#
# Licensed to the Apache Software Foundation (ASF) under one or more
# contributor license agreements.  See the NOTICE file distributed with
# this work for additional information regarding copyright ownership.
# The ASF licenses this file to You under the Apache License, Version 2.0
# (the "License"); you may not use this file except in compliance with
# the License.  You may obtain a copy of the License at
#
#    http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#

import collections
import datetime
import functools
import json
import logging
import os
import pprint
import re
from collections.abc import Iterable
from collections.abc import Mapping
from typing import Any

import jinja2
import yaml

import apache_beam as beam
from apache_beam.io.filesystems import FileSystems
from apache_beam.options.pipeline_options import GoogleCloudOptions
from apache_beam.transforms.fully_qualified_named_transform import FullyQualifiedNamedTransform
from apache_beam.typehints import schemas
from apache_beam.typehints import typehints
from apache_beam.yaml import json_utils
from apache_beam.yaml import yaml_provider
from apache_beam.yaml import yaml_utils
from apache_beam.yaml.yaml_combine import normalize_combine
from apache_beam.yaml.yaml_mapping import Validate
from apache_beam.yaml.yaml_mapping import normalize_mapping
from apache_beam.yaml.yaml_mapping import validate_generic_expressions
from apache_beam.yaml.yaml_utils import SafeLineLoader

__all__ = ["YamlTransform"]

_LOGGER = logging.getLogger(__name__)
yaml_provider.fix_pycallable()

try:
  import jsonschema
except ImportError:
  jsonschema = None


@functools.lru_cache
def pipeline_schema(strictness):
  with open(yaml_utils.locate_data_file('pipeline.schema.yaml')) as yaml_file:
    pipeline_schema = yaml.safe_load(yaml_file)
  if strictness == 'per_transform':
    transform_schemas_path = yaml_utils.locate_data_file(
        'transforms.schema.yaml')
    if not os.path.exists(transform_schemas_path):
      raise RuntimeError(
          "Please run "
          "python -m apache_beam.yaml.generate_yaml_docs "
          f"--schema_file='{transform_schemas_path}' "
          "to run with transform-specific validation.")
    with open(transform_schemas_path) as fin:
      pipeline_schema['$defs']['transform']['allOf'].extend(yaml.safe_load(fin))
  return pipeline_schema


def _closest_line(o, path):
  best_line = SafeLineLoader.get_line(o)
  for step in path:
    o = o[step]
    maybe_line = SafeLineLoader.get_line(o)
    if maybe_line != 'unknown':
      best_line = maybe_line
  return best_line


def validate_against_schema(pipeline, strictness):
  try:
    jsonschema.validate(pipeline, pipeline_schema(strictness))
  except jsonschema.ValidationError as exn:
    exn.message += f" around line {_closest_line(pipeline, exn.path)}"
    # validation message for chain-type transform
    if (exn.schema_path[-1] == 'not' and
        exn.schema_path[-2] in ['input', 'output']):
      exn.message = (
          f"'{exn.schema_path[-2]}' should not be used "
          "along with 'chain' type transforms. " + exn.message)
    raise exn


def memoize_method(func):
  def wrapper(self, *args):
    if not hasattr(self, '_cache'):
      self._cache = {}
    key = func.__name__, args
    if key not in self._cache:
      self._cache[key] = func(self, *args)
    return self._cache[key]

  return wrapper


def only_element(xs):
  x, = xs
  return x


# These allow a user to explicitly pass no input to a transform (i.e. use it
# as a root transform) without an error even if the transform is not known to
# handle it.
def explicitly_empty():
  return {'__explicitly_empty__': None}


def is_explicitly_empty(io):
  return io == explicitly_empty()


def is_empty(io):
  return not io or is_explicitly_empty(io)


def empty_if_explicitly_empty(io):
  if is_explicitly_empty(io):
    return {}
  else:
    return io


class LightweightScope(object):
  def __init__(self, transforms):
    self._transforms = transforms
    self._transforms_by_uuid = {t['__uuid__']: t for t in self._transforms}
    self._uuid_by_name = collections.defaultdict(set)
    for spec in self._transforms:
      if 'name' in spec:
        self._uuid_by_name[spec['name']].add(spec['__uuid__'])
      if 'type' in spec:
        self._uuid_by_name[spec['type']].add(spec['__uuid__'])

  def get_transform_id_and_output_name(self, name):
    if '.' in name:
      transform_name, output = name.rsplit('.', 1)
    else:
      transform_name, output = name, None
    return self.get_transform_id(transform_name), output

  def get_transform_id(self, transform_name):
    if transform_name in self._transforms_by_uuid:
      return transform_name
    else:
      candidates = self._uuid_by_name[transform_name]
      if not candidates:
        raise ValueError(
            f'Unknown transform at line '
            f'{SafeLineLoader.get_line(transform_name)}: {transform_name}')
      elif len(candidates) > 1:
        raise ValueError(
            f'Ambiguous transform at line '
            f'{SafeLineLoader.get_line(transform_name)}: {transform_name}')
      else:
        return only_element(candidates)

  def get_transform_spec(self, transform_name_or_id):
    return self._transforms_by_uuid[self.get_transform_id(transform_name_or_id)]


class Scope(LightweightScope):
  """To look up PCollections (typically outputs of prior transforms) by name."""
  def __init__(
      self,
      root,
      inputs: Mapping[str, Any],
      transforms: Iterable[dict],
      providers: Mapping[str, Iterable[yaml_provider.Provider]],
      input_providers: Iterable[yaml_provider.Provider]):
    super().__init__(transforms)
    self.root = root
    self._inputs = inputs
    self.providers = providers
    self._seen_names: set[str] = set()
    self.input_providers = input_providers
    self._all_followers = None

  def followers(self, transform_name):
    if self._all_followers is None:
      self._all_followers = collections.defaultdict(list)
      # TODO(yaml): Also trace through outputs and composites.
      for transform in self._transforms:
        if transform['type'] != 'composite':
          for input in empty_if_explicitly_empty(transform['input']).values():
            if input not in self._inputs:
              transform_id, _ = self.get_transform_id_and_output_name(input)
              self._all_followers[transform_id].append(transform['__uuid__'])
    return self._all_followers[self.get_transform_id(transform_name)]

