PaddleClas/ppcls/modeling/architectures/efficientnet.py

import paddle
from paddle import ParamAttr
import paddle.nn as nn
import paddle.nn.functional as F
from paddle.nn import Conv2D, BatchNorm, Linear, Dropout
from paddle.nn import AdaptiveAvgPool2D, MaxPool2D, AvgPool2D
import math
import collections
import re
import copy

__all__ = [
    'EfficientNet', 'EfficientNetB0_small', 'EfficientNetB0', 'EfficientNetB1',
    'EfficientNetB2', 'EfficientNetB3', 'EfficientNetB4', 'EfficientNetB5',
    'EfficientNetB6', 'EfficientNetB7'
]

GlobalParams = collections.namedtuple('GlobalParams', [
    'batch_norm_momentum',
    'batch_norm_epsilon',
    'dropout_rate',
    'num_classes',
    'width_coefficient',
    'depth_coefficient',
    'depth_divisor',
    'min_depth',
    'drop_connect_rate',
])

BlockArgs = collections.namedtuple('BlockArgs', [
    'kernel_size', 'num_repeat', 'input_filters', 'output_filters',
    'expand_ratio', 'id_skip', 'stride', 'se_ratio'
])

GlobalParams.__new__.__defaults__ = (None, ) * len(GlobalParams._fields)
BlockArgs.__new__.__defaults__ = (None, ) * len(BlockArgs._fields)


def efficientnet_params(model_name):
    """ Map EfficientNet model name to parameter coefficients. """
    params_dict = {
        # Coefficients:   width,depth,resolution,dropout
        'efficientnet-b0': (1.0, 1.0, 224, 0.2),
        'efficientnet-b1': (1.0, 1.1, 240, 0.2),
        'efficientnet-b2': (1.1, 1.2, 260, 0.3),
        'efficientnet-b3': (1.2, 1.4, 300, 0.3),
        'efficientnet-b4': (1.4, 1.8, 380, 0.4),
        'efficientnet-b5': (1.6, 2.2, 456, 0.4),
        'efficientnet-b6': (1.8, 2.6, 528, 0.5),
        'efficientnet-b7': (2.0, 3.1, 600, 0.5),
    }
    return params_dict[model_name]


def efficientnet(width_coefficient=None,
                 depth_coefficient=None,
                 dropout_rate=0.2,
                 drop_connect_rate=0.2):
    """ Get block arguments according to parameter and coefficients. """
    blocks_args = [
        'r1_k3_s11_e1_i32_o16_se0.25',
        'r2_k3_s22_e6_i16_o24_se0.25',
        'r2_k5_s22_e6_i24_o40_se0.25',
        'r3_k3_s22_e6_i40_o80_se0.25',
        'r3_k5_s11_e6_i80_o112_se0.25',
        'r4_k5_s22_e6_i112_o192_se0.25',
        'r1_k3_s11_e6_i192_o320_se0.25',
    ]
    blocks_args = BlockDecoder.decode(blocks_args)

    global_params = GlobalParams(
        batch_norm_momentum=0.99,
        batch_norm_epsilon=1e-3,
        dropout_rate=dropout_rate,
        drop_connect_rate=drop_connect_rate,
        num_classes=1000,
        width_coefficient=width_coefficient,
        depth_coefficient=depth_coefficient,
        depth_divisor=8,
        min_depth=None)

    return blocks_args, global_params


def get_model_params(model_name, override_params):
    """ Get the block args and global params for a given model """
    if model_name.startswith('efficientnet'):
        w, d, _, p = efficientnet_params(model_name)
        blocks_args, global_params = efficientnet(
            width_coefficient=w, depth_coefficient=d, dropout_rate=p)
    else:
        raise NotImplementedError('model name is not pre-defined: %s' %
                                  model_name)
    if override_params:
        global_params = global_params._replace(**override_params)
    return blocks_args, global_params


def round_filters(filters, global_params):
    """ Calculate and round number of filters based on depth multiplier. """
    multiplier = global_params.width_coefficient
    if not multiplier:
        return filters
    divisor = global_params.depth_divisor
    min_depth = global_params.min_depth
    filters *= multiplier
    min_depth = min_depth or divisor
    new_filters = max(min_depth,
                      int(filters + divisor / 2) // divisor * divisor)
    if new_filters < 0.9 * filters:  # prevent rounding by more than 10%
        new_filters += divisor
    return int(new_filters)


def round_repeats(repeats, global_params):
    """ Round number of filters based on depth multiplier. """
    multiplier = global_params.depth_coefficient
    if not multiplier:
        return repeats
    return int(math.ceil(multiplier * repeats))


class BlockDecoder(object):
    """
    Block Decoder, straight from the official TensorFlow repository.
    """

