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oleibman
ec96ef3a6c
Fix #689. XIRR is calculated by making guesses which are hopefully better with each iteration. It is not guaranteed to succeed for Excel, PhpSpreadsheet, or any other implementation. PhpSpreadsheet uses the Newton-Raphson method for its guesses. So does Python package xirr (https://github.com/tarioch/xirr/), but, if Newton-Raphson fails to converge, Python tries Brent's method as an alternative. Two sets of non-converging data are noted in 689. For both, a solution does converge in Excel. For the first of the problems, a solution converges in Python with Newton-Raphson; but, for the second, a solution converges which requires Brent. For the Java package https://github.com/RayDeCampo/java-xirr on which Python was based, and which uses only Newton-Raphson, a solution converges for the first, and does not converge for the second. To try to match the good results of the others, I added an alternate algorithm if Newton-Raphson fails. Brent's algorithm seems difficult to implement to me. I might have gone there regardless, but I first tried a slightly simpler alternative, bisection. This solved the problem for both of the cases in 689. Perhaps someone will one day report a problem that doesn't converge for Newton-Raphson or bisection, but does for Brent. We can review this decision then. The new code causes 3 changes in the unit test. In all 3 tests, Excel and PhpSpreadsheet had not converged, but Python and/or Java had. I now believe that Python/Java is correct in those cases, and Excel is not. The new code aligns PhpSpreadsheet with Python/Java for those tests. It is, of course, impossible to know when Excel's implementation doesn't converge, so we aren't guaranteed to match its results in those hopefully rare situations.