# Copyright (c) Meta Platforms, Inc. and affiliates
from typing import cast, List, Optional, Sequence, Tuple

import torch

from torch.distributed._tensor._utils import compute_local_shape
from torch.distributed._tensor.op_schema import (
    OpSchema,
    OpStrategy,
    OutputSharding,
    PlacementStrategy,
    RuntimeSchemaInfo,
    StrategyType,
)
from torch.distributed._tensor.ops.common_rules import pointwise_rule
from torch.distributed._tensor.ops.utils import (
    is_tensor_dim_sharded,
    is_tensor_partial,
    normalize_dim,
    prod,
    register_op_strategy,
    register_prop_rule,
)
from torch.distributed._tensor.placement_types import (
    _Partial,
    DTensorSpec,
    Placement,
    Replicate,
    Shard,
)
from torch.distributed.device_mesh import DeviceMesh


aten = torch.ops.aten


def default_strategy(mesh: DeviceMesh, op_schema: OpSchema) -> StrategyType:
    # Default strategy by default just propagate the first input strategy
    select_strategy = op_schema.args_schema[0]
    assert isinstance(select_strategy, OpStrategy)
    default_strategy = []
    for strategy in select_strategy.strategies:
        # we create new DTensorSpecs even for default strategy to assure that
        # the tensor metas are distinct between the arguments and outputs
        default_strategy.append(
            PlacementStrategy(
                output_spec=DTensorSpec(
                    mesh=strategy.output_spec.mesh,
                    placements=strategy.output_spec.placements,
                )
            )
        )
    return OpStrategy(default_strategy)


register_op_strategy(
    [
        aten.clone.default,
        aten.contiguous.default,
        aten.copy_.default,
        aten.detach.default,
        aten.fill_.Scalar,
        aten.zero_.default,
    ]
)(default_strategy)

register_op_strategy(
    aten._to_copy.default, schema_info=RuntimeSchemaInfo(static_kwargkey=["dtype"])
)(default_strategy)


@register_op_strategy(
    [
        aten.equal.default,
        aten.is_same_size.default,
    ]
)
def equal_strategy(mesh: DeviceMesh, op_schema: OpSchema) -> StrategyType:
    # equal_strategy deals with ops that comparing two tensor, we need to make sure
    # sharding layout the same with two operands, we choose to follow the arg with max
    # num of shards, still keep is_same_size here for completeness as they share the
    # same strategy in theory.
    self_strategy, other_strategy = op_schema.args_schema
    assert isinstance(self_strategy, OpStrategy)
    assert isinstance(other_strategy, OpStrategy)

    select_strategy = (
        self_strategy
        if self_strategy.max_num_shards() >= other_strategy.max_num_shards()
        else other_strategy
    )
    equal_strategy = OpStrategy([])

    for arg_strategy in select_strategy.strategies:
        arg_spec = arg_strategy.output_spec
        if is_tensor_partial(arg_spec):
            # if the arg_spec have partial, reshard to replicate
            # otherwise local shard tensor comparison would be invalid
            output_spec = DTensorSpec(
                mesh=arg_spec.mesh,
                placements=tuple(
                    Replicate() if isinstance(p, _Partial) else p
                    for p in arg_spec.placements
                ),
            )
            equal_strategy.strategies.append(PlacementStrategy(output_spec=output_spec))
        else:
            equal_strategy.strategies.append(PlacementStrategy(arg_spec))
    return equal_strategy


