Converting angle to indexes, or vector -> angle -> indexes.. I have done this at least 4 times in my life for various games/projects before, yet have to do it each time again. So here is a gist for it

vector to direction index for 8 directional movement and sprite state settings

float degangle = atan2f(t_dir.x, t_dir.y)*180/M_PI;

//pushing it down, so that -22.5 becomes the new 0
degangle += 22.5f;

//now making the final range as 0 - 360
if (degangle < 0) degangle += 360;

//now this var is ranged 0 - 7, with -22.5f to 22.5f being 0 and so on...
m_DirectionIndex = (int)floorf(degangle/22.5f);