  def compute_all(self):
    for transform_id in self._transforms_by_uuid.keys():
      self.compute_outputs(transform_id)

  def get_pcollection(self, name):
    if name in self._inputs:
      return self._inputs[name]
    elif '.' in name:
      transform, output = name.rsplit('.', 1)
      outputs = self.get_outputs(transform)
      if output in outputs:
        return outputs[output]
      elif len(outputs) == 1 and outputs[next(iter(outputs))].tag == output:
        return outputs[next(iter(outputs))]
      else:
        raise ValueError(
            f'Unknown output {repr(output)} '
            f'at line {SafeLineLoader.get_line(name)}: '
            f'{transform} only has outputs {list(outputs.keys())}')
    else:
      outputs = self.get_outputs(name)
      if len(outputs) == 1:
        return only_element(outputs.values())
      else:
        error_output = self._transforms_by_uuid[self.get_transform_id(
            name)]['config'].get('error_handling', {}).get('output')
        if error_output and error_output in outputs and len(outputs) == 2:
          return next(
              output for tag, output in outputs.items() if tag != error_output)
        raise ValueError(
            f'Ambiguous output at line {SafeLineLoader.get_line(name)}: '
            f'{name} has outputs {list(outputs.keys())}')

  def get_outputs(self, transform_name):
    return self.compute_outputs(self.get_transform_id(transform_name))

  @memoize_method
  def compute_outputs(self, transform_id):
    return expand_transform(self._transforms_by_uuid[transform_id], self)

  def best_provider(
      self, t, input_providers: yaml_provider.Iterable[yaml_provider.Provider]):
    if isinstance(t, dict):
      spec = t
    else:
      spec = self._transforms_by_uuid[self.get_transform_id(t)]
    possible_providers = []
    unavailable_provider_messages = []
    for p in self.providers[spec['type']]:
      is_available = p.available()
      if is_available:
        possible_providers.append(p)
      else:
        reason = getattr(is_available, 'reason', 'no reason given')
        unavailable_provider_messages.append(
            f'{p.__class__.__name__} ({reason})')
    if not possible_providers:
      if unavailable_provider_messages:
        unavailable_provider_message = (
            '\nThe following providers were found but not available: ' +
            '\n'.join(unavailable_provider_messages))
      else:
        unavailable_provider_message = ''
      raise ValueError(
          'No available provider for type %r at %s%s' %
          (spec['type'], identify_object(spec), unavailable_provider_message))
    # From here on, we have the invariant that possible_providers is not empty.

    # Only one possible provider, no need to rank further.
    if len(possible_providers) == 1:
      return possible_providers[0]

    def best_matches(
        possible_providers: Iterable[yaml_provider.Provider],
        adjacent_provider_options: Iterable[Iterable[yaml_provider.Provider]]
    ) -> list[yaml_provider.Provider]:
      """Given a set of possible providers, and a set of providers for each
      adjacent transform, returns the top possible providers as ranked by
      affinity to the adjacent transforms' providers.
      """
      providers_by_score = collections.defaultdict(list)
      for p in possible_providers:
        # The sum of the affinity of the best provider
        # for each adjacent transform.
        providers_by_score[sum(
            max(p.affinity(ap) for ap in apo)
            for apo in adjacent_provider_options)].append(p)
      return providers_by_score[max(providers_by_score.keys())]

    # If there are any inputs, prefer to match them.
    if input_providers:
      possible_providers = best_matches(
          possible_providers, [[p] for p in input_providers])

    # Without __uuid__ we can't find downstream operations.
    if '__uuid__' not in spec:
      return possible_providers[0]

    # Match against downstream transforms, continuing until there is no tie
    # or we run out of downstream transforms.
    if len(possible_providers) > 1:
      adjacent_transforms = list(self.followers(spec['__uuid__']))
      while adjacent_transforms:
        # This is a list of all possible providers for each adjacent transform.
        adjacent_provider_options = [[
            p for p in self.providers[self._transforms_by_uuid[t]['type']]
            if p.available()
        ] for t in adjacent_transforms]
        if any(not apo for apo in adjacent_provider_options):
          # One of the transforms had no available providers.
          # We will throw an error later, doesn't matter what we return.
          break
        # Filter down the set of possible providers to the best ones.
        possible_providers = best_matches(
            possible_providers, adjacent_provider_options)
        # If we are down to one option, no need to go further.
        if len(possible_providers) == 1:
          break
        # Go downstream one more step.
        adjacent_transforms = sum(
            [list(self.followers(t)) for t in adjacent_transforms], [])

    return possible_providers[0]

  # A method on scope as providers may be scoped...
  def create_ptransform(self, spec, input_pcolls):
    def maybe_with_resource_hints(transform):
      if 'resource_hints' in spec:
        return transform.with_resource_hints(
            **SafeLineLoader.strip_metadata(spec['resource_hints']))
      else:
        return transform

    if 'type' not in spec:
      raise ValueError(f'Missing transform type: {identify_object(spec)}')

    if spec['type'] == 'composite':

      class _CompositeTransformStub(beam.PTransform):
        @staticmethod
        def expand(pcolls):
          if isinstance(pcolls, beam.PCollection):
            pcolls = {'input': pcolls}
          elif isinstance(pcolls, beam.pvalue.PBegin):
            pcolls = {}

          inner_scope = Scope(
              self.root,
              pcolls,
              spec['transforms'],
              self.providers,
              self.input_providers)
          inner_scope.compute_all()
          if '__implicit_outputs__' in spec['output']:
            return inner_scope.get_outputs(
                spec['output']['__implicit_outputs__'])
          else:
            return {
                key: inner_scope.get_pcollection(value)
                for (key, value) in spec['output'].items()
            }

      return maybe_with_resource_hints(_CompositeTransformStub())

    if spec['type'] not in self.providers:
      raise ValueError(
          'Unknown transform type %r at %s' %
          (spec['type'], identify_object(spec)))