@register_op_strategy(
    [
        aten.empty_like.default,
        aten.ones_like.default,
        aten.rand_like.default,
        aten.randn_like.default,
        aten.zeros_like.default,
    ],
    schema_info=RuntimeSchemaInfo(1, ["dtype"]),
)
@register_op_strategy(
    [aten.full_like.default],
    schema_info=RuntimeSchemaInfo(2, ["dtype"]),
)
@register_op_strategy(
    [
        aten.randint_like.default,
        aten.randint_like.low_dtype,
        aten.randint_like.low_dtype_out,
    ],
    schema_info=RuntimeSchemaInfo(3, ["dtype"]),
)
def create_like_strategy(mesh: DeviceMesh, op_schema: OpSchema) -> StrategyType:
    # create_like_strategy deals with ops that creating tensors with same
    # shape as input, but with specific content that does not depend on
    # the input, we can propagate sharding, but we have to make sure we
    # move from partial to replicated.
    select_strategy = op_schema.args_schema[0]
    create_like_strategy = OpStrategy([])
    assert isinstance(select_strategy, OpStrategy)
    for arg_strategy in select_strategy.strategies:
        arg_spec = arg_strategy.output_spec
        if is_tensor_partial(arg_spec):
            # if the arg_spec have partial, accept partial
            # in the input_specs but output replicate for
            # those corresponding mesh dims
            output_spec = DTensorSpec(
                mesh=arg_spec.mesh,
                placements=tuple(
                    Replicate() if isinstance(p, _Partial) else p
                    for p in arg_spec.placements
                ),
            )
            create_like_strategy.strategies.append(
                PlacementStrategy(output_spec=output_spec, input_specs=(arg_spec,))
            )

        else:
            create_like_strategy.strategies.append(PlacementStrategy(arg_spec))

    return create_like_strategy


@register_op_strategy(
    [
        aten.new_empty.default,
        aten.new_full.default,
        aten.new_ones.default,
        aten.new_zeros.default,
        aten.new_empty_strided.default,  # TODO: re-think new_empty_strided
    ],
    schema_info=RuntimeSchemaInfo(1, ["dtype"]),
)
def new_factory_strategy(mesh: DeviceMesh, _) -> StrategyType:
    # TODO: maybe we should generate all possible shardings intead of just stay
    # replicated for new factory methods
    replica_spec = DTensorSpec(mesh, tuple([Replicate()] * mesh.ndim))
    return OpStrategy([PlacementStrategy(replica_spec)])


@register_op_strategy(aten.bucketize.Tensor)
def gen_bucketize_strategy(mesh: DeviceMesh, op_schema: OpSchema) -> StrategyType:
    """Just propagate input sharding, but expect replicated for boundaries input."""
    input_strategy = op_schema.args_schema[0]
    bucketize_strategy = OpStrategy([])
    assert isinstance(input_strategy, OpStrategy)
    for arg_strategy in input_strategy.strategies:
        arg_spec = DTensorSpec(mesh, arg_strategy.output_spec.placements)
        replica_spec = DTensorSpec(mesh, tuple([Replicate()] * mesh.ndim))
        bucketize_strategy.strategies.append(
            PlacementStrategy(
                output_spec=arg_spec, input_specs=(arg_spec, replica_spec)
            )
        )

    return bucketize_strategy


@register_op_strategy(aten.slice.Tensor, schema_info=RuntimeSchemaInfo(1))
def gen_slice_strategy(mesh: DeviceMesh, op_schema: OpSchema) -> StrategyType:
    """Forward all shardings except the slice dimension."""
    defaults = (None, 0, None, None, 1)
    input_strategy, dim, start, end, step = (
        op_schema.args_schema + defaults[len(op_schema.args_schema) :]
    )
    assert isinstance(input_strategy, OpStrategy)
    input_shape = input_strategy.output_shape
    input_ndim = input_strategy.output_ndim
    assert isinstance(dim, int)
    if start is None:
        start = 0
    if end is None or end > input_shape[dim]:
        end = input_shape[dim]
    assert isinstance(start, int)
    assert isinstance(end, int)
    assert isinstance(step, int)

    # normalize args
    slice_dim = normalize_dim(dim, input_ndim)
    start = normalize_dim(start, input_shape[dim])
    end = normalize_dim(end, input_shape[dim])

    redundant_slice = start == 0 and end == input_shape[dim] and step == 1

    slice_strategy = OpStrategy([])

    for arg_strategy in input_strategy.strategies:
        arg_spec = arg_strategy.output_spec
        if not is_tensor_dim_sharded(arg_spec, dim=slice_dim) or redundant_slice:
            # only add the strategy if the slice dim is not sharded
            out_spec = DTensorSpec(mesh, arg_spec.placements)
            slice_strategy.strategies.append(PlacementStrategy(output_spec=out_spec))
    if not slice_strategy.strategies:
        # if all strategies are filtered out, unsharding all specs on slice dim
        # of the input strategy, and use that as the op strategy
        for arg_strategy in input_strategy.strategies:
            arg_spec = arg_strategy.output_spec
            unshard_spec = DTensorSpec(
                mesh, unshard_tensor_dim(arg_spec.placements, dim=slice_dim)
            )
            slice_strategy.strategies.append(
                PlacementStrategy(output_spec=unshard_spec)
            )
    return slice_strategy


