# Copyright 2024 Bytedance Ltd. and/or its affiliates
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
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#     http://www.apache.org/licenses/LICENSE-2.0
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"""
Utilities for DeepSpeed Ulysses Sequence Parallelism.
DeepSpeed Ulysses Paper: https://arxiv.org/abs/2309.14509
Inspired from: https://github.com/deepspeedai/DeepSpeed/blob/master/deepspeed/sequence/layer.py
"""

from typing import Any, Optional, Tuple

import torch
import torch.distributed as dist
from torch import Tensor
from torch.distributed import ProcessGroup

_ULYSSES_SEQUENCE_PARALLEL_GROUP = None


def set_ulysses_sequence_parallel_group(group: dist.ProcessGroup):
    """
    Set ulysses sequence parallel process group.
    """
    global _ULYSSES_SEQUENCE_PARALLEL_GROUP
    _ULYSSES_SEQUENCE_PARALLEL_GROUP = group


def get_ulysses_sequence_parallel_group() -> Optional[dist.ProcessGroup]:
    """
    Get ulysses sequence parallel process group.
    """
    global _ULYSSES_SEQUENCE_PARALLEL_GROUP
    return _ULYSSES_SEQUENCE_PARALLEL_GROUP


def get_ulysses_sequence_parallel_world_size(group: ProcessGroup = None) -> int:
    """
    Get ulysses sequence parallel world size.
    """
    group = get_ulysses_sequence_parallel_group() if group is None else group
    return dist.get_world_size(group) if group else 1


def get_ulysses_sequence_parallel_rank(group: ProcessGroup = None) -> int:
    """
    Get ulysses sequence parallel rank.
    """
    group = get_ulysses_sequence_parallel_group() if group is None else group
    return dist.get_rank(group) if group else 0


def gather_seq_scatter_heads(
    x: Tensor,
    seq_dim: int,
    head_dim: int,
    unpadded_dim_size: int = 0,
    group: ProcessGroup = None,
) -> Tensor:
    """
    A func to sync embedding input with alltoall in sequence parallel
    gather sequence dimension and scatter head dim:
    e.g. seq_dim: 1, head_dim: 2
    [bsz, seq/n, h, ...] -> [bsz, seq, h/n, ...]
    """
    group = get_ulysses_sequence_parallel_group() if group is None else group
    if not group:
        return x
    sp_world = get_ulysses_sequence_parallel_world_size(group)
    x = SeqAllToAll.apply(group, x, head_dim, seq_dim)
    if unpadded_dim_size and unpadded_dim_size % sp_world != 0:
        padding_size = x.size(seq_dim) - unpadded_dim_size
        x = _unpad_tensor(x, seq_dim, padding_size)
    return x


def gather_heads_scatter_seq(x: Tensor, head_dim: int, seq_dim: int, group: ProcessGroup = None) -> Tensor:
    """
    A func to sync attention result with alltoall in sequence parallel
    gather head dimension and scatter seq dim:
    e.g. seq_dim: 1, head_dim: 2
    [bsz, seq, h/n, ...] -> [bsz, seq/n, h, ...]
    """
    group = get_ulysses_sequence_parallel_group() if group is None else group
    if not group:
        return x
    dim_size = x.size(seq_dim)
    sp_world = get_ulysses_sequence_parallel_world_size(group)
    if dim_size % sp_world != 0:
        padding_size = sp_world - (dim_size % sp_world)
        x = _pad_tensor(x, seq_dim, padding_size)
    return SeqAllToAll.apply(group, x, seq_dim, head_dim, False)


def _pad_tensor(x: Tensor, dim: int, padding_size: int) -> Tensor:
    shape = list(x.shape)
    shape[dim] = padding_size
    pad = torch.zeros(shape, dtype=x.dtype, device=x.device)
    return torch.cat([x, pad], dim=dim)


def _unpad_tensor(x: Tensor, dim: int, padding_size: int) -> Tensor:
    slc = [slice(None)] * len(x.shape)
    slc[dim] = slice(0, -padding_size)
    return x[slc]


