"""
Title: Image classification with EANet (External Attention Transformer)
Author: [ZhiYong Chang](https://github.com/czy00000)
Date created: 2021/10/19
Last modified: 2021/10/19
Description: Image classification with a Transformer that leverages external attention.
Accelerator: GPU
"""

"""
## Introduction

This example implements the [EANet](https://arxiv.org/abs/2105.02358)
model for image classification, and demonstrates it on the CIFAR-100 dataset.
EANet introduces a novel attention mechanism
named ***external attention***, based on two external, small, learnable, and
shared memories, which can be implemented easily by simply using two cascaded
linear layers and two normalization layers. It conveniently replaces self-attention
as used in existing architectures. External attention has linear complexity, as it only
implicitly considers the correlations between all samples.

This example requires TensorFlow 2.5 or higher, as well as
[TensorFlow Addons](https://www.tensorflow.org/addons/overview) package,
which can be installed using the following command:

```python
pip install -U tensorflow-addons
```
"""

"""
## Setup
"""

import numpy as np
import tensorflow as tf
from tensorflow import keras
from tensorflow.keras import layers
import tensorflow_addons as tfa
import matplotlib.pyplot as plt


"""
## Prepare the data
"""

num_classes = 100
input_shape = (32, 32, 3)

(x_train, y_train), (x_test, y_test) = keras.datasets.cifar100.load_data()
y_train = keras.utils.to_categorical(y_train, num_classes)
y_test = keras.utils.to_categorical(y_test, num_classes)
print(f"x_train shape: {x_train.shape} - y_train shape: {y_train.shape}")
print(f"x_test shape: {x_test.shape} - y_test shape: {y_test.shape}")

"""
## Configure the hyperparameters
"""

weight_decay = 0.0001
learning_rate = 0.001
label_smoothing = 0.1
validation_split = 0.2
batch_size = 128
num_epochs = 50
patch_size = 2  # Size of the patches to be extracted from the input images.
num_patches = (input_shape[0] // patch_size) ** 2  # Number of patch
embedding_dim = 64  # Number of hidden units.
mlp_dim = 64
dim_coefficient = 4
num_heads = 4
attention_dropout = 0.2
projection_dropout = 0.2
num_transformer_blocks = 8  # Number of repetitions of the transformer layer

print(f"Patch size: {patch_size} X {patch_size} = {patch_size ** 2} ")
print(f"Patches per image: {num_patches}")


"""
## Use data augmentation
"""

data_augmentation = keras.Sequential(
    [
        layers.Normalization(),
        layers.RandomFlip("horizontal"),
        layers.RandomRotation(factor=0.1),
        layers.RandomContrast(factor=0.1),
        layers.RandomZoom(height_factor=0.2, width_factor=0.2),
    ],
    name="data_augmentation",
)
# Compute the mean and the variance of the training data for normalization.
data_augmentation.layers[0].adapt(x_train)

"""
## Implement the patch extraction and encoding layer
"""


class PatchExtract(layers.Layer):
    def __init__(self, patch_size, **kwargs):
        super().__init__(**kwargs)
        self.patch_size = patch_size

    def call(self, images):
        batch_size = tf.shape(images)[0]
        patches = tf.image.extract_patches(
            images=images,
            sizes=(1, self.patch_size, self.patch_size, 1),
            strides=(1, self.patch_size, self.patch_size, 1),
            rates=(1, 1, 1, 1),
            padding="VALID",
        )
        patch_dim = patches.shape[-1]
        patch_num = patches.shape[1]
        return tf.reshape(patches, (batch_size, patch_num * patch_num, patch_dim))


class PatchEmbedding(layers.Layer):
    def __init__(self, num_patch, embed_dim, **kwargs):
        super().__init__(**kwargs)
        self.num_patch = num_patch
        self.proj = layers.Dense(embed_dim)
        self.pos_embed = layers.Embedding(input_dim=num_patch, output_dim=embed_dim)

    def call(self, patch):
        pos = tf.range(start=0, limit=self.num_patch, delta=1)
        return self.proj(patch) + self.pos_embed(pos)