    @staticmethod
    def _decode_block_string(block_string):
        """ Gets a block through a string notation of arguments. """
        assert isinstance(block_string, str)

        ops = block_string.split('_')
        options = {}
        for op in ops:
            splits = re.split(r'(\d.*)', op)
            if len(splits) >= 2:
                key, value = splits[:2]
                options[key] = value

        # Check stride
        cond_1 = ('s' in options and len(options['s']) == 1)
        cond_2 = ((len(options['s']) == 2) and
                  (options['s'][0] == options['s'][1]))
        assert (cond_1 or cond_2)

        return BlockArgs(
            kernel_size=int(options['k']),
            num_repeat=int(options['r']),
            input_filters=int(options['i']),
            output_filters=int(options['o']),
            expand_ratio=int(options['e']),
            id_skip=('noskip' not in block_string),
            se_ratio=float(options['se']) if 'se' in options else None,
            stride=[int(options['s'][0])])

    @staticmethod
    def _encode_block_string(block):
        """Encodes a block to a string."""
        args = [
            'r%d' % block.num_repeat, 'k%d' % block.kernel_size, 's%d%d' %
            (block.strides[0], block.strides[1]), 'e%s' % block.expand_ratio,
            'i%d' % block.input_filters, 'o%d' % block.output_filters
        ]
        if 0 < block.se_ratio <= 1:
            args.append('se%s' % block.se_ratio)
        if block.id_skip is False:
            args.append('noskip')
        return '_'.join(args)

    @staticmethod
    def decode(string_list):
        """
        Decode a list of string notations to specify blocks in the network.

        string_list: list of strings, each string is a notation of block
        return
            list of BlockArgs namedtuples of block args
        """
        assert isinstance(string_list, list)
        blocks_args = []
        for block_string in string_list:
            blocks_args.append(BlockDecoder._decode_block_string(block_string))
        return blocks_args

    @staticmethod
    def encode(blocks_args):
        """
        Encodes a list of BlockArgs to a list of strings.

        :param blocks_args: a list of BlockArgs namedtuples of block args
        :return: a list of strings, each string is a notation of block
        """
        block_strings = []
        for block in blocks_args:
            block_strings.append(BlockDecoder._encode_block_string(block))
        return block_strings


def initial_type(name, use_bias=False):
    param_attr = ParamAttr(name=name + "_weights")
    if use_bias:
        bias_attr = ParamAttr(name=name + "_offset")
    else:
        bias_attr = False
    return param_attr, bias_attr


def init_batch_norm_layer(name="batch_norm"):
    param_attr = ParamAttr(name=name + "_scale")
    bias_attr = ParamAttr(name=name + "_offset")
    return param_attr, bias_attr


def init_fc_layer(name="fc"):
    param_attr = ParamAttr(name=name + "_weights")
    bias_attr = ParamAttr(name=name + "_offset")
    return param_attr, bias_attr


def cal_padding(img_size, stride, filter_size, dilation=1):
    """Calculate padding size."""
    if img_size % stride == 0:
        out_size = max(filter_size - stride, 0)
    else:
        out_size = max(filter_size - (img_size % stride), 0)
    return out_size // 2, out_size - out_size // 2


inp_shape = {
    "b0_small": [224, 112, 112, 56, 28, 14, 14, 7],
    "b0": [224, 112, 112, 56, 28, 14, 14, 7],
    "b1": [240, 120, 120, 60, 30, 15, 15, 8],
    "b2": [260, 130, 130, 65, 33, 17, 17, 9],
    "b3": [300, 150, 150, 75, 38, 19, 19, 10],
    "b4": [380, 190, 190, 95, 48, 24, 24, 12],
    "b5": [456, 228, 228, 114, 57, 29, 29, 15],
    "b6": [528, 264, 264, 132, 66, 33, 33, 17],
    "b7": [600, 300, 300, 150, 75, 38, 38, 19]
}


