add a fast likelihood function

This commit adds a fast function to calculate the expected number of PE at a
PMT without numerically integrating over the track. This calculation is *much*
faster than integrating over the track (~30 ms compared to several seconds) and
so we use it during the "quick" minimization phase of the fit to quickly find
the best position.

1 files changed, 23 insertions, 0 deletions

diff --git a/src/test.c b/src/test.c
index fb788ba..fa4256f 100644
--- a/src/test.c
+++ b/src/test.c
@@ -467,6 +467,28 @@ err:
     return 1;
 }
 
+int test_lnfact(char *err)
+{
+    /* Tests the lnfact() function. */
+    size_t i;
+    double x, expected;
+
+    for (i = 0; i < 1000; i++) {
+        x = lnfact(i);
+        expected = gsl_sf_lnfact(i);
+
+        if (!isclose(x, expected, 1e-9, 1e-9)) {
+            sprintf(err, "lnfact(%zu) returned %.5g, but expected %.5g", i, x, expected);
+            goto err;
+        }
+    }
+
+    return 0;
+
+err:
+    return 1;
+}
+
 struct tests {
     testFunction *test;
     char *name;
@@ -482,6 +504,7 @@ struct tests {
     {test_path, "test_path"},
     {test_interp1d, "test_interp1d"},
     {test_kahan_sum, "test_kahan_sum"},
+    {test_lnfact, "test_lnfact"},
 };
 
 int main(int argc, char **argv)