From b330dd7e0cfa2f661373346cc599c3d25200df12 Mon Sep 17 00:00:00 2001
From: Jacob Hass <jacobhass8@gmail.com>
Date: Fri, 2 Jan 2026 11:09:54 -0800
Subject: [PATCH] Updated code to match non_central_chi_squared syntax

---
 .../boost/math/distributions/non_central_f.hpp   | 16 +++++++++-------
 test/test_nc_f.cpp                               |  4 ++--
 2 files changed, 11 insertions(+), 9 deletions(-)

diff --git a/include/boost/math/distributions/non_central_f.hpp b/include/boost/math/distributions/non_central_f.hpp
index 63736a2b5..8bad55c45 100644
--- a/include/boost/math/distributions/non_central_f.hpp
+++ b/include/boost/math/distributions/non_central_f.hpp
@@ -44,23 +44,23 @@ namespace boost
          };
          
          template <class RealType, class Policy>
-         inline RealType find_non_centrality_f(const RealType dfn, const RealType dfd, const RealType p, const RealType x, const Policy& pol)
+         inline RealType find_non_centrality_f(RealType x, const RealType dfn, const RealType dfd, const RealType p, const RealType q, const Policy& pol)
          {
             constexpr auto function = "non_central_f<%1%>::find_non_centrality";
 
-            if ( p <= 0 || p >= 1) {
+            if ( p == 0 || q == 0) {
                return policies::raise_domain_error<RealType>(function, "Can't find non centrality parameter when the probability is <=0 or >=1, only possible answer is %1%", // LCOV_EXCL_LINE
                   RealType(boost::math::numeric_limits<RealType>::quiet_NaN()), Policy()); // LCOV_EXCL_LINE
             }
-            
+
+            non_centrality_finder_f<RealType, Policy> f(x, dfn, dfd, p < q ? p : q, p < q ? false : true);
             RealType guess = RealType(10);                       // Starting guess.
             RealType factor = 1;                                 // How big steps to take when searching.
             boost::math::uintmax_t max_iter = policies::get_max_root_iterations<Policy>();
             tools::eps_tolerance<RealType> tol(policies::digits<RealType, Policy>());
 
             std::pair<RealType, RealType> result_bracket = tools::bracket_and_solve_root(
-                                 detail::non_centrality_finder_f<RealType, Policy>(x, dfn, dfd, p, false), guess, factor,
-                                 false, tol, max_iter, pol);
+                                 f, guess, factor, false, tol, max_iter, pol);
             
             RealType result = result_bracket.first + (result_bracket.second - result_bracket.first)/2;
             if (max_iter >= policies::get_max_root_iterations<Policy>()) {
@@ -107,7 +107,7 @@ namespace boost
          { // Private data getter function.
             return ncp;
          }
-         static RealType find_non_centrality(const RealType dfn, const RealType dfd, const RealType p, const RealType x)
+         static RealType find_non_centrality(const RealType x, const RealType dfn, const RealType dfd, const RealType p)
          {
             constexpr auto function = "non_central_f_distribution<%1%>::find_non_centrality";
             typedef typename policies::evaluation<RealType, Policy>::type eval_type;
@@ -118,10 +118,11 @@ namespace boost
                policies::discrete_quantile<>,
                policies::assert_undefined<> >::type forwarding_policy;
             eval_type result = detail::find_non_centrality_f(
+               static_cast<eval_type>(x),
                static_cast<eval_type>(dfn),
                static_cast<eval_type>(dfd),
                static_cast<eval_type>(p),
-               static_cast<eval_type>(x),
+               static_cast<eval_type>(1-p),
                forwarding_policy());
             return policies::checked_narrowing_cast<RealType, forwarding_policy>(
                result,
@@ -143,6 +144,7 @@ namespace boost
                static_cast<eval_type>(c.param1),
                static_cast<eval_type>(c.param2),
                static_cast<eval_type>(c.param3),
+               static_cast<eval_type>(1-c.param3),
                forwarding_policy());
             return policies::checked_narrowing_cast<RealType, forwarding_policy>(
                result,
diff --git a/test/test_nc_f.cpp b/test/test_nc_f.cpp
index 064e9eb88..6000a051c 100644
--- a/test/test_nc_f.cpp
+++ b/test/test_nc_f.cpp
@@ -142,9 +142,9 @@ void test_spot(
       BOOST_CHECK_CLOSE(
             quantile(complement(dist, Q)), x, tol * 10);
       BOOST_CHECK_CLOSE(
-            dist.find_non_centrality(a, b, P, x), ncp, tol * 10);
+            dist.find_non_centrality(x, a, b, P), ncp, tol * 10);
       BOOST_CHECK_CLOSE(
-            dist.find_non_centrality(boost::math::complement(a, b, P, x)), ncp, tol * 10);
+            dist.find_non_centrality(boost::math::complement(x, a, b, P)), ncp, tol * 10);
    }
    if(boost::math::tools::digits<RealType>() > 50)
    {