Merging vs Adding and Sorting 2 pre-sorted collections

bm_merge_vs_add_and_sort.rb

require 'benchmark'

# ------------------------------------------------------------------------------
# Add 2 collections together and sort the result
def add_and_sort(left, right)
  (left + right).sort { |a, b| b[:updated_at] <=> a[:updated_at] }
end

# ------------------------------------------------------------------------------
# Merge 2 previously ordered collections together
def merge(left, right)
  result = []

  remainder = loop do
    (result << left.shift) until lesser(left, right)
    break right if left.empty?

    (result << right.shift) until lesser(right, left)
    break left if right.empty?
  end

  result + remainder
end

def lesser(one, other)
  one.empty? ||
    other.empty? ||
    one.first[:updated_at] < other.first[:updated_at]
end

# ------------------------------------------------------------------------------
# SETUP

def build_arrays(size)
  now = Time.now

  a = Array.new(size) { |i| { updated_at: now - i*2 } }
  b = Array.new(size) { |i| { updated_at: now - (i*2+1) } }

  return [a,b]
end

def test(times, method)
  a,b = build_arrays(times)
  send(method, a, b)
end

# ------------------------------------------------------------------------------
# Running tests...

Benchmark.bmbm(15) do |x|
  x.report('merge      10x') { test(10, :merge) }
  x.report('a+s        10x') { test(10, :add_and_sort) }
  x.report('merge    1000x') { test(1000, :merge) }
  x.report('a+s      1000x') { test(1000, :add_and_sort) }
  x.report('merge  100000x') { test(100000, :merge) }
  x.report('a+s    100000x') { test(100000, :add_and_sort) }
  x.report('merge 1000000x') { test(1000000, :merge) }
  x.report('a+s   1000000x') { test(1000000, :add_and_sort) }
end

results.txt

Rehearsal ---------------------------------------------------
merge      10x    0.000000   0.000000   0.000000 (  0.000033)
a+s        10x    0.000000   0.000000   0.000000 (  0.000187)
merge    1000x    0.000000   0.000000   0.000000 (  0.002411)
a+s      1000x    0.010000   0.000000   0.010000 (  0.004957)
merge  100000x    0.230000   0.020000   0.250000 (  0.255833)
a+s    100000x    0.780000   0.020000   0.800000 (  0.791663)
merge 1000000x    3.540000   0.200000   3.740000 (  3.743085)
a+s   1000000x   10.030000   0.400000  10.430000 ( 10.437322)
----------------------------------------- total: 15.230000sec

                      user     system      total        real
merge      10x    0.000000   0.000000   0.000000 (  0.000043)
a+s        10x    0.000000   0.000000   0.000000 (  0.000049)
merge    1000x    0.000000   0.000000   0.000000 (  0.002536)
a+s      1000x    0.010000   0.000000   0.010000 (  0.004930)
merge  100000x    0.220000   0.010000   0.230000 (  0.235227)
a+s    100000x    0.700000   0.050000   0.750000 (  0.759551)
merge 1000000x    3.240000   0.650000   3.890000 (  3.899715)
a+s   1000000x    9.380000   1.170000  10.550000 ( 10.554972)