Add per_unit and reverse_per_unit methods to Money class

lib/money/money.rb

@@ -342,6 +342,66 @@ def calculate_splits(num)
     Splitter.new(self, num).split.dup
   end
 
+  # Calculate the unit price based on a total quantity of units.
+  # A number of units to take can be optionally provided to get the sum of
+  # their unit prices with correct rounding.
+  #
+  # The higher-valued splits are used first (i.e., units with the "extra penny").
+  #
+  # @param quantity [Integer] total number of units
+  # @param take [Integer] number of unit prices to sum (default: 1)
+  #
+  # @return [Money]
+  #
+  # @example Get the unit price for 1 unit out of 3
+  #   Money.new(1.00, 'USD').per_unit(3)
+  #   # => Money.new(0.34, 'USD')  # the higher-valued split
+  #
+  # @example Get total for 3 units out of 3 (returns original amount)
+  #   Money.new(1.00, 'USD').per_unit(3, take: 3)
+  #   # => Money.new(1.00, 'USD')
+  def per_unit(quantity, take: 1)
+    raise ArgumentError, "take should be positive" if take < 0
+    raise ArgumentError, "quantity should be positive" if quantity <= 0
+    raise ArgumentError, "take cannot be greater than quantity" if take > quantity
+
+    calculate_splits(quantity).sum(Money.new(0, currency)) do |value, count|
+      count = [take, count].min
+      take -= count
+      value * count
+    end
+  end
+
+  # Calculate the unit price based on a total quantity of units.
+  # Uses the lesser-valued splits first (i.e., units without the "extra penny").
+  #
+  # A number of units to take can be optionally provided to get the sum of
+  # their unit prices with correct rounding.
+  #
+  # @param quantity [Integer] total number of units
+  # @param take [Integer] number of unit prices to sum (default: 1)
+  #
+  # @return [Money]
+  #
+  # @example Get the unit price for 1 unit out of 3 (lower value)
+  #   Money.new(1.00, 'USD').reverse_per_unit(3)
+  #   # => Money.new(0.33, 'USD')  # the lower-valued split
+  #
+  # @example Get total for 3 units out of 3 (returns original amount)
+  #   Money.new(1.00, 'USD').reverse_per_unit(3, take: 3)
+  #   # => Money.new(1.00, 'USD')
+  def reverse_per_unit(quantity, take: 1)
+    raise ArgumentError, "quantity should be positive" if quantity < 0
+    raise ArgumentError, "take should be positive" if take <= 0
+    raise ArgumentError, "take cannot be greater than quantity" if take > quantity
+
+    calculate_splits(quantity).reverse_each.sum(Money.new(0, currency)) do |value, count|
+      count = [take, count].min
+      take -= count
+      value * count
+    end
+  end
+
   # Clamps the value to be within the specified minimum and maximum. Returns
   # self if the value is within bounds, otherwise a new Money object with the
   # closest min or max value.

spec/splitter_spec.rb

@@ -107,4 +107,78 @@
       })
     end
   end
+
+  describe "per_unit" do
+    specify "#per_unit returns the higher-valued split by default" do
+      # $1 split 3 ways: 34¢, 33¢, 33¢
+      expect(Money.new(1.00, 'USD').per_unit(3)).to eq(Money.new(0.34, 'USD'))
+    end
+
+    specify "#per_unit with take returns sum of unit prices" do
+      # Take 2 units: 34¢ + 33¢ = 67¢
+      expect(Money.new(1.00, 'USD').per_unit(3, take: 2)).to eq(Money.new(0.67, 'USD'))
+    end
+
+    specify "#per_unit with take: quantity returns original amount" do
+      expect(Money.new(1.00, 'USD').per_unit(3, take: 3)).to eq(Money.new(1.00, 'USD'))
+    end
+
+    specify "#per_unit works with non-decimal currencies" do
+      # 100 JPY split 3 ways: 34, 33, 33
+      expect(Money.new(100, 'JPY').per_unit(3)).to eq(Money.new(34, 'JPY'))
+      expect(Money.new(100, 'JPY').per_unit(3, take: 2)).to eq(Money.new(67, 'JPY'))
+    end
+
+    specify "#per_unit raises for invalid take" do
+      expect { Money.new(1.00, 'USD').per_unit(3, take: -1) }.to raise_error(ArgumentError, "take should be positive")
+    end
+
+    specify "#per_unit raises for invalid quantity" do
+      expect { Money.new(1.00, 'USD').per_unit(0) }.to raise_error(ArgumentError, "quantity should be positive")
+      expect { Money.new(1.00, 'USD').per_unit(-1) }.to raise_error(ArgumentError, "quantity should be positive")
+    end
+
+    specify "#per_unit raises when take exceeds quantity" do
+      expect { Money.new(1.00, 'USD').per_unit(3, take: 4) }.to raise_error(ArgumentError, "take cannot be greater than quantity")
+    end
+
+    specify "#per_unit returns zero when take is 0" do
+      expect(Money.new(1.00, 'USD').per_unit(3, take: 0)).to eq(Money.new(0, 'USD'))
+    end
+  end
+
+  describe "reverse_per_unit" do
+    specify "#reverse_per_unit returns the lower-valued split by default" do
+      # $1 split 3 ways: 34¢, 33¢, 33¢ (reversed: 33¢, 33¢, 34¢)
+      expect(Money.new(1.00, 'USD').reverse_per_unit(3)).to eq(Money.new(0.33, 'USD'))
+    end
+
+    specify "#reverse_per_unit with take returns sum of lower-valued unit prices" do
+      # Take 2 units (from reversed): 33¢ + 33¢ = 66¢
+      expect(Money.new(1.00, 'USD').reverse_per_unit(3, take: 2)).to eq(Money.new(0.66, 'USD'))
+    end
+
+    specify "#reverse_per_unit with take: quantity returns original amount" do
+      expect(Money.new(1.00, 'USD').reverse_per_unit(3, take: 3)).to eq(Money.new(1.00, 'USD'))
+    end
+
+    specify "#reverse_per_unit works with non-decimal currencies" do
+      # 100 JPY split 3 ways: 34, 33, 33 (reversed: 33, 33, 34)
+      expect(Money.new(100, 'JPY').reverse_per_unit(3)).to eq(Money.new(33, 'JPY'))
+      expect(Money.new(100, 'JPY').reverse_per_unit(3, take: 2)).to eq(Money.new(66, 'JPY'))
+    end
+
+    specify "#reverse_per_unit raises for invalid take" do
+      expect { Money.new(1.00, 'USD').reverse_per_unit(3, take: 0) }.to raise_error(ArgumentError, "take should be positive")
+      expect { Money.new(1.00, 'USD').reverse_per_unit(3, take: -1) }.to raise_error(ArgumentError, "take should be positive")
+    end
+
+    specify "#reverse_per_unit raises for invalid quantity" do
+      expect { Money.new(1.00, 'USD').reverse_per_unit(-1) }.to raise_error(ArgumentError, "quantity should be positive")
+    end
+
+    specify "#reverse_per_unit raises when take exceeds quantity" do
+      expect { Money.new(1.00, 'USD').reverse_per_unit(3, take: 4) }.to raise_error(ArgumentError, "take cannot be greater than quantity")
+    end
+  end
 end