# SPDX-License-Identifier: Apache-2.0
# SPDX-FileCopyrightText: Copyright contributors to the vLLM project

import torch

from vllm.distributed import (
    get_tensor_model_parallel_world_size,
    tensor_model_parallel_all_reduce,
)
from vllm.model_executor.layers.fused_moe.layer import FusedMoE


# TODO(bnell): Add shared + fused combo function? e.g. +
class SharedFusedMoE(FusedMoE):
    """
    A FusedMoE operation that also computes the results of shared experts.
    If an all2all communicator is being used the shared expert computation
    can be interleaved with the fused all2all dispatch communication step.
    """

    def __init__(
        self,
        shared_experts: torch.nn.Module | None,
        gate: torch.nn.Module | None = None,
        use_overlapped: bool = True,
        routed_input_transform: torch.nn.Module | None = None,
        **kwargs,
    ):
        super().__init__(**kwargs)
        self._shared_experts = shared_experts
        self._routed_input_transform = routed_input_transform

        # Disable shared expert overlap if:
        #   - we are using eplb with non-default backend, because of correctness issues
        #   - we are using flashinfer with DP, since there nothing to gain
        #   - we are using marlin kernels
        backend = self.moe_parallel_config.all2all_backend
        self.use_overlapped = (
            use_overlapped
            and not (
                (self.enable_eplb and backend != "allgather_reducescatter")
                or self.moe_parallel_config.use_fi_all2allv_kernels
            )
            and self._shared_experts is not None
        )

        self._gate = gate

    @property
    def shared_experts(self) -> torch.nn.Module | None:
        return self._shared_experts if self.use_overlapped else None

    @property
    def gate(self) -> torch.nn.Module | None:
        return self._gate if self.use_overlapped else None

    @property
    def is_internal_router(self) -> bool:
        return self.gate is not None

    def apply_routed_input_transform(self, hidden_states: torch.Tensor) -> torch.Tensor:
        """Apply transform for routed experts (e.g., latent projection).

        This is called by FusedMoE.forward_native. The original hidden_states
        is saved separately so shared experts get [S, hidden_size] while
        routed experts get the transformed [S, moe_latent_size].

        TODO: For latent MoE bandwidth optimization, fc2_latent_proj could be
        moved inside SharedFusedMoE to all-reduce on the smaller latent
        dimension.
        """
        if self._routed_input_transform is not None:
            result = self._routed_input_transform(hidden_states)
            # ReplicatedLinear returns (output, extra_bias) tuple.
            # We only need the output tensor; extra_bias is not used here.
            if isinstance(result, tuple):
                return result[0]
            return result
        return hidden_states

    def forward(
        self,
        hidden_states: torch.Tensor,
        router_logits: torch.Tensor,
    ) -> tuple[torch.Tensor, torch.Tensor]:
        if not self.use_overlapped:
            if self._shared_experts is not None:
                shared_out = self._shared_experts(hidden_states)

                # Reduce shared expert outputs if necessary, since the MLP
                # should have been created with reduce_results=False.
                if (
                    self.reduce_results
                    and get_tensor_model_parallel_world_size() > 1
                    and self.must_reduce_shared_expert_outputs()
                ):
                    shared_out = tensor_model_parallel_all_reduce(shared_out)
            else:
                shared_out = None

            fused_out = super().forward(
                hidden_states=hidden_states,
                router_logits=router_logits,
            )
        else:
            shared_out, fused_out = super().forward(
                hidden_states=hidden_states,
                router_logits=router_logits,
            )
            # ensure early TP reduction of shared expert outputs when required
            if (
                shared_out is not None
                and self.reduce_results
                and get_tensor_model_parallel_world_size() > 1
                and self.must_reduce_shared_expert_outputs()
            ):
                shared_out = tensor_model_parallel_all_reduce(shared_out)
        return shared_out, fused_out