def slice_input_tensor(x: Tensor, dim: int, padding: bool = True, group: ProcessGroup = None) -> Tensor:
    group = get_ulysses_sequence_parallel_group() if group is None else group
    sp_world_size = dist.get_world_size(group)
    sp_rank = get_ulysses_sequence_parallel_rank()
    dim_size = x.size(dim)
    # pad before slice
    if padding and dim_size % sp_world_size:
        padding_size = sp_world_size - (dim_size % sp_world_size)
        x = _pad_tensor(x, dim, padding_size)
    # slice the input tensor
    parts = x.size(dim) // sp_world_size
    slc = [slice(None)] * len(x.shape)
    slc[dim] = slice(sp_rank * parts, (sp_rank + 1) * parts)
    return x[slc].contiguous()


def all_to_all_tensor(
    local_input: Tensor,
    scatter_dim: int,
    gather_dim: int,
    group: Optional[dist.ProcessGroup] = None,
    async_op: bool = False,
):
    group = get_ulysses_sequence_parallel_group() if group is None else group
    seq_world_size = dist.get_world_size(group)
    input_list = [t.contiguous() for t in torch.tensor_split(local_input, seq_world_size, scatter_dim)]
    output_list = [torch.empty_like(input_list[0]) for _ in range(seq_world_size)]
    comm = dist.all_to_all(output_list, input_list, group=group, async_op=async_op)
    if async_op:

        def wait():
            comm.wait()
            return torch.cat(output_list, dim=gather_dim).contiguous()

        return wait
    return torch.cat(output_list, dim=gather_dim).contiguous()


def all_gather_tensor(local_tensor: Tensor, group: Optional[dist.ProcessGroup] = None, async_op: bool = False):
    group = get_ulysses_sequence_parallel_group() if group is None else group
    sp_world_size = dist.get_world_size(group=group)
    output_shape = list(local_tensor.shape)
    output_shape[0] = output_shape[0] * sp_world_size
    output = torch.empty(output_shape, dtype=local_tensor.dtype, device=local_tensor.device)
    dist.all_gather_into_tensor(output, local_tensor, group=group, async_op=async_op)
    return output


class SeqAllToAll(torch.autograd.Function):
    @staticmethod
    def forward(
        ctx: Any,
        group: dist.ProcessGroup,
        local_input: Tensor,
        scatter_dim: int,
        gather_dim: int,
        async_op: bool = False,
    ) -> Tensor:
        ctx.group = group
        ctx.scatter_dim = scatter_dim
        ctx.gather_dim = gather_dim
        ctx.async_op = async_op
        return all_to_all_tensor(local_input, scatter_dim, gather_dim, group, async_op)

    @staticmethod
    def backward(ctx: Any, *grad_output: Tensor) -> Tuple[None, Tensor, None, None]:
        input_t = torch.cat(grad_output[1:], dim=ctx.gather_dim).contiguous() if ctx.async_op else grad_output[0]
        return (
            None,
            all_to_all_tensor(input_t, ctx.gather_dim, ctx.scatter_dim, ctx.group, False),
            None,
            None,
            None,
            None,
        )


class Gather(torch.autograd.Function):
    @staticmethod
    def forward(
        ctx: Any,
        group: dist.ProcessGroup,
        local_tensor: Tensor,
        gather_dim: int,
        grad_scaler: bool = True,
        async_op=False,
    ) -> Tensor:
        ctx.group = group
        ctx.gather_dim = gather_dim
        ctx.grad_scaler = grad_scaler
        ctx.async_op = async_op

        sp_world_size = dist.get_world_size(group=group)
        ctx.sp_world_size = sp_world_size

        sp_rank = dist.get_rank(group=group)
        ctx.sp_rank = sp_rank

        local_shape = list(local_tensor.size())
        split_size = local_shape[0]
        part_size = local_shape[gather_dim]  # store original size
        ctx.part_size = part_size

        output = all_gather_tensor(local_tensor, group, async_op)
        return torch.cat(output.split(split_size, dim=0), dim=gather_dim)

    @staticmethod
    def backward(ctx: Any, grad_output: Tensor) -> Any:
        if ctx.grad_scaler:
            grad_output = grad_output * ctx.sp_world_size
        return (
            None,
            grad_output.split(ctx.part_size, dim=ctx.gather_dim)[ctx.sp_rank].contiguous(),
            None,
            None,
            None,
            None,
        )


def gather_outpus_and_unpad(
    x: Tensor,
    gather_dim: int,
    unpad_dim: int = None,
    padding_size: int = 0,
    grad_scaler: bool = True,
    group: Optional[dist.ProcessGroup] = None,
):
    """
    Gather a tensor across a process group and optionally unpad its padded elements.