def unshard_tensor_dim(
    placements: Sequence[Placement], dim: int
) -> Tuple[Placement, ...]:
    """Disallow the given tensor dimension to be sharded."""
    return tuple(
        p if (not isinstance(p, Shard) or p.dim != dim) else Replicate()
        for p in placements
    )


def replicate_tensor_dim(
    placements: Sequence[Placement], dim: int
) -> Tuple[Placement, ...]:
    """Force the given tensor dimension to be replicated."""
    # Not using p.is_shard() to avoid mypy complain about Placement not having
    # attribute dim.
    return tuple(
        Replicate() if p.is_partial() or isinstance(p, Shard) and p.dim == dim else p
        for p in placements
    )


@register_op_strategy(aten.slice_scatter.default, schema_info=RuntimeSchemaInfo(2))
def gen_slice_scatter_strategy(mesh: DeviceMesh, op_schema: OpSchema) -> StrategyType:
    # 1. number of dimensions in input and src need to match.
    # 2. number of elements on all non-dim need to match between input and src.
    # 3. numer of elements in src in dim need to match the slice size.
    # Given the above:
    # - We suggest for src to follow the sharding of input, except on the scatter dimension,
    #   where our best bet for now is to make them replicated as a fall-back.
    #   TODO: Ideally we'd like to make sure the output is re-sharded afterwards to keep input sharding.

    input_strategy = op_schema.args_schema[0]
    assert isinstance(input_strategy, OpStrategy)
    input_ndim = input_strategy.output_ndim
    slice_dim = (
        cast(int, op_schema.args_schema[2]) if len(op_schema.args_schema) > 2 else 0
    )
    slice_dim = normalize_dim(slice_dim, input_ndim)

    slice_scatter_strategy = OpStrategy([])
    # by default follow the input strategy for both input and src
    for arg_strategy in input_strategy.strategies:
        arg_spec = arg_strategy.output_spec
        if not (
            is_tensor_dim_sharded(arg_spec, dim=slice_dim)
            or is_tensor_partial(arg_spec)
        ):
            # only add the strategy if the slice_scatter dim is not sharded or partial
            slice_scatter_strategy.strategies.append(
                PlacementStrategy(output_spec=arg_spec)
            )

    if not slice_scatter_strategy.strategies:
        # if all strategies are filtered out, replicating all specs on slice_scatter dim
        # of the input strategy, and use that as the op strategy
        for arg_strategy in input_strategy.strategies:
            arg_spec = arg_strategy.output_spec
            replicate_spec = DTensorSpec(
                mesh, replicate_tensor_dim(arg_spec.placements, dim=slice_dim)
            )
            slice_scatter_strategy.strategies.append(
                PlacementStrategy(output_spec=replicate_spec)
            )
    return slice_scatter_strategy


@register_op_strategy(aten._local_scalar_dense.default)
def replica_only_strategy(mesh: DeviceMesh, op_schema: OpSchema) -> StrategyType:
    """Only allow replication on the input/ouput."""
    replicate_spec = DTensorSpec(mesh, tuple([Replicate()] * mesh.ndim))
    return OpStrategy([PlacementStrategy(replicate_spec)])


@register_prop_rule(aten.index_select.default, schema_info=RuntimeSchemaInfo(1))
def prop_index_select(op_schema: OpSchema) -> OutputSharding:
    values_spec, dim, indices_spec = op_schema.args_schema

    assert isinstance(values_spec, DTensorSpec)
    assert isinstance(dim, int)
    assert isinstance(indices_spec, DTensorSpec)

    all_indices_spec: List[Optional[DTensorSpec]] = [
        indices_spec if dim == i else None for i in range(values_spec.ndim)
    ]

    result = prop_index(
        OpSchema(
            op=op_schema.op,
            args_schema=(values_spec, all_indices_spec),
            kwargs_schema=op_schema.kwargs_schema,
        )
    )
    if result.schema_suggestions:
        result.schema_suggestions = [
            OpSchema(
                op=op_schema.op,
                args_schema=(s.args_schema[0], dim, s.args_schema[1][dim]),
                kwargs_schema=op_schema.kwargs_schema,
            )
            for s in result.schema_suggestions
        ]
    return result


