为什么使用String#count比使用Ruby中的String #chars更快地计算

def create_genome
  "gattaca" * 100
end

def count_frequency_using_chars(sequence)
  100000.times do
    sequence.chars.group_by{|x| x}.map{|letter,array| [letter,array.count]}
  end
end

def count_frequency_using_count(sequence)
  100000.times do
    ["a","c","g","t"].map{|letter| sequence.count(letter)}
  end
end

sequence = create_genome
count_frequency_using_chars(sequence)
count_frequency_using_count(sequence)

我发现,在基于C的Ruby中,对于1.8和1.9.2,使用String #count(字母)比使用Enumerable#group_by和Array#count对它们进行排序和计算快约50倍.我对此感到有些惊讶,因为String#count方法每次迭代读取字符串四次,而后者只读取一次.

我尝试在ruby-prof和perftools.rb下运行代码,并且它们都只是表明String #chars占用了90％的时间,没有分解花费90％的时间.

如果我不得不猜测为什么会有差异,我会说创造7000万个单字符字符串会很昂贵,但我怎么能知道呢？ (更新：字符串#chars不是罪魁祸首 – 请参阅mostly_execute_a_trivial_block的基准测试)

编辑：使用1.9.2补丁级别180的当前基准：

require 'pp'
require 'benchmark'

def create_genome
  "gattaca" * 100
end

ZILLION = 100000

def count_frequency_using_count(sequence)
  ZILLION.times do
    ["a","t"].map{|letter| sequence.count(letter)}
  end
end

def count_frequency_using_chars(sequence)
  ZILLION.times do
    sequence.chars.group_by{|x| x}.map{|letter,array.count]}
  end
end

def count_frequency_using_inject_hash(sequence)
  ZILLION.times do
     sequence.chars.inject(Hash.new(0)) { |h,e| h[e] += 1 ; h }
  end
end

def count_frequency_using_each_with_object(sequence)
  ZILLION.times do
     sequence.chars.each_with_object(Hash.new(0)) { |char,hash| hash[char] += 1}
  end
end


def just_group_by(sequence)
  ZILLION.times do
    sequence.chars.group_by{|x| x}
  end
end

def just_chars_and_trivial_block(sequence)
  ZILLION.times do
    sequence.chars() {}
  end
end

def mainly_execute_a_trivial_block(sequence)
  ZILLION.times do
    sequence.length.times() {}
  end
end

def execute_an_empty_loop_instead(sequence)
  ZILLION.times do
    i = 0
    max = sequence.length
    until i == max
      i += 1
    end
  end
end

sequence = create_genome

puts RUBY_VERSION

Benchmark.bm do |benchmark|
  benchmark.report do
    count_frequency_using_count(sequence)
  end
  benchmark.report do
    count_frequency_using_chars(sequence)
  end
  benchmark.report do
    count_frequency_using_inject_hash(sequence)
  end
  benchmark.report do
    count_frequency_using_each_with_object(sequence)
  end
  benchmark.report do
    just_group_by(sequence)
  end
  benchmark.report do
    just_chars_and_trivial_block(sequence)
  end
  benchmark.report do
    mainly_execute_a_trivial_block(sequence)
  end
  benchmark.report do
    execute_an_empty_for_loop_instead(sequence)
  end
end

结果：

user     system      total        real
 0.510000   0.000000   0.510000 (  0.508499) # count_frequency_using_count
23.470000   0.030000  23.500000 ( 23.575487) # count_frequency_using_chars
32.770000   0.050000  32.820000 ( 32.874634) # count_frequency_using_inject_hash
31.880000   0.040000  31.920000 ( 31.942437) # count_frequency_using_each_with_object
22.950000   0.030000  22.980000 ( 22.970500) # just_group_by
13.300000   0.020000  13.320000 ( 13.314466) # just_chars_and_trivial_block
 5.660000   0.000000   5.660000 (  5.661516) # mainly_execute_a_trivial_block
 1.930000   0.010000   1.940000 (  1.934861) # execute_an_empty_loop_instead