def _drop_connect(inputs, prob, is_test):
    if is_test:
        return inputs
    keep_prob = 1.0 - prob
    inputs_shape = paddle.shape(inputs)
    random_tensor = keep_prob + paddle.rand(shape=[inputs_shape[0], 1, 1, 1])
    binary_tensor = paddle.floor(random_tensor)
    output = inputs / keep_prob * binary_tensor
    return output


class Conv2ds(nn.Layer):
    def __init__(self,
                 input_channels,
                 output_channels,
                 filter_size,
                 stride=1,
                 padding=0,
                 groups=None,
                 name="conv2d",
                 act=None,
                 use_bias=False,
                 padding_type=None,
                 model_name=None,
                 cur_stage=None):
        super(Conv2ds, self).__init__()
        assert act in [None, "swish", "sigmoid"]
        self.act = act

        param_attr, bias_attr = initial_type(name=name, use_bias=use_bias)

        def get_padding(filter_size, stride=1, dilation=1):
            padding = ((stride - 1) + dilation * (filter_size - 1)) // 2
            return padding

        inps = 1 if model_name == None and cur_stage == None else inp_shape[
            model_name][cur_stage]
        self.need_crop = False
        if padding_type == "SAME":
            top_padding, bottom_padding = cal_padding(inps, stride,
                                                      filter_size)
            left_padding, right_padding = cal_padding(inps, stride,
                                                      filter_size)
            height_padding = bottom_padding
            width_padding = right_padding
            if top_padding != bottom_padding or left_padding != right_padding:
                height_padding = top_padding + stride
                width_padding = left_padding + stride
                self.need_crop = True
            padding = [height_padding, width_padding]
        elif padding_type == "VALID":
            height_padding = 0
            width_padding = 0
            padding = [height_padding, width_padding]
        elif padding_type == "DYNAMIC":
            padding = get_padding(filter_size, stride)
        else:
            padding = padding_type

        groups = 1 if groups is None else groups
        self._conv = Conv2D(
            input_channels,
            output_channels,
            filter_size,
            groups=groups,
            stride=stride,
            #             act=act,
            padding=padding,
            weight_attr=param_attr,
            bias_attr=bias_attr)

    def forward(self, inputs):
        x = self._conv(inputs)
        if self.act == "swish":
            x = F.swish(x)
        elif self.act == "sigmoid":
            x = F.sigmoid(x)

        if self.need_crop:
            x = x[:, :, 1:, 1:]
        return x


class ConvBNLayer(nn.Layer):
    def __init__(self,
                 input_channels,
                 filter_size,
                 output_channels,
                 stride=1,
                 num_groups=1,
                 padding_type="SAME",
                 conv_act=None,
                 bn_act="swish",
                 use_bn=True,
                 use_bias=False,
                 name=None,
                 conv_name=None,
                 bn_name=None,
                 model_name=None,
                 cur_stage=None):
        super(ConvBNLayer, self).__init__()

        self._conv = Conv2ds(
            input_channels=input_channels,
            output_channels=output_channels,
            filter_size=filter_size,
            stride=stride,
            groups=num_groups,
            act=conv_act,
            padding_type=padding_type,
            name=conv_name,
            use_bias=use_bias,
            model_name=model_name,
            cur_stage=cur_stage)
        self.use_bn = use_bn
        if use_bn is True:
            bn_name = name + bn_name
            param_attr, bias_attr = init_batch_norm_layer(bn_name)

            self._bn = BatchNorm(
                num_channels=output_channels,
                act=bn_act,
                momentum=0.99,
                epsilon=0.001,
                moving_mean_name=bn_name + "_mean",
                moving_variance_name=bn_name + "_variance",
                param_attr=param_attr,
                bias_attr=bias_attr)

    def forward(self, inputs):
        if self.use_bn:
            x = self._conv(inputs)
            x = self._bn(x)
            return x
        else:
            return self._conv(inputs)


class ExpandConvNorm(nn.Layer):
    def __init__(self,
                 input_channels,
                 block_args,
                 padding_type,
                 name=None,
                 model_name=None,
                 cur_stage=None):
        super(ExpandConvNorm, self).__init__()

        self.oup = block_args.input_filters * block_args.expand_ratio
        self.expand_ratio = block_args.expand_ratio