@register_prop_rule(aten.index.Tensor, schema_info=RuntimeSchemaInfo(needs_pytree=True))
def prop_index(op_schema: OpSchema) -> OutputSharding:
    """
    Expect replicated on the first input; _mostly_ pointwise on the second input.

    TODO: exception: when the dtype of second input is "bool", then a torch.nonzero needs to be triggered first.
    """
    # Current sharding constraints:
    # For values:
    #   1. We currently require that the dimension of values_spec be replicated or partial
    #      if they are being indexed on.
    #   2. Other dimensions of values_spec can remain sharded if they are so.
    # For indices:
    #   Indices can be either sharded or replicated. All index tensors need to be sharded
    #   in a compatible way, following the pointwise rule (including resolving _Partial
    #   into either sharded or replicated)

    values_spec, multi_indices_spec = op_schema.args_schema
    assert isinstance(values_spec, DTensorSpec)
    assert isinstance(multi_indices_spec, list)
    multi_indices_spec = cast(List[Optional[DTensorSpec]], multi_indices_spec)
    valid_indices_spec: List[Tuple[int, DTensorSpec]] = [
        (i, a) for i, a in enumerate(multi_indices_spec) if a is not None
    ]

    # 1. All indices have to be sharded equally. Moreover, indices can be broadcast.
    #    Here, we piggyback on the pointwise sharding rule for indices.
    indices_out = pointwise_rule(
        OpSchema(
            op=op_schema.op,
            args_schema=tuple(v[1] for v in valid_indices_spec),
            kwargs_schema={},
        )
    )
    need_reshard_on_indices = indices_out.output_spec is None

    if not need_reshard_on_indices:
        # this means that our inputs are already sharded properly and we will use that as our indices_spec
        assert isinstance(indices_out.output_spec, DTensorSpec)
        indices_spec: DTensorSpec = indices_out.output_spec
    else:
        assert indices_out.schema_suggestions is not None
        valid_indices_suggestion = indices_out.schema_suggestions[0]
        for i, v in enumerate(valid_indices_suggestion.args_spec):
            multi_indices_spec[valid_indices_spec[i][0]] = v
        # we'll need to call pointwise_rule again to see what's our ideal indices_spec and then
        # use that to compute our ideal values_spec
        indices_output_spec = pointwise_rule(valid_indices_suggestion).output_spec
        assert isinstance(indices_output_spec, DTensorSpec)
        indices_spec = indices_output_spec

    lookup_dims = {v[0] for v in valid_indices_spec}

    need_reshard_on_values = tuple(
        (isinstance(vp, Shard) and (vp.dim in lookup_dims or isinstance(ip, Shard)))
        for vp, ip in zip(values_spec.placements, indices_spec.placements)
    )

    if not need_reshard_on_indices and not any(need_reshard_on_values):
        value_placements = values_spec.placements

        all_dims_consecutive = all(
            b[0] - a[0] == 1
            for b, a in zip(valid_indices_spec[1:], valid_indices_spec[:-1])
        )
        if all_dims_consecutive:
            # if all index vectors are consecutives, insert at the dimension of the first index
            insert_dim: int = valid_indices_spec[0][0]
        else:
            # else, insert on the first dimension
            insert_dim = 0

        def place(vp: Placement, ip: Placement) -> Placement:
            if isinstance(vp, Shard):
                return Shard(
                    vp.dim
                    if vp.dim < insert_dim
                    # accounts for the offset in output dimensions
                    else vp.dim
                    + indices_spec.ndim
                    - sum(1 if vp.dim > v[0] else 0 for v in valid_indices_spec)
                )
            if isinstance(ip, Shard):
                return Shard(ip.dim + insert_dim)
            # _Partial or Replicated
            return vp

