Document Ruby::Enum performance overhead in README

Add benchmarks/basic.rb comparing basic Ruby::Enum operations
(constant access, value, key, key?, keys, values) against their
plain Ruby equivalents (constants, Hash), and a rake benchmark:basic
task to run it.

Add a new 'Performance' subsection under Benchmarks in README.md
documenting, with real numbers, that constant access has no
measurable overhead while hash-backed lookups carry roughly 3-5x
overhead vs. a plain Hash, that this overhead does not grow with
subclass depth (see rake benchmark:inheritance), and that
Ruby::Enum::Case is the one notable outlier at 50-100x slower than
a native case statement (see rake benchmark:case).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: dblock

README.md

@@ -27,6 +27,7 @@ Enum-like behavior for Ruby, heavily inspired by [this](http://www.rubyfleebie.c
   - [Exhaustive case matcher](#exhaustive-case-matcher)
   - [I18n support](#i18n-support)
 - [Benchmarks](#benchmarks)
+  - [Performance](#performance)
 - [Contributing](#contributing)
 - [Copyright and License](#copyright-and-license)
 - [Related Projects](#related-projects)
@@ -335,10 +336,21 @@ gem "i18n"
 Benchmark scripts are defined in the [`benchmarks`](benchmarks) folder and can be run with Rake:
 
 ```console
+rake benchmark:basic
 rake benchmark:case
 rake benchmark:inheritance
 ```
 
+### Performance
+
+Constant access (e.g. `Colors::RED`) has no measurable overhead versus a plain Ruby constant, since it's just a constant lookup either way. Basic operations backed by a hash lookup - `value`, `key`, `key?`, `value?`, `keys`, `values` - carry some overhead (roughly 3-5x) compared to using a plain `Hash` directly, due to the extra method dispatch and object wrapping `Ruby::Enum` does internally. Run `rake benchmark:basic` to measure this on your own machine.
+
+This overhead is constant regardless of how deep a subclass hierarchy is - `keys`, `key?`, `value?`, `key`, `value`, `to_h`, `parse` and `each` merge and memoize enums inherited from superclasses, so a subclass' lookups are effectively as fast as the base class' (see `rake benchmark:inheritance`).
+
+The one notable exception is `Ruby::Enum::Case`, whose exhaustive `case`-like matcher is significantly slower (on the order of 50-100x, see `rake benchmark:case`) than a native Ruby `case`/`when` statement, since it builds and evaluates lambdas on every call rather than being optimized by the Ruby VM. Prefer a native `case` statement in hot code paths and reserve `Ruby::Enum::Case` for cases where its exhaustiveness check is worth the overhead.
+
+For most applications this overhead is negligible in absolute terms (low single-digit microseconds per call), but it's worth being aware of in very hot code paths.
+
 ## Contributing
 
 You're encouraged to contribute to ruby-enum. See [CONTRIBUTING](CONTRIBUTING.md) for details.

Rakefile

@@ -18,6 +18,11 @@ RuboCop::RakeTask.new(:rubocop)
 task default: %i[rubocop spec]
 
 namespace :benchmark do
+  desc 'Run benchmark for basic Ruby::Enum operations vs. plain Ruby equivalents'
+  task :basic do
+    require_relative 'benchmarks/basic'
+  end
+
   desc 'Run benchmark for the Ruby::Enum::Case'
   task :case do
     require_relative 'benchmarks/case'

benchmarks/basic.rb

@@ -0,0 +1,61 @@
+# frozen_string_literal: true
+
+$LOAD_PATH.unshift(File.join(File.dirname(__FILE__), '..', 'lib'))
+
+require 'benchmark'
+require 'ruby-enum'
+
+##
+# Ruby::Enum equivalent of a plain Hash/constant lookup.
+class Colors
+  include Ruby::Enum
+
+  define :RED, 'red'
+  define :GREEN, 'green'
+  define :BLUE, 'blue'
+end
+
+# Plain Ruby equivalents, doing the same lookups without Ruby::Enum.
+PLAIN_HASH = { RED: 'red', GREEN: 'green', BLUE: 'blue' }.freeze
+
+module PlainConstants
+  RED = 'red'
+  GREEN = 'green'
+  BLUE = 'blue'
+end
+
+n = 1_000_000
+
+def benchmark(label, iterations, &block)
+  time = Benchmark.realtime { iterations.times(&block) }
+  puts "#{label}: #{time.round(4)}"
+  time
+end
+
+puts "Running #{n} iterations of each scenario below\n\n"
+
+puts '--- Constant access ---'
+enum_const_time = benchmark('Ruby::Enum constant (Colors::RED)', n) { Colors::RED }
+plain_const_time = benchmark('plain Ruby constant (PlainConstants::RED)', n) { PlainConstants::RED }
+
+puts "\n--- Key to value lookup ---"
+enum_value_time = benchmark('Ruby::Enum (Colors.value(:RED))', n) { Colors.value(:RED) }
+hash_value_time = benchmark('plain Hash (PLAIN_HASH[:RED])', n) { PLAIN_HASH[:RED] }
+
+puts "\n--- Value to key lookup ---"
+benchmark('Ruby::Enum (Colors.key(\'red\'))', n) { Colors.key('red') }
+benchmark('plain Hash (PLAIN_HASH.key(\'red\'))', n) { PLAIN_HASH.key('red') }
+
+puts "\n--- Existence checks ---"
+benchmark('Ruby::Enum (Colors.key?(:RED))', n) { Colors.key?(:RED) }
+benchmark('plain Hash (PLAIN_HASH.key?(:RED))', n) { PLAIN_HASH.key?(:RED) }
+
+puts "\n--- Enumerating all keys/values ---"
+benchmark('Ruby::Enum (Colors.keys)', n) { Colors.keys }
+benchmark('plain Hash (PLAIN_HASH.keys)', n) { PLAIN_HASH.keys }
+benchmark('Ruby::Enum (Colors.values)', n) { Colors.values }
+benchmark('plain Hash (PLAIN_HASH.values)', n) { PLAIN_HASH.values }
+
+puts "\n--- Comparison ---"
+puts "Ruby::Enum constant access is #{(enum_const_time / plain_const_time).round(2)}x plain Ruby constant access"
+puts "Ruby::Enum .value is #{(enum_value_time / hash_value_time).round(2)}x plain Hash#[]"