import torch

class RotaryEmbedding(torch.nn.Module):
    
    def __init__(self, dim, base=10000):
        super().__init__()
        inv_freq = 1. / (base ** (torch.arange(0, dim, 2).float() / dim))
        self.register_buffer('inv_freq', inv_freq)
        self.seq_len_cached = 0 
        self.cos_cached = None
        self.sin_cached = None

    def forward(self, x, seq_dim=1):
        seq_len = x.shape[seq_dim]
        if seq_len != self.seq_len_cached:
            #if seq_len > self.seq_len_cached:
            self.seq_len_cached = seq_len
            t = torch.arange(x.shape[seq_dim], device=x.device).type_as(self.inv_freq)
            freqs = torch.einsum('i,j->ij', t, self.inv_freq)
            emb = torch.cat((freqs, freqs), dim=-1).to(x.device)
            self.cos_cached = emb.cos()[None,:, None, :]
            self.sin_cached = emb.sin()[None,:, None, :]
            #else:
            #    cos_return = self.cos_cached[..., :seq_len]
            #    sin_return = self.sin_cached[..., :seq_len]
            #    return cos_return, sin_return
                
        return self.cos_cached, self.sin_cached


# rotary pos emb helpers:

def rotate_half(x):
    x1, x2 = x[..., :x.shape[-1] // 2], x[..., x.shape[-1] // 2:]
    return torch.cat((-x2, x1), dim=x1.ndim - 1) # dim=-1 triggers a bug in earlier torch versions

@torch.jit.script
def apply_rotary_pos_emb(q, k, cos, sin):
    return (q * cos) + (rotate_half(q) * sin), (k * cos) + (rotate_half(k) * sin)
