New Bisection Method

CHANGE_LOG.md

@@ -1,8 +1,11 @@
+## Version 0.2.6
+
+* New Bisection method to avoid not converging to negative IRRs if not enough precision
+
 ## Version 0.2.5
 
 * Default XIRR does not raise an Exception if cashflow is invalid
 
-
 ## Version 0.2.4
 
 * Cashflow Invalid Messages are now public.

lib/xirr/bisection.rb

@@ -20,8 +20,7 @@ def xirr(midpoint = nil)
       while ((right - left).abs > Xirr::EPS && runs < Xirr.config.iteration_limit.to_i) do
 
         runs += 1
-        npv_positive?(midpoint) == npv_positive?(left) ? left = midpoint : right = midpoint
-        midpoint = format_irr(left, right)
+        left, midpoint, right = bisection(left, midpoint, right)
 
       end
 
@@ -35,6 +34,24 @@ def xirr(midpoint = nil)
 
     private
 
+    # @param left [BigDecimal]
+    # @param midpoint [BigDecimal]
+    # @param right [BigDecimal]
+    # @return [Array]
+    # Calculates the Bisections
+    def bisection(left, midpoint, right)
+      _left, _mid = npv_positive?(left), npv_positive?(midpoint)
+      if _left && _mid
+        return left, left, left if npv_positive?(right) # Not Enough Precision in the left to find the IRR
+      else
+        if _left == _mid
+          return midpoint, format_irr(midpoint, right), right # Result is to the Right
+        else
+          return left, format_irr(left, midpoint), midpoint # Result is to the Left
+        end
+      end
+    end
+
     # @param midpoint [Float]
     # @return [Bolean]
     # Returns true if result is to the right ot the range

lib/xirr/version.rb

@@ -1,4 +1,4 @@
 module Xirr
   # Version of the Gem
-  VERSION = "0.2.5"
+  VERSION = "0.2.6"
 end