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, 4 insertions, 5 deletions

diff --git a/src/sno_charge.c b/src/sno_charge.c
index fe1e553..31de608 100644
--- a/src/sno_charge.c
+++ b/src/sno_charge.c
@@ -101,7 +101,7 @@ double pq(double q, int n)
     return gsl_spline_eval(splines[n-1], q, acc);
 }
 
-double get_pmiss(int n)
+double get_log_pmiss(int n)
 {
     if (!initialized) {
         fprintf(stderr, "charge interpolation hasn't been initialized!\n");
@@ -109,10 +109,9 @@ double get_pmiss(int n)
     }
 
     if (n == 0) {
-        return 1.0;
-    } else if (n > MAX_PE) {
-        /* Assume the distribution is gaussian by the central limit theorem. */
         return 0.0;
+    } else if (n > MAX_PE) {
+        return -INFINITY;
     }
 
     return pmiss[n-1];
@@ -223,7 +222,7 @@ void init_charge(void)
     for (i = 1; i <= MAX_PE; i++) {
         params[0] = i;
         gsl_integration_cquad(&F, 0, MEAN_THRESH/MEAN_HIPT, 0, 1e-9, w, &result, &error, &nevals);
-        pmiss[i-1] = result;
+        pmiss[i-1] = log(result);
     }
 
     F.function = &gsl_smear;