# Copyright (c) 2024, NVIDIA CORPORATION. All rights reserved.

from contextlib import nullcontext
from dataclasses import dataclass
from typing import List, Optional, Union

import torch
from torch import Tensor

from megatron.core import InferenceParams, parallel_state, tensor_parallel
from megatron.core.dist_checkpointing.mapping import ShardedStateDict
from megatron.core.dist_checkpointing.utils import replace_prefix_for_sharding
from megatron.core.fusions.fused_layer_norm import FusedLayerNorm
from megatron.core.packed_seq_params import PackedSeqParams
from megatron.core.transformer.module import MegatronModule
from megatron.core.transformer.spec_utils import ModuleSpec, build_module
from megatron.core.transformer.transformer_config import TransformerConfig
from megatron.core.transformer.transformer_layer import BaseTransformerLayer
from megatron.core.transformer.utils import sharded_state_dict_default
from megatron.core.utils import make_viewless_tensor

try:
    from megatron.core.extensions.transformer_engine import (
        TEDelayedScaling,
        TENorm,
        get_cpu_offload_context,
        te_checkpoint,
    )

    HAVE_TE = True
    LayerNormImpl = TENorm
except ImportError:
    HAVE_TE = False
    get_cpu_offload_context = None

    try:
        import apex  # pylint: disable=unused-import

        LayerNormImpl = FusedLayerNorm

    except ImportError:
        from megatron.core.transformer.torch_layer_norm import WrappedTorchLayerNorm

        LayerNormImpl = WrappedTorchLayerNorm


def get_num_layers_to_build(config: TransformerConfig) -> int:
    """
    Determine the number of transformer layers to build for the current pipeline stage.
    Args:
        config (TransformerConfig): Configuration object containing transformer model parameters.

    Returns:
        int: The number of layers to be built for the current pipeline stage.
    """
    if config.first_pipeline_num_layers is not None or config.last_pipeline_num_layers is not None:
        assert (
            parallel_state.get_virtual_pipeline_model_parallel_world_size() is None
        ), "Uneven number of layer not compatible with interleaved pipeline schedule"

        # Number of layers to distribute over rest of pipeline stages
        layers_to_distribute = config.num_layers
        # Number of pipeline stages left for distributing transformer layers
        pipeline_stages_left = parallel_state.get_pipeline_model_parallel_world_size()

        if config.first_pipeline_num_layers is not None:
            layers_to_distribute -= config.first_pipeline_num_layers
            pipeline_stages_left -= 1
            if parallel_state.is_pipeline_first_stage():
                return config.first_pipeline_num_layers

        if config.last_pipeline_num_layers is not None:
            layers_to_distribute -= config.last_pipeline_num_layers
            pipeline_stages_left -= 1
            if parallel_state.is_pipeline_last_stage():
                return config.last_pipeline_num_layers

        assert (
            layers_to_distribute % pipeline_stages_left == 0
        ), "With uneven pipelineing the left over layers must be divisible by left over stages"
        num_layers_per_pipeline_rank = layers_to_distribute // pipeline_stages_left
    else:
        pipeline_ranks = config.pipeline_model_parallel_size
        num_layers_per_pipeline_rank = config.num_layers // pipeline_ranks

    if parallel_state.get_virtual_pipeline_model_parallel_world_size() is not None:
        # Interleaved pipeline parallelism:
        # Number of layers in each model chunk is the number of layers in the stage,
        # divided by the number of model chunks in a stage.
        # With 8 layers, 2 stages, and 4 model chunks, we want an assignment of
        # layers to stages like (each list is a model chunk):
        # Stage 0: [0]  [2]  [4]  [6]
        # Stage 1: [1]  [3]  [5]  [7]
        # With 8 layers, 2 stages, and 2 virtual stages, we want an assignment of
        # layers to stages like (each list is a model chunk):
        # Stage 0: [0, 1]  [4, 5]
        # Stage 1: [2, 3]  [6, 7]

        vp_size = parallel_state.get_virtual_pipeline_model_parallel_world_size()

        num_layers_per_virtual_rank = num_layers_per_pipeline_rank // vp_size

        num_layers_to_build = num_layers_per_virtual_rank

    else:
        # Non-interleaved pipeline parallelism:
        # Each stage gets a contiguous set of layers.

        num_layers_to_build = num_layers_per_pipeline_rank

    return num_layers_to_build


@dataclass
class TransformerBlockSubmodules:
    """
    Dataclass for specifying the submodules of a transformer block.

