by_weight.rb

# Calculates what percentage of 'n' each element of 'arr' should have, by weight.
#
# @param arr [Array<#amount>] the array of objects to spread n across.
# @param n [Number] the amount to spread accross arr.
def adj(arr, n)
  total = arr.inject(BigDecimal(0)) { |acc, memo| acc += memo.amount }
  results = arr.map { |elem| (elem.amount / total).round(2) }
  # ensure the last entry accounts for rounding
  results[-1] = total - results[0...-1].inject(&:+)
  arr.zip(results)
end