fix: memoise key parsing in RubyDataParser to fix combiner performance regression

CHANGELOG.md

@@ -1,3 +1,9 @@
+Unreleased
+==========
+
+## Performance
+* Fix 5x performance regression on report combining (introduced in `1.0.0` as a result of using `Ripper#parse` in a hot path) by adding parsed key memoisation to `RubyDataParser.call`.
+
 1.0.2 (2026-07-18)
 ==================
 

lib/simplecov/source_file/ruby_data_parser.rb

@@ -15,10 +15,26 @@ module RubyDataParser
 
       # Tests use the real data structures (except for integration tests)
       # so no need to put them through here.
+      #
+      # String parses are memoized: `Combine::BranchesCombiner` and
+      # `Combine::MethodsCombiner` derive a merge identity from every key of
+      # both sides on every pairwise merge, so collating N resultsets parses
+      # each key string N-1 times — and Ripper dominates the wall time of a
+      # large collate.
+      # Key strings repeat across folds and within report building, while the
+      # set of unique keys is bounded by the project's branch and method
+      # count, so a permanent cache stays small. Cached arrays are frozen:
+      # every caller destructures without mutating, and sharing one array
+      # across callers must stay that way.
       def call(structure)
         return structure if structure.is_a?(Array)
 
-        parse_array_string(structure.to_s)
+        string = structure.to_s
+        parse_cache[string] ||= parse_array_string(string).freeze
+      end
+
+      def parse_cache
+        @parse_cache ||= {} #: Hash[String, untyped]
       end
 
       # Parse a string like '[:if, 0, 3, 4, 3, 21]' or

sig/internal/simplecov/source_file/ruby_data_parser.rbs

@@ -7,10 +7,16 @@ module SimpleCov
     # #801). The grammar covers symbols, strings, integers, unary minus,
     # and constant paths — every shape Coverage ever emits.
     module RubyDataParser
+      # Memoizes string parses; see the implementation comment on `call`.
+      self.@parse_cache: Hash[String, untyped]?
+      @parse_cache: Hash[String, untyped]?
+
       # Tests use the real data structures (except for integration tests)
       # so no need to put them through here.
       def self?.call: (untyped structure) -> untyped
 
+      def self?.parse_cache: () -> Hash[String, untyped]
+
       # Parse a string like '[:if, 0, 3, 4, 3, 21]' or
       # '["ClassName", :method1, 2, 2, 5, 5]' back into a Ruby array.
       def self?.parse_array_string: (untyped str) -> untyped

spec/source_file_spec.rb

@@ -1316,6 +1316,27 @@ def build(coverage_data, source_lines)
         expect { described_class.parse_array_string("42") }.to raise_error(ArgumentError, /array literal/)
       end
     end
+
+    describe ".call" do
+      it "parses a stringified tuple into its array form" do
+        expect(described_class.call("[:if, 0, 3, 4, 3, 21]")).to eq([:if, 0, 3, 4, 3, 21])
+      end
+
+      it "returns arrays untouched and unfrozen" do
+        tuple = [:then, 4, 8, 6, 8, 12]
+
+        expect(described_class.call(tuple)).to be(tuple)
+        expect(tuple).not_to be_frozen
+      end
+
+      it "memoizes string parses, returning one frozen array for equal keys" do
+        first = described_class.call("[:while, 1, 5, 2, 7, 5]")
+        second = described_class.call(+"[:while, 1, 5, 2, 7, 5]")
+
+        expect(second).to be(first)
+        expect(first).to be_frozen
+      end
+    end
   end
 
   describe "method-coverage round-trip with a dynamic-symbol method name" do