# 31_fused_moe_fwd
# Expert-parallel (EP) fused MoE forward — BASE case.
#
# This is the generic fused MoE forward pass: router (softmax + top-k) -> token
# permutation -> all_to_all dispatch -> per-expert SiLU MLP -> all_to_all combine ->
# weighted unpermute. Here the expert count is fixed (num_experts = 8) regardless of
# world size, so the EP load pattern depends on the launch configuration.
from typing import List, Optional, Tuple, Union

import torch
import torch.distributed as dist


class _AllToAll(torch.autograd.Function):
    @staticmethod
    def forward(ctx, group, input, output_split_sizes, input_split_sizes):
        ctx.group = group
        ctx.output_split_sizes = output_split_sizes
        ctx.input_split_sizes = input_split_sizes
        if dist.get_world_size(group=group) == 1:
            return input.contiguous()
        input = input.contiguous()
        if output_split_sizes is None:
            output = torch.empty_like(input)
        else:
            output = torch.empty(
                size=(sum(output_split_sizes), input.size(1)),
                dtype=input.dtype,
                device=input.device,
            )
        dist.all_to_all_single(
            output,
            input,
            output_split_sizes=output_split_sizes,
            input_split_sizes=input_split_sizes,
            group=group,
        )
        return output

    @staticmethod
    def backward(ctx, grad_output):
        return (
            None,
            _AllToAll.apply(
                ctx.group, grad_output, ctx.input_split_sizes, ctx.output_split_sizes
            ),
            None,
            None,
        )


def _all_to_all(
    group: dist.ProcessGroup,
    input: torch.Tensor,
    output_split_sizes: Optional[List[int]],
    input_split_sizes: Optional[List[int]],
) -> torch.Tensor:
    return _AllToAll.apply(group, input, output_split_sizes, input_split_sizes)


def _preprocess(
    expert_mask: torch.Tensor,
    num_experts: int,
    ep_group: dist.ProcessGroup,
) -> Tuple[List[int], List[int], torch.Tensor, torch.Tensor]:
    ep_size = ep_group.size()
    num_local_experts = num_experts // ep_size
    rank = dist.get_rank(ep_group)
    num_local_tokens_per_expert = expert_mask.sum(dim=(1, 2))
    input_splits = (
        num_local_tokens_per_expert.reshape(ep_size, num_local_experts).sum(dim=1).tolist()
    )
    num_local_tokens_per_expert_flat = num_local_tokens_per_expert.contiguous().view(-1)
    output_size = ep_size * num_local_tokens_per_expert_flat.numel()
    num_global_tokens_per_expert_flat = torch.empty(
        output_size,
        dtype=num_local_tokens_per_expert.dtype,
        device=num_local_tokens_per_expert.device,
    )
    dist.all_gather_into_tensor(
        num_global_tokens_per_expert_flat, num_local_tokens_per_expert_flat, group=ep_group
    )
    num_global_tokens_per_expert = num_global_tokens_per_expert_flat.view(
        ep_size, num_local_tokens_per_expert.size(0)
    )
    start_idx, end_idx = rank * num_local_experts, (rank + 1) * num_local_experts
    num_global_tokens_per_local_expert = num_global_tokens_per_expert[
        :, start_idx:end_idx
    ].contiguous()
    output_splits = num_global_tokens_per_local_expert.sum(dim=1).tolist()
    num_global_sum_tokens_per_local_expert = num_global_tokens_per_local_expert.sum(
        dim=0
    ).to(torch.device("cpu"), non_blocking=True)
    num_global_tokens_per_local_expert = num_global_tokens_per_local_expert.view(
        -1, num_local_experts
    ).to(torch.device("cpu"), non_blocking=True)
    return (
        input_splits,
        output_splits,
        num_global_tokens_per_local_expert,
        num_global_sum_tokens_per_local_expert,
    )


def _permute(
    tokens: torch.Tensor, routing_map: torch.Tensor
) -> Tuple[torch.Tensor, torch.Tensor]:
    num_tokens, _ = tokens.shape
    num_experts = routing_map.shape[0]
    routing_map = routing_map.bool()
    token_indices = (
        torch.arange(num_tokens, device=routing_map.device)
        .unsqueeze(0)
        .expand(num_experts, -1)
    )
    sorted_indices = token_indices.masked_select(routing_map)
    permuted_input = tokens.index_select(0, sorted_indices)
    return permuted_input, sorted_indices


def _sort_chunks_by_idxs(
    input: torch.Tensor,
    split_sizes: Union[torch.Tensor, List[int]],
    sorted_idxs: List[int],
) -> torch.Tensor:
    if isinstance(split_sizes, torch.Tensor):
        split_sizes = split_sizes.tolist()
    chunks = torch.split(input, split_sizes, dim=0)
    return torch.cat([chunks[i] for i in sorted_idxs], dim=0)


def _generate_weights_idx(
    routing_weights: torch.Tensor,
    selected_experts: torch.Tensor,
    num_experts: int,
) -> torch.Tensor:
    num_tokens, topk = routing_weights.shape
    weights_idx = torch.zeros(
        (num_tokens, num_experts),
        dtype=routing_weights.dtype,
        device=routing_weights.device,
    )
    weights_idx.scatter_add_(1, selected_experts, routing_weights)
    return weights_idx