    # TODO(yaml): Perhaps we can do better than a greedy choice here.
    # TODO(yaml): Figure out why this is needed.
    providers_by_input = {k: v for k, v in self.input_providers.items()}
    input_providers = [
        providers_by_input[pcoll] for pcoll in input_pcolls
        if pcoll in providers_by_input
    ]
    provider = self.best_provider(spec, input_providers)
    extra_dependencies, spec = extract_extra_dependencies(spec)
    if extra_dependencies:
      provider = provider.with_extra_dependencies(frozenset(extra_dependencies))

    config = SafeLineLoader.strip_metadata(spec.get('config', {}))
    if not isinstance(config, dict):
      raise ValueError(
          'Config for transform at %s must be a mapping.' %
          identify_object(spec))

    if (not input_pcolls and not is_explicitly_empty(spec.get('input', {})) and
        provider.requires_inputs(spec['type'], config)):
      raise ValueError(
          f'Missing inputs for transform at {identify_object(spec)}')

    try:
      if spec['type'].endswith('-generic'):
        # Centralize the validation rather than require every implementation
        # to do it.
        validate_generic_expressions(
            spec['type'].rsplit('-', 1)[0], config, input_pcolls)

      # pylint: disable=undefined-loop-variable
      ptransform = maybe_with_resource_hints(
          provider.create_transform(
              spec['type'],
              config,
              lambda config, input_pcolls=input_pcolls: self.create_ptransform(
                  config, input_pcolls)))
      # TODO(robertwb): Should we have a better API for adding annotations
      # than this?
      annotations = {
          **{
              'yaml_type': spec['type'],
              'yaml_args': json.dumps(config),
              'yaml_provider': json.dumps(provider.to_json())
          },
          **ptransform.annotations()
      }
      ptransform.annotations = lambda: annotations
      original_expand = ptransform.expand

      def recording_expand(pvalue):
        result = original_expand(pvalue)

        def record_providers(pvalueish):
          if isinstance(pvalueish, (tuple, list)):
            for p in pvalueish:
              record_providers(p)
          elif isinstance(pvalueish, dict):
            for p in pvalueish.values():
              record_providers(p)
          elif isinstance(pvalueish, beam.PCollection):
            if pvalueish not in self.input_providers:
              self.input_providers[pvalueish] = provider

        record_providers(result)
        return result

      ptransform.expand = recording_expand
      return ptransform
    except Exception as exn:
      if isinstance(exn, TypeError):
        # Create a slightly more generic error message for argument errors.
        msg = str(exn).replace('positional', '').replace('keyword', '')
        msg = re.sub(r'\S+lambda\S+', '', msg)
        msg = re.sub('  +', ' ', msg).strip()
      else:
        msg = str(exn)
      raise ValueError(
          f'Invalid transform specification at {identify_object(spec)}: {msg}'
      ) from exn

  def unique_name(self, spec, ptransform, strictness=0):
    if 'name' in spec:
      name = spec['name']
      strictness += 1
    elif ('ExternalTransform' not in ptransform.label and
          not ptransform.label.startswith('_')):
      # The label may have interesting information.
      name = ptransform.label
    else:
      name = spec['type']
    if name in self._seen_names:
      if strictness >= 2:
        raise ValueError(f'Duplicate name at {identify_object(spec)}: {name}')
      else:
        name = f'{name}@{SafeLineLoader.get_line(spec)}'
    self._seen_names.add(name)
    return name


def expand_transform(spec, scope):
  if 'type' not in spec:
    raise TypeError(
        f'Missing type parameter for transform at {identify_object(spec)}')
  type = spec['type']
  if type == 'composite':
    return expand_composite_transform(spec, scope)
  else:
    return expand_leaf_transform(spec, scope)


def expand_leaf_transform(spec, scope):
  spec = spec.copy()

  # Check for optional output_schema to verify on.
  # The idea is to pass this output_schema config to the ValidateWithSchema
  # transform.
  output_schema_spec = {}
  if 'output_schema' in spec.get('config', {}):
    output_schema_spec = spec.get('config').pop('output_schema')

  spec = normalize_inputs_outputs(spec)
  inputs_dict = {
      key: scope.get_pcollection(value)
      for (key, value) in empty_if_explicitly_empty(spec['input']).items()
  }
  input_type = spec.get('input_type', 'default')
  if input_type == 'list':
    inputs = tuple(inputs_dict.values())
  elif input_type == 'map':
    inputs = inputs_dict
  else:
    if len(inputs_dict) == 0:
      inputs = scope.root
    elif len(inputs_dict) == 1:
      inputs = next(iter(inputs_dict.values()))
    else:
      inputs = inputs_dict
  _LOGGER.info("Expanding %s ", identify_object(spec))
  ptransform = scope.create_ptransform(spec, inputs_dict.values())
  try:
    # TODO: Move validation to construction?
    with FullyQualifiedNamedTransform.with_filter('*'):
      outputs = inputs | scope.unique_name(spec, ptransform) >> ptransform
  except Exception as exn:
    raise ValueError(
        f"Error applying transform {identify_object(spec)}: {exn}") from exn

  # Optional output_schema was found, so lets expand on that before returning.
  if output_schema_spec:
    error_handling_spec = {}
    # Obtain original transform error_handling_spec, so that all validate
    # schema errors use that.
    if 'error_handling' in spec.get('config', None):
      error_handling_spec = spec.get('config').get('error_handling', {})

    outputs = expand_output_schema_transform(
        spec=output_schema_spec,
        outputs=outputs,
        error_handling_spec=error_handling_spec)

  if isinstance(outputs, dict):
    # TODO: Handle (or at least reject) nested case.
    return outputs
  elif isinstance(outputs, (tuple, list)):
    return {f'out{ix}': pcoll for (ix, pcoll) in enumerate(outputs)}
  elif isinstance(outputs, beam.PCollection):
    return {'out': outputs}
  elif outputs is None or isinstance(outputs, beam.pvalue.PDone):
    return {}
  else:
    raise ValueError(
        f'Transform {identify_object(spec)} returned an unexpected type '
        f'{type(outputs)}')


def expand_output_schema_transform(spec, outputs, error_handling_spec):
  """Applies a `Validate` transform to the output of another transform.

  This function is called when an `output_schema` is defined on a transform.
  It wraps the original transform's output(s) with a `Validate` transform
  to ensure the data conforms to the specified schema.

  If the original transform has error handling configured, validation errors
  will be routed to the specified error output. If not, validation failures
  will cause the pipeline to fail.