    Args:
        x (Tensor): Input tensor to gather.
        gather_dim (int): Dimension along which to gather across ranks.
        unpad_dim (int, optional): Dimension from which to remove padding. If None, no unpadding.
        padding_size (int): Number of padding elements to remove on `unpad_dim`. Defaults to 0.
        grad_scaler (bool): Whether to apply gradient scaling during gather. Defaults to True.
        group (ProcessGroup, optional): Process group for gathering. If None, uses
            `get_ulysses_sequence_parallel_group()`. If still None, returns `x` unchanged.

    Returns:
        Tensor: The gathered tensor, with padding removed if requested.
    """
    group = get_ulysses_sequence_parallel_group() if group is None else group
    if group is None:
        return x
    x = Gather.apply(group, x, gather_dim, grad_scaler)
    if unpad_dim is not None:
        assert isinstance(padding_size, int), "padding size is not given or is not an integer"
        if padding_size == 0:
            return x
        x = _unpad_tensor(x, unpad_dim, padding_size)
    return x


def ulysses_pad(input_ids_rmpad: torch.Tensor, position_ids_rmpad: Optional[torch.Tensor] = None, sp_size: int = 1):
    if position_ids_rmpad is not None:
        assert position_ids_rmpad.size(-2) == 1
        assert input_ids_rmpad.size(-1) == position_ids_rmpad.size(-1)
    if sp_size <= 1:
        return input_ids_rmpad, position_ids_rmpad, 0
    _, total_seq_len = input_ids_rmpad.shape
    pad_size = (sp_size - total_seq_len % sp_size) % sp_size
    if pad_size > 0:
        input_ids_rmpad = torch.nn.functional.pad(input_ids_rmpad, (0, pad_size), value=0)
        if position_ids_rmpad is not None:
            pad_pos_ids = torch.arange(pad_size, device=position_ids_rmpad.device).unsqueeze(0)
            if position_ids_rmpad.dim() == 3:
                pad_pos_ids = pad_pos_ids.unsqueeze(0).repeat(3, 1, 1)
            position_ids_rmpad = torch.cat((position_ids_rmpad, pad_pos_ids), dim=-1)
    return input_ids_rmpad, position_ids_rmpad, pad_size


def ulysses_pad_and_slice_inputs(
    input_ids_rmpad: torch.Tensor, position_ids_rmpad: Optional[torch.Tensor] = None, sp_size: int = 1
):
    """
    Pad and slice input_ids to be divisible by sp_size
    Pad position_ids to be divisible by sp_size.

    Note both input_ids_rmpad and position_ids_rmpad will be padded and sliced.

    The is the utility of pre-forward for ulysses sequence parallelism

    Args:
        input_ids_rmpad: shape of [bsz, seqlen]
        position_ids_rmpad: shape of [bsz, seqlen], where bsz must be 1
        sp_size (int): ulysses sequence parallelism size

    Returns:
        torch.Tensor: padded and sliced input_ids
        torch.Tensor: padded and sliced position_ids
        int: pad size
    """
    input_ids_rmpad, position_ids_rmpad, pad_size = ulysses_pad(input_ids_rmpad, position_ids_rmpad, sp_size)
    input_ids_rmpad = slice_input_tensor(input_ids_rmpad, dim=1, padding=False)
    if position_ids_rmpad is not None:
        position_ids_rmpad = slice_input_tensor(position_ids_rmpad, dim=1, padding=False)
    return input_ids_rmpad, position_ids_rmpad, pad_size


def validate_ulysses_config(num_heads, ulysses_sequence_size):
    if ulysses_sequence_size > 1:
        assert num_heads % ulysses_sequence_size == 0, (
            f"num_heads ({num_heads}) must be divisible by ulysses sequence size({ulysses_sequence_size})"
        )
