Add Minitest benchmarking file for sequential vs parallel execution

Co-authored-by: KOTP <[email protected]>

exercises/practice/parallel-letter-frequency/.docs/instructions.append.md

@@ -1,7 +1,19 @@
 # Instructions Append
+
+## Benchmarking
+
+If you are solving this exercise locally, there is an included `parallel_letter_frequency_benchmark_test.rb` file which will compare your parallel solution to a sequential one.
+You can run it via `ruby parallel_letter_frequency_benchmark_test.rb`.
+The output will show execution times of each implementation over a range of large text counts in a tabular format.
+Feel free to change the test cases.
+You may want to investigate what performance impact length of text vs number of texts has on the execution time for each implementation.
+
+**Note:** For smaller sets of text, the sequential count _may_ be faster due to processing overhead costs.
+
 ## Further Reading
 
 - [Ruby `Thread` Documentation](https://docs.ruby-lang.org/en/master/Thread.html)
 - [Ruby `Thread::Queue` Documentation](https://docs.ruby-lang.org/en/master/Thread/Queue.html)
 - [Ruby `Fiber` Documentation](https://docs.ruby-lang.org/en/master/Fiber.html)
 - [Ruby `Ractor` Documentation](https://docs.ruby-lang.org/en/master/Ractor.html)
+- [`Minitest::Benchmark` Documentation](https://ruby-doc.org/3.0.6/gems/minitest/Minitest/Benchmark.html)

exercises/practice/parallel-letter-frequency/parallel_letter_frequency_benchmark_test.rb

@@ -0,0 +1,39 @@
+require 'minitest/autorun'
+require 'minitest/benchmark'
+require_relative 'parallel_letter_frequency'
+
+class ParallelLetterFrequencyBenchmarkTest < Minitest::Benchmark
+  def self.bench_range
+    bench_exp(1, 100_000)
+  end
+
+  def setup
+    @strings = {}
+    self.class.bench_range.each do |n|
+      @strings[n] = Array.new(10, 'a' * n)
+    end
+  end
+
+  def bench_sequential
+    assert_performance ->(_, _) { true } do |n|
+      sequential_letter_frequency(@strings[n])
+    end
+  end
+
+  def bench_parallel
+    assert_performance ->(_, _) { true } do |n|
+      ParallelLetterFrequency.count(@strings[n])
+    end
+  end
+
+  def sequential_letter_frequency(texts)
+    tally = Hash.new(0)
+    texts.each do |text|
+      text.each_grapheme_cluster do |cluster|
+        tally[cluster] += 1
+      end
+    end
+
+    tally
+  end
+end