c_skew.c

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#include <math.h>
 
#include <grass/gis.h>
#include <grass/raster.h>
 
void c_skew(DCELL *result, DCELL *values, int n, const void *closure UNUSED)
{
    DCELL sum, ave, sumsq, sumcb, sdev;
    int count;
    int i;
 
    sum = 0.0;
    count = 0;
 
    for (i = 0; i < n; i++) {
        if (Rast_is_d_null_value(&values[i]))
            continue;
 
        sum += values[i];
        count++;
    }
 
    if (count == 0) {
        Rast_set_d_null_value(result, 1);
        return;
    }
 
    ave = sum / count;
 
    sumsq = 0;
    sumcb = 0;
 
    for (i = 0; i < n; i++) {
        DCELL d;
 
        if (Rast_is_d_null_value(&values[i]))
            continue;
 
        d = values[i] - ave;
        sumsq += d * d;
        sumcb += d * d * d;
    }
 
    sdev = sqrt(sumsq / count);
 
    *result = sumcb / (count * sdev * sdev * sdev);
}
 
void w_skew(DCELL *result, DCELL (*values)[2], int n,
            const void *closure UNUSED)
{
    DCELL sum, ave, sumsq, sumcb, sdev;
    DCELL count;
    int i;
 
    sum = 0.0;
    count = 0;
 
    for (i = 0; i < n; i++) {
        if (Rast_is_d_null_value(&values[i][0]))
            continue;
 
        sum += values[i][0] * values[i][1];
        count += values[i][1];
    }
 
    if (count == 0) {
        Rast_set_d_null_value(result, 1);
        return;
    }
 
    ave = sum / count;
 
    sumsq = 0;
    sumcb = 0;
 
    for (i = 0; i < n; i++) {
        DCELL d;
 
        if (Rast_is_d_null_value(&values[i][0]))
            continue;
 
        d = values[i][0] - ave;
        sumsq += d * d * values[i][1];
        sumcb += d * d * d * values[i][1];
    }
 
    sdev = sqrt(sumsq / count);
 
    *result = sumcb / (count * sdev * sdev * sdev);
}