    This class defines the structure for configuring the layers and normalization
    within a transformer block, allowing for flexible and customizable architecture designs.

    Args:
        layer_specs (List[ModuleSpec], optional): A list of module specifications for
            the layers within the transformer block. Each specification typically
            defines a complete transformer layer (e.g., self-attention, feed-forward network).
        layer_norm (Optional[Union[ModuleSpec, torch.nn.Module]], optional): Specification
            or instance of the layer normalization to be applied.
    """

    layer_specs: List[ModuleSpec] = None
    layer_norm: Optional[Union[ModuleSpec, torch.nn.Module]] = None


def _get_block_submodules(
    config: TransformerConfig, spec: Union[TransformerBlockSubmodules, ModuleSpec]
) -> TransformerBlockSubmodules:
    """
    Retrieve or construct TransformerBlockSubmodules based on the provided specification.

    Args:
        config (TransformerConfig): Configuration object for the transformer model.
        spec (Union[TransformerBlockSubmodules, ModuleSpec]): Specification for the
            transformer block submodules. Can be either a TransformerBlockSubmodules
            instance or a ModuleSpec.

    Returns:
        TransformerBlockSubmodules: The submodules for the transformer block.
    """

    # Transformer block submodules.
    if isinstance(spec, TransformerBlockSubmodules):
        return spec

    # ModuleSpec here is generally assumed to be for a transformer layer that
    # is implemented in `transformer_layer.py` or if it subclasses
    # `BaseTransformerLayer` from the `transformer_layer.py` file.
    elif isinstance(spec, ModuleSpec):
        if issubclass(spec.module, TransformerBlock):
            return spec.submodules
        elif issubclass(spec.module, BaseTransformerLayer):
            num_layers = get_num_layers_to_build(config)
            return TransformerBlockSubmodules(
                layer_specs=[spec] * num_layers, layer_norm=LayerNormImpl
            )
        else:
            raise Exception(f"specialize for {spec.module.__name__}.")
    else:
        raise Exception(f"specialize for {type(spec).__name__}.")


class TransformerBlock(MegatronModule):
    """Transformer class."""

    def __init__(
        self,
        config: TransformerConfig,
        spec: Union[TransformerBlockSubmodules, ModuleSpec],
        post_layer_norm: bool = True,
        pre_process: bool = True,
        post_process: bool = True,
    ):
        super().__init__(config=config)

        self.submodules = _get_block_submodules(config, spec)
        self.post_layer_norm = post_layer_norm
        self.pre_process = pre_process
        self.post_process = post_process
        # Dictionary to store CUDA graphs. Number of items in the dictionary = len(self.layers).
        # Item `i` in the dictionary is a list of `N` CUDA graphs for layer 'i' where N is the
        # number of microbatches. Multiple CUDA graphs per layer is required to support
        # pipelining which requires running FWD graph of multiple microbatches before BWD graph.
        self.cuda_graphs = {}
        self.current_microbatch = -1

        # required for pipeline parallel schedules
        self.input_tensor = None

        self.checkpoint_core_attention = self.config.recompute_granularity == 'selective'

        if get_cpu_offload_context is not None:
            (self.offload_context, self.group_prefetch_offload_commit_async) = (
                get_cpu_offload_context(
                    self.config.cpu_offloading,
                    self.config.cpu_offloading_num_layers,
                    self.config.num_layers,
                    self.config.cpu_offloading_activations,
                    self.config.cpu_offloading_weights,
                )
            )
            self.config._cpu_offloading_context = (
                self.offload_context if self.config.cpu_offloading else None
            )
        else:
            assert (
                self.config.cpu_offloading is False
            ), "CPU Offloading is enabled when TE is not present"

            self.offload_context, self.group_prefetch_offload_commit_async = nullcontext(), None
            self.config._cpu_offloading_context = None

        self._build_layers()
        self.num_layers_per_pipeline_rank = len(self.layers)
        self.tp_only_amax_red = config.tp_only_amax_red

    def _build_layers(self):
        # Transformer layers.
        # @jcasper can we improve how we deal with layer_number?
        # currently it's only used in CoreAttention?
        # if self.apply_query_key_layer_scaling:
        #     coeff = self.layer_number
        #     self.norm_factor *= coeff
        def build_layer(layer_spec, layer_number):
            return build_module(layer_spec, config=self.config, layer_number=layer_number)

