From 8c2d4b4d58d6845b50c47d166a0a9eb64a3551fe Mon Sep 17 00:00:00 2001
From: Keith O'Hara <kth.ohr@gmail.com>
Date: Sun, 30 Jul 2023 09:30:35 -0400
Subject: [PATCH] add C++14 version of erf; update erf tests

---
 include/gcem_incl/erf.hpp | 54 ++++++++++++++++++++++++++++++++++++---
 tests/erf.cpp             |  5 ++++
 2 files changed, 56 insertions(+), 3 deletions(-)

diff --git a/include/gcem_incl/erf.hpp b/include/gcem_incl/erf.hpp
index d0bc83a..ce920fb 100644
--- a/include/gcem_incl/erf.hpp
+++ b/include/gcem_incl/erf.hpp
@@ -31,6 +31,28 @@ namespace internal
 // see
 // http://functions.wolfram.com/GammaBetaErf/Erf/10/01/0007/
 
+#if __cplusplus >= 201402L // C++14 version
+
+template<typename T>
+constexpr
+T
+erf_cf_large_recur(const T x, const int depth_end)
+noexcept
+{
+    int depth = GCEM_ERF_MAX_ITER - 1;
+    T res = x;
+
+    while (depth > depth_end - 1) {
+        res = x + 2 * depth / res;
+
+        --depth;
+    }
+
+    return res;
+}
+
+#else // C++11 version
+
 template<typename T>
 constexpr
 T
@@ -39,11 +61,13 @@ noexcept
 {
     return( depth < GCEM_ERF_MAX_ITER ? \
             // if
-                x + 2*depth/erf_cf_large_recur(x,depth+1) :
+                x + 2 * depth / erf_cf_large_recur(x,depth+1) :
             // else
                 x );
 }
 
+#endif
+
 template<typename T>
 constexpr
 T
@@ -57,6 +81,28 @@ noexcept
 // see
 // http://functions.wolfram.com/GammaBetaErf/Erf/10/01/0005/
 
+#if __cplusplus >= 201402L // C++14 version
+
+template<typename T>
+constexpr
+T
+erf_cf_small_recur(const T xx, const int depth_end)
+noexcept
+{
+    int depth = GCEM_ERF_MAX_ITER - 1;
+    T res = T(2*(depth+1) - 1) - 2 * xx;
+
+    while (depth > depth_end - 1) {
+        res = T(2*depth - 1) - 2 * xx + 4 * depth * xx / res;
+
+        --depth;
+    }
+
+    return res;
+}
+
+#else // C++11 version
+
 template<typename T>
 constexpr
 T
@@ -65,12 +111,14 @@ noexcept
 {
     return( depth < GCEM_ERF_MAX_ITER ? \
             // if
-                (2*depth - T(1)) - 2*xx \
-                    + 4*depth*xx / erf_cf_small_recur(xx,depth+1) :
+                (2*depth - T(1)) - 2 * xx \
+                    + 4 * depth * xx / erf_cf_small_recur(xx,depth+1) :
             // else
                 (2*depth - T(1)) - 2*xx );
 }
 
+#endif
+
 template<typename T>
 constexpr
 T
diff --git a/tests/erf.cpp b/tests/erf.cpp
index d090817..242cc9c 100644
--- a/tests/erf.cpp
+++ b/tests/erf.cpp
@@ -37,7 +37,12 @@ int main()
     GCEM_TEST_COMPARE_VALS(gcem::erf, std::erf, -2.099L);
     GCEM_TEST_COMPARE_VALS(gcem::erf, std::erf, -2.0L);
     GCEM_TEST_COMPARE_VALS(gcem::erf, std::erf, -1.3L);
+    GCEM_TEST_COMPARE_VALS(gcem::erf, std::erf, -0.7L);
+    GCEM_TEST_COMPARE_VALS(gcem::erf, std::erf, -0.1L);
     GCEM_TEST_COMPARE_VALS(gcem::erf, std::erf,  0.0L);
+    GCEM_TEST_COMPARE_VALS(gcem::erf, std::erf,  0.1L);
+    GCEM_TEST_COMPARE_VALS(gcem::erf, std::erf,  0.7L);
+    GCEM_TEST_COMPARE_VALS(gcem::erf, std::erf,  1.3L);
     GCEM_TEST_COMPARE_VALS(gcem::erf, std::erf,  1.3L);
     GCEM_TEST_COMPARE_VALS(gcem::erf, std::erf,  2.0L);
     GCEM_TEST_COMPARE_VALS(gcem::erf, std::erf,  2.099L);