Project Euler #08 with simpler alternative

pe_08_with_simple.rb

# Project Euler #8 - Largest product in a series
# https://projecteuler.net/problem=8
#
# Find the thirteen adjacent digits in the 1000-digit number that have the
# greatest product. What is the value of this product?

def largest_product_in_series(series, adjacency_length = 13)
  factors = []
  largest_product = 0
  current_product = 1
  series.to_s.each_char do |new_factor|
    # this assumes we can trust our input will only contain digits. if we can
    # safely assume this, calling string#ord and then subtracting the ordinal
    # of the string '0' will work faster than string#to_i.
    new_factor = new_factor.to_i
    if new_factor.zero?
      # if our new_factor is zero, we know that the product of anything
      # currently in our collection of factors will be zero. so, rather than
      # work through that, just drop the current set of factors, drop the
      # zero, and reset our current product.
      factors.clear
      current_product = 1
    else
      factors << new_factor
      current_product *= new_factor
    end
    next if factors.length < adjacency_length
    largest_product = current_product if current_product > largest_product
    current_product /= factors.shift
  end
  largest_product
end

def simpler_largest_product_in_series(numbers)
  digits = numbers.to_s.split('').map!(&:to_i)
  largest_product_factors = digits.each_cons(13).max_by { |factors| factors.inject(:*) }
  largest_product_factors.inject(:*)
end

require 'benchmark/ips'
NUMBER = 7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450

Benchmark.ips do |bm|
  bm.report('probably_overly_complex') do
    raise unless largest_product_in_series(NUMBER) == 23514624000
  end

  bm.report('simpler_largest_product_in_series') do
    raise unless simpler_largest_product_in_series(NUMBER) == 23514624000
  end
end

# When using String#split with empty string
#
# Calculating -------------------------------------
# probably_overly_complex           277.000  i/100ms
# simpler_largest_product_in_series  47.000  i/100ms
# -------------------------------------------------
# probably_overly_complex              2.806k (± 3.6%) i/s -     14.127k
# simpler_largest_product_in_series  470.173  (± 4.7%) i/s -      2.350k


# When using split without argument:
#
# pe_08.rb:37:in `simpler_largest_product_in_series': undefined method `inject' for nil:NilClass (NoMethodError)
# from pe_08.rb:49:in `block (2 levels) in <main>'
# from /usr/local/rvm/gems/ruby-2.2.0/gems/benchmark-ips-2.1.1/lib/benchmark/ips/job.rb:72:in `call'
# from /usr/local/rvm/gems/ruby-2.2.0/gems/benchmark-ips-2.1.1/lib/benchmark/ips/job.rb:72:in `call_times'
# from /usr/local/rvm/gems/ruby-2.2.0/gems/benchmark-ips-2.1.1/lib/benchmark/ips/job.rb:221:in `block in run_warmup'
# from /usr/local/rvm/gems/ruby-2.2.0/gems/benchmark-ips-2.1.1/lib/benchmark/ips/job.rb:206:in `each'
# from /usr/local/rvm/gems/ruby-2.2.0/gems/benchmark-ips-2.1.1/lib/benchmark/ips/job.rb:206:in `run_warmup'
# from /usr/local/rvm/gems/ruby-2.2.0/gems/benchmark-ips-2.1.1/lib/benchmark/ips.rb:49:in `ips'
# from pe_08.rb:43:in `<main>'