        # offset is implicit in TransformerLayer
        self.layers = torch.nn.ModuleList(
            [
                build_layer(layer_spec, i + 1)
                for i, layer_spec in enumerate(self.submodules.layer_specs)
            ]
        )

        # @TODO: add back standalone_embedding_stage (see issue #293)
        # In pipeline parallelism, we want to add this LN only to the last stage of the pipeline
        # self.post_process and self.post_layer_norm guide this behavior
        if self.submodules.layer_norm and self.post_process and self.post_layer_norm:
            self.final_layernorm = build_module(
                self.submodules.layer_norm,
                config=self.config,
                hidden_size=self.config.hidden_size,
                eps=self.config.layernorm_epsilon,
            )
        else:
            self.final_layernorm = None  # Either this or nn.Identity

    def _get_layer(self, layer_number: int):
        return self.layers[layer_number]

    def _checkpointed_forward(
        self,
        hidden_states: Tensor,
        attention_mask: Tensor,
        context: Tensor,
        context_mask: Tensor,
        rotary_pos_emb: Tensor,
        packed_seq_params: PackedSeqParams,
    ):
        """Forward method with activation checkpointing."""

        def custom(start: int, end: int):
            def custom_forward(
                hidden_states, attention_mask, context, context_mask, rotary_pos_emb
            ):
                for index in range(start, end):
                    layer = self._get_layer(index)
                    hidden_states, context = layer(
                        hidden_states=hidden_states,
                        attention_mask=attention_mask,
                        context=context,
                        context_mask=context_mask,
                        rotary_pos_emb=rotary_pos_emb,
                        inference_params=None,
                        packed_seq_params=packed_seq_params,
                    )
                return hidden_states, context

            return custom_forward

        def checkpoint_handler(forward_func):
            if self.config.fp8:
                return te_checkpoint(
                    forward_func,
                    self.config.distribute_saved_activations,
                    tensor_parallel.random.get_cuda_rng_tracker,
                    parallel_state.get_tensor_model_parallel_group(),
                    hidden_states,
                    attention_mask,
                    context,
                    context_mask,
                    rotary_pos_emb,
                )
            else:
                return tensor_parallel.checkpoint(
                    forward_func,
                    self.config.distribute_saved_activations,
                    hidden_states,
                    attention_mask,
                    context,
                    context_mask,
                    rotary_pos_emb,
                )

        if self.config.recompute_method == 'uniform':
            # Uniformly divide the total number of Transformer layers and checkpoint
            # the input activation of each divided chunk.
            # A method to further reduce memory usage reducing checkpoints.
            layer_idx = 0
            while layer_idx < self.num_layers_per_pipeline_rank:
                hidden_states, context = checkpoint_handler(
                    custom(layer_idx, layer_idx + self.config.recompute_num_layers)
                )

                layer_idx += self.config.recompute_num_layers

        elif self.config.recompute_method == 'block':
            # Checkpoint the input activation of only a set number of individual
            # Transformer layers and skip the rest.
            # A method fully use the device memory removing redundant re-computation.
            recompute_skip_num_layers = 0
            for layer_idx in range(self.num_layers_per_pipeline_rank):
                # Skip recomputation when input grad computation is not needed.
                # Need to have at least one input tensor with gradient computation
                # for re-enterant autograd engine.
                if self.config.fp8 and not hidden_states.requires_grad:
                    recompute_skip_num_layers += 1
                if (
                    layer_idx >= recompute_skip_num_layers
                    and layer_idx < self.config.recompute_num_layers + recompute_skip_num_layers
                ):
                    hidden_states, context = checkpoint_handler(custom(layer_idx, layer_idx + 1))
                else:
                    hidden_states, context = custom(layer_idx, layer_idx + 1)(
                        hidden_states, attention_mask, context, context_mask, rotary_pos_emb
                    )
        else:
            raise ValueError("Invalid activation recompute method.")

        return hidden_states

    def set_input_tensor(self, input_tensor: Tensor):
        """Set input tensor to be used instead of forward()'s input.

