import torch


def get_dihedral(c1, c2, c3, c4):
    """ Returns the dihedral angle in radians.
        Will use atan2 formula from: 
        https://en.wikipedia.org/wiki/Dihedral_angle#In_polymer_physics
        Inputs: 
        * c1: (batch, 3) or (3,)
        * c2: (batch, 3) or (3,)
        * c3: (batch, 3) or (3,)
        * c4: (batch, 3) or (3,)
    """
    u1 = c2 - c1
    u2 = c3 - c2
    u3 = c4 - c3

    return torch.atan2( ( (torch.norm(u2, dim=-1, keepdim=True) * u1) * torch.cross(u2,u3, dim=-1) ).sum(dim=-1) ,  
                        (  torch.cross(u1,u2, dim=-1) * torch.cross(u2, u3, dim=-1) ).sum(dim=-1) )