content)|
+          content_size = content.size
+
+          if action == :equal
+            new_pointer += content_size
+            old_pointer += content_size
+          end
+
+          if action == :delete
+            old_diffs << (old_pointer..(old_pointer + content_size - 1))
+            old_pointer += content_size
+          end
+
+          if action == :insert
+            new_diffs << (new_pointer..(new_pointer + content_size - 1))
+            new_pointer += content_size
+          end
+        end
+
+        [old_diffs, new_diffs]
+      end
+
       def to_html
         @changes.map do |op, text|
           %{<span class="#{html_class_names(op)}">#{ERB::Util.html_escape(text)}</span>}

lib/gitlab/diff/inline_diff.rb

@@ -31,20 +31,7 @@ module Gitlab
         # Skip inline diff if empty line was replaced with content
         return if old_line == ""
 
-        lcp = longest_common_prefix(old_line, new_line)
-        lcs = longest_common_suffix(old_line[lcp..-1], new_line[lcp..-1])
-
-        lcp += offset
-        old_length = old_line.length + offset
-        new_length = new_line.length + offset
-
-        old_diff_range = lcp..(old_length - lcs - 1)
-        new_diff_range = lcp..(new_length - lcs - 1)
-
-        old_diffs = [old_diff_range] if old_diff_range.begin <= old_diff_range.end
-        new_diffs = [new_diff_range] if new_diff_range.begin <= new_diff_range.end
-
-        [old_diffs, new_diffs]
+        CharDiff.new(old_line, new_line).changed_ranges(offset: offset)
       end
 
       class << self

spec/helpers/diff_helper_spec.rb

@@ -169,9 +169,9 @@ RSpec.describe DiffHelper do
     it "returns strings with marked inline diffs" do
       marked_old_line, marked_new_line = mark_inline_diffs(old_line, new_line)
 
-      expect(marked_old_line).to eq(%q{abc <span class="idiff left right deletion">&#39;def&#39;</span>})
+      expect(marked_old_line).to eq(%q{abc <span class="idiff left deletion">&#39;</span>def<span class="idiff right deletion">&#39;</span>})
       expect(marked_old_line).to be_html_safe
-      expect(marked_new_line).to eq(%q{abc <span class="idiff left right addition">&quot;def&quot;</span>})
+      expect(marked_new_line).to eq(%q{abc <span class="idiff left addition">&quot;</span>def<span class="idiff right addition">&quot;</span>})
       expect(marked_new_line).to be_html_safe
     end
 

ee/spec/lib/gitlab/diff/char_diff_spec.rb → spec/lib/gitlab/diff/char_diff_spec.rb

@@ -7,7 +7,7 @@ RSpec.describe Gitlab::Diff::CharDiff do
   let(:old_string) { "Helo \n Worlld" }
   let(:new_string) { "Hello \n World" }
 
-  subject { described_class.new(old_string, new_string) }
+  subject(:diff) { described_class.new(old_string, new_string) }
 
   describe '#generate_diff' do
     context 'when old string is nil' do
@@ -39,6 +39,28 @@ RSpec.describe Gitlab::Diff::CharDiff do
     end
   end
 
+  describe '#changed_ranges' do
+    subject { diff.changed_ranges }
+
+    context 'when old string is nil' do
+      let(:old_string) { nil }
+
+      it 'returns lists of changes' do
+        old_diffs, new_diffs = subject
+
+        expect(old_diffs).to eq([])
+        expect(new_diffs).to eq([0..12])
+      end
+    end
+
+    it 'returns ranges of changes' do
+      old_diffs, new_diffs = subject
+
+      expect(old_diffs).to eq([11..11])
+      expect(new_diffs).to eq([3..3])
+    end
+  end
+
   describe '#to_html' do
     it 'returns an HTML representation of the diff' do
       subject.generate_diff

spec/lib/gitlab/diff/inline_diff_spec.rb

@@ -37,6 +37,22 @@ RSpec.describe Gitlab::Diff::InlineDiff do
     it 'can handle unchanged empty lines' do
       expect { described_class.for_lines(['- bar', '+ baz', '']) }.not_to raise_error
     end
+
+    context 'when lines have multiple changes' do
+      let(:diff) do
+        <<~EOF
+        - Hello, how are you?
+        + Hi, how are you doing?
+        EOF
+      end
+
+      let(:subject) { described_class.for_lines(diff.lines) }
+
+      it 'finds all inline diffs' do
+        expect(subject[0]).to eq([3..6])
+        expect(subject[1]).to eq([3..3, 17..22])
+      end
+    end
   end
 
   describe "#inline_diffs" do