MicroDexed/third-party/AudioEffectDynamics/fast_log.h

/* ----------------------------------------------------------------------
* https://community.arm.com/tools/f/discussions/4292/cmsis-dsp-new-functionality-proposal/22621#22621
* Fast approximation to the log2() function.  It uses a two step
* process.  First, it decomposes the floating-point number into
* a fractional component F and an exponent E.  The fraction component
* is used in a polynomial approximation and then the exponent added
* to the result.  A 3rd order polynomial is used and the result
* when computing db20() is accurate to 7.984884e-003 dB.
** ------------------------------------------------------------------- */

float log2f_approx_coeff[4] = {1.23149591368684f, -4.11852516267426f, 6.02197014179219f, -3.13396450166353f};

float log2f_approx(float X)
{
  float *C = &log2f_approx_coeff[0];
  float Y;
  float F;
  int E;

  // This is the approximation to log2()
  F = frexpf(fabsf(X), &E);

  //  Y = C[0]*F*F*F + C[1]*F*F + C[2]*F + C[3] + E;
  Y = *C++;
  Y *= F;
  Y += (*C++);
  Y *= F;
  Y += (*C++);
  Y *= F;
  Y += (*C++);
  Y += E;
  return(Y);
}

// https://codingforspeed.com/using-faster-exponential-approximation/
inline float expf_approx(float x) {
  x = 1.0f + x / 1024;
  x *= x; x *= x; x *= x; x *= x;
  x *= x; x *= x; x *= x; x *= x;
  x *= x; x *= x;
  return x;
}

inline float unitToDb(float unit) {
	return 6.02f * log2f_approx(unit);
}

inline float dbToUnit(float db) {
	return expf_approx(db * 2.302585092994046f * 0.05f);
}