1 files changed, 36 insertions, 0 deletions
diff --git a/lib/correlation.h b/lib/correlation.h
new file mode 100644
index 0000000..5722aab
--- /dev/null
+++ b/lib/correlation.h
@@ -0,0 +1,36 @@
+
+#pragma once
+
+#include "types.h"
+#include "state.h"
+
+#include <fftw3.h>
+
+template <class R_t, class X_t>
+double correlation_length(const state_t <R_t, X_t> *s) {
+  double *data = (double *)fftw_malloc(s->nv * sizeof(double));
+  int rank = s->D;
+  int *n = (int *)malloc(rank * sizeof(int));
+  fftw_r2r_kind *kind = (fftw_r2r_kind *)malloc(rank * sizeof(fftw_r2r_kind));
+  for (D_t i = 0; i < rank; i++) {
+    n[i] = s->L;
+    kind[i] = FFTW_R2HC;
+  }
+  fftw_plan plan = fftw_plan_r2r(rank, n, data, data, kind, 0);
+
+  for (v_t i = 0; i < s->nv; i++) {
+    data[i] = correlation_component(s->spins[i]);
+  }
+
+  fftw_execute(plan);
+
+  double length = pow(data[0], 2);
+
+  fftw_destroy_plan(plan);
+  fftw_free(data);
+  free(n);
+  free(kind);
+
+  return length;
+}
+