From dd95de97f2bebece3db11a0aeeb71b8891fea34b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marek=20Ne=C4=8Dada?= <marek@necada.org>
Date: Sat, 18 Aug 2018 08:50:04 +0000
Subject: [PATCH] Test direct quadrature of the sigma2 integral in Ewald sums.
 C

Former-commit-id: 50b7bf83971c9693b59256e6f02b99acb2e84581
---
 tests/gsl-int/quadratures.c | 55 +++++++++++++++++++++++++++++++++++++
 tests/gsl-int/values.in     |  4 +++
 2 files changed, 59 insertions(+)
 create mode 100644 tests/gsl-int/quadratures.c
 create mode 100644 tests/gsl-int/values.in

diff --git a/tests/gsl-int/quadratures.c b/tests/gsl-int/quadratures.c
new file mode 100644
index 0000000..4819e9b
--- /dev/null
+++ b/tests/gsl-int/quadratures.c
@@ -0,0 +1,55 @@
+#include <stdio.h>
+#include <math.h>
+#include <gsl/gsl_integration.h>
+#include <gsl/gsl_errno.h>
+
+#define EPSABS 0
+#define EPSREL 1e-8
+#define LIMIT 8192 //???
+#define R0 1
+
+/* Relevant quadrature methods from gsl:
+ * gsl_integration_qagiu
+ * ... and that's probably it.
+ */
+
+//gsl_function sigma2_integrand;
+
+struct sigma2_integrand_params {
+  int n;
+  double k, R;
+};
+
+static inline double sq(double x) {return x * x;}
+
+double sigma2_integrand(double ksi, void *params) {
+  struct sigma2_integrand_params *p = (struct sigma2_integrand_params *) params;
+  return exp(-sq(p->R*ksi) + sq(p->k/ksi/2)) * pow(ksi, 2*p->n);
+}
+
+
+int main(int argc, char **argv) {
+  struct sigma2_integrand_params p;
+  gsl_function F;
+  F.function = sigma2_integrand;
+  F.params = &p;
+  gsl_integration_workspace *workspace = gsl_integration_workspace_alloc(LIMIT);
+  gsl_error_handler_t * old_handler=gsl_set_error_handler_off();
+  double eta, eta_orig, k_orig, R_orig;
+  while (scanf("%d %lf %lf %lf", &(p.n), &k_orig, &R_orig, &eta_orig) == 4) {
+    eta = eta_orig * R0;
+    p.k = k_orig * R0;
+    p.R = R_orig / R0;
+    double result, abserr;
+    int retval = gsl_integration_qagiu(&F, eta, EPSABS, EPSREL,
+        LIMIT, workspace, &result, &abserr);
+    double normfac = pow(R0, -2*p.n - 1);
+    result *= normfac;
+    abserr *= normfac;
+    printf("%d %.16g %.16g %.16g %.16g %.16g %d\n", p.n, k_orig, R_orig, 
+        eta_orig, result, abserr, retval);
+  }
+  gsl_integration_workspace_free(workspace);
+}
+
+
diff --git a/tests/gsl-int/values.in b/tests/gsl-int/values.in
new file mode 100644
index 0000000..9e4bc09
--- /dev/null
+++ b/tests/gsl-int/values.in
@@ -0,0 +1,4 @@
+1 1 1 1
+1 5e7 504e-9 0
+1 5e7 504e-9 4e7
+