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diff --git a/spec/tooling/danger/commit_linter_spec.rb b/spec/tooling/danger/commit_linter_spec.rb
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+++ b/spec/tooling/danger/commit_linter_spec.rb
@@ -0,0 +1,241 @@
+# frozen_string_literal: true
+
+require 'rspec-parameterized'
+require_relative 'danger_spec_helper'
+
+require_relative '../../../tooling/danger/commit_linter'
+
+RSpec.describe Tooling::Danger::CommitLinter do
+  using RSpec::Parameterized::TableSyntax
+
+  let(:total_files_changed) { 2 }
+  let(:total_lines_changed) { 10 }
+  let(:stats) { { total: { files: total_files_changed, lines: total_lines_changed } } }
+  let(:diff_parent) { Struct.new(:stats).new(stats) }
+  let(:commit_class) do
+    Struct.new(:message, :sha, :diff_parent)
+  end
+
+  let(:commit_message) { 'A commit message' }
+  let(:commit_sha) { 'abcd1234' }
+  let(:commit) { commit_class.new(commit_message, commit_sha, diff_parent) }
+
+  subject(:commit_linter) { described_class.new(commit) }
+
+  describe '#fixup?' do
+    where(:commit_message, :is_fixup) do
+      'A commit message' | false
+      'fixup!' | true
+      'fixup! A commit message' | true
+      'squash!' | true
+      'squash! A commit message' | true
+    end
+
+    with_them do
+      it 'is true when commit message starts with "fixup!" or "squash!"' do
+        expect(commit_linter.fixup?).to be(is_fixup)
+      end
+    end
+  end
+
+  describe '#suggestion?' do
+    where(:commit_message, :is_suggestion) do
+      'A commit message' | false
+      'Apply suggestion to' | true
+      'Apply suggestion to "A commit message"' | true
+    end
+
+    with_them do
+      it 'is true when commit message starts with "Apply suggestion to"' do
+        expect(commit_linter.suggestion?).to be(is_suggestion)
+      end
+    end
+  end
+
+  describe '#merge?' do
+    where(:commit_message, :is_merge) do
+      'A commit message' | false
+      'Merge branch' | true
+      'Merge branch "A commit message"' | true
+    end
+
+    with_them do
+      it 'is true when commit message starts with "Merge branch"' do
+        expect(commit_linter.merge?).to be(is_merge)
+      end
+    end
+  end
+
+  describe '#revert?' do
+    where(:commit_message, :is_revert) do
+      'A commit message' | false
+      'Revert' | false
+      'Revert "' | true
+      'Revert "A commit message"' | true
+    end
+
+    with_them do
+      it 'is true when commit message starts with "Revert \""' do
+        expect(commit_linter.revert?).to be(is_revert)
+      end
+    end
+  end
+
+  describe '#multi_line?' do
+    where(:commit_message, :is_multi_line) do
+      "A commit message" | false
+      "A commit message\n" | false
+      "A commit message\n\n" | false
+      "A commit message\n\nSigned-off-by: User Name <user@name.me>" | false
+      "A commit message\n\nWith details" | true
+    end
+
+    with_them do
+      it 'is true when commit message contains details' do
+        expect(commit_linter.multi_line?).to be(is_multi_line)
+      end
+    end
+  end
+
+  shared_examples 'a valid commit' do
+    it 'does not have any problem' do
+      commit_linter.lint
+
+      expect(commit_linter.problems).to be_empty
+    end
+  end
+
+  describe '#lint' do
+    describe 'separator' do
+      context 'when separator is missing' do
+        let(:commit_message) { "A B C\n" }
+
+        it_behaves_like 'a valid commit'
+      end
+
+      context 'when separator is a blank line' do
+        let(:commit_message) { "A B C\n\nMore details." }
+
+        it_behaves_like 'a valid commit'
+      end
+
+      context 'when separator is missing' do
+        let(:commit_message) { "A B C\nMore details." }
+
+        it 'adds a problem' do
+          expect(commit_linter).to receive(:add_problem).with(:separator_missing)
