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# For licensing see accompanying LICENSE file.
# Copyright (c) 2025 Apple Inc. Licensed under MIT License.
#
# Started from https://github.com/facebookresearch/esm/tree/main,
# licensed under MIT License, Copyright (c) Meta Platforms, Inc. and affiliates.
import mlx.core as mx
import mlx.nn as nn
def rotate_half(x):
x1, x2 = x.split(2, axis=-1)
return mx.concatenate([-x2, x1], axis=-1)
def apply_rotary_pos_emb(x, cos, sin):
cos = cos[:, : x.shape[-2], :]
sin = sin[:, : x.shape[-2], :]
return (x * cos) + (rotate_half(x) * sin)
class RotaryEmbedding(nn.Module):
"""
The rotary position embeddings from RoFormer_ (Su et. al).
A crucial insight from the method is that the query and keys are
transformed by rotation matrices which depend on the relative positions.
Other implementations are available in the Rotary Transformer repo_ and in
GPT-NeoX_, GPT-NeoX was an inspiration
.. _RoFormer: https://arxiv.org/abs/2104.09864
.. _repo: https://github.com/ZhuiyiTechnology/roformer
.. _GPT-NeoX: https://github.com/EleutherAI/gpt-neox
.. warning: Please note that this embedding is not registered on purpose, as it is transformative
(it does not create the embedding dimension) and will likely be picked up (imported) on a ad-hoc basis
"""
def __init__(self, dim: int, *_, **__):
super().__init__()
# Generate and save the inverse frequency buffer (non trainable)
self.inv_freq = mx.array(
1.0 / (10000 ** (mx.arange(0, dim, 2).astype(mx.float32) / dim))
)
self._seq_len_cached = None
self._cos_cached = None
self._sin_cached = None
def _update_cos_sin_tables(self, x, seq_dimension=1):
seq_len = x.shape[seq_dimension]
# Reset the tables if the sequence length has changed,
# or if we're on a new device (possibly due to tracing for instance)
if seq_len != self._seq_len_cached:
self._seq_len_cached = seq_len
t = mx.arange(x.shape[seq_dimension]).astype(self.inv_freq.dtype)
freqs = mx.einsum("i,j->ij", t, self.inv_freq)
emb = mx.concatenate([freqs, freqs], axis=-1)
self._cos_cached = emb.cos()[None, :, :]
self._sin_cached = emb.sin()[None, :, :]
return self._cos_cached, self._sin_cached
def __call__(self, q, k):
self._cos_cached, self._sin_cached = self._update_cos_sin_tables(
k, seq_dimension=-2
)
return (
apply_rotary_pos_emb(q, self._cos_cached, self._sin_cached),
apply_rotary_pos_emb(k, self._cos_cached, self._sin_cached),
)
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