        value_placements = tuple(
            place(vp, ip)
            for vp, ip in zip(values_spec.placements, indices_spec.placements)
        )
        result = OutputSharding(
            output_spec=DTensorSpec(
                mesh=values_spec.mesh,
                placements=value_placements,
            )
        )
        return result
    else:
        result = OutputSharding(
            output_spec=None,
            schema_suggestions=[
                OpSchema(
                    op=op_schema.op,
                    args_schema=(
                        DTensorSpec(
                            mesh=values_spec.mesh,
                            placements=tuple(
                                [
                                    Replicate() if need_reshard_on_values[i] else v
                                    for i, v in enumerate(values_spec.placements)
                                ]
                            ),
                            tensor_meta=values_spec.tensor_meta,
                        ),
                        multi_indices_spec,
                    ),
                    kwargs_schema=op_schema.kwargs_schema,
                )
            ],
        )
        return result


@register_prop_rule(
    aten.cat.default, schema_info=RuntimeSchemaInfo(1, needs_pytree=True)
)
def cat_rule(op_schema: OpSchema) -> OutputSharding:
    # torch.cat requires all tensors must either have the same shape (except
    # in the concatenating dimension) or be "empty". "Empty" here strictly means
    # tensor.shape is torch.Size([0]). When tensor.ndim > 1, it will be treated
    # as a non-empty tensor and the shape must match on non-cat dimensions.
    def is_empty(spec: DTensorSpec) -> bool:
        return list(spec.shape) == [0]

    # the first arg is a list of input tensor specs
    tensor_list_specs = cast(List[DTensorSpec], op_schema.args_schema[0])
    assert len(tensor_list_specs) > 0, "torch.cat expects a non-empty list of tensors"
    non_empty_specs = [spec for spec in tensor_list_specs if not is_empty(spec)]

    if len(non_empty_specs) == 0:
        # all tensors are empty, we can return any output sharding
        return OutputSharding(
            output_spec=DTensorSpec(
                mesh=tensor_list_specs[0].mesh,
                placements=tensor_list_specs[0].placements,
            )
        )

    assert all(
        spec.ndim == non_empty_specs[0].ndim for spec in non_empty_specs
    ), f"Expect all tensors to have same shape or empty, but got {tensor_list_specs}"
    assert all(
        spec.mesh == tensor_list_specs[0].mesh for spec in tensor_list_specs
    ), f"Expect all tensors to have same mesh, but got {tensor_list_specs}"

    # ndim will also be the result's ndim
    ndim = 1
    for spec in tensor_list_specs:
        ndim = max(ndim, spec.ndim)

    dim = 0  # default dim = 0
    if len(op_schema.args_schema) > 1:
        dim = cast(int, op_schema.args_schema[1])
    dim = normalize_dim(dim, ndim)

    # Make sure all tensors are replciated on cat dimension
    need_reshard = False
    tensor_list_specs_after: List[DTensorSpec] = []
    for spec in tensor_list_specs:
        if not is_empty(spec) and (
            is_tensor_dim_sharded(spec, dim=dim) or is_tensor_partial(spec)
        ):
            need_reshard = True
            tensor_list_specs_after.append(
                DTensorSpec(
                    mesh=spec.mesh,
                    placements=replicate_tensor_dim(spec.placements, dim=dim),
                    tensor_meta=spec.tensor_meta,
                )
            )
        else:
            tensor_list_specs_after.append(spec)

