From 97d5b680455401ec8d9997e228e2e3c7a8ddfd1d Mon Sep 17 00:00:00 2001
From: Ahmet Inan <inan@aicodix.de>
Date: Mon, 21 Jan 2019 11:36:50 +0100
Subject: [PATCH] added quick and dirty decibel approximation

---
 utils.hh | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/utils.hh b/utils.hh
index 16ea615..f5165bb 100644
--- a/utils.hh
+++ b/utils.hh
@@ -43,7 +43,27 @@ TYPE delta(TYPE x)
 template <typename TYPE>
 TYPE decibel(TYPE v)
 {
+#if 0
 	return TYPE(10) * log10(v);
+#else
+	static constexpr TYPE
+		// 2*1024*10/l(10)
+		scale = 8894.350989378597430295120259412072085389250678859091274024013479236L,
+		inv1 = TYPE(1) / TYPE(1),
+		inv3 = TYPE(1) / TYPE(3),
+		inv5 = TYPE(1) / TYPE(5),
+		inv7 = TYPE(1) / TYPE(7),
+		inv9 = TYPE(1) / TYPE(9),
+		inv11 = TYPE(1) / TYPE(11),
+		inv13 = TYPE(1) / TYPE(13);
+	TYPE x = v;
+	for (int i = 0; i < 10; ++i)
+		x = sqrt(x);
+	x = (x - TYPE(1)) / (x + TYPE(1));
+	TYPE xx = x * x;
+	return scale * x * (xx * (xx * (xx * (xx * (xx * (xx *
+		inv13 + inv11) + inv9) + inv7) + inv5) + inv3) + inv1);
+#endif
 }
 
 }