Implement Finance::Loan#ppmt & fix Finance::Loan#pmt

Author: wowinter13

README.md

@@ -14,7 +14,7 @@ which are as follows:
 | fv                           |   ✅    |   Computes the  future value|
 | ipmt                         |   ✅   |   Computes interest payment for a loan|
 | pmt                          |  ✅    |   Computes the fixed periodic payment(principal + interest) made against a loan amount|
-| ppmt                         |       |   Computes principal payment for a loan|
+| ppmt                         |   ✅   |   Computes principal payment for a loan|
 | nper                         |       |    Computes the number of periodic payments|
 | pv                           |       |   Computes the present value of a payment|
 | rate                         |     |    Computes the rate of interest per period|

lib/finance/loan.rb

@@ -80,7 +80,7 @@ def pmt
           (factor - 1) * (1 + monthly_rate * ptype) / monthly_rate
         end
 
-      (-future_value + amount * factor) / second_factor
+      -((future_value + amount * factor) / second_factor)
     end
 
     # IPmt computes interest payment for a loan under a given period.
@@ -111,6 +111,27 @@ def ipmt
       end
     end
 
+    # PPmt computes principal payment for a loan under a given period.
+    #
+    #   Required Loan arguments: period, nominal_rate, duration, amount, future_value*
+    #
+    # @return [Float] Principal payment for a loan under a given period.
+    #
+    # @example
+    #   require 'finance_rb'
+    #   Finance::Loan.new(nominal_rate: 0.0824, duration: 12, amount: 2500, period: 1).ppmt
+    #   #=> -200.58192368678277
+    #
+    # @see http://www.oasis-open.org/committees/documents.php?wg_abbrev=office-formulaOpenDocument-formula-20090508.odt
+    # @see [WRW] Wheeler, D. A., E. Rathke, and R. Weir (Eds.) (2009, May).
+    #   Open Document Format for Office Applications (OpenDocument)v1.2,
+    #   Part 2: Recalculated Formula (OpenFormula) Format - Annotated Version,
+    #   Pre-Draft 12. Organization for the Advancement of Structured Information
+    #   Standards (OASIS). Billerica, MA, USA. [ODT Document].
+    def ppmt
+      pmt - ipmt
+    end
+
     # Fv computes future value at the end of some periods (duration).
     #   Required Loan arguments: nominal_rate, duration, payment, amount*
     #
@@ -156,7 +177,7 @@ def remaining_balance
       self.class.new(
         nominal_rate: nominal_rate.to_f, duration: period - 1.0,
         amount: amount.to_f, ptype: PAYMENT_TYPE_MAPPING.key(ptype)
-      ).fv(payment: -pmt)
+      ).fv(payment: pmt)
     end
   end
 end

spec/finance/loan_spec.rb

@@ -5,37 +5,37 @@
     context 'w/o a full set of params' do
       it 'calculates correct pmt value w/o :ptype' do
         loan = Finance::Loan.new(nominal_rate: 0.1, duration: 12, amount: 1000)
-        expect(loan.pmt).to eq(87.9158872300099)
+        expect(loan.pmt).to eq(-87.9158872300099)
       end
 
       it 'calculates correct pmt value w/o :nominal_rate' do
         loan = Finance::Loan.new(duration: 12, amount: 1200, ptype: :end)
-        expect(loan.pmt).to eq(100)
+        expect(loan.pmt).to eq(-100)
       end
     end
 
     context 'with zero rates' do
       it 'calculates correct pmt value for 3 years' do
         loan = Finance::Loan.new(nominal_rate: 0, duration: 36, amount: 10_000, ptype: :end)
-        expect(loan.pmt).to eq(277.77777777777777)
+        expect(loan.pmt).to eq(-277.77777777777777)
       end
 
       it 'calculates correct pmt value for 6 months' do
         loan = Finance::Loan.new(nominal_rate: 0, duration: 6, amount: 10_000, ptype: :end)
-        expect(loan.pmt).to eq(1666.6666666666667)
+        expect(loan.pmt).to eq(-1666.6666666666667)
       end
     end
 
     context 'with :beginning ptype' do
       it 'calculates correct pmt value' do
         loan = Finance::Loan.new(nominal_rate: 0.12, duration: 6, amount: 1000, ptype: :beginning)
-        expect(loan.pmt).to eq(170.8399670404763)
+        expect(loan.pmt).to eq(-170.8399670404763)
       end
     end
 
     it 'calculates correct pmt value' do
       loan = Finance::Loan.new(nominal_rate: 0.13, duration: 90, amount: 1_000_000, ptype: :end)
-      expect(loan.pmt).to eq(17_449.90775727763)
+      expect(loan.pmt).to eq(-17_449.90775727763)
     end
   end
 
@@ -99,4 +99,51 @@
       end
     end
   end
+
+  describe '#ppmt' do
+    context 'when 1 period' do
+      it 'calculates correct ppmt value' do
+        loan = Finance::Loan.new(
+          nominal_rate: 0.0824, duration: 12, amount: 2500, period: 1
+        )
+        expect(loan.ppmt).to eq(-200.58192368678277)
+      end
+    end
+
+    context 'when 2 periods' do
+      it 'calculates correct ppmt value' do
+        loan = Finance::Loan.new(
+          nominal_rate: 0.0824, duration: 12, amount: 2500, period: 2
+        )
+        expect(loan.ppmt).to eq(-201.95925289609866)
+      end
+    end
+
+    context 'when 3 periods' do
+      it 'calculates correct ppmt value' do
+        loan = Finance::Loan.new(
+          nominal_rate: 0.0824, duration: 12, amount: 2500, period: 3
+        )
+        expect(loan.ppmt).to eq(-203.34603976598518)
+      end
+    end
+
+    context 'when 4 periods' do
+      it 'calculates correct ppmt value' do
+        loan = Finance::Loan.new(
+          nominal_rate: 0.0824, duration: 12, amount: 2500, period: 4
+        )
+        expect(loan.ppmt).to eq(-204.7423492390449)
+      end
+    end
+
+    context 'when 5 periods' do
+      it 'calculates correct ppmt value' do
+        loan = Finance::Loan.new(
+          nominal_rate: 0.0824, duration: 12, amount: 2500, period: 5
+        )
+        expect(loan.ppmt).to eq(-206.1482467038197)
+      end
+    end
+  end
 end