# Find the greatest product of five consecutive digits in the 1000-digit number.
# 73167176531330624919225119674426574742355349194934
# 96983520312774506326239578318016984801869478851843
# 85861560789112949495459501737958331952853208805511
# 12540698747158523863050715693290963295227443043557
# 66896648950445244523161731856403098711121722383113
# 62229893423380308135336276614282806444486645238749
# 30358907296290491560440772390713810515859307960866
# 70172427121883998797908792274921901699720888093776
# 65727333001053367881220235421809751254540594752243
# 52584907711670556013604839586446706324415722155397
# 53697817977846174064955149290862569321978468622482
# 83972241375657056057490261407972968652414535100474
# 82166370484403199890008895243450658541227588666881
# 16427171479924442928230863465674813919123162824586
# 17866458359124566529476545682848912883142607690042
# 24219022671055626321111109370544217506941658960408
# 07198403850962455444362981230987879927244284909188
# 84580156166097919133875499200524063689912560717606
# 05886116467109405077541002256983155200055935729725
# 71636269561882670428252483600823257530420752963450

# Naive approach: iterate through all 996 combinations, find the greatest
# product.
#  We also know the potentially "winning" string: 99999, we could probably
#  look for that match and work backwards until we find it.


number = "73167176531330624919225119674426574742355349194934
          96983520312774506326239578318016984801869478851843
          85861560789112949495459501737958331952853208805511
          12540698747158523863050715693290963295227443043557
          66896648950445244523161731856403098711121722383113
          62229893423380308135336276614282806444486645238749
          30358907296290491560440772390713810515859307960866
          70172427121883998797908792274921901699720888093776
          65727333001053367881220235421809751254540594752243
          52584907711670556013604839586446706324415722155397
          53697817977846174064955149290862569321978468622482
          83972241375657056057490261407972968652414535100474
          82166370484403199890008895243450658541227588666881
          16427171479924442928230863465674813919123162824586
          17866458359124566529476545682848912883142607690042
          24219022671055626321111109370544217506941658960408
          07198403850962455444362981230987879927244284909188
          84580156166097919133875499200524063689912560717606
          05886116467109405077541002256983155200055935729725
          71636269561882670428252483600823257530420752963450"
number.gsub!(/\s/, '')

max = 0
number.length.times do |i|
  if i + 5 < number.length
    product = number[i...(i+5)].split(//).map(&:to_i).inject(1, &:*)
    max = product if product > max
  end
end

puts max