        When doing pipeline parallelism the input from the previous
        stage comes from communication, not from the input, so the
        model's forward_step_func won't have it. This function is thus
        used by internal code to bypass the input provided by the
        forward_step_func"""
        self.input_tensor = input_tensor

    def forward(
        self,
        hidden_states: Tensor,
        attention_mask: Tensor,
        context: Tensor = None,
        context_mask: Tensor = None,
        rotary_pos_emb: Tensor = None,
        inference_params: InferenceParams = None,
        packed_seq_params: PackedSeqParams = None,
    ):
        """
        Perform the forward pass through the transformer block.

        This method handles the core computation of the transformer, including
        self-attention, optional cross-attention, and feed-forward operations.

        Args:
            hidden_states (Tensor): Input tensor of shape [s, b, h] where s is the
                sequence length, b is the batch size, and h is the hidden size.
            attention_mask (Tensor): Boolean tensor of shape [1, 1, s, s] for masking
                self-attention.
            context (Tensor, optional): Context tensor for cross-attention.
            context_mask (Tensor, optional): Mask for cross-attention context
            rotary_pos_emb (Tensor, optional): Rotary positional embeddings.
            inference_params (InferenceParams, optional): Parameters for inference-time
                optimizations.
            packed_seq_params (PackedSeqParams, optional): Parameters for packed sequence
                processing.

        Returns:
            Union[Tensor, Tuple[Tensor, Tensor]]: The output hidden states tensor of shape
            [s, b, h], and optionally the updated context tensor if cross-attention is used.
        """

        if not self.pre_process:
            # See set_input_tensor()
            hidden_states = self.input_tensor

        # Viewless tensor.
        # - We only need to create a viewless tensor in the case of micro batch
        #   size (mbs) == 1, since in this case, 'hidden_states.transpose()'
        #   above creates a view tensor, and '.contiguous()' is a pass-through.
        #   For mbs >= 2, '.contiguous()' creates a new tensor, eliminating
        #   the need to make it viewless.
        #
        #   However, we don't explicitly check mbs == 1 here because
        #   make_viewless_tensor() has negligible overhead when its input
        #   is already viewless.
        #
        # - For the 'else' case above, calling make_viewless_tensor() here is
        #   likely redundant, since p2p_communication.py (likely originator)
        #   already creates viewless tensors. That said, make_viewless_tensor()
        #   is called here to be future-proof and corner-case-proof.
        hidden_states = make_viewless_tensor(inp=hidden_states, requires_grad=True, keep_graph=True)

        if self.config.sequence_parallel:
            rng_context = tensor_parallel.get_cuda_rng_tracker().fork()
        else:
            rng_context = nullcontext()

        if self.config.fp8:
            import transformer_engine  # To keep out TE dependency when not training in fp8

            if self.config.fp8 == "e4m3":
                fp8_format = transformer_engine.common.recipe.Format.E4M3
            elif self.config.fp8 == "hybrid":
                fp8_format = transformer_engine.common.recipe.Format.HYBRID
            else:
                raise ValueError("E4M3 and HYBRID are the only supported FP8 formats.")

            fp8_recipe = TEDelayedScaling(
                config=self.config,
                fp8_format=fp8_format,
                override_linear_precision=(False, False, not self.config.fp8_wgrad),
            )
            fp8_group = None
            if parallel_state.model_parallel_is_initialized():
                fp8_group = parallel_state.get_amax_reduction_group(
                    with_context_parallel=True, tp_only_amax_red=self.tp_only_amax_red
                )
            fp8_context = transformer_engine.pytorch.fp8_autocast(
                enabled=True, fp8_recipe=fp8_recipe, fp8_group=fp8_group
            )
        else:
            fp8_context = nullcontext()

