From ebbbd37865b9e83bd3145f00b89ee6053f73f863 Mon Sep 17 00:00:00 2001
From: device111 <48546979+device111@users.noreply.github.com>
Date: Sun, 24 May 2020 12:28:08 +0200
Subject: [PATCH] add private pow function in lib for future use

alternate pow function for future use in Tasmota to calculate normalized Lux and normalized white values.
---
 lib/Adafruit_VEML7700/Adafruit_VEML7700.cpp | 27 +++++++++++++++++++--
 lib/Adafruit_VEML7700/Adafruit_VEML7700.h   |  6 +++++
 2 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/lib/Adafruit_VEML7700/Adafruit_VEML7700.cpp b/lib/Adafruit_VEML7700/Adafruit_VEML7700.cpp
index 001d4995e..b719af9fa 100644
--- a/lib/Adafruit_VEML7700/Adafruit_VEML7700.cpp
+++ b/lib/Adafruit_VEML7700/Adafruit_VEML7700.cpp
@@ -76,6 +76,29 @@ boolean Adafruit_VEML7700::begin(TwoWire *theWire) {
   return true;
 }
 
+float Adafruit_VEML7700::alternate_pow(float a, float b)
+{
+  // https://martin.ankerl.com/2012/01/25/optimized-approximative-pow-in-c-and-cpp/
+  // calculate approximation with fraction of the exponent
+  int e = abs((int)b);
+  union {
+    double d;
+    int x[2];
+  } u = { a };
+  u.x[1] = (int)((b - e) * (u.x[1] - 1072632447) + 1072632447);
+  u.x[0] = 0;
+  // exponentiation by squaring with the exponent's integer part
+  // double r = u.d makes everything much slower, not sure why
+  double r = 1.0;
+  while (e) {
+    if (e & 1) {
+      r *= a;
+    }
+    a *= a;
+    e >>= 1;
+  }
+  return r * u.d;
+}
 
 float Adafruit_VEML7700::normalize_resolution(float value) {
   // adjust for gain (1x is normalized)
@@ -123,7 +146,7 @@ float Adafruit_VEML7700::readLuxNormalized() {
   // user-provided correction for non-linearities at high lux/white values:
   // https://forums.adafruit.com/viewtopic.php?f=19&t=152997&p=758582#p759346
   if ((getGain() == VEML7700_GAIN_1_8) && (getIntegrationTime() == VEML7700_IT_25MS)){
-    lux = 6.0135e-13*pow(lux,4) -  9.3924e-9*pow(lux,3) + 8.1488e-5*pow(lux,2) + 1.0023*lux;
+    lux = 6.0135e-13*alternate_pow(lux,4) -  9.3924e-9*alternate_pow(lux,3) + 8.1488e-5*alternate_pow(lux,2) + 1.0023*lux;
   }
  
   return lux;  
@@ -156,7 +179,7 @@ float Adafruit_VEML7700::readWhiteNormalized() {
   // user-provided correction for non-linearities at high lux values:
   // https://forums.adafruit.com/viewtopic.php?f=19&t=152997&p=758582#p759346
   if ((getGain() == VEML7700_GAIN_1_8) && (getIntegrationTime() == VEML7700_IT_25MS)){
-    white = 2E-15*pow(white,4) + 4E-12*pow(white,3) + 9E-06*pow(white,2) + 1.0179*white - 11.052;
+    white = 2E-15*alternate_pow(white,4) + 4E-12*alternate_pow(white,3) + 9E-06*alternate_pow(white,2) + 1.0179*white - 11.052;
   }
 
   return white;
diff --git a/lib/Adafruit_VEML7700/Adafruit_VEML7700.h b/lib/Adafruit_VEML7700/Adafruit_VEML7700.h
index b842a7bc1..13a0bded5 100644
--- a/lib/Adafruit_VEML7700/Adafruit_VEML7700.h
+++ b/lib/Adafruit_VEML7700/Adafruit_VEML7700.h
@@ -14,6 +14,11 @@
  *	BSD license (see license.txt)
  */
 
+/* 
+ * change from device111 for Tasmota
+ * Add alternativ Pow function for readLuxNormalized() and readWhiteNormalized()
+ */
+
 #ifndef _ADAFRUIT_VEML7700_H
 #define _ADAFRUIT_VEML7700_H
 
@@ -105,6 +110,7 @@ private:
     *PowerSave_Enable, *PowerSave_Mode;
 
   float normalize_resolution(float value);
+  float alternate_pow(float a, float b);
 
   Adafruit_I2CDevice *i2c_dev;