def palindromic_numbers_v1
  Enumerator.new do |y|
    y << 0
    (1..Float::INFINITY).lazy.map {|k| [10 ** (k - 1), 10 ** k - 1].map(&:to_s)}.each do |b, e|
      (b..e).each {|s| y << (s + s.reverse[1..-1]).to_i}
      (b..e).each {|s| y << (s + s.reverse).to_i}
    end
  end
end
def palindromic_numbers
  arevb = -> a, b {b == 0 ? a : arevb.(a * 10 + b % 10, b / 10)}
  Enumerator.new do |y|
    y << 0
    (1..Float::INFINITY).lazy.map {|k| [10 ** (k - 1), 10 ** k - 1]}.each do |b, e|
      (b..e).each {|x| y << arevb.(x, x / 10)}
      (b..e).each {|x| y << arevb.(x, x)}
    end
  end
end
f = -> n {
  ps = palindromic_numbers.each_cons(2).find {|a, b| a <= n && n <= b}
  ps.map {|x| [(n - x).abs, x]}.sort.uniq.group_by(&:first).first.last.map(&:last)
}
p [0, 17, 100, 1000, 10000].map {|n| [n, f.(n)]}
 
require 'benchmark'
Benchmark.benchmark(Benchmark::CAPTION, 12) do |x|
  n = 500000
  x.report("v1(#{n}): ") {palindromic_numbers_v1.take(n)}
  x.report("v2(#{n}): ") {palindromic_numbers.take(n)}
  x.report("v1(#{n}): ") {palindromic_numbers_v1.take(n)}
  x.report("v2(#{n}): ") {palindromic_numbers.take(n)}
  p palindromic_numbers_v1.take(n) == palindromic_numbers.take(n)
end
#p palindromic_numbers_v1.lazy.zip(palindromic_numbers.lazy).map {|v1, v2| [v1 - v2, v1, v2]}.take(50).force.sort
 

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