Functions
float32_t	arm_cos_f32 (float32_t x)
	Fast approximation to the trigonometric cosine function for floating-point data.

q15_t	arm_cos_q15 (q15_t x)
	Fast approximation to the trigonometric cosine function for Q15 data.

q31_t	arm_cos_q31 (q31_t x)
	Fast approximation to the trigonometric cosine function for Q31 data.

Description

Computes the trigonometric cosine function using a combination of table lookup and cubic interpolation. There are separate functions for Q15, Q31, and floating-point data types. The input to the floating-point version is in radians while the fixed-point Q15 and Q31 have a scaled input with the range [0 +0.9999] mapping to [0 2*pi). The fixed-point range is chosen so that a value of 2*pi wraps around to 0.

The implementation is based on table lookup using 256 values together with cubic interpolation. The steps used are:

Calculation of the nearest integer table index
Fetch the four table values a, b, c, and d
Compute the fractional portion (fract) of the table index.
Calculation of wa, wb, wc, wd
The final result equals a*wa + b*wb + c*wc + d*wd

where

    
   a=Table[index-1];    
   b=Table[index+0];    
   c=Table[index+1];    
   d=Table[index+2];

and

    
   wa=-(1/6)*fract.^3 + (1/2)*fract.^2 - (1/3)*fract;    
   wb=(1/2)*fract.^3 - fract.^2 - (1/2)*fract + 1;    
   wc=-(1/2)*fract.^3+(1/2)*fract.^2+fract;    
   wd=(1/6)*fract.^3 - (1/6)*fract;