Module: Philiprehberger::Money::Allocation

Included in:
Philiprehberger::Money
Defined in:
lib/philiprehberger/money/allocation.rb

Overview

Fair allocation of money across ratios using the largest remainder method

Instance Method Summary collapse

Instance Method Details

#allocate(ratios) ⇒ Array<Money>

Split money by ratios, distributing remainder cents fairly

Uses the largest remainder method: calculate exact shares, floor each, then distribute remaining cents to shares with the largest fractional parts.

Parameters:

  • ratios (Array<Numeric>) —

    the ratios to split by (e.g. [0.5, 0.3, 0.2])

Returns:

  • (Array<Money>) —

    array of Money objects that sum to the original amount



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# File 'lib/philiprehberger/money/allocation.rb', line 14

def allocate(ratios)
  total_ratio = ratios.sum(BigDecimal('0'))
  exact_shares = ratios.map { |r| BigDecimal(cents.to_s) * BigDecimal(r.to_s) / total_ratio }
  floored = exact_shares.map { |s| s.floor.to_i }
  remainder = cents - floored.sum

  # Distribute remaining cents to shares with the largest fractional parts
  fractional_parts = exact_shares.each_with_index.map { |s, i| [s - s.floor, i] }
  fractional_parts.sort_by! { |frac, _i| -frac }

  remainder.abs.times do |n|
    idx = fractional_parts[n][1]
    floored[idx] += remainder.positive? ? 1 : -1
  end

  floored.map { |c| self.class.new(c, currency.code) }
end