transaction_fee_calculation.rb

# 1. Transaction Fee Caculation

# We have a ladder fee structure. Please write a function to calculate total transaction fee we can get given the trading volume.

# Volume Fee
# 0 - 10 btc  2%
# 10 - 50 btc 1%
# 50 - 100 btc 0.5%
# 100 - 1000 btc 0.25%
# 1000 - 5000 btc 0.1%
# 5000 - 10000 btc 0.05%
# >= 10000 btc 0%

# examples:
# f(5) = 5*0.02 = 0.1
# f(10) = 10*0.02 = 0.2
# f(20) = 10*0.02 + 10*0.01 = 0.3
# f(60) = 10*0.02 + 40*0.01 + 10*0.005 = 0.65

require 'bigdecimal'

class FeeCalculator
  BITCOIN_TAX = {
    10000..Float::INFINITY => 0,
    5001..10000 => 0.05,
    1001..5000 => 0.1,
    101..1000 => 0.25,
    51..100 => 0.5,
    11..50 => 1,
    1..10 => 2
  }

  def self.for(bitcoins)
    return self.tax_for(bitcoins)
  end

  private
  def self.tax_for(bitcoins)
    total = BigDecimal(0)
    BITCOIN_TAX.each do |range, tax|
      while range.include? bitcoins and bitcoins > 0 do
        total += (bitcoins - range.min + 1) * (tax / 100.0)
        bitcoins = range.min - 1
      end
    end
    total
  end
end