Distance between vertices:

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static inline GLfloat Vertex3DDistanceBetweenVertices
(Vertex3D vertex1, Vertex3D vertex2)
{
    GLfloat deltaX, deltaY, deltaZ;

    deltaX = vertex2.x - vertex1.x;
    deltaY = vertex2.y - vertex1.y;
    deltaZ = vertex2.z - vertex1.z;

    return sqrtf((deltaX * deltaX) +
        (deltaY * deltaY) +
        (deltaZ * deltaZ));
}

Magnitude of a vector:

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static inline GLfloat Vector3DMagnitude(Vector3D vector)
{
    return sqrtf((vector.x * vector.x) +
        (vector.y * vector.y) +
        (vector.z * vector.z));
}