"""
## Implement the external attention block
"""


def external_attention(
    x, dim, num_heads, dim_coefficient=4, attention_dropout=0, projection_dropout=0
):
    _, num_patch, channel = x.shape
    assert dim % num_heads == 0
    num_heads = num_heads * dim_coefficient

    x = layers.Dense(dim * dim_coefficient)(x)
    # create tensor [batch_size, num_patches, num_heads, dim*dim_coefficient//num_heads]
    x = tf.reshape(
        x, shape=(-1, num_patch, num_heads, dim * dim_coefficient // num_heads)
    )
    x = tf.transpose(x, perm=[0, 2, 1, 3])
    # a linear layer M_k
    attn = layers.Dense(dim // dim_coefficient)(x)
    # normalize attention map
    attn = layers.Softmax(axis=2)(attn)
    # dobule-normalization
    attn = attn / (1e-9 + tf.reduce_sum(attn, axis=-1, keepdims=True))
    attn = layers.Dropout(attention_dropout)(attn)
    # a linear layer M_v
    x = layers.Dense(dim * dim_coefficient // num_heads)(attn)
    x = tf.transpose(x, perm=[0, 2, 1, 3])
    x = tf.reshape(x, [-1, num_patch, dim * dim_coefficient])
    # a linear layer to project original dim
    x = layers.Dense(dim)(x)
    x = layers.Dropout(projection_dropout)(x)
    return x


"""
## Implement the MLP block
"""


def mlp(x, embedding_dim, mlp_dim, drop_rate=0.2):
    x = layers.Dense(mlp_dim, activation=tf.nn.gelu)(x)
    x = layers.Dropout(drop_rate)(x)
    x = layers.Dense(embedding_dim)(x)
    x = layers.Dropout(drop_rate)(x)
    return x


"""
## Implement the Transformer block
"""


def transformer_encoder(
    x,
    embedding_dim,
    mlp_dim,
    num_heads,
    dim_coefficient,
    attention_dropout,
    projection_dropout,
    attention_type="external_attention",
):
    residual_1 = x
    x = layers.LayerNormalization(epsilon=1e-5)(x)
    if attention_type == "external_attention":
        x = external_attention(
            x,
            embedding_dim,
            num_heads,
            dim_coefficient,
            attention_dropout,
            projection_dropout,
        )
    elif attention_type == "self_attention":
        x = layers.MultiHeadAttention(
            num_heads=num_heads, key_dim=embedding_dim, dropout=attention_dropout
        )(x, x)
    x = layers.add([x, residual_1])
    residual_2 = x
    x = layers.LayerNormalization(epsilon=1e-5)(x)
    x = mlp(x, embedding_dim, mlp_dim)
    x = layers.add([x, residual_2])
    return x


"""
## Implement the EANet model
"""

"""
The EANet model leverages external attention.
The computational complexity of traditional self attention is `O(d * N ** 2)`,
where `d` is the embedding size, and `N` is the number of patch.
the authors find that most pixels are closely related to just a few other
pixels, and an `N`-to-`N` attention matrix may be redundant.
So, they propose as an alternative an external
attention module where the computational complexity of external attention is `O(d * S * N)`.
As `d` and `S` are hyper-parameters,
the proposed algorithm is linear in the number of pixels. In fact, this is equivalent
to a drop patch operation, because a lot of information contained in a patch
in an image is redundant and unimportant.
"""


def get_model(attention_type="external_attention"):
    inputs = layers.Input(shape=input_shape)
    # Image augment
    x = data_augmentation(inputs)
    # Extract patches.
    x = PatchExtract(patch_size)(x)
    # Create patch embedding.
    x = PatchEmbedding(num_patches, embedding_dim)(x)
    # Create Transformer block.
    for _ in range(num_transformer_blocks):
        x = transformer_encoder(
            x,
            embedding_dim,
            mlp_dim,
            num_heads,
            dim_coefficient,
            attention_dropout,
            projection_dropout,
            attention_type,
        )

    x = layers.GlobalAvgPool1D()(x)
    outputs = layers.Dense(num_classes, activation="softmax")(x)
    model = keras.Model(inputs=inputs, outputs=outputs)
    return model


"""
## Train on CIFAR-100

"""


model = get_model(attention_type="external_attention")

model.compile(
    loss=keras.losses.CategoricalCrossentropy(label_smoothing=label_smoothing),
    optimizer=tfa.optimizers.AdamW(
        learning_rate=learning_rate, weight_decay=weight_decay
    ),
    metrics=[
        keras.metrics.CategoricalAccuracy(