        if self.expand_ratio != 1:
            self._conv = ConvBNLayer(
                input_channels,
                1,
                self.oup,
                bn_act=None,
                padding_type=padding_type,
                name=name,
                conv_name=name + "_expand_conv",
                bn_name="_bn0",
                model_name=model_name,
                cur_stage=cur_stage)

    def forward(self, inputs):
        if self.expand_ratio != 1:
            return self._conv(inputs)
        else:
            return inputs


class DepthwiseConvNorm(nn.Layer):
    def __init__(self,
                 input_channels,
                 block_args,
                 padding_type,
                 name=None,
                 model_name=None,
                 cur_stage=None):
        super(DepthwiseConvNorm, self).__init__()

        self.k = block_args.kernel_size
        self.s = block_args.stride
        if isinstance(self.s, list) or isinstance(self.s, tuple):
            self.s = self.s[0]
        oup = block_args.input_filters * block_args.expand_ratio

        self._conv = ConvBNLayer(
            input_channels,
            self.k,
            oup,
            self.s,
            num_groups=input_channels,
            bn_act=None,
            padding_type=padding_type,
            name=name,
            conv_name=name + "_depthwise_conv",
            bn_name="_bn1",
            model_name=model_name,
            cur_stage=cur_stage)

    def forward(self, inputs):
        return self._conv(inputs)


class ProjectConvNorm(nn.Layer):
    def __init__(self,
                 input_channels,
                 block_args,
                 padding_type,
                 name=None,
                 model_name=None,
                 cur_stage=None):
        super(ProjectConvNorm, self).__init__()

        final_oup = block_args.output_filters

        self._conv = ConvBNLayer(
            input_channels,
            1,
            final_oup,
            bn_act=None,
            padding_type=padding_type,
            name=name,
            conv_name=name + "_project_conv",
            bn_name="_bn2",
            model_name=model_name,
            cur_stage=cur_stage)

    def forward(self, inputs):
        return self._conv(inputs)


class SEBlock(nn.Layer):
    def __init__(self,
                 input_channels,
                 num_squeezed_channels,
                 oup,
                 padding_type,
                 name=None,
                 model_name=None,
                 cur_stage=None):
        super(SEBlock, self).__init__()

        self._pool = AdaptiveAvgPool2D(1)
        self._conv1 = Conv2ds(
            input_channels,
            num_squeezed_channels,
            1,
            use_bias=True,
            padding_type=padding_type,
            act="swish",
            name=name + "_se_reduce")

        self._conv2 = Conv2ds(
            num_squeezed_channels,
            oup,
            1,
            act="sigmoid",
            use_bias=True,
            padding_type=padding_type,
            name=name + "_se_expand")

    def forward(self, inputs):
        x = self._pool(inputs)
        x = self._conv1(x)
        x = self._conv2(x)
        return paddle.multiply(inputs, x)


class MbConvBlock(nn.Layer):
    def __init__(self,
                 input_channels,
                 block_args,
                 padding_type,
                 use_se,
                 name=None,
                 drop_connect_rate=None,
                 model_name=None,
                 cur_stage=None):
        super(MbConvBlock, self).__init__()

        oup = block_args.input_filters * block_args.expand_ratio
        self.block_args = block_args
        self.has_se = use_se and (block_args.se_ratio is not None) and (
            0 < block_args.se_ratio <= 1)
        self.id_skip = block_args.id_skip
        self.expand_ratio = block_args.expand_ratio
        self.drop_connect_rate = drop_connect_rate

        if self.expand_ratio != 1:
            self._ecn = ExpandConvNorm(
                input_channels,
                block_args,
                padding_type=padding_type,
                name=name,
                model_name=model_name,
                cur_stage=cur_stage)

        self._dcn = DepthwiseConvNorm(
            input_channels * block_args.expand_ratio,
            block_args,
            padding_type=padding_type,
            name=name,
            model_name=model_name,
            cur_stage=cur_stage)

        if self.has_se:
            num_squeezed_channels = max(
                1, int(block_args.input_filters * block_args.se_ratio))
            self._se = SEBlock(
                input_channels * block_args.expand_ratio,
                num_squeezed_channels,
                oup,
                padding_type=padding_type,
                name=name,
                model_name=model_name,
                cur_stage=cur_stage)

