[PATCH 3/8] util: add util_exponential_decay

This is copied from network.c that uses a static table to lookup
exponential decay values by index (generated from 1/pow(n, 0.3)).
network.c uses this for network ranking but it can be useful for
BSS ranking as well if you need to apply some exponential backoff
to a value.
---
 src/util.c | 28 ++++++++++++++++++++++++++++
 src/util.h |  2 ++
 2 files changed, 30 insertions(+)

diff --git a/src/util.c b/src/util.c
index a6ab9d85..b673b002 100644
--- a/src/util.c
+++ b/src/util.c
@@ -636,3 +636,31 @@ struct scan_freq_set *scan_freq_set_clone(const struct scan_freq_set *set,
 
 	return new;
 }
+
+/* First 64 entries calculated by 1 / pow(n, 0.3) for n >= 1 */
+static const double rankmod_table[] = {
+	1.0000000000, 0.8122523964, 0.7192230933, 0.6597539554,
+	0.6170338627, 0.5841906811, 0.5577898253, 0.5358867313,
+	0.5172818580, 0.5011872336, 0.4870596972, 0.4745102806,
+	0.4632516708, 0.4530661223, 0.4437850034, 0.4352752816,
+	0.4274303178, 0.4201634287, 0.4134032816, 0.4070905315,
+	0.4011753236, 0.3956154062, 0.3903746872, 0.3854221125,
+	0.3807307877, 0.3762772797, 0.3720410580, 0.3680040435,
+	0.3641502401, 0.3604654325, 0.3569369365, 0.3535533906,
+	0.3503045821, 0.3471812999, 0.3441752105, 0.3412787518,
+	0.3384850430, 0.3357878061, 0.3331812996, 0.3306602598,
+	0.3282198502, 0.3258556179, 0.3235634544, 0.3213395618,
+	0.3191804229, 0.3170827751, 0.3150435863, 0.3130600345,
+	0.3111294892, 0.3092494947, 0.3074177553, 0.3056321221,
+	0.3038905808, 0.3021912409, 0.3005323264, 0.2989121662,
+	0.2973291870, 0.2957819051, 0.2942689208, 0.2927889114,
+	0.2913406263, 0.2899228820, 0.2885345572, 0.2871745887,
+};
+
+double util_exponential_decay(unsigned int n)
+{
+	if (n > L_ARRAY_SIZE(rankmod_table))
+		return rankmod_table[L_ARRAY_SIZE(rankmod_table) - 1];
+
+	return rankmod_table[n];
+}
diff --git a/src/util.h b/src/util.h
index 9f3f0a57..8aef2985 100644
--- a/src/util.h
+++ b/src/util.h
@@ -141,4 +141,6 @@ struct scan_freq_set *scan_freq_set_clone(const struct scan_freq_set *set,
 
 DEFINE_CLEANUP_FUNC(scan_freq_set_free);
 
+double util_exponential_decay(unsigned int n);
+
 #endif /* __UTIL_H */
-- 
2.34.1