Merge pull request #16 from wowinter13/feat/loan_pv

Implement Finance::Loan#pv

Author: wowinter13

README.md

@@ -16,7 +16,7 @@ which are as follows:
 | pmt                          |  ✅    |   Computes the fixed periodic payment(principal + interest) made against a loan amount|
 | ppmt                         |   ✅   |   Computes principal payment for a loan|
 | nper                         |    ✅   |    Computes the number of periodic payments|
-| pv                           |       |   Computes the present value of a payment|
+| pv                           |    ✅  |   Computes the present value of a payment|
 | rate                         |     |    Computes the rate of interest per period|
 | irr                          |    ✅    |    Computes the internal rate of return|
 | npv                          |  ✅   |   Computes the net present value of a series of cash flow|

lib/finance/loan.rb

@@ -25,6 +25,7 @@ class Loan
 
     # @return [Float] The number of periods to be compounded for. (I.e. Nper())
     #   Defaults to 1.
+    #   You can use #nper method to calculate value if param is not defined.
     attr_accessor :duration
 
     # @return [Float] Future value.
@@ -178,6 +179,29 @@ def fv(payment: nil)
       -((amount * factor) + (final_payment.to_f * second_factor))
     end
 
+    # Pv computes present value.
+    #   Required Loan arguments: nominal_rate, duration, payment, future_value, *ptype
+    #
+    # @return [Float] The present value.
+    #
+    # @example
+    #   require 'finance_rb'
+    #   Finance::Loan.new(nominal_rate: 0.24, duration: 12, future_value: 1000, payment: -300, ptype: :ending).pv
+    #   #=> 2384.1091906935
+    #
+    # @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 pv
+      factor = (1.0 + monthly_rate)**duration
+      second_factor = (factor - 1) * (1 + monthly_rate * ptype) / monthly_rate
+
+      -(future_value + (payment.to_f * second_factor)) / factor
+    end
+
     private
 
     def initialize_payment_type(ptype)

spec/finance/loan_spec.rb

@@ -71,6 +71,26 @@
     end
   end
 
+  describe '#pv' do
+    context 'when :ptype == beginning' do
+      it 'calculates correct pv value' do
+        loan = Finance::Loan.new(
+          nominal_rate: 0.24, duration: 12, future_value: 1000, payment: -300, ptype: :beginning
+        )
+        expect(loan.pv).to eq(2447.5612380190028)
+      end
+    end
+
+    context 'when :ptype == ending' do
+      it 'calculates correct pv value' do
+        loan = Finance::Loan.new(
+          nominal_rate: 0.24, duration: 12, future_value: 1000, payment: -300, ptype: :ending
+        )
+        expect(loan.pv).to eq(2384.1091906935)
+      end
+    end
+  end
+
   describe '#ipmt' do
     context 'when 1 period' do
       it 'calculates correct ipmt value' do