// Standard Error
//
// standard_error
//
// Standard Error of the Mean
//
// 
//
// Inputs:    1
// Outputs:   1
// Constants: 1


#include <math.h>
#include "ts.h"

typedef struct {
    uint_t k;
} ts_standard_error_params_t;

ts_parameter_t const ts_standard_error_p = { .n_inputs = 1, .n_outputs = 1, .n_constants = 1, .params_size = sizeof(ts_standard_error_params_t) };

uint_t ts_standard_error_offset( ts_constant_t const *c )
{
	( void )c;
	return ( c ? ((ts_standard_error_params_t *)c)->k > 0 ? ((ts_standard_error_params_t *)c)->k - 1 : 0 : 0 );
}

int ts_standard_error(
	fp_t * const *y, uint_t y_n,
	fp_t const * const *x, uint_t x_n,
	uint_t n, ts_constant_t const *c
)
{
	TS_PROLOGUE( ts_standard_error_p );
	if ( !c ) return -1;
    uint_t k = ((ts_standard_error_params_t *)c)->k;
    if (k < 2) return -1;

    for( uint_t i = k - 1; i < n; ++i )
    {
        fp_t sum_x = 0, sum_y = 0, sum_xy = 0, sum_x2 = 0, sum_y2 = 0;
        for( uint_t j = 0; j < k; ++j )
        {
            fp_t xv = (fp_t)j;
            fp_t yv = x[0][i - k + 1 + j];
            sum_x += xv;
            sum_y += yv;
            sum_xy += xv * yv;
            sum_x2 += xv * xv;
            sum_y2 += yv * yv;
        }
        fp_t nk = (fp_t)k;
        fp_t denom = nk * sum_x2 - sum_x * sum_x;
        if( denom == 0 )
        {
            y[0][i] = 0;
        }
        else
        {
            fp_t ss_res = sum_y2 - (sum_y * sum_y) / nk - ((nk * sum_xy - sum_x * sum_y) * (nk * sum_xy - sum_x * sum_y)) / (nk * denom);
            y[0][i] = ss_res > 0 ? sqrt( ss_res / (nk - 2) ) : 0;
        }
    }
	return 0;
}