1 files changed, 19 insertions, 0 deletions
diff --git a/src/misc.c b/src/misc.c
index e6939f7..f705c74 100644
--- a/src/misc.c
+++ b/src/misc.c
@@ -862,3 +862,22 @@ void get_dir(double *dir, double theta, double phi)
     dir[1] = sin_theta*sin_phi;
     dir[2] = cos_theta;
 }
+
+/* Fast version of acos() which uses a lookup table computed on the first call. */
+double fast_acos(double x)
+{
+    size_t i;
+    static int initialized = 0;
+    static double xs[N_ACOS];
+    static double ys[N_ACOS];
+
+    if (!initialized) {
+        for (i = 0; i < LEN(xs); i++) {
+            xs[i] = -1.0 + 2.0*i/(LEN(xs)-1);
+            ys[i] = acos(xs[i]);
+        }
+        initialized = 1;
+    }
+
+    return interp1d(x,xs,ys,LEN(xs));
+}