From 5365ae97d5e7f42bbf9a30a16da37ced03da391c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marek=20Ne=C4=8Dada?= <marek@necada.org>
Date: Mon, 14 May 2018 18:38:49 +0000
Subject: [PATCH] hexlattice ewald: lattice generation

Former-commit-id: 1b7494125603332d5bb2ffb3e607095452661c50
---
 qpms/apps/hexlattice_ewald.c | 78 ++++++++++++++++++++++++++++++++++++
 1 file changed, 78 insertions(+)
 create mode 100644 qpms/apps/hexlattice_ewald.c

diff --git a/qpms/apps/hexlattice_ewald.c b/qpms/apps/hexlattice_ewald.c
new file mode 100644
index 0000000..3917fd7
--- /dev/null
+++ b/qpms/apps/hexlattice_ewald.c
@@ -0,0 +1,78 @@
+#include <stdlib.h>
+#include <math.h>
+#include <stdio.h>
+#include "kahansum.h"
+#include "vectors.h"
+
+static const double s3 = 1.732050807568877293527446341505872366942805253810380628055;
+
+// For sorting the points by distance from origin / radius
+int cart2_cmpr (const void *p1, const void *p2) {
+  const  cart2_t *p1t = (const  cart2_t *)p1;
+  const  cart2_t *p2t = (const  cart2_t *)p2;
+  double r21 = cart2norm(*p1t);
+  double r22 = cart2norm(*p2t);
+  if (r21 < r22) return -1;
+  else if (r21 > r22) return 1;
+  else return 0;
+}
+
+typedef struct {
+  int npoints; // number of lattice points.
+  int capacity; // for how much points memory is allocated
+  double maxR; // circle radius, points <= R
+   cart2_t *points;
+} latticepoints_circle_t;
+
+void sort_cart2points_by_r(cart2_t *points, size_t nmemb) {
+  qsort(points, nmemb, sizeof(cart2_t), cart2_cmpr);
+}
+
+void latticepoints_circle_free(latticepoints_circle_t *c) {
+  free(c->points);
+  c->capacity = 0;
+}
+
+// "horizontal" orientation of the adjacent A, B points
+latticepoints_circle_t generate_hexpoints_hor(double h, double R, cart2_t offset /* if zero, an A is in the origin */) {
+  latticepoints_circle_t lat;
+  lat.maxR = R;
+  lat.npoints = 0;
+  int nmax = R / (1.5 * h) + 2; // max no of lattice shifts in each direction (with reserve)
+  double unitcellS = s3 * 3 / 2 * h * h; // unit cell area
+  lat.capacity = 5 + 2 * (R + 1) * (R + 1) * M_PI / unitcellS; // should be enough with some random reserve
+  lat.points = malloc(lat.capacity *sizeof(cart2_t));
+
+  cart2_t BAoffset = {h, 0};
+
+  cart2_t a1 = {-1.5*h, s3/2 *h};
+  cart2_t a2 = {1.5*h, s3/2 *h};
+
+  for (int i1 = -nmax; i1 <= nmax; ++i1)
+    for (int i2 = -nmax; i2 <= nmax; ++i2) {
+      cart2_t Apoint = cart2_add(offset, cart2_add(cart2_scale(i1, a1), cart2_scale(i2, a2)));
+      if (cart2norm(Apoint) <= R) {
+        assert(lat.npoints < lat.capacity);
+        lat.points[lat.npoints] = Apoint;
+        lat.npoints++;
+      }
+      cart2_t Bpoint = cart2_add(Apoint, BAoffset);
+      if (cart2norm(Bpoint) <= R) {
+        assert(lat.npoints < lat.capacity);
+        lat.points[lat.npoints] = Bpoint;
+        lat.npoints++;
+      }
+    }
+  sort_cart2points_by_r(lat.points, lat.npoints);
+  return lat;
+}
+
+int main (int argc, char **argv) {
+  double h = 1.2;
+  cart2_t offset = {0,0};
+
+  latticepoints_circle_t lat = generate_hexpoints_hor(h, 5, offset);
+  for (int i = 0; i < lat.npoints; ++i) 
+    printf("%g %g\n", lat.points[i].x, lat.points[i].y);
+  latticepoints_circle_free(&lat);
+}