import itertools
from dataclasses import dataclass

from torch.distributed.device_mesh import DeviceMesh
from torch.distributed.tensor._dtensor_spec import DTensorSpec
from torch.distributed.tensor._op_schema import OpStrategy, PlacementStrategy
from torch.distributed.tensor.placement_types import (
    Partial,
    Placement,
    Replicate,
    Shard,
)


@dataclass
class EinsumDims:
    contracting_dims: list[str]
    batch_dims: list[str]
    lhs_out_only_dims: list[str]
    rhs_out_only_dims: list[str]

    @classmethod
    def parse_equation(cls, equation: str) -> tuple[list[str], str]:
        # parse einop equation and extract arg specs
        """
        Parse the einsum equation str to input dim chars and output dim char
        """
        inputs, outputs = equation.split("->")
        input_dims, output_dims = inputs.split(","), outputs.split(",")

        # NOTE: only support at most two inputs, and single output
        # extend to support more inputs if needed in future
        assert len(input_dims) <= 2, "Only support at most two inputs"
        assert len(output_dims) == 1, "Only support single output"
        output_dim = output_dims[0]
        return input_dims, output_dim

    @classmethod
    def parse_dims(cls, input_dims: list[str], output_dim: str) -> "EinsumDims":
        """
        Parse the dims and extract the contracting, batch, and free dimensions
        for the left and right hand sides.
        """
        dim_char_set: set[str] = set()
        for input_dim in input_dims:
            dim_char_set.update(input_dim)

        # get a determinisitc order of all dim chars
        all_dim_chars = sorted(dim_char_set)

        # parse input and output dimensions
        lhs_out_only_dims, rhs_out_only_dims = [], []
        batch_dims, contracting_dims = [], []

        for dim_char in all_dim_chars:
            if dim_char not in output_dim:
                contracting_dims.append(dim_char)
            else:
                is_batch_dim = True
                for input_dim in input_dims:
                    is_batch_dim = is_batch_dim and dim_char in input_dim

                if is_batch_dim:
                    batch_dims.append(dim_char)
                else:
                    assert len(input_dims) == 2, (
                        "free dimension only supported for two inputs!"
                    )
                    lhs, rhs = input_dims
                    if dim_char in lhs:
                        lhs_out_only_dims.append(dim_char)
                    elif dim_char in rhs:
                        rhs_out_only_dims.append(dim_char)
                    else:
                        raise RuntimeError("Invalid dimension character")

        return cls(
            contracting_dims=contracting_dims,
            batch_dims=batch_dims,
            lhs_out_only_dims=lhs_out_only_dims,
            rhs_out_only_dims=rhs_out_only_dims,
        )


def gen_einsum_strategies(
    equation: str,
    mesh: DeviceMesh,
    *,
    linearity: bool = False,
) -> OpStrategy:
    """
    Generate a strategy list for the ops that follow einsum style notation.
    """
    # parse einop equation and extract dims
    input_dims, output_dim = EinsumDims.parse_equation(equation)
    edims = EinsumDims.parse_dims(input_dims, output_dim)

    all_mesh_dim_strategies = []

    # generate strategies for each mesh dim
    for mesh_dim in range(mesh.ndim):
        mesh_dim_strategies = []

        # placement list stores placements of [output, input1, input2, ...]
        # first we always have replicate all for inputs and output
        placement_list: list[Placement] = [Replicate()] * (len(input_dims) + 1)
        mesh_dim_strategies.append(placement_list)

        # split batch dim
        for batch_dim in edims.batch_dims:
            output_batch_dim = output_dim.index(batch_dim)
            placement_list = [Shard(output_batch_dim)]
            for input_dim in input_dims:
                input_batch_dim = input_dim.index(batch_dim)
                placement_list.append(Shard(input_batch_dim))

            mesh_dim_strategies.append(placement_list)

        # split contracting dim
        for contracting_dim in edims.contracting_dims:
            placement_list = [Partial()]
            for input_dim in input_dims:
                input_contracting_dim = input_dim.index(contracting_dim)
                placement_list.append(Shard(input_contracting_dim))

            mesh_dim_strategies.append(placement_list)

        # split lhs free dim
        for lhs_dim in edims.lhs_out_only_dims:
            lhs_free_dim = output_dim.index(lhs_dim)
            # this means split the lhs input and output
            # i.e. S(0), R -> S(0)
            lhs_placement_list: list[Placement] = [
                Shard(lhs_free_dim),
                Shard(lhs_free_dim),
                Replicate(),
            ]
            mesh_dim_strategies.append(lhs_placement_list)

        # split rhs free dim
        for rhs_dim in edims.rhs_out_only_dims:
            rhs_free_dim = output_dim.index(rhs_dim)
            rhs_placement_list: list[Placement] = [
                Shard(rhs_free_dim),
                Replicate(),
                Shard(rhs_free_dim),
            ]
            mesh_dim_strategies.append(rhs_placement_list)

        # linearity strategy
        if linearity:
            linearity_placement_list: list[Placement] = [Partial()]
            for input_dim in input_dims:
                linearity_placement_list.append(Partial())
            mesh_dim_strategies.append(linearity_placement_list)

        all_mesh_dim_strategies.append(mesh_dim_strategies)

    # generate strategies for entire mesh
    strategy_combs = itertools.product(*all_mesh_dim_strategies)

    # TODO: filter out invalid strategies, at this point we generate
    # all possible strategies without considering the whether the tensor
    # dim could be sharded or not, we would need to filter out invalid
    # strategies base on the actual tensor shape
    # (i.e. for Shard, tensor dim size must > mesh size)
    all_strategies = []
    for strategy_comb in strategy_combs:
        spec_list = [DTensorSpec(mesh, tuple(specs)) for specs in zip(*strategy_comb)]
        strat = PlacementStrategy(output_specs=spec_list[0], input_specs=spec_list[1:])
        all_strategies.append(strat)

    return OpStrategy(all_strategies)
