运行时间 Array#unshift 与 Array#shift

Question

我预计Array#shift 和Array#unshift 的运行时间都是Θ(n)。原因是机器需要循环遍历每个数组成员并将其分配给左侧或右侧的键。

在Array#unshift的情况下，假设只有一个值作为参数传入并且有很多数组成员，我假设array[0]的值分配对运行没有显着影响时间。换句话说，当数组成员的数量多而传递给Array#unshift的变量数量少时，我希望Array#shift和Array#unshift具有相同的运行时间。

在 Ruby 2.1.2 上运行基准测试时，这些假设不成立。为什么？

代码：

require 'benchmark'

GC.disable    

number_of_elements = 25_600_000

a1 =[]
a2 = []
a3 = []
a4 = []
q1 = Queue.new
q2 = Queue.new

puts number_of_elements

number_of_elements.times do
  q1.enq(true)
  q2.enq(true)
  a1 << true
  a2 << true
  a3 << true
  a4 << true
end

number_of_operations = 1

Benchmark.bm do |bm|
  puts "Queue#enq('test')"
  bm.report do    
    number_of_operations.times { q1.enq('test') }
  end

  puts "Queue#deq"
  bm.report do    
    number_of_operations.times { q2.deq }
  end

  puts "Array#shift"
  bm.report do
    number_of_operations.times { a1.shift }
  end 

  puts "Array#unshift"
  bm.report do
    number_of_operations.times { a2.unshift('test') }    
  end 

  puts "Array#pop"
  bm.report do
    number_of_operations.times { a3.pop }
  end

  puts "Array#<<"
  bm.report do
    number_of_operations.times { a4 << 'test' }
  end      
end

结果：

25600000
       user     system      total        real
Queue#enq('test')
   0.000000   0.000000   0.000000 (  0.000006)
Queue#deq
   0.010000   0.020000   0.030000 (  0.029928)
Array#shift
   0.010000   0.020000   0.030000 (  0.032203)
Array#unshift
   0.080000   0.060000   0.140000 (  0.143272)
Array#pop
   0.000000   0.000000   0.000000 (  0.000004)
Array#<<
   0.000000   0.000000   0.000000 (  0.000007)

Answer 1

在 MRI Ruby 2.1.2 中，unshift 会重新分配数组并完全复制它：

              static VALUE
rb_ary_unshift_m(int argc, VALUE *argv, VALUE ary)
{
    long len = RARRAY_LEN(ary);

    [...]

    ary_ensure_room_for_unshift(ary, argc);
    ary_memcpy(ary, 0, argc, argv);
    ARY_SET_LEN(ary, len + argc);
    return ary;
}

shift 显然并不总是这样做：

              static VALUE
rb_ary_shift_m(int argc, VALUE *argv, VALUE ary)
{
    VALUE result;
    long n;

    [...]

    rb_ary_modify_check(ary);
    result = ary_take_first_or_last(argc, argv, ary, ARY_TAKE_FIRST);
    n = RARRAY_LEN(result);
    if (ARY_SHARED_P(ary)) {
        if (ARY_SHARED_OCCUPIED(ARY_SHARED(ary))) {
            ary_mem_clear(ary, 0, n);
        }
        ARY_INCREASE_PTR(ary, n);
    }
    else {
        RARRAY_PTR_USE(ary, ptr, {
            MEMMOVE(ptr, ptr + n, VALUE, RARRAY_LEN(ary)-n);
        }); /* WB: no new reference */
    }
    ARY_INCREASE_LEN(ary, -n);

    return result;
}