slice+freeze vs slice!

app.rb

class FIFO
  def initialize(from_encoding, encoding)
    @from_encoding = from_encoding
    @encoding = encoding
    @need_enc = from_encoding != encoding
    @buffer = ''.force_encoding(from_encoding)
    @eol = "\n".encode(from_encoding).freeze
  end

  attr_reader :from_encoding, :encoding, :buffer

  def <<(chunk)
    orig_encoding = chunk.encoding
    chunk.force_encoding(from_encoding)
    @buffer << chunk
    # Thus the encoding needs to be reverted back here
    chunk.force_encoding(orig_encoding)
  end

  def convert(s)
    if @need_enc
      s.encode!(@encoding, @from_encoding)
    else
      s
    end
  rescue
    s.encode!(@encoding, @from_encoding, :invalid => :replace, :undef => :replace)
  end

  def next_line
    idx = @buffer.index(@eol)
    convert(@buffer.slice!(0, idx + 1)) unless idx.nil?
  end

  def next_line2
    idx = @buffer.index(@eol)

    unless idx.nil?
      @buffer.freeze
      rbuf = @buffer.slice(0, idx + 1)
      @buffer = @buffer.slice(idx + 1, @buffer.size)
      convert(rbuf)
    end
  end

  def bytesize
    @buffer.bytesize
  end
end

require 'benchmark'

log = "this is a line of log\n"

f1 = FIFO.new(Encoding::ASCII_8BIT, Encoding::ASCII_8BIT)
f2 = FIFO.new(Encoding::ASCII_8BIT, Encoding::ASCII_8BIT)

N = 40000

N.times do
  f1 << log
  f2 << log
end

puts "Ruby version: #{RUBY_VERSION}"

Benchmark.bmbm do |x|
  x.report('slice!') do
    (N/2).times do
      if f1.next_line.nil?
        raise
      end
    end
  end

  x.report('slice') do
    (N/2).times do
      if f2.next_line2.nil?
        raise
      end
    end
  end
end

result.md

ruby 2.1.10

$ ruby app.rb
Ruby version: 2.1.10
Rehearsal ------------------------------------------
slice!   2.210000   2.390000   4.600000 (  4.614415)
slice    0.010000   0.000000   0.010000 (  0.008950)
--------------------------------- total: 4.610000sec

             user     system      total        real
slice!   0.920000   1.060000   1.980000 (  1.990008)
slice    0.010000   0.000000   0.010000 (  0.007840)

ruby 2.4.6

$ ruby app.rb
Ruby version: 2.4.6
Rehearsal ------------------------------------------
slice!   2.240000   2.450000   4.690000 (  4.715899)
slice    0.010000   0.000000   0.010000 (  0.009899)
--------------------------------- total: 4.700000sec

             user     system      total        real
slice!   0.870000   1.090000   1.960000 (  1.957490)
slice    0.010000   0.000000   0.010000 (  0.013238)

ruby 2.6.3

$ ruby app.rb
Ruby version: 2.6.3
Rehearsal ------------------------------------------
slice!   2.221282   2.468471   4.689753 (  4.701863)
slice    0.009605   0.001857   0.011462 (  0.011467)
--------------------------------- total: 4.701215sec

             user     system      total        real
slice!   0.867567   1.087543   1.955110 (  1.958745)
slice    0.007350   0.000164   0.007514 (  0.007529)