8-largest-product.rb

# The four adjacent digits in the 1000-digit number that have the greatest product are 9 × 9 × 8 × 9 = 5832.
# Find the thirteen adjacent digits in the 1000-digit number that have the greatest product. What is the value of this product?
require 'benchmark'

series = 
"73167176531330624919225119674426574742355349194934\
96983520312774506326239578318016984801869478851843\
85861560789112949495459501737958331952853208805511\
12540698747158523863050715693290963295227443043557\
66896648950445244523161731856403098711121722383113\
62229893423380308135336276614282806444486645238749\
30358907296290491560440772390713810515859307960866\
70172427121883998797908792274921901699720888093776\
65727333001053367881220235421809751254540594752243\
52584907711670556013604839586446706324415722155397\
53697817977846174064955149290862569321978468622482\
83972241375657056057490261407972968652414535100474\
82166370484403199890008895243450658541227588666881\
16427171479924442928230863465674813919123162824586\
17866458359124566529476545682848912883142607690042\
24219022671055626321111109370544217506941658960408\
07198403850962455444362981230987879927244284909188\
84580156166097919133875499200524063689912560717606\
05886116467109405077541002256983155200055935729725\
71636269561882670428252483600823257530420752963450"

series = series.split("").map(&:to_i)


def big_product_in_series_of(length, series)
  winner = {index: 0, product: 0}
  series.each_with_index do |item, index|
    product = series[index...index+length].reduce(:+)
    if product > winner[:product]
      winner[:index]    = index
      winner[:product]  = product
    end
  end
  winner
end


#Abstracted more

product = lambda {|x,y| x * y}
sum     = lambda {|x,y| x + y}
minus   = lambda {|x,y| x - y}

def big_lambda_in_series_of(length, series, lambda)
  winner = {index: 0, product: 0}
  series.each_with_index do |item, index|
    product = series[index...index+length].inject(&lambda)
    if product > winner[:product]
      winner[:index]    = index
      winner[:product]  = product
    end
  end
  winner
end

Benchmark.realtime { 1000.times do
  big_lambda_in_series_of(13, series, sum)
end
}

Benchmark.realtime { 1000.times do
  big_lambda_in_series_of(13, series, product)
end
}

Benchmark.realtime { 1000.times do
  big_lambda_in_series_of(13, series, minus)
end
}


# Not using lambdas is a 1/5 faster
Benchmark.realtime { 1000.times do
  big_product_in_series_of(13, series)
end
}

Benchmark.realtime { 1000.times do
  big_product_in_series_of(13, series)
end
}

Benchmark.realtime { 1000.times do
  big_product_in_series_of(13, series)
end
}