Euler Challenge: maximum product from 5 consecutive digits in a 1000-digit integer

max-product.rb

#! /usr/bin/env ruby
#
# PROBLEM:
# 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
#
# SOLUTION:

NUMBER =
  '73167176531330624919225119674426574742355349194934' +
  '96983520312774506326239578318016984801869478851843' +
  '85861560789112949495459501737958331952853208805511' +
  '12540698747158523863050715693290963295227443043557' +
  '66896648950445244523161731856403098711121722383113' +
  '62229893423380308135336276614282806444486645238749' +
  '30358907296290491560440772390713810515859307960866' +
  '70172427121883998797908792274921901699720888093776' +
  '65727333001053367881220235421809751254540594752243' +
  '52584907711670556013604839586446706324415722155397' +
  '53697817977846174064955149290862569321978468622482' +
  '83972241375657056057490261407972968652414535100474' +
  '82166370484403199890008895243450658541227588666881' +
  '16427171479924442928230863465674813919123162824586' +
  '17866458359124566529476545682848912883142607690042' +
  '24219022671055626321111109370544217506941658960408' +
  '07198403850962455444362981230987879927244284909188' +
  '84580156166097919133875499200524063689912560717606' +
  '05886116467109405077541002256983155200055935729725' +
  '71636269561882670428252483600823257530420752963450'

# WRITE YOUR CODE HERE
#
# This technique uses regular expressions to find the best solution.
# It starts by looking for any occurrences of /[9]{5}/ (five consecutive
# '9' numerals), then /[98]{5}/ (five digits of either 8 or 9), and
# so on.  It stops as soon as it finds a pattern that has matches, and
# picks the best match by calculating the products.
#
itercount = 0
maxproduct = 0
digits = '987654321'
dcount = 1

while (dcount < digits.length)
  itercount += 1
  pattern = Regexp.new("[#{digits[0,dcount]}]{5}")
  matches = NUMBER.scan(pattern)
  dcount += 1
  next if (matches.empty?)
  bestval = 0
  matches.each do |str|
    val = str.split(//).map { |c| c.to_i }.inject(1) { |m,i| m *= i ; m }
    bestval = [ bestval, val ].max
  end
  maxproduct = bestval
  break
end

puts(maxproduct)
puts("Iterations: #{itercount}")

#
# The best case will find the solution in 1 iteration: when the number has
# one or more occurrences of '99999'.  The worst case will require 8 iterations
# and occurs when the number is all zeroes.
#