+
+          commit_linter.lint
+        end
+      end
+    end
+
+    describe 'details' do
+      context 'when details are valid' do
+        let(:commit_message) { "A B C\n\nMore details." }
+
+        it_behaves_like 'a valid commit'
+      end
+
+      context 'when no details are given and many files are changed' do
+        let(:total_files_changed) { described_class::MAX_CHANGED_FILES_IN_COMMIT + 1 }
+
+        it_behaves_like 'a valid commit'
+      end
+
+      context 'when no details are given and many lines are changed' do
+        let(:total_lines_changed) { described_class::MAX_CHANGED_LINES_IN_COMMIT + 1 }
+
+        it_behaves_like 'a valid commit'
+      end
+
+      context 'when no details are given and many files and lines are changed' do
+        let(:total_files_changed) { described_class::MAX_CHANGED_FILES_IN_COMMIT + 1 }
+        let(:total_lines_changed) { described_class::MAX_CHANGED_LINES_IN_COMMIT + 1 }
+
+        it 'adds a problem' do
+          expect(commit_linter).to receive(:add_problem).with(:details_too_many_changes)
+
+          commit_linter.lint
+        end
+      end
+
+      context 'when details exceeds the max line length' do
+        let(:commit_message) { "A B C\n\n" + 'D' * (described_class::MAX_LINE_LENGTH + 1) }
+
+        it 'adds a problem' do
+          expect(commit_linter).to receive(:add_problem).with(:details_line_too_long)
+
+          commit_linter.lint
+        end
+      end
+
+      context 'when details exceeds the max line length including URLs' do
+        let(:commit_message) do
+          "A B C\n\nsome message with https://example.com and https://gitlab.com" + 'D' * described_class::MAX_LINE_LENGTH
+        end
+
+        it_behaves_like 'a valid commit'
+      end
+    end
+
+    describe 'message' do
+      context 'when message includes a text emoji' do
+        let(:commit_message) { "A commit message :+1:" }
+
+        it 'adds a problem' do
+          expect(commit_linter).to receive(:add_problem).with(:message_contains_text_emoji)
+
+          commit_linter.lint
+        end
+      end
+
+      context 'when message includes a unicode emoji' do
+        let(:commit_message) { "A commit message 🚀" }
+
+        it 'adds a problem' do
+          expect(commit_linter).to receive(:add_problem).with(:message_contains_unicode_emoji)
+
+          commit_linter.lint
+        end
+      end
+
+      context 'when message includes a value that is surrounded by backticks' do
+        let(:commit_message) { "A commit message `%20`" }
+
+        it 'does not add a problem' do
+          expect(commit_linter).not_to receive(:add_problem)
+
+          commit_linter.lint
+        end
+      end
+
+      context 'when message includes a short reference' do
+        [
+          'A commit message to fix #1234',
+          'A commit message to fix !1234',
+          'A commit message to fix &1234',
+          'A commit message to fix %1234',
+          'A commit message to fix gitlab#1234',
+          'A commit message to fix gitlab!1234',
+          'A commit message to fix gitlab&1234',
+          'A commit message to fix gitlab%1234',
+          'A commit message to fix gitlab-org/gitlab#1234',
+          'A commit message to fix gitlab-org/gitlab!1234',
+          'A commit message to fix gitlab-org/gitlab&1234',
+          'A commit message to fix gitlab-org/gitlab%1234',
+          'A commit message to fix "gitlab-org/gitlab%1234"',
+          'A commit message to fix `gitlab-org/gitlab%1234'
+        ].each do |message|
+          let(:commit_message) { message }
+
+          it 'adds a problem' do
+            expect(commit_linter).to receive(:add_problem).with(:message_contains_short_reference)
+
+            commit_linter.lint
+          end
+        end
+      end
+    end
+  end
+end