Solutions to Project Euler Problem #1

euler_1.rb

require 'benchmark'

def plain_map n
  # Enter your code here. Read input from STDIN. Print output to STDOUT
  three_or_five = -> i {i % 3 == 0 || i % 5 == 0}

  total = (0...n.to_i).inject(0) do |sum, i|
      next sum unless three_or_five.call(i)
      sum + i
  end
end

def threes_and_non_three_fives n
    threes = (3...n).step(3).inject(&:+)
    fives = (5...n).step(5).select{|i| i % 3 != 0}.inject(&:+)
    threes + fives
end

def threes_fives_no_fifteens n
    threes = (3...n).step(3).inject(0, &:+)
    fives = (5...n).step(5).inject(0, &:+)
    fifteens = (15...n).step(15).inject(0, &:+)

    threes + fives - fifteens
end

def partial_sum(base, max)
    top = max / base
    sum = Rational(top, 2) * (top + 1)
    sum.floor * base
end

def computed n
    threes = partial_sum 3, n
    fives = partial_sum 5, n
    fifteens = partial_sum 15, n

    threes + fives - fifteens
end

n = 10_000_000
Benchmark.bm(30) do |bm|
  bm.report('plain_map') { plain_map n }
  bm.report('threes_and_non_three_fives') { threes_and_non_three_fives n }
  bm.report('threes_fives_no_fifteens') { threes_fives_no_fifteens n }
  bm.report('computed') { computed n }
end


#                                     user     system      total        real
#plain_map                        2.750000   0.020000   2.770000 (  2.833699)
#threes_and_non_three_fives       0.720000   0.010000   0.730000 (  0.738672)
#threes_fives_no_fifteens         0.540000   0.000000   0.540000 (  0.555927)
#computed                         0.000000   0.000000   0.000000 (  0.000019)