From: daniel@...42.com
Date: 2020-08-14T15:14:53+00:00
Subject: [ruby-core:99588] [Ruby master Feature#17115] Optimize String#casecmp? for ASCII strings

Issue #17115 has been updated by Dan0042 (Daniel DeLorme).


In the benchmark you'd need to change the regexp from `/\Afoo\Z/i` to `/\Aconnection\z/i`; if you do so you'll find the regexp performance is similar to `casecmp?`

+1 for special-casing ASCII strings though.

Related: #13750, #14055

----------------------------------------
Feature #17115: Optimize String#casecmp? for ASCII strings
https://bugs.ruby-lang.org/issues/17115#change-87065

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
----------------------------------------
Patch: https://github.com/ruby/ruby/pull/3369

`casecmp?` is kind of a performance trap as it's much slower than using a case insensitive regexp or just `casecmp == 0`.

```
str = "Connection"
cmp = "connection"
Benchmark.ips do |x|
  x.report('/\A\z/i.match?') { /\Afoo\Z/i.match?(str) }
  x.report('casecmp?') { cmp.casecmp?(str) }
  x.report('casecmp') { cmp.casecmp(str) == 0 }
  x.compare!
end
Calculating -------------------------------------
      /\A\z/i.match?     11.447M (� 1.3%) i/s -     57.814M in   5.051489s
            casecmp?      6.197M (� 0.9%) i/s -     31.138M in   5.025252s
             casecmp     12.753M (� 1.2%) i/s -     64.636M in   5.069195s

Comparison:
             casecmp: 12752791.6 i/s
      /\A\z/i.match?: 11446996.1 i/s - 1.11x  (� 0.00) slower
            casecmp?:  6196886.0 i/s - 2.06x  (� 0.00) slower
```

This is because contrary to the others it tries to be correct in regards to unicode case folding.

However there are cases where fast case insentive equality check of known ASCII strings is useful. For instance for matching HTTP headers.

This patch check if both strings use a single byte encoding, and if so then do a simple iterative comparison with `TOLOWER()`.

This makes casecmp? sligthly faster than `casecmp == 0` when both strings are ASCII.

```
|                        |compare-ruby|built-ruby|
|:-----------------------|-----------:|---------:|
|casecmp-1               |     11.618M|   10.757M|
|                        |       1.08x|         -|
|casecmp-10              |      1.849M|    1.723M|
|                        |       1.07x|         -|
|casecmp-100             |    204.490k|  186.798k|
|                        |       1.09x|         -|
|casecmp-1000            |     20.413k|   20.184k|
|                        |       1.01x|         -|
|casecmp-nonascii1       |     19.541M|   20.100M|
|                        |           -|     1.03x|
|casecmp-nonascii10      |     19.489M|   19.914M|
|                        |           -|     1.02x|
|casecmp-nonascii100     |     19.479M|   20.155M|
|                        |           -|     1.03x|
|casecmp-nonascii1000    |     19.462M|   20.064M|
|                        |           -|     1.03x|
|casecmp_p-1             |      2.214M|   12.030M|
|                        |           -|     5.43x|
|casecmp_p-10            |      1.373M|    2.150M|
|                        |           -|     1.57x|
|casecmp_p-100           |    249.292k|  231.041k|
|                        |       1.08x|         -|
|casecmp_p-1000          |     16.173k|   23.592k|
|                        |           -|     1.46x|
|casecmp_p-nonascii1     |    651.921k|  650.572k|
|                        |       1.00x|         -|
|casecmp_p-nonascii10    |    108.253k|  109.006k|
|                        |           -|     1.01x|
|casecmp_p-nonascii100   |     11.749k|   11.889k|
|                        |           -|     1.01x|
|casecmp_p-nonascii1000  |      1.140k|    1.138k|
|  
```



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