replace fx2 with c function without fma.

Author: peterkrack

pineappl/Cargo.toml

@@ -12,6 +12,9 @@ repository.workspace = true
 rust-version.workspace = true
 version.workspace = true
 
+[build-dependencies]
+cc = "1.0"
+
 [lints]
 workspace = true
 

pineappl/src/fx2.c

@@ -0,0 +1,25 @@
+#include <math.h>
+#include <float.h>
+
+double fx2(double y) {
+    double x, yp, deltap, delta, deriv;
+    int i;
+
+    yp = y;
+    deltap = DBL_MAX;
+
+    for (i = 0; i <= 10; i=i+1) {
+        x = exp(-yp);
+        delta = (1.0 - x) * (-5.0) + (y - yp);
+
+        if (fabs(delta) < 1e-15 && delta >= deltap) {
+            return x;
+        }
+
+        deriv = -5.0 * x - 1.0;
+        yp = yp - delta / deriv;
+        deltap = delta;
+    }
+
+    return -1;
+}

pineappl/src/lib.rs

@@ -33,6 +33,13 @@
 //! [special keys]: https://nnpdf.github.io/pineappl/docs/metadata.html
 //! [CLI tutorial]: https://nnpdf.github.io/pineappl/docs/cli-tutorial.html
 
+extern "C" {
+    fn fx2(y: f64) -> f64;
+}
+
+pub(crate) fn fx2_rust(y: f64) -> f64 {
+    unsafe { fx2(y) }
+}
 mod convert;
 mod v0;
 
@@ -47,3 +54,4 @@ pub mod packed_array;
 pub mod pids;
 pub mod reference;
 pub mod subgrid;
+