normal of triangle surface

//-std=c99 -Wall -Wextra -Wpedantic -Wshadow
//linker -lm

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <math.h>
#include <float.h>  //DBL_EPSILON

typedef struct{
    double x;
    double y;
    double z;
}Vector3;

typedef struct{
    double x;
    double y;
    double z;
}Point3;

void printVector(Vector3 v)
{
    printf("%f %f %f\n", v.x, v.y, v.z);
}

bool isVectAlmZero(Vector3 v)
{

    //if(fabs(v.x) > DBL_EPSILON || fabs(v.y) > DBL_EPSILON || fabs(v.z) > DBL_EPSILON) return false;
    //return true;

    return !(fabs(v.x) > DBL_EPSILON || fabs(v.y) > DBL_EPSILON || fabs(v.z) > DBL_EPSILON);

}

Vector3 normalizeVec3(Vector3 v)
{

    Vector3 retVec;

    double length = sqrt((v.x * v.x) + (v.y * v.y) + (v.z * v.z));

    retVec.x = v.x / length;
    retVec.y = v.y / length;
    retVec.z = v.z / length;

    return retVec;
}


//normal of triangle surface
Vector3 triangleSurfaceNormal(Point3 a, Point3 b, Point3 c)
{
    Vector3 retVec;

    Vector3 edge1;
    Vector3 edge2;

    edge1.x = b.x - a.x;
    edge1.y = b.y - a.y;
    edge1.z = b.z - a.z;

    edge2.x = c.x - a.x;
    edge2.y = c.y - a.y;
    edge2.z = c.z - a.z;

    //cross product
    retVec.x = edge1.y * edge2.z - edge1.z * edge2.y;
    retVec.y = edge1.z * edge2.x - edge1.x * edge2.z;
    retVec.z = edge1.x * edge2.y - edge1.y * edge2.x;

    return retVec;
}

int main (int argc, char **argv)
{

    Point3 tstP1 = {0.0,0.0,0.0};
    Point3 tstP2 = {0.5,0.0,0.0};
    Point3 tstP3 = {0.5,0.5,0.0};

    Vector3 resultV;

    printf("CW: ");

    //CW, clockwise
    resultV = triangleSurfaceNormal(tstP1,tstP2,tstP3);
    printVector(resultV);

    if(isVectAlmZero(resultV))
    {
        printf("error: zero vector cannot be normalized.\n");
        return 1;
    }

    printf("CW (normalized): ");
    printVector(normalizeVec3(resultV));


    printf("CCW: ");

    //CCW, counter-clockwise
    resultV = triangleSurfaceNormal(tstP1,tstP3,tstP2);
    printVector(resultV);

    if(isVectAlmZero(resultV))
    {
        printf("error: zero vector cannot be normalized.\n");
        return 1;
    }

    printf("CCW (normalized): ");
    printVector(normalizeVec3(resultV));

    printf("\n");


return 0;
}

/*
output:

CW: 0.000000 0.000000 0.250000
CW (normalized): 0.000000 0.000000 1.000000
CCW: 0.000000 0.000000 -0.250000
CCW (normalized): 0.000000 0.000000 -1.000000

*/