Array#sort_byのマニュアルを見ていたら便利そうなKernel.testの記述がありました。
Array#sortのマニュアルより抜粋
まずはsortとsort_byのbenchmark
require 'benchmark' a = (1..100000).map {rand(100000)} Benchmark.bm(10) do |b| b.report("Sort") { a.sort } b.report("Sort by") { a.sort_by {|a| a} } end produces: user system total real Sort 0.180000 0.000000 0.180000 ( 0.175469) Sort by 1.980000 0.040000 2.020000 ( 2.013586)
sort_by遅いね。
しかし、、、
ファイルのmtimeでのsort
However, consider the case where comparing the keys is a non-trivial operation. The following code sorts some files on modification time using the basic sort method. files = Dir["*"] sorted = files.sort {|a,b| File.new(a).mtime <=> File.new(b).mtime} sorted #=> ["mon", "tues", "wed", "thurs"]
mtimeで比較となると、まぁ私もFileオブジェクト使っちゃいますね。
Kernel#test の登場
しかし、これはFileオブジェクトを毎回2つ作っちゃうので効率的でない。
そこで登場するのがKernel#test。
へー知らんかった。めっちゃ便利。
This sort is inefficient: it generates two new File objects during every comparison. A slightly better technique is to use the Kernel#test method to generate the modification times directly. files = Dir["*"] sorted = files.sort { |a,b| test(?M, a) <=> test(?M, b) } sorted #=> ["mon", "tues", "wed", "thurs"]
まだ無駄がある
そらまぁ、Fileオブジェクトは毎回作らなくてもTimeオブジェクトは作成されます。
そこで登場するのが昨日のSchwartzian Transform（シュワルツ変換）ですね。
参考：rubyのsort_by / shuffle から学ぶシュワルツ変換とフィッシャー - イェーツのシャッフル - rochefort's blog
This still generates many unnecessary Time objects. A more efficient technique is to cache the sort keys (modification times in this case) before the sort. Perl users often call this approach a Schwartzian Transform, after Randal Schwartz. We construct a temporary array, where each element is an array containing our sort key along with the filename. We sort this array, and then extract the filename from the result. sorted = Dir["*"].collect { |f| [test(?M, f), f] }.sort.collect { |f| f[1] } sorted #=> ["mon", "tues", "wed", "thurs"]
そしてこれを内部的にやっているのがsort_by
This is exactly what sort_by does internally. sorted = Dir["*"].sort_by {|f| test(?M, f)} sorted #=> ["mon", "tues", "wed", "thurs"]
sort_byはシュワルツ変換なんです。sort_by が良くなる局面も多々有ります。
Kernel#testのオプション
たくさん便利なのがありました。
module function Kernel.#test (Ruby 2.3.0)
Test Returns Meaning
"A" | Time | Last access time for file1
"b" | boolean | True if file1 is a block device
"c" | boolean | True if file1 is a character device
"C" | Time | Last change time for file1
"d" | boolean | True if file1 exists and is a directory
"e" | boolean | True if file1 exists
"f" | boolean | True if file1 exists and is a regular file
"g" | boolean | True if file1 has the \CF{setgid} bit
| | set (false under NT)
"G" | boolean | True if file1 exists and has a group
| | ownership equal to the caller's group
"k" | boolean | True if file1 exists and has the sticky bit set
"l" | boolean | True if file1 exists and is a symbolic link
"M" | Time | Last modification time for file1
"o" | boolean | True if file1 exists and is owned by
| | the caller's effective uid
"O" | boolean | True if file1 exists and is owned by
| | the caller's real uid
"p" | boolean | True if file1 exists and is a fifo
"r" | boolean | True if file1 is readable by the effective
| | uid/gid of the caller
"R" | boolean | True if file is readable by the real
| | uid/gid of the caller
"s" | int/nil | If file1 has nonzero size, return the size,
| | otherwise return nil
"S" | boolean | True if file1 exists and is a socket
"u" | boolean | True if file1 has the setuid bit set
"w" | boolean | True if file1 exists and is writable by
| | the effective uid/gid
"W" | boolean | True if file1 exists and is writable by
| | the real uid/gid
"x" | boolean | True if file1 exists and is executable by
| | the effective uid/gid
"X" | boolean | True if file1 exists and is executable by
| | the real uid/gid
"z" | boolean | True if file1 exists and has a zero length
Tests that take two files:
"-" | boolean | True if file1 and file2 are identical
"=" | boolean | True if the modification times of file1
| | and file2 are equal
"<" | boolean | True if the modification time of file1
| | is prior to that of file2
">" | boolean | True if the modification time of file1
| | is after that of file2