  Args:
    spec (dict): The `output_schema` specification from the YAML config.
    outputs (beam.PCollection or dict[str, beam.PCollection]): The output(s)
      from the transform to be validated.
    error_handling_spec (dict): The `error_handling` configuration from the
      original transform.

  Returns:
    The validated PCollection(s). If error handling is enabled, this will be a
    dictionary containing the 'good' output and any error outputs.

  Raises:
    ValueError: If `error_handling` is incorrectly specified within the
      `output_schema` spec itself, or if the main output of a multi-output
      transform cannot be determined.
  """
  if 'error_handling' in spec:
    raise ValueError(
        'error_handling config is not supported directly in '
        'the output_schema. Please use error_handling config in '
        'the transform, if possible, or use ValidateWithSchema transform '
        'instead.')

  # Strip metadata such as __line__ and __uuid__ as these will interfere with
  # the validation downstream.
  clean_schema = SafeLineLoader.strip_metadata(spec)

  # If no error handling is specified for the main transform, warn the user
  # that the pipeline may fail if any output data fails the output schema
  # validation.
  if not error_handling_spec:
    _LOGGER.warning("Output_schema config is attached to a transform that has "\
    "no error_handling config specified. Any failures validating on output" \
    "schema will fail the pipeline unless the user specifies an" \
    "error_handling config on a capable transform. Alternatively, you can " \
    "remove the output_schema config on this transform and add a " \
    "ValidateWithSchema transform with separate error handling downstream of " \
    "the current transform.")

  # The transform produced outputs with a single beam.PCollection
  if isinstance(outputs, beam.PCollection):
    outputs = _enforce_schema(
        outputs, 'EnforceOutputSchema', error_handling_spec, clean_schema)
    if isinstance(outputs, dict):
      main_tag = error_handling_spec.get('main_tag', 'good')
      main_output = outputs.pop(main_tag)
      if error_handling_spec:
        error_output_tag = error_handling_spec.get('output')
        if error_output_tag in outputs:
          return {
              'output': main_output,
              error_output_tag: outputs.pop(error_output_tag)
          }
      return main_output

  # The transform produced outputs with many named PCollections and need to
  # determine which PCollection should be validated on.
  elif isinstance(outputs, dict):
    main_output_key = get_main_output_key(spec, outputs, error_handling_spec)

    validation_result = _enforce_schema(
        outputs[main_output_key],
        f'EnforceOutputSchema_{main_output_key}',
        error_handling_spec,
        clean_schema)
    outputs = _integrate_validation_results(
        outputs, validation_result, main_output_key, error_handling_spec)

  return outputs


def get_main_output_key(spec, outputs, error_handling_spec):
  """Determines the main output key from a dictionary of PCollections.

  This is used to identify which output of a multi-output transform should be
  validated against an `output_schema`.

  The main output is determined using the following precedence:
  1. An output with the key 'output'.
  2. An output with the key 'good'.
  3. The single output if there is only one.

  Args:
    spec: The transform specification, used for creating informative error
      messages.
    outputs: A dictionary mapping output tags to their corresponding
      PCollections.
    error_handling_spec (dict): The `error_handling` configuration from the
      original transform.

  Returns:
    The key of the main output PCollection.

  Raises:
    ValueError: If a main output cannot be determined because there are
      multiple outputs and none are named 'output' or 'good'.
  """
  main_output_key = 'output'
  if main_output_key not in outputs:
    if 'good' in outputs:
      main_output_key = 'good'
    elif len(outputs) == 1:
      main_output_key = next(iter(outputs.keys()))
    else:
      raise ValueError(
          f"Transform {identify_object(spec)} has outputs "
          f"{list(outputs.keys())}, but none are named 'output' or 'good'. To "
          "apply an 'output_schema', please ensure the transform has exactly "
          "one output, or that the main output is named 'output' or 'good'.")

  if len(outputs) >= 3 or \
    (len(outputs) == 2 and error_handling_spec.get('output') not in outputs):
    _LOGGER.warning(
        "There are currently %s outputs: %s. Only the main output will be "
        "validated.",
        len(outputs),
        outputs)

  return main_output_key


def _integrate_validation_results(
    outputs, validation_result, main_output_key, error_handling_spec):
  """
  Integrates the results of a validation transform back into the outputs of
  the original transform.

  This function handles merging the "good" and "bad" outputs from a
  `Validate` transform with the existing outputs of the transform that was
  validated.

  Args:
    outputs: The original dictionary of output PCollections from the transform.
    validation_result: The output of the `Validate` transform. This can be a
      single PCollection (if all elements passed) or a dictionary of
      PCollections (if error handling was enabled for validation).
    main_output_key: The key in the `outputs` dictionary corresponding to the
      PCollection that was validated.
    error_handling_spec: The error handling configuration of the original
      transform.

  Returns:
    The updated dictionary of output PCollections, with validation results
    integrated.

  Raises:
    ValueError: If the validation transform produces unexpected outputs.
  """
  if not isinstance(validation_result, dict):
    outputs[main_output_key] = validation_result
    return outputs

  # The main output from validation is the good output.
  main_tag = error_handling_spec.get('main_tag', 'good')
  outputs[main_output_key] = validation_result.pop(main_tag)

  if error_handling_spec:
    error_output_tag = error_handling_spec['output']
    if error_output_tag in validation_result:
      schema_error_pcoll = validation_result.pop(error_output_tag)
      # The original transform also had an error output. Merge them.
      outputs[error_output_tag] = (
          (outputs[error_output_tag], schema_error_pcoll)
          | f'FlattenErrors_{main_output_key}' >> beam.Flatten())

    # There should be no other outputs from validation.
    if validation_result:
      raise ValueError(
          "Unexpected outputs from validation: "
          f"{list(validation_result.keys())}")

  return outputs


def _enforce_schema(pcoll, label, error_handling_spec, clean_schema):
  """Applies schema to PCollection elements if necessary, then validates.

  This function ensures that the input PCollection conforms to a specified
  schema. If the PCollection is schemaless (i.e., its element_type is Any),
  it attempts to convert its elements into schema-aware `beam.Row` objects
  based on the provided `clean_schema`. After ensuring the PCollection has
  a defined schema, it applies a `Validate` transform to perform the actual
  schema validation.

