# Problem 1
def multiply_three_or_five(from, to)
  (from...to).select { |i| i%3==0 || i%5==0 }.inject(:+)
end
 
"Result:"
#=> pry(main)> multiply_three_or_five(0, 1000)
#=> 233168
 
#Problem 2
def find_fibonachi_array(start, last_number)
  while start[-2] + start[-1] < last_number
    start << start[-2] + start[-1]
  end
  start
end
 
def sum(fibonachi_array)
  fibonachi_array.inject(0) {|sum, num| num.even? ? sum+num : sum }
end
 
def even_fibonachi_numbers_sum(start, last_number)
  sum(find_fibonachi_array(start, last_number))
end
 
#=>pry(main)> even_fibonachi_numbers_sum([1,2], 4_000_000)
#=> 4613732
 
#Problem 3
def prime_factors(last_number)
  return [] if last_number == 1
  factor = (2..last_number).find { |number| last_number%number == 0 }
  #puts "factor is: " + factor.to_s
  #puts "prime_factor is: " + prime_factors(last_number / factor).to_s
  [factor] + prime_factors(last_number / factor)
end  
  
#=> prime_factors(600_851_475_143)  
#=> 6857
 
# Problem 4
def polindrome_number(first_num, second_num=first_num)
  product = first_num * second_num
  return product if number_is_polindrome?(product)
  first_num, second_num = find_pair_of_numbers(first_num, second_num)
  polindrome_number(first_num, second_num)
end
 
def number_is_polindrome? number
  number.to_s == number.to_s.reverse
end
 
def find_pair_of_numbers(num1, num2)
  if num2 > 900
    num2 -= 1
  else
    num1 -= 1
    num2 = num1
  end
  [num1, num2]
end
 
puts polindrome_number(999)  
 
# Problem 5
def smallest_multiply
  (from..to).inject(1) { |result_number, number| result_number.lcm(number) }
end
 
#=> smallest_multiply(1,20)
#=> 232792560
 
# Problem 6
def difference(from, to)
  square_sum(from, to) - sum_of_squares(from, to)
end
 
def square_sum(from,to)
  sum = (from..to).inject(:+)
  sum * sum
end
 
def sum_of_squares(from, to)
  (from..to).inject(0) { |sum, num| sum+num**2}
end
 
#=> difference(1,10)
#=> 2640
#=> difference(1,100)
#=> 25164150