        with rng_context and fp8_context:
            # Forward pass.
            if self.config.recompute_granularity == 'full' and self.training:
                hidden_states = self._checkpointed_forward(
                    hidden_states=hidden_states,
                    attention_mask=attention_mask,
                    context=context,
                    context_mask=context_mask,
                    rotary_pos_emb=rotary_pos_emb,
                    packed_seq_params=packed_seq_params,
                )
            else:
                for l_no, layer in enumerate(self.layers):
                    with self.offload_context:
                        layer.use_cudagraph = True
                        if (len(self.cuda_graphs) == 0) or (not self.training):
                            hidden_states, context = layer(
                                hidden_states=hidden_states,
                                attention_mask=attention_mask,
                                context=context,
                                context_mask=context_mask,
                                rotary_pos_emb=rotary_pos_emb,
                                inference_params=inference_params,
                                packed_seq_params=packed_seq_params,
                            )
                        else:
                            # CUDA graph replay for layer `l_no` and microbatch
                            # `self.current_microbatch`
                            # CUDA graph requires positional arguments with the exception
                            # of is_first_microbatch.
                            # Also CUDA graph accepts only Tensor inputs and outputs.
                            # Hence, the arg list and returned list is limited to `hidden_states`.
                            assert (len(self.cuda_graphs) > l_no) and (
                                self.current_microbatch < len(self.cuda_graphs[l_no])
                            )
                            hidden_states = self.cuda_graphs[l_no][self.current_microbatch](
                                hidden_states, is_first_microbatch=(self.current_microbatch == 0)
                            )

                    if (
                        torch.is_grad_enabled()
                        and self.config.cpu_offloading
                        and self.group_prefetch_offload_commit_async is not None
                    ):
                        hidden_states = self.group_prefetch_offload_commit_async(hidden_states)

        # Final layer norm.
        if self.final_layernorm is not None:
            hidden_states = self.final_layernorm(hidden_states)
            # TENorm produces a "viewed" tensor. This will result in schedule.py's
            # deallocate_output_tensor() throwing an error, so a viewless tensor is
            # created to prevent this.
            hidden_states = make_viewless_tensor(
                inp=hidden_states, requires_grad=True, keep_graph=True
            )

        return hidden_states

    def sharded_state_dict(
        self, prefix: str = '', sharded_offsets: tuple = (), metadata: dict = None
    ) -> ShardedStateDict:
        """
        Generate a sharded state dictionary for the transformer block.

        Args:
            prefix (str, optional): Prefix to be added to all keys in the state dict.
                Defaults to an empty string.
            sharded_offsets (tuple, optional): Tuple of sharding offsets.
            metadata (dict, optional): Additional metadata for sharding.
                Can specify if layers are non-homogeneous. Defaults to None.

        Returns:
            ShardedStateDict: A dictionary containing the sharded state of the model.
        """
        assert not sharded_offsets, "Unexpected sharded offsets"
        non_homogeneous_layers = metadata is not None and metadata.get(
            'non_homogeneous_layers', False
        )
        sharded_state_dict = {}

        layer_prefix = f'{prefix}layers.'
        num_layers = self.config.num_layers
        for layer in self.layers:
            offset = layer._get_layer_offset()

            global_layer_offset = layer.layer_number - 1  # self.layer_number starts at 1
            state_dict_prefix = f'{layer_prefix}{global_layer_offset - offset}.'  # module list index in TransformerBlock # pylint: disable=line-too-long
            if non_homogeneous_layers:
                sharded_prefix = f'{layer_prefix}{global_layer_offset}.'
                sharded_pp_offset = []
            else:
                sharded_prefix = layer_prefix
                sharded_pp_offset = [
                    (0, global_layer_offset, num_layers)
                ]  # PP sharding offset for ShardedTensors
            layer_sharded_state_dict = layer.sharded_state_dict(
                state_dict_prefix, sharded_pp_offset, metadata
            )
            replace_prefix_for_sharding(layer_sharded_state_dict, state_dict_prefix, sharded_prefix)

            sharded_state_dict.update(layer_sharded_state_dict)

        # Add modules other than self.layers
        for name, module in self.named_children():
            if not module is self.layers:
                sharded_state_dict.update(
                    sharded_state_dict_default(
                        module, f'{prefix}{name}.', sharded_offsets, metadata
                    )
                )

        return sharded_state_dict