  Args:
    pcoll: The input PCollection to be schema-enforced and validated.
    label: A string label to be used for the Beam transforms created within this
    function.
    error_handling_spec: A dictionary specifying how to handle validation
    errors.
    clean_schema: A dictionary representing the schema to enforce and validate
    against.

  Returns:
    A PCollection (or PCollectionTuple if error handling is enabled) resulting
    from the `Validate` transform.
  """
  if pcoll.element_type == typehints.Any:
    _LOGGER.info(
        "PCollection for %s has no schema (element_type=Any). "
        "Converting elements to beam.Row based on provided output_schema.",
        label)
    try:
      # Attempt to confer the schemaless elements into schema-aware beam.Row
      # objects
      beam_schema = json_utils.json_schema_to_beam_schema(clean_schema)
      row_type_constraint = schemas.named_tuple_from_schema(beam_schema)

      def to_row(element):
        """
        Convert a single element into the row type constraint type.
        """
        if isinstance(element, dict):
          return row_type_constraint(**element)
        elif hasattr(element, '_asdict'):  # Handle NamedTuple, beam.Row
          return row_type_constraint(**element._asdict())
        else:
          raise TypeError(
              f"Cannot convert element of type {type(element)} to beam.Row "
              f"for validation in {label}. Element: {element}")

      pcoll = pcoll | f'{label}_ConvertToRow' >> beam.Map(
          to_row).with_output_types(row_type_constraint)
    except Exception as e:
      raise ValueError(
          f"Failed to prepare schemaless PCollection for \
            validation in {label}: {e}") from e

  # Add Validation step downstream of current transform
  return pcoll | label >> Validate(
      schema=clean_schema, error_handling=error_handling_spec)


def expand_composite_transform(spec, scope):
  spec = normalize_inputs_outputs(normalize_source_sink(spec))

  original_transforms = spec['transforms']
  # Check if any transform has a NON-EMPTY explicit input or output.
  # Note: {} (empty dict) means "no explicit input specified" and should
  # NOT count as having explicit io.
  # However, if the composite has no input, we can't do implicit chaining.
  has_explicit_io = any(
      io is not None and not is_empty(t.get(io, {}))
      for t in original_transforms for io in ('input', 'output'))

  # If the composite has no input, we can't do implicit chaining
  composite_has_input = not is_empty(spec.get('input', {}))

  # Only do implicit chaining if:
  # 1. No transform has explicit io, AND
  # 2. The composite has an input to chain from
  if not has_explicit_io and composite_has_input:
    new_transforms = []
    for ix, transform in enumerate(original_transforms):
      transform = dict(transform)
      if ix == 0:
        composite_input = spec.get('input', {})
        if is_explicitly_empty(composite_input):
          transform['input'] = composite_input
        elif is_empty(composite_input):
          # No explicit input - the composite input IS the pipeline input.
          # Reference the 'input' key from the Scope's inputs.
          transform['input'] = 'input'
        else:
          transform['input'] = {key: key for key in composite_input.keys()}
      else:
        transform['input'] = new_transforms[-1]['__uuid__']
      new_transforms.append(transform)

    if new_transforms:
      spec = dict(spec, transforms=new_transforms)
      # Check if output is empty, not just present (normalization sets it to {})
      if is_empty(spec.get('output', {})):
        spec['output'] = {
            '__implicit_outputs__': new_transforms[-1]['__uuid__']
        }

  # Compute the inputs for the inner scope.
  # If the composite has an empty input dict ({}), it means the composite
  # should use the parent scope's inputs directly.
  composite_input = spec.get('input', {})

  if is_empty(composite_input):
    # No explicit input - use the parent scope's inputs directly
    inner_scope_inputs = dict(scope._inputs)
  else:
    # The composite has explicit input references
    # They can reference either:
    # 1. A parent scope input (e.g., 'input' key in scope._inputs)
    # 2. A transform output (e.g., 'uuid' -> the output of a transform)
    inner_scope_inputs = {}
    for key, value in composite_input.items():
      if isinstance(value, str) and value in scope._inputs:
        # Reference to a parent scope input
        inner_scope_inputs[key] = scope._inputs[value]
      else:
        # Reference to a transform output
        inner_scope_inputs[key] = scope.get_pcollection(value)

  inner_scope = Scope(
      scope.root,
      inner_scope_inputs,
      spec['transforms'],
      # TODO(robertwb): Are scoped providers ever used? Worth supporting?
      yaml_provider.merge_providers(
          yaml_provider.parse_providers('', spec.get('providers', [])),
          scope.providers),
      scope.input_providers)

  class CompositePTransform(beam.PTransform):
    @staticmethod
    def expand(inputs):
      inner_scope.compute_all()
      if '__implicit_outputs__' in spec['output']:
        result = inner_scope.get_outputs(spec['output']['__implicit_outputs__'])
        return result
      else:
        return {
            key: inner_scope.get_pcollection(value)
            for (key, value) in spec['output'].items()
        }

  transform = CompositePTransform()
  if 'resource_hints' in spec:
    transform = transform.with_resource_hints(
        **SafeLineLoader.strip_metadata(spec['resource_hints']))

  # Always set a name for the composite to ensure proper return value
  if 'name' not in spec:
    spec['name'] = 'Composite'
  if spec['name'] is None:  # top-level pipeline, don't nest
    return transform.expand(None)
  else:
    _LOGGER.info("Expanding %s ", identify_object(spec))
    # When the input references a scope input (not a transform output),
    # we need to use the scope's inputs directly
    input_dict = {}
    for key, value in empty_if_explicitly_empty(spec['input']).items():
      if isinstance(value, str) and value in scope._inputs:
        # Reference to a scope input
        input_dict[key] = scope._inputs[value]
      else:
        # Reference to a transform output
        input_dict[key] = scope.get_pcollection(value)
    return (input_dict or
            scope.root) | scope.unique_name(spec, None) >> transform


def expand_chain_transform(spec, scope):
  return expand_composite_transform(chain_as_composite(spec), scope)


def chain_as_composite(spec):
  def is_not_output_of_last_transform(new_transforms, value):
    return (
        ('name' in new_transforms[-1] and
         value != new_transforms[-1]['name']) or
        ('type' in new_transforms[-1] and value != new_transforms[-1]['type']))

