From 052cfbc11de1fc6d2fc0d6723a6ec2643770ff6d Mon Sep 17 00:00:00 2001
From: Theo Arends <11044339+arendst@users.noreply.github.com>
Date: Sat, 18 May 2019 18:24:17 +0200
Subject: [PATCH] Save 1k code space by using TaylorSeries log function

Save 1k code space by using TaylorSeries log function
---
 sonoff/support.ino        | 18 ++++++++++++++++++
 sonoff/xsns_02_analog.ino |  2 +-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/sonoff/support.ino b/sonoff/support.ino
index c10c81eb0..7c0bbb1a2 100644
--- a/sonoff/support.ino
+++ b/sonoff/support.ino
@@ -680,6 +680,24 @@ float FastPrecisePowf(const float x, const float y)
   return (float)FastPrecisePow(x, y);
 }
 
+double TaylorLog(double x)
+{
+  // https://stackoverflow.com/questions/46879166/finding-the-natural-logarithm-of-a-number-using-taylor-series-in-c
+
+  double z = (x + 1) / (x - 1);                              // We start from power -1, to make sure we get the right power in each iteration;
+  double step = ((x - 1) * (x - 1)) / ((x + 1) * (x + 1));   // Store step to not have to calculate it each time
+  double totalValue = 0;
+  double powe = 1;
+  double y;
+  for (int count = 0; count < 10; count++) {
+    z *= step;
+    y = (1 / powe) * z;
+    totalValue = totalValue + y;
+    powe = powe + 2;
+  }
+  return 2 * totalValue;
+}
+
 uint32_t SqrtInt(uint32_t num)
 {
   if (num <= 1) {
diff --git a/sonoff/xsns_02_analog.ino b/sonoff/xsns_02_analog.ino
index e93377165..85560e3d3 100644
--- a/sonoff/xsns_02_analog.ino
+++ b/sonoff/xsns_02_analog.ino
@@ -100,7 +100,7 @@ void AdcEverySecond(void)
     int adc = AdcRead(2);
     // Steinhart-Hart equation for thermistor as temperature sensor
     double Rt = (adc * ANALOG_NTC_BRIDGE_RESISTANCE) / (1024.0 * ANALOG_V33 - (double)adc);
-    double T = ANALOG_NTC_B_COEFFICIENT / (ANALOG_NTC_B_COEFFICIENT / ANALOG_T0 + log(Rt / ANALOG_NTC_RESISTANCE));
+    double T = ANALOG_NTC_B_COEFFICIENT / (ANALOG_NTC_B_COEFFICIENT / ANALOG_T0 + TaylorLog(Rt / ANALOG_NTC_RESISTANCE));
     adc_temp = ConvertTemp(TO_CELSIUS(T));
   }
 }