Fixing Issue #38

finance.gemspec

@@ -15,7 +15,4 @@ SPEC = Gem::Specification.new do |s|
   s.add_development_dependency 'activesupport', '>= 4.0.0'
   s.add_development_dependency 'pry'
   s.files = `git ls-files`.split("\n")
-
-  s.has_rdoc = true
-  s.extra_rdoc_files = ['README.md', 'COPYING', 'COPYING.LESSER', 'HISTORY']
 end

lib/finance/cashflows.rb

@@ -22,7 +22,7 @@ class Function
 
       values.each do |key, value|
         define_method key do
-          BigDecimal.new value
+          Kernel.BigDecimal(value)
         end
       end
 
@@ -33,7 +33,18 @@ def initialize(transactions, function)
 
       def values(x)
         value = @transactions.send(@function, Flt::DecNum.new(x[0].to_s))
-        [ BigDecimal.new(value.to_s) ]
+
+        # Due to the fact that somewhere along the way we are taking negative
+        # values to a fractional power, +value+ ends up being a +Complex+ number.
+        # String representation (+value.to_s+) of a +Complex+ number looks like "12.34+56.78i",
+        # and when you take +BigDecimal.new()+ of that, the imaginary part is completely
+        # ignored, guiding the Newtonian into entirely wrong direction (see Issue #38).
+        # Instead, we should get the magnitude of complex number and ensure it points
+        # the vector points in the right direction, too.
+        value_direction = (value.real <=>0)
+        value_direction = (value.imaginary <=> 0) if value_direction == 0
+
+        [ Kernel.BigDecimal((value.magnitude * value_direction).to_s) ]
       end
     end
 

lib/finance/decimal.rb

@@ -7,7 +7,7 @@
 end
 
 DecNum.context.define_conversion_to(BigDecimal) do |x|
-  BigDecimal.new(x.to_s)
+  Kernel.BigDecimal(x.to_s)
 end
 
 class Numeric

lib/finance/transaction.rb

@@ -71,7 +71,7 @@ def interest?
 
     # @api public
     def inspect
-      "Transaction(#{@amount})"
+      "Transaction(#{@amount}#{date && date.strftime(' <%F>')})"
     end
 
     # Modify a Transaction's amount by passing a block

test/test_cashflows.rb

@@ -28,5 +28,32 @@
     it "should have a Net Present Value" do
       assert_equal D("-937.41"), @xactions.xnpv(0.6).round(2)
     end
+
+    it "should properly calculate IRR when Complex numbers arise from calculations" do
+      # Note that times must be in UTC (+00:00) here, so
+      # the local system's DST settings don't screw us up.
+      arr = [ Finance::Transaction.new( 70, :date => Time.new(2015, 7, 31, 0, 0, 0, '+00:00')),
+              Finance::Transaction.new(-90, :date => Time.new(2021, 1, 13, 0, 0, 0, '+00:00')),
+              Finance::Transaction.new(-20, :date => Time.new(2021, 3, 31, 0, 0, 0, '+00:00')) ]
+      assert_equal D("0.085677"), arr.xirr.effective.round(6)
+    end
+
+    it "IRR should decrease as payback moves further and further into the future" do
+      prev_max = 99999
+
+      (1..30).each do |n|
+        arr = [ Finance::Transaction.new(100, :date => Time.new(2001, 1, 1, 0, 0, 0, '+00:00')),
+                Finance::Transaction.new(-50, :date => Time.new(2001, 2, 1, 0, 0, 0, '+00:00')),
+                Finance::Transaction.new(-60, :date => Time.new(2001, 2, 1, 0, 0, 0, '+00:00') + (n * 30*24*3600)) ]
+
+        irr = arr.xirr.effective.round(6)
+
+        assert(irr > 0, "IRR should not be negative; encountered #{irr}")
+        assert(irr < prev_max, "IRR should only decrease, but it increased from #{prev_max} to #{irr}")
+
+        prev_max = irr
+      end
+    end
+
   end
 end