  # A chain is simply a composite transform where all inputs and outputs
  # are implicit.
  spec = normalize_source_sink(spec)
  if 'transforms' not in spec:
    raise TypeError(
        f"Chain at {identify_object(spec)} missing transforms property.")
  has_explicit_outputs = 'output' in spec
  composite_spec = dict(normalize_inputs_outputs(tag_explicit_inputs(spec)))
  new_transforms = []
  for ix, transform in enumerate(composite_spec['transforms']):
    transform = dict(transform)
    if any(io in transform for io in ('input', 'output')):
      if (ix == 0 and 'input' in transform and 'output' not in transform and
          is_explicitly_empty(transform['input'])):
        # This is OK as source clause sets an explicitly empty input.
        pass
      else:
        raise ValueError(
            f'Transform {identify_object(transform)} is part of a chain. '
            'Cannot define explicit inputs on chain pipeline')
    if ix == 0:
      if is_explicitly_empty(transform.get('input', None)):
        pass
      elif is_explicitly_empty(composite_spec['input']):
        transform['input'] = composite_spec['input']
      elif is_empty(composite_spec['input']):
        del composite_spec['input']
      else:
        transform['input'] = {
            key: key
            for key in composite_spec['input'].keys()
        }
    else:
      transform['input'] = new_transforms[-1]['__uuid__']
    new_transforms.append(transform)
  new_transforms.extend(spec.get('extra_transforms', []))
  composite_spec['transforms'] = new_transforms

  last_transform = new_transforms[-1]['__uuid__']
  if has_explicit_outputs:
    for (key, value) in composite_spec['output'].items():
      if is_not_output_of_last_transform(new_transforms, value):
        raise ValueError(
            f"Explicit output {identify_object(value)} of the chain transform"
            f" is not an output of the last transform.")

    composite_spec['output'] = {
        key: f'{last_transform}.{value}'
        for (key, value) in composite_spec['output'].items()
    }
  else:
    composite_spec['output'] = {'__implicit_outputs__': last_transform}
  if 'name' not in composite_spec:
    composite_spec['name'] = 'Chain'
  composite_spec['type'] = 'composite'
  return composite_spec


def preprocess_chain(spec):
  if spec['type'] == 'chain':
    return chain_as_composite(spec)
  else:
    return spec


def pipeline_as_composite(spec):
  if isinstance(spec, list):
    return {
        'type': 'composite',
        'name': None,
        'transforms': spec,
        '__line__': spec[0]['__line__'],
        '__uuid__': SafeLineLoader.create_uuid(),
    }
  else:
    return dict(spec, name=None, type=spec.get('type', 'composite'))


def normalize_source_sink(spec):
  if 'source' not in spec and 'sink' not in spec:
    return spec
  spec = dict(spec)
  spec['transforms'] = list(spec.get('transforms', []))
  if 'source' in spec:
    if 'input' not in spec['source']:
      spec['source']['input'] = explicitly_empty()
    spec['transforms'].insert(0, spec.pop('source'))
  if 'sink' in spec:
    spec['transforms'].append(spec.pop('sink'))
  return spec


def preprocess_source_sink(spec):
  if spec['type'] in ('chain', 'composite'):
    return normalize_source_sink(spec)
  else:
    return spec


def tag_explicit_inputs(spec):
  if 'input' in spec and not SafeLineLoader.strip_metadata(spec['input']):
    return dict(spec, input=explicitly_empty())
  else:
    return spec


def normalize_inputs_outputs(spec):
  spec = dict(spec)

  def normalize_io(tag):
    io = spec.get(tag, {})
    if isinstance(io, (str, list)):
      return {tag: io}
    else:
      return SafeLineLoader.strip_metadata(io, tagged_str=False)

  return dict(spec, input=normalize_io('input'), output=normalize_io('output'))


def identify_object(spec):
  line = SafeLineLoader.get_line(spec)
  name = extract_name(spec)
  if name:
    return f'"{name}" at line {line}'
  else:
    return f'at line {line}'


def extract_name(spec):
  if isinstance(spec, dict):
    if 'name' in spec:
      return spec['name']
    elif 'id' in spec:
      return spec['id']
    elif 'type' in spec:
      return spec['type']
    elif len(spec) == 1:
      return extract_name(next(iter(spec.values())))
    else:
      return ''
  elif isinstance(spec, str):
    return spec
  else:
    return ''


def extract_extra_dependencies(spec):
  deps = spec.get('config', {}).get('dependencies', [])
  if not deps:
    return [], spec
  if not isinstance(deps, list):
    raise TypeError(f'Dependencies must be a list of strings, got {deps}')
  return deps, dict(
      spec,
      config={k: v for k, v in spec['config'].items() if k != 'dependencies'})


def push_windowing_to_roots(spec):
  scope = LightweightScope(spec['transforms'])
  consumed_outputs_by_transform = collections.defaultdict(set)
  for transform in spec['transforms']:
    for _, input_ref in empty_if_explicitly_empty(transform['input']).items():
      try:
        transform_id, output = scope.get_transform_id_and_output_name(input_ref)
        consumed_outputs_by_transform[transform_id].add(output)
      except ValueError:
        # Could be an input or an ambiguity we'll raise later.
        pass

  for transform in spec['transforms']:
    if is_empty(transform['input']) and 'windowing' not in transform:
      transform['windowing'] = spec['windowing']
      transform['__consumed_outputs'] = consumed_outputs_by_transform[
          transform['__uuid__']]

  return spec


def preprocess_windowing(spec):
  if spec['type'] == 'WindowInto':
    # This is the transform where it is actually applied.
    if 'windowing' in spec:
      spec['config'] = spec.get('config', {})
      spec['config']['windowing'] = spec.pop('windowing')

    if spec.get('config', {}).get('windowing'):
      windowing_config = spec['config']['windowing']
      if isinstance(windowing_config, str):
        try:
          # PyYAML can load a JSON string - one-line and multi-line.
          # Without this code, multi-line is not supported.
          parsed_config = yaml.safe_load(windowing_config)
          if not isinstance(parsed_config, dict):
            raise TypeError('Windowing config string must be a YAML/JSON map.')
          spec['config']['windowing'] = parsed_config
        except Exception as e:
          raise ValueError(
              f'Error parsing windowing config string at \
                {identify_object(spec)}: {e}') from e
    return spec
  elif 'windowing' not in spec:
    # Nothing to do.
    return spec

