add private pow function in VEML7700 lib for future use

[device111]

alternate pow function in VEML7700 libary for future use in Tasmota to calculate normalized Lux and normalized white values.

Description:

Related issue (if applicable): fixes #

Checklist:

• The pull request is done against the latest dev branch
• Only relevant files were touched
• Only one feature/fix was added per PR.
• The code change is tested and works on core ESP8266 V.2.7.1
• The code change is tested and works on core ESP32 V.1.12.0
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass

[s-hadinger]

@device111 It's the same implementation as FastPrecisePow() in support_float.ino. Could you please use this instead to avoid duplication of code?

[arendst]

I think the problem is accessing the function from the library won't work.

[device111]

yes, the problem is calling from the libary. My first thought was to delete the functions from lib and implement the affected pow functions in the driver, but that would break the idea of libarys.

[device111]

@theo
is it planed in the future, to transfer the related (pow, log, ..) functions in support_float.ino to a c++ lib?

[s-hadinger]

Actually it works if you add the following in 'Adafruit_VEML7700.h`:

Before the class definitio:

extern float FastPrecisePowf(const float x, const float y);

Then replace the definition of the function with:

  static inline float alternate_pow(float a, float b) { return FastPrecisePowf(a, b); }

You can then completely remove the Adafruit_VEML7700::alternate_pow() implementation.

[device111]

yes, the pow changes are only made for the future developement of the driver.
but the code from above are good. I will change it.
thanks!

[s-hadinger]

By the way, if it does not hurt the precision, replace all floating points literal with floats. Otherwise calculation is done by the compiler using double and code goes huge.

For ex, replace

    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;

with

    lux = 6.0135e-13f*alternate_pow(lux,4) -  9.3924e-9f*alternate_pow(lux,3) + 8.1488e-5f*alternate_pow(lux,2) + 1.0023f*lux;

[device111]

ok, in the next pr i will change it.
thanks a lot!