[gnome-software/gnome-3-20] ubuntu-reviews: Fix Wilson score calculation

[Robert Ancell <rancell src gnome org>]

commit 47520845a7c01bf28789473ad4e8991fe1ff233b
Author: Robert Ancell <robert ancell canonical com>
Date:   Thu Oct 5 17:05:31 2017 +1300

    ubuntu-reviews: Fix Wilson score calculation

 src/plugins/gs-plugin-ubuntu-reviews.c |   22 +++++++++++++++-------
 1 files changed, 15 insertions(+), 7 deletions(-)
---
diff --git a/src/plugins/gs-plugin-ubuntu-reviews.c b/src/plugins/gs-plugin-ubuntu-reviews.c
index 1d17d53..764d414 100644
--- a/src/plugins/gs-plugin-ubuntu-reviews.c
+++ b/src/plugins/gs-plugin-ubuntu-reviews.c
@@ -230,19 +230,27 @@ wilson_score (gdouble value, gint n, gdouble power)
 static gint
 get_rating (gint64 one_star_count, gint64 two_star_count, gint64 three_star_count, gint64 four_star_count, 
gint64 five_star_count)
 {
+       gdouble val;
        gint n_ratings;
 
        n_ratings = one_star_count + two_star_count + three_star_count + four_star_count + five_star_count;
        if (n_ratings == 0)
                return -1;
 
-       // Use a Wilson score which is a method of ensuring small numbers of ratings don't give high scores
-       // https://en.wikipedia.org/wiki/Binomial_proportion_confidence_interval
-       return ((wilson_score (one_star_count, n_ratings, 0.1) * 20) +
-               (wilson_score (two_star_count, n_ratings, 0.1) * 40) +
-               (wilson_score (three_star_count, n_ratings, 0.1) * 60) +
-               (wilson_score (four_star_count, n_ratings, 0.1) * 80) +
-               (wilson_score (five_star_count, n_ratings, 0.1) * 100));
+       /* get score */
+       val =  (wilson_score (one_star_count, n_ratings, 0.2) * -2);
+       val += (wilson_score (two_star_count, n_ratings, 0.2) * -1);
+       val += (wilson_score (four_star_count, n_ratings, 0.2) * 1);
+       val += (wilson_score (five_star_count, n_ratings, 0.2) * 2);
+
+       /* normalize from -2..+2 to 0..5 */
+       val += 3;
+
+       /* multiply to a percentage */
+       val *= 20;
+
+       /* return rounded up integer */
+       return (gint) ceil (val);
 }
 
 static gboolean