Forumla -- Total Extant Bitcoins at a Given Block

getExtantCoins

# bitcoin example, halving schedule
initial_reward = 50
halving_period = 210000

# given a block height, calculate number of bitcoins minted thus far
blocks = `bitcoind getblockcount`

# number of eras -- rounded up to nearest int
era_count = (blocks / halving_period).ceil
total_coins = 0

# loop over each era
(1..era_count).each do |era|

  if era == era_count
	  era_blocks = blocks % halving_period
  else
	  era_blocks = halving_period
  end

  # reward halves each new era
  total_coins = total_coins + (initial_reward * 0.5^(era - 1) * era_blocks)

end

puts total_coins