    tensor_list_specs = tensor_list_specs_after

    # align non-cat dimensions placements based on reshard cost
    non_empty_specs = [spec for spec in tensor_list_specs if not is_empty(spec)]
    mesh = non_empty_specs[0].mesh
    ndim = non_empty_specs[0].ndim
    new_placements: List[Placement] = []
    for mesh_dim in range(mesh.ndim):
        # compute the minimum cost of resharding on this mesh_dim
        if any(
            spec.placements[mesh_dim] != non_empty_specs[0].placements[mesh_dim]
            for spec in non_empty_specs
        ):
            # only reshard if there is a mismatch
            need_reshard = True
            reshard_cost = []
            for shard_dim in range(ndim):
                # compute the cost of resharding on this shard_dim
                cost: float = 0.0
                for spec in non_empty_specs:
                    global_shape = spec.shape
                    if global_shape[shard_dim] < mesh.size(mesh_dim):
                        # found one tensor where the shard_dim is smaller than
                        # mesh_dim. In this case, we cannot shard on this shard_dim,
                        # and hence set cost to infinity.
                        cost = +float("inf")
                    elif (
                        is_tensor_dim_sharded(spec, dim=shard_dim)
                        or prod(global_shape) == 0
                    ):
                        continue
                    else:
                        local_shape = compute_local_shape(
                            global_shape, spec.mesh, spec.placements
                        )
                        cost += prod(local_shape) * spec.mesh.size(mesh_dim)
                reshard_cost.append(cost)
            best_dim = reshard_cost.index(min(reshard_cost))
            new_placements.append(Shard(best_dim))
        else:
            # no mismatch, keep the original placement
            new_placements.append(non_empty_specs[0].placements[mesh_dim])

    if need_reshard:
        tensor_list_specs_after = []
        for spec in tensor_list_specs:
            if is_empty(spec):
                tensor_list_specs_after.append(spec)
            else:
                tensor_list_specs_after.append(
                    DTensorSpec(
                        mesh=spec.mesh,
                        placements=tuple(new_placements),
                        tensor_meta=spec.tensor_meta,
                    )
                )

        return OutputSharding(
            output_spec=None,
            schema_suggestions=[
                OpSchema(
                    op=op_schema.op,
                    args_schema=(
                        tuple(tensor_list_specs_after),
                        *op_schema.args_schema[1:],
                    ),
                    kwargs_schema=op_schema.kwargs_schema,
                ),
            ],
        )
    else:
        # at this point, the cat dim is not sharded,
        return OutputSharding(
            output_spec=DTensorSpec(
                mesh=non_empty_specs[0].mesh,
                placements=non_empty_specs[0].placements,
            ),
        )


@register_prop_rule(
    [aten.split.Tensor, aten.split_with_sizes.default], schema_info=RuntimeSchemaInfo(1)
)
def split_rule(op_schema: OpSchema) -> OutputSharding:
    output_spec_list: List[DTensorSpec] = []
    input_spec = cast(DTensorSpec, op_schema.args_schema[0])
    ndim = input_spec.ndim
    split_size_or_sections = op_schema.args_schema[1]
    dim = cast(int, op_schema.args_schema[2]) if len(op_schema.args_schema) > 2 else 0
    dim = normalize_dim(dim, ndim)

    # TODO: tensor to split cannot have _Partial
    # in its placements for now. Will need to
    # support in future.
    if input_spec.sums:
        raise NotImplementedError(
            f"splitting distributed tensor with "
            f"_Partial placement is not implemented!\n"
            f"DTensorSpec={input_spec}"
        )

    # TODO: just like slice op, split replicates before
    # splitting on a sharded dimension
    need_reshard = False
    if is_tensor_dim_sharded(input_spec, dim=dim):
        need_reshard = True
        input_spec = DTensorSpec(
            mesh=input_spec.mesh,
            placements=unshard_tensor_dim(input_spec.placements, dim=dim),
            tensor_meta=input_spec.tensor_meta,
        )

    if need_reshard:
        return OutputSharding(
            None,
            schema_suggestions=[
                OpSchema(
                    op=op_schema.op,
                    args_schema=(input_spec,) + op_schema.args_schema[1:],
                    kwargs_schema=op_schema.kwargs_schema,
                ),
            ],
        )

    def size_split(N, i):
        # Last chunk will be smaller if the tensor size N
        # along the given dimension dim is not divisible by i.
        assert i > 0
        return [i] * (N // i) + ([N % i] if N % i != 0 else [])

    output_size_list = (
        size_split(input_spec.shape[dim], split_size_or_sections)
        if isinstance(split_size_or_sections, int)
        else split_size_or_sections
    )
    output_spec_list = [
        DTensorSpec(
            mesh=input_spec.mesh,
            placements=input_spec.placements,
        )
        for _ in range(len(output_size_list))
    ]
    return OutputSharding(output_spec_list)
