Change precision on findroot to 1e-6. Fix #27.

Author: jcoliz

src/ExcelFinancialFunctions/common.fs

@@ -112,7 +112,7 @@ module internal Common =
     // If the result is sensible (it exist and has the same sign as guess), then return it, else try bisection.
     // I'm sure more complex way to pick algos exist (i.e. Brent's method). But I favor simplicity here ...         
     let findRoot f guess =
-        let precision = 0.0000001 // Excel precision on this, from docs
+        let precision = 0.000001 // Excel precision on this, from docs
         let newtValue = newton f guess 0 precision
         if newtValue.IsSome && sign guess = sign newtValue.Value
         then newtValue.Value

tests/ExcelFinancialFunctions.Tests/spottests.fs

@@ -15,6 +15,13 @@ module SpotTests =
         Financial.Yield param
         |> shouldEqual (sprintf "YieldIssue8(%A)" param) -0.67428578540657702
 
+    [<Test>]
+    let XirrIssue27() =
+        let values = [ -177900000.; 8799805.85 ] 
+        let dates = [ DateTime(2020,7,3); DateTime(2021,2,25) ]
+        Financial.XIrr (values, dates)
+        |> shouldEqual (sprintf "XirrIssue27(%A,%A)" values dates) -0.990247691899517
+
     [<Test>]
     let spotYield() =
         let param =