        self._pcn = ProjectConvNorm(
            input_channels * block_args.expand_ratio,
            block_args,
            padding_type=padding_type,
            name=name,
            model_name=model_name,
            cur_stage=cur_stage)

    def forward(self, inputs):
        x = inputs
        if self.expand_ratio != 1:
            x = self._ecn(x)
            x = F.swish(x)
        x = self._dcn(x)
        x = F.swish(x)
        if self.has_se:
            x = self._se(x)
        x = self._pcn(x)
        if self.id_skip and \
                self.block_args.stride == 1 and \
                self.block_args.input_filters == self.block_args.output_filters:
            if self.drop_connect_rate:
                x = _drop_connect(x, self.drop_connect_rate, not self.training)
            x = paddle.add(x, inputs)
        return x


class ConvStemNorm(nn.Layer):
    def __init__(self,
                 input_channels,
                 padding_type,
                 _global_params,
                 name=None,
                 model_name=None,
                 cur_stage=None):
        super(ConvStemNorm, self).__init__()

        output_channels = round_filters(32, _global_params)
        self._conv = ConvBNLayer(
            input_channels,
            filter_size=3,
            output_channels=output_channels,
            stride=2,
            bn_act=None,
            padding_type=padding_type,
            name="",
            conv_name="_conv_stem",
            bn_name="_bn0",
            model_name=model_name,
            cur_stage=cur_stage)

    def forward(self, inputs):
        return self._conv(inputs)


class ExtractFeatures(nn.Layer):
    def __init__(self,
                 input_channels,
                 _block_args,
                 _global_params,
                 padding_type,
                 use_se,
                 model_name=None):
        super(ExtractFeatures, self).__init__()

        self._global_params = _global_params

        self._conv_stem = ConvStemNorm(
            input_channels,
            padding_type=padding_type,
            _global_params=_global_params,
            model_name=model_name,
            cur_stage=0)

        self.block_args_copy = copy.deepcopy(_block_args)
        idx = 0
        block_size = 0
        for block_arg in self.block_args_copy:
            block_arg = block_arg._replace(
                input_filters=round_filters(block_arg.input_filters,
                                            _global_params),
                output_filters=round_filters(block_arg.output_filters,
                                             _global_params),
                num_repeat=round_repeats(block_arg.num_repeat, _global_params))
            block_size += 1
            for _ in range(block_arg.num_repeat - 1):
                block_size += 1

        self.conv_seq = []
        cur_stage = 1
        for block_args in _block_args:
            block_args = block_args._replace(
                input_filters=round_filters(block_args.input_filters,
                                            _global_params),
                output_filters=round_filters(block_args.output_filters,
                                             _global_params),
                num_repeat=round_repeats(block_args.num_repeat,
                                         _global_params))

            drop_connect_rate = self._global_params.drop_connect_rate
            if drop_connect_rate:
                drop_connect_rate *= float(idx) / block_size

            _mc_block = self.add_sublayer(
                "_blocks." + str(idx) + ".",
                MbConvBlock(
                    block_args.input_filters,
                    block_args=block_args,
                    padding_type=padding_type,
                    use_se=use_se,
                    name="_blocks." + str(idx) + ".",
                    drop_connect_rate=drop_connect_rate,
                    model_name=model_name,
                    cur_stage=cur_stage))
            self.conv_seq.append(_mc_block)
            idx += 1
            if block_args.num_repeat > 1:
                block_args = block_args._replace(
                    input_filters=block_args.output_filters, stride=1)
            for _ in range(block_args.num_repeat - 1):
                drop_connect_rate = self._global_params.drop_connect_rate
                if drop_connect_rate:
                    drop_connect_rate *= float(idx) / block_size
                _mc_block = self.add_sublayer(
                    "block." + str(idx) + ".",
                    MbConvBlock(
                        block_args.input_filters,
                        block_args,
                        padding_type=padding_type,
                        use_se=use_se,
                        name="_blocks." + str(idx) + ".",
                        drop_connect_rate=drop_connect_rate,
                        model_name=model_name,
                        cur_stage=cur_stage))
                self.conv_seq.append(_mc_block)
                idx += 1
            cur_stage += 1

    def forward(self, inputs):
        x = self._conv_stem(inputs)
        x = F.swish(x)
        for _mc_block in self.conv_seq:
            x = _mc_block(x)
        return x


