Is there any references to the algorithm of celt_exp2/celt_log2 function?

I can't understand the algorithm of celt_exp2/celt_log2 function.
Is there any references to this algorithm ?
Thank you!

/** Base-2 exponential approximation (2^x). */
static OPUS_INLINE float celt_exp2(float x)
{
   int integer;
   float frac;
   union {
      float f;
      opus_uint32 i;
   } res;
   integer = floor(x);
   if (integer < -50)
      return 0;
   frac = x-integer;
   /* K0 = 1, K1 = log(2), K2 = 3-4*log(2), K3 = 3*log(2) - 2 */
   res.f = 0.99992522f + frac * (0.69583354f
           + frac * (0.22606716f + 0.078024523f*frac));
   res.i = (res.i + ((opus_uint32)integer<<23)) & 0x7fffffff;
   return res.f;
}

K0 = 1
K1 = log(2) = 0.69314718056
K2 = 3 - 4log(2) = 0.22741127776
K3 = 3log(2) - 2 = 0.07944154167

why its different from the next line?

Start at the celt_exp2 and celt_log2 entry points in the Opus source and trace the surrounding comments and constants. Look for referenced algorithm notes or documentation explaining the approximation; done means documenting the coefficient source and why the listed K values differ from the implementation.