[kilk@klkpc ruby_small]$ rb comp.rb
Elapsed time for Brute Force : 5.804927 Primes = 78498
Elapsed time for Sieve of Eratosthenes: 0.482605 Primes = 78498
Elapsed time for Sieve of Sundaram : 0.650439 Primes = 78498

[kilk@klkpc crystal]$ bin/crystal test1.cr --run
Elapsed time for Brute Force : 0.327009 Primes = 78498
Elapsed time for Sieve of Eratosthenes: 0.0609334 Primes = 78498
Elapsed time for Sieve of Sundaram : 0.0452578 Primes = 78498



def bruteForce(topCandidate)
  totalCount = 1
  isPrime = true

  3.step(topCandidate, 2) do |i|
    j=3

    while j*j <= i && isPrime
      isPrime = false if i%j==0
      j += 2
    end

    isPrime ? (totalCount += 1) : (isPrime = true)
  end

totalCount
end

def sieveOfEratosthenes(topCandidate)
  myBA1 = Array.new(topCandidate + 1) {true}
  myBA1[0] = myBA1[1] = false
  thisFactor = 2

  while thisFactor * thisFactor <= topCandidate
    mark = thisFactor + thisFactor
    mark.step(topCandidate, thisFactor) {|n| myBA1[n] = false}

    thisFactor += 1

    while (!myBA1[thisFactor])
      thisFactor += 1
    end

  end

  myBA1.count(true)
end

def sieveOfSundaram(topCandidate)
  k = topCandidate / 2
  myBA1 = Array.new(k + 1) {true}
  myBA1[0] = myBA1[k] = false

  (1..k).each do |i|
    denominator = (i << 1) + 1
    maxVal = (k - i) / denominator
    i.step(maxVal, 1) {|n| myBA1[i + n * denominator] = false}

    # this version takes .20 seconds longer to run 1M iterations!
    # for n in i..maxVal+1 do
      # myBA1[i + n * denominator] = false
    # end
  end

myBA1.count(true) + 1
end

def main
  max = 1000000
  startTime = Time.now()
  primes = bruteForce(max)
  elapsed = Time.now() - startTime
  puts("Elapsed time for Brute Force : #{elapsed} Primes = #{primes}")

  startTime = Time.now()
  primes = sieveOfEratosthenes(max)
  endTime = Time.now()
  elapsed = endTime - startTime
  puts("Elapsed time for Sieve of Eratosthenes: #{elapsed} Primes = #{primes}")

  startTime = Time.now()
  primes = sieveOfSundaram(max)
  endTime = Time.now()
  elapsed = endTime - startTime
  puts("Elapsed time for Sieve of Sundaram : #{elapsed} Primes = #{primes}")
end

main