optimize result merger

lib/simplecov/result_merger.rb

@@ -32,11 +32,16 @@ def merge_results(*file_paths, ignore_timeout: false)
         # of data. Reading them all in easily produces Gigabytes of memory consumption which
         # we want to avoid.
 
-        results = file_paths.map { |path| valid_results(path, ignore_timeout: ignore_timeout) }
-        merge_coverage(results)
+        file_paths = file_paths.dup
+        initial_result = merge_file_results(file_paths.shift, ignore_timeout: ignore_timeout)
+
+        file_paths.reduce(initial_result) do |memo, path|
+          file_result = merge_file_results(path, ignore_timeout: ignore_timeout)
+          merge_coverage([memo, file_result])
+        end
       end
 
-      def valid_results(file_path, ignore_timeout: false)
+      def merge_file_results(file_path, ignore_timeout:)
         raw_results = parse_file(file_path)
         results = Result.from_hash(raw_results)
         merge_valid_results(results, ignore_timeout: ignore_timeout)

lib/simplecov/result_serialization.rb

@@ -24,7 +24,7 @@ def deserialize(hash) # rubocop:disable Metrics/MethodLength
         hash.map do |command_name, data|
           coverage = {}
 
-          data["coverage"].each do |file_name, file_data|
+          data.fetch("coverage").each do |file_name, file_data|
             parsed_file_data = {}
 
             file_data = {lines: file_data} if file_data.is_a?(Array)
@@ -39,7 +39,7 @@ def deserialize(hash) # rubocop:disable Metrics/MethodLength
 
           result = SimpleCov::Result.new(coverage)
           result.command_name = command_name
-          result.created_at = Time.at(data["timestamp"])
+          result.created_at = Time.at(data.fetch("timestamp"))
           result
         end
       end