  if spec['type'] == 'composite':
    # Apply the windowing to any reads, creates, etc. in this transform
    # TODO(robertwb): Better handle the case where a read is followed by a
    # setting of the timestamps. We should be careful of sliding windows
    # in particular.
    spec = push_windowing_to_roots(spec)

  windowing = spec.pop('windowing')
  if not is_empty(spec['input']):
    # Apply the windowing to all inputs by wrapping it in a transform that
    # first applies windowing and then applies the original transform.
    original_inputs = spec['input']
    windowing_transforms = [{
        'type': 'WindowInto',
        'name': f'WindowInto[{key}]',
        'windowing': windowing,
        'input': {
            'input': key
        },
        '__line__': spec['__line__'],
        '__uuid__': SafeLineLoader.create_uuid(),
    } for key in original_inputs.keys()]
    windowed_inputs = {
        key: t['__uuid__']
        for (key, t) in zip(original_inputs.keys(), windowing_transforms)
    }
    modified_spec = dict(
        spec, input=windowed_inputs, __uuid__=SafeLineLoader.create_uuid())
    return {
        'type': 'composite',
        'name': spec.get('name', None) or spec['type'],
        'transforms': [modified_spec] + windowing_transforms,
        'input': spec['input'],
        'output': modified_spec['__uuid__'],
        'config': {
            'error_handling': spec.get('config', {}).get('error_handling', {})
        },
        '__line__': spec['__line__'],
        '__uuid__': spec['__uuid__'],
    }

  elif spec['type'] == 'composite':
    # Pushing the windowing down was sufficient.
    return spec

  else:
    # No inputs, apply the windowing to all outputs.
    consumed_outputs = list(spec.pop('__consumed_outputs', {None}))
    modified_spec = dict(spec, __uuid__=SafeLineLoader.create_uuid())
    windowing_transforms = [{
        'type': 'WindowInto',
        'name': f'WindowInto[{out}]',
        'windowing': windowing,
        'input': {
            'input': modified_spec['__uuid__'] + ('.' + out if out else '')
        },
        '__line__': spec['__line__'],
        '__uuid__': SafeLineLoader.create_uuid(),
    } for out in consumed_outputs]
    if consumed_outputs == [None]:
      windowed_outputs = only_element(windowing_transforms)['__uuid__']
    else:
      windowed_outputs = {
          out: t['__uuid__']
          for (out, t) in zip(consumed_outputs, windowing_transforms)
      }
    return {
        'type': 'composite',
        'name': spec.get('name', None) or spec['type'],
        'transforms': [modified_spec] + windowing_transforms,
        'output': windowed_outputs,
        'config': {
            'error_handling': spec.get('config', {}).get('error_handling', {})
        },
        '__line__': spec['__line__'],
        '__uuid__': spec['__uuid__'],
    }


def preprocess_flattened_inputs(spec):
  if spec['type'] != 'composite':
    return spec

  # Prefer to add the flattens as sibling operations rather than nesting
  # to keep graph shape consistent when the number of inputs goes from
  # one to multiple.
  new_transforms = []
  for t in spec['transforms']:
    if t['type'] == 'Flatten':
      # Don't flatten before explicit flatten.
      # But we do have to expand list inputs into singleton inputs.
      def all_inputs(t):
        for key, values in t.get('input', {}).items():
          if isinstance(values, list):
            for ix, value in enumerate(values):
              yield f'{key}{ix}', value
          else:
            yield key, values

      inputs_dict = {}
      for key, value in all_inputs(t):
        while key in inputs_dict:
          key += '_'
        inputs_dict[key] = value
      t = dict(t, input=inputs_dict)
    else:
      replaced_inputs = {}
      for key, values in t.get('input', {}).items():
        if isinstance(values, list):
          flatten_id = SafeLineLoader.create_uuid()
          new_transforms.append({
              'type': 'Flatten',
              'name': '%s-Flatten[%s]' % (t.get('name', t['type']), key),
              'input': {
                  f'input{ix}': value
                  for (ix, value) in enumerate(values)
              },
              '__line__': spec['__line__'],
              '__uuid__': flatten_id,
          })
          replaced_inputs[key] = flatten_id
      if replaced_inputs:
        t = dict(t, input={**t['input'], **replaced_inputs})
    new_transforms.append(t)
  return dict(spec, transforms=new_transforms)


def ensure_transforms_have_types(spec):
  if 'type' not in spec:
    raise ValueError(f'Missing type specification in {identify_object(spec)}')
  return spec


def ensure_errors_consumed(spec):
  if spec['type'] == 'composite':
    scope = LightweightScope(spec['transforms'])
    to_handle = {}
    consumed = set(
        scope.get_transform_id_and_output_name(output)
        for output in spec['output'].values())
    for t in spec['transforms']:
      config = t.get('config', t)
      if 'error_handling' in config:
        if 'output' not in config['error_handling']:
          raise ValueError(
              f'Missing output in error_handling of {identify_object(t)}')
        to_handle[t['__uuid__'], config['error_handling']['output']] = t
      for _, input in empty_if_explicitly_empty(t['input']).items():
        if input not in spec['input']:
          consumed.add(scope.get_transform_id_and_output_name(input))
    for error_pcoll, t in to_handle.items():
      if error_pcoll not in consumed:
        config = t.get('config', t)
        transform_name = t.get('name', t.get('type'))
        error_output_name = config['error_handling']['output']
        raise ValueError(
            f'Unconsumed error output for {identify_object(t)}. '
            f'The output named {transform_name}.{error_output_name} '
            'must be used as an input to some other transform. '
            'See https://beam.apache.org/documentation/sdks/yaml-errors')
  return spec


def lift_config(spec):
  if 'config' not in spec:
    common_params = 'name', 'type', 'input', 'output', 'transforms'
    return {
        'config': {
            k: v
            for (k, v) in spec.items() if k not in common_params
        },
        **{
            k: v
            for (k, v) in spec.items()  #
            if k in common_params or k in ('__line__', '__uuid__')
        }
    }
  else:
    return spec