def _unpermute(
    tokens: torch.Tensor,
    routing_weights: torch.Tensor,
    hidden_states_shape: torch.Size,
    permutation_mapping: torch.Tensor,
    routing_map: torch.Tensor,
) -> torch.Tensor:
    tokens_weight = routing_weights.T.contiguous().masked_select(routing_map.bool())
    tokens = tokens * tokens_weight.unsqueeze(-1)
    hidden_dim = hidden_states_shape[-1]
    unpermuted_tokens = torch.zeros(
        hidden_states_shape, device=tokens.device, dtype=tokens.dtype
    )
    expanded_mapping = permutation_mapping.unsqueeze(1).expand(-1, hidden_dim)
    unpermuted_tokens.scatter_add_(0, expanded_mapping, tokens)
    return unpermuted_tokens


def token_pre_all2all(
    hidden_states: torch.Tensor,
    expert_mask: torch.Tensor,
    num_experts: int,
    input_splits: List[int],
    output_splits: List[int],
    num_global_tokens_per_local_expert: torch.Tensor,
    group: Optional[dist.ProcessGroup] = None,
) -> Tuple[torch.Tensor, torch.Tensor, torch.Tensor, torch.Size]:
    group = group or dist.group.WORLD
    hidden_dim = hidden_states.size(-1)
    hidden_states = hidden_states.reshape(-1, hidden_dim)
    org_hidden_states_shape = hidden_states.shape
    routing_map = expert_mask.sum(dim=1)

    local_permuted_hidden_states, local_input_permutation_mapping = _permute(
        hidden_states, routing_map
    )
    expected_tokens = sum(input_splits)
    actual_tokens = local_permuted_hidden_states.shape[0]
    if expected_tokens != actual_tokens:
        raise RuntimeError(
            f"EP split mismatch: input_splits sum ({expected_tokens}) != "
            f"permuted tokens ({actual_tokens})"
        )

    global_permuted_hidden_states = _all_to_all(
        group, local_permuted_hidden_states, output_splits, input_splits
    )
    num_local_experts = num_experts // dist.get_world_size(group)
    permute_order = (
        torch.arange(num_experts).reshape(-1, num_local_experts).T.ravel().tolist()
    )
    split_sizes = num_global_tokens_per_local_expert.ravel().tolist()
    global_permuted_hidden_states = _sort_chunks_by_idxs(
        global_permuted_hidden_states, split_sizes, permute_order
    )
    return (
        global_permuted_hidden_states,
        routing_map,
        local_input_permutation_mapping,
        org_hidden_states_shape,
    )


def tokens_post_all2all(
    expert_outputs: torch.Tensor,
    routing_weights: torch.Tensor,
    selected_experts: torch.Tensor,
    num_experts: int,
    input_splits: List[int],
    output_splits: List[int],
    num_global_tokens_per_local_expert: torch.Tensor,
    routing_map: torch.Tensor,
    local_input_permutation_mapping: torch.Tensor,
    org_hidden_states_shape: torch.Size,
    group: Optional[dist.ProcessGroup] = None,
) -> torch.Tensor:
    group = group or dist.group.WORLD
    num_local_experts = num_experts // dist.get_world_size(group)
    unpermute_order = (
        torch.arange(num_experts).reshape(num_local_experts, -1).T.ravel().tolist()
    )
    split_sizes = num_global_tokens_per_local_expert.T.ravel().tolist()
    expert_outputs = _sort_chunks_by_idxs(
        expert_outputs, split_sizes, unpermute_order
    )
    unpermute_outputs = _all_to_all(group, expert_outputs, input_splits, output_splits)
    weights_idx = _generate_weights_idx(routing_weights, selected_experts, num_experts)
    unpermute_outputs = _unpermute(
        unpermute_outputs,
        weights_idx,
        org_hidden_states_shape,
        local_input_permutation_mapping,
        routing_map,
    )
    return unpermute_outputs


def expert_forward(
    x: torch.Tensor,
    gate_proj: torch.nn.Linear,
    up_proj: torch.nn.Linear,
    down_proj: torch.nn.Linear,
) -> torch.Tensor:
    gate = torch.nn.functional.silu(gate_proj(x))
    up = up_proj(x)
    return down_proj(gate * up)


def solution(
    hidden_states: torch.Tensor,
    gate_weight: torch.Tensor,
    gate_bias: Optional[torch.Tensor],
    gate_proj: torch.nn.Linear,
    up_proj: torch.nn.Linear,
    down_proj: torch.nn.Linear,
    num_experts: int,
    top_k: int,
    group: Optional[dist.ProcessGroup] = None,
) -> torch.Tensor:
    group = group or dist.group.WORLD
    hidden_dim = hidden_states.size(-1)
    num_tokens = hidden_states.reshape(-1, hidden_dim).size(0)

    router_logits = torch.nn.functional.linear(
        hidden_states.reshape(-1, hidden_dim), gate_weight, gate_bias
    )
    routing_weights, selected_experts = torch.topk(
        torch.softmax(router_logits, dim=-1), top_k, dim=-1
    )
    expert_mask = torch.nn.functional.one_hot(
        selected_experts, num_classes=num_experts
    ).permute(2, 1, 0)

    input_splits, output_splits, num_global_tokens_per_local_expert, _ = _preprocess(
        expert_mask, num_experts, group
    )

    (
        global_permuted_hidden_states,
        routing_map,
        local_input_permutation_mapping,
        org_hidden_states_shape,
    ) = token_pre_all2all(
        hidden_states,
        expert_mask,
        num_experts,
        input_splits,
        output_splits,
        num_global_tokens_per_local_expert,
        group,
    )

    expert_outputs = expert_forward(
        global_permuted_hidden_states, gate_proj, up_proj, down_proj
    )

    out = tokens_post_all2all(
        expert_outputs,
        routing_weights,
        selected_experts,
        num_experts,
        input_splits,
        output_splits,
        num_global_tokens_per_local_expert,
        routing_map,
        local_input_permutation_mapping,
        org_hidden_states_shape,
        group,
    )
    return out