FS#8967: Fix autoscore computation overflow when the playtime is huge.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@24471 a1c6a512-1295-4272-9138-f99709370657

1 files changed, 13 insertions, 3 deletions

diff --git a/apps/tagcache.c b/apps/tagcache.c
index 79c96ff186..9ffb6f86fe 100644
--- a/apps/tagcache.c
+++ b/apps/tagcache.c
@@ -772,9 +772,19 @@ static long check_virtual_tags(int tag, const struct index_entry *idx)
             }
             else
             {
-                data = 100 * idx->tag_seek[tag_playtime]
-                    / idx->tag_seek[tag_length]
-                    / idx->tag_seek[tag_playcount];
+                /* A straight calculus gives:
+                     autoscore = 100 * playtime / length / playcout (1)
+                   Now, consider the euclidian division of playtime by length:
+                     playtime = alpha * length + beta
+                   With:
+                     0 <= beta < length
+                   Now, (1) becomes:
+                     autoscore = 100 * (alpha / playcout + beta / length / playcount)
+                   Both terms should be small enough to avoid any overflow
+                */
+                data = 100 * (idx->tag_seek[tag_playtime] / idx->tag_seek[tag_length])
+                     + (100 * (idx->tag_seek[tag_playtime] % idx->tag_seek[tag_length])) / idx->tag_seek[tag_length];
+                data /=  idx->tag_seek[tag_playcount];
             }
             break;