new cop: Single Line Complexity

changelog/new_single_line_complexity_cop.md

@@ -0,0 +1 @@
+* [#13595](https://github.com/rubocop/rubocop/pull/13595): Add a new `Metrics/SingleLineComplexity` cop. ([@kallin][])

config/default.yml

@@ -2742,6 +2742,12 @@ Metrics/PerceivedComplexity:
   AllowedPatterns: []
   Max: 8
 
+Metrics/SingleLineComplexity:
+  Description: 'Metrics/AbcSize, calculated per line instead of per method.'
+  Enabled: true
+  VersionAdded: '<<next>>'
+  Max: 14
+
 ################## Migration #############################
 
 Migration/DepartmentName:

lib/rubocop.rb

@@ -445,6 +445,7 @@
 require_relative 'rubocop/cop/metrics/module_length'
 require_relative 'rubocop/cop/metrics/parameter_lists'
 require_relative 'rubocop/cop/metrics/perceived_complexity'
+require_relative 'rubocop/cop/metrics/single_line_complexity'
 
 require_relative 'rubocop/cop/naming/accessor_method_name'
 require_relative 'rubocop/cop/naming/ascii_identifiers'

lib/rubocop/cop/metrics/single_line_complexity.rb

@@ -0,0 +1,104 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Metrics
+      # Checks that the ABC size of each individual line is not higher than the
+      # configured maximum. Please see the `Metrics/AbcSize` cop for more
+      # information on how the ABC size is calculated.
+      #
+      class SingleLineComplexity < Base
+        MSG = 'Assignment Branch Condition size too high for line %<line>d. ' \
+              '[%<abc_vector>s %<complexity>.4g/%<max>.4g]'
+
+        def on_new_investigation
+          return unless (root = processed_source.ast)
+
+          top_level_nodes_per_line = top_level_nodes_per_line(root)
+
+          top_level_nodes_per_line.each do |line, nodes|
+            complexity, abc_vector = abc_score_for_nodes(nodes)
+
+            next unless complexity > max
+
+            msg = format(
+              self.class::MSG, line: line, complexity: complexity, abc_vector: abc_vector, max: max
+            )
+
+            add_offense(nodes.first, message: msg)
+          end
+        end
+
+        private
+
+        def abc_score_for_nodes(nodes)
+          scores = nodes.map do |node|
+            Utils::AbcSizeCalculator.calculate(node)
+          end
+
+          # in cases such as `a = 1; b = 2` there are multiple 'top-level' nodes on the same line
+          # we sum the scores of all nodes on the line
+          sum_complexity_scores(scores)
+        end
+
+        def sum_complexity_scores(scores)
+          vector_sums = []
+
+          scores.each do |score|
+            _, vector = score
+
+            # Parse and sum vector components
+            vector_values = vector[1..-2].split(',').map(&:to_f) # Remove < > and split by comma
+            vector_values.each_with_index do |value, i|
+              vector_sums[i] = (vector_sums[i] || 0) + value
+            end
+          end
+
+          total_score = Math.sqrt(vector_sums.sum { |sum| sum**2 }).round(2)
+
+          [total_score, "<#{vector_sums.join(', ')}>"]
+        end
+
+        def top_level_nodes_per_line(root)
+          nodes_per_line = nodes_per_line(root)
+
+          nodes_per_line.transform_values do |nodes_on_line|
+            nodes_on_line.reject do |node|
+              # if the node's parent is on this line, then this node is not top-level for the line
+              nodes_on_line.include?(node.parent)
+            end
+          end
+        end
+
+        def nodes_per_line(root)
+          nodes_per_line = {}
+
+          root.each_node do |child|
+            next unless child.source_range
+
+            line_range = child.source_range.first_line..child.source_range.last_line
+
+            # Skip nodes that span multiple lines
+            next if line_range.size > 1
+
+            next if heredoc_node?(child)
+
+            line_range.each do |line|
+              nodes_per_line[line] ||= []
+              nodes_per_line[line] << child
+            end
+          end
+          nodes_per_line
+        end
+
+        def heredoc_node?(node)
+          node.dstr_type?
+        end
+
+        def max
+          cop_config['Max']
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/metrics/single_line_complexity_spec.rb

@@ -0,0 +1,71 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Metrics::SingleLineComplexity, :config do
+  let(:cop_config) { { 'Max' => 2 } }
+
+  context 'single line code' do
+    it 'registers an offense when ABC per line > 2' do
+      expect_offense(<<~RUBY)
+        a ? b : c
+        ^^^^^^^^^ Assignment Branch Condition size too high for line 1. [<0.0, 3.0, 1.0> 3.16/2]
+      RUBY
+    end
+  end
+
+  context 'multi line code' do
+    it 'registers two offenses when 2 of 4 lines have ABC > 2' do
+      expect_offense(<<~RUBY)
+        a ? b : c
+        ^^^^^^^^^ Assignment Branch Condition size too high for line 1. [<0.0, 3.0, 1.0> 3.16/2]
+        x = 1
+        c ? d : e
+        ^^^^^^^^^ Assignment Branch Condition size too high for line 3. [<0.0, 3.0, 1.0> 3.16/2]
+        y = 2
+      RUBY
+    end
+
+    it 'accepts all lines when ABC per line always <= 2' do
+      expect_no_offenses(<<~RUBY)
+        x = 1 + 1
+        y = 2
+      RUBY
+    end
+  end
+
+  context 'inline statements' do
+    it 'rejects an offense when ABC > 2' do
+      expect_offense(<<~RUBY)
+        x = 1; y = 2; z = 3;
+        ^^^^^^^^^^^^^^^^^^^ Assignment Branch Condition size too high for line 1. [<3.0, 0.0, 0.0> 3/2]
+      RUBY
+    end
+
+    it 'accepts when ABC == 2' do
+      expect_no_offenses(<<~RUBY)
+        x = 1; y = 2;
+      RUBY
+    end
+  end
+
+  it 'does not register an offense for a multiline heredoc' do
+    expect_no_offenses(<<~'RUBY')
+      <<~STRING
+        #{foo}
+        #{bar}
+        #{baz}
+        #{quux}
+      STRING
+    RUBY
+  end
+
+  it 'registers an offense for a complex line within a heredoc' do
+    expect_offense(<<~'RUBY')
+      <<~STRING
+
+        Heredoc with multiple lines
+        #{foo.bar.baz.quux}
+        ^^^^^^^^^^^^^^^^^^^ Assignment Branch Condition size too high for line 4. [<0.0, 4.0, 0.0> 4/2]
+      STRING
+    RUBY
+  end
+end