def ensure_config(spec):
  if 'config' not in spec:
    spec['config'] = {}
  return spec


def apply_phase(phase, spec):
  spec = phase(spec)
  if spec['type'] in {'composite', 'chain'} and 'transforms' in spec:
    spec = dict(
        spec, transforms=[apply_phase(phase, t) for t in spec['transforms']])
  return spec


def preprocess(spec, verbose=False, known_transforms=None):
  if verbose:
    pprint.pprint(spec)

  if known_transforms:
    known_transforms = set(known_transforms).union(['chain', 'composite'])

  def ensure_transforms_have_providers(spec):
    if known_transforms:
      if spec['type'] not in known_transforms:
        raise ValueError(
            'Unknown type or missing provider '
            f'for type {spec["type"]} for {identify_object(spec)}')
    return spec

  def preprocess_languages(spec):
    if spec['type'] in ('AssignTimestamps',
                        'Combine',
                        'Filter',
                        'MapToFields',
                        'Partition'):
      language = spec.get('config', {}).get('language', 'generic')
      new_type = spec['type'] + '-' + language
      if known_transforms and new_type not in known_transforms:
        if language == 'generic':
          raise ValueError(f'Missing language for {identify_object(spec)}')
        else:
          raise ValueError(
              f'Unknown language {language} for {identify_object(spec)}')
      return dict(spec, type=new_type, name=spec.get('name', spec['type']))
    else:
      return spec

  def validate_transform_references(spec):
    name = spec.get('name', '')
    transform_type = spec.get('type')
    inputs = spec.get('input').get('input', [])

    if not is_empty(inputs):
      input_values = [inputs] if isinstance(inputs, str) else inputs
      for input_value in input_values:
        if input_value in (name, transform_type):
          raise ValueError(
              f"Circular reference detected: Transform {name} "
              f"references itself as input in {identify_object(spec)}")

    return spec

  for phase in [
      ensure_transforms_have_types,
      normalize_mapping,
      normalize_combine,
      preprocess_languages,
      ensure_transforms_have_providers,
      preprocess_source_sink,
      preprocess_chain,
      tag_explicit_inputs,
      normalize_inputs_outputs,
      validate_transform_references,
      preprocess_flattened_inputs,
      ensure_errors_consumed,
      preprocess_windowing,
      # TODO(robertwb): Consider enabling this by default, or as an option.
      # lift_config,
      ensure_config,
  ]:
    spec = apply_phase(phase, spec)
    if verbose:
      print('=' * 20, phase, '=' * 20)
      pprint.pprint(spec)
  return spec


class _BeamFileIOLoader(jinja2.BaseLoader):
  def get_source(self, environment, path):
    with FileSystems.open(path) as fin:
      source = fin.read().decode()
    return source, path, lambda: True


def expand_jinja(
    jinja_template: str, jinja_variables: Mapping[str, Any]) -> str:
  return (  # keep formatting
      jinja2.Environment(
          undefined=jinja2.StrictUndefined, loader=_BeamFileIOLoader())
      .from_string(jinja_template)
      .render(datetime=datetime, **jinja_variables))


class YamlTransform(beam.PTransform):
  def __init__(self, spec, providers={}):  # pylint: disable=dangerous-default-value
    if isinstance(spec, str):
      spec = yaml.load(spec, Loader=SafeLineLoader)
    if isinstance(providers, dict):
      providers = {
          key: yaml_provider.as_provider_list(key, value)
          for (key, value) in providers.items()
      }
    # TODO(BEAM-26941): Validate as a transform.
    self._providers = yaml_provider.merge_providers(
        providers, yaml_provider.standard_providers())
    self._spec = preprocess(spec, known_transforms=self._providers.keys())
    self._was_chain = spec['type'] == 'chain'

  def expand(self, pcolls):
    if isinstance(pcolls, beam.pvalue.PBegin):
      root = pcolls
      pipeline = root.pipeline
      pcolls = {}
    elif isinstance(pcolls, beam.PCollection):
      root = pcolls.pipeline
      pipeline = root
      pcolls = {'input': pcolls}
      if not self._spec['input']:
        self._spec['input'] = {'input': 'input'}
        if self._was_chain and self._spec['transforms']:
          # This should have been copied as part of the composite-to-chain.
          self._spec['transforms'][0]['input'] = self._spec['input']
    else:
      root = next(iter(pcolls.values())).pipeline
      pipeline = root
      if not self._spec['input']:
        self._spec['input'] = {name: name for name in pcolls.keys()}
    python_provider = yaml_provider.InlineProvider({})

    # Label goog-dataflow-yaml if job is started using Beam YAML.
    options = pipeline.options.view_as(GoogleCloudOptions)
    yaml_version = ('beam-yaml=' + beam.version.__version__.replace('.', '_'))
    if not options.labels:
      options.labels = []
    if yaml_version not in options.labels:
      options.labels.append(yaml_version)

    result = expand_transform(
        self._spec,
        Scope(
            root,
            pcolls,
            transforms=[self._spec],
            providers=self._providers,
            input_providers={
                pcoll: python_provider
                for pcoll in pcolls.values()
            }))
    if len(result) == 1:
      return only_element(result.values())
    else:
      return result


def expand_pipeline(
    pipeline,
    pipeline_spec,
    providers=None,
    validate_schema='generic' if jsonschema is not None else None,
    pipeline_path=''):
  if isinstance(pipeline, beam.pvalue.PBegin):
    root = pipeline
  else:
    root = beam.pvalue.PBegin(pipeline)
  if isinstance(pipeline_spec, str):
    pipeline_spec = yaml.load(pipeline_spec, Loader=SafeLineLoader)
  # TODO(robertwb): It's unclear whether this gives as good of errors, but
  # this could certainly be handy as a first pass when Beam is not available.
  if validate_schema and validate_schema != 'none':
    validate_against_schema(pipeline_spec, validate_schema)
  # Calling expand directly to avoid outer layer of nesting.
  return YamlTransform(
      pipeline_as_composite(pipeline_spec['pipeline']),
      yaml_provider.merge_providers(
          yaml_provider.parse_providers(
              pipeline_path, pipeline_spec.get('providers', [])),
          providers or {})).expand(root)