class EfficientNet(nn.Layer):
    def __init__(self,
                 name="b0",
                 padding_type="SAME",
                 override_params=None,
                 use_se=True,
                 class_dim=1000):
        super(EfficientNet, self).__init__()

        model_name = 'efficientnet-' + name
        self.name = name
        self._block_args, self._global_params = get_model_params(
            model_name, override_params)
        self.padding_type = padding_type
        self.use_se = use_se

        self._ef = ExtractFeatures(
            3,
            self._block_args,
            self._global_params,
            self.padding_type,
            self.use_se,
            model_name=self.name)

        output_channels = round_filters(1280, self._global_params)
        if name == "b0_small" or name == "b0" or name == "b1":
            oup = 320
        elif name == "b2":
            oup = 352
        elif name == "b3":
            oup = 384
        elif name == "b4":
            oup = 448
        elif name == "b5":
            oup = 512
        elif name == "b6":
            oup = 576
        elif name == "b7":
            oup = 640
        self._conv = ConvBNLayer(
            oup,
            1,
            output_channels,
            bn_act="swish",
            padding_type=self.padding_type,
            name="",
            conv_name="_conv_head",
            bn_name="_bn1",
            model_name=self.name,
            cur_stage=7)
        self._pool = AdaptiveAvgPool2D(1)

        if self._global_params.dropout_rate:
            self._drop = Dropout(
                p=self._global_params.dropout_rate, mode="upscale_in_train")

        param_attr, bias_attr = init_fc_layer("_fc")
        self._fc = Linear(
            output_channels,
            class_dim,
            weight_attr=param_attr,
            bias_attr=bias_attr)

    def forward(self, inputs):
        x = self._ef(inputs)
        x = self._conv(x)
        x = self._pool(x)
        if self._global_params.dropout_rate:
            x = self._drop(x)
        x = paddle.squeeze(x, axis=[2, 3])
        x = self._fc(x)
        return x


def EfficientNetB0_small(padding_type='DYNAMIC',
                         override_params=None,
                         use_se=False,
                         **args):
    model = EfficientNet(
        name='b0',
        padding_type=padding_type,
        override_params=override_params,
        use_se=use_se,
        **args)
    return model


def EfficientNetB0(padding_type='SAME',
                   override_params=None,
                   use_se=True,
                   **args):
    model = EfficientNet(
        name='b0',
        padding_type=padding_type,
        override_params=override_params,
        use_se=use_se,
        **args)
    return model


def EfficientNetB1(padding_type='SAME',
                   override_params=None,
                   use_se=True,
                   **args):
    model = EfficientNet(
        name='b1',
        padding_type=padding_type,
        override_params=override_params,
        use_se=use_se,
        **args)
    return model


def EfficientNetB2(padding_type='SAME',
                   override_params=None,
                   use_se=True,
                   **args):
    model = EfficientNet(
        name='b2',
        padding_type=padding_type,
        override_params=override_params,
        use_se=use_se,
        **args)
    return model


def EfficientNetB3(padding_type='SAME',
                   override_params=None,
                   use_se=True,
                   **args):
    model = EfficientNet(
        name='b3',
        padding_type=padding_type,
        override_params=override_params,
        use_se=use_se,
        **args)
    return model


def EfficientNetB4(padding_type='SAME',
                   override_params=None,
                   use_se=True,
                   **args):
    model = EfficientNet(
        name='b4',
        padding_type=padding_type,
        override_params=override_params,
        use_se=use_se,
        **args)
    return model


def EfficientNetB5(padding_type='SAME',
                   override_params=None,
                   use_se=True,
                   **args):
    model = EfficientNet(
        name='b5',
        padding_type=padding_type,
        override_params=override_params,
        use_se=use_se,
        **args)
    return model


def EfficientNetB6(padding_type='SAME',
                   override_params=None,
                   use_se=True,
                   **args):
    model = EfficientNet(
        name='b6',
        padding_type=padding_type,
        override_params=override_params,
        use_se=use_se,
        **args)
    return model


def EfficientNetB7(padding_type='SAME',
                   override_params=None,
                   use_se=True,
                   **args):
    model = EfficientNet(
        name='b7',
        padding_type=padding_type,
        override_params=override_params,
        use_se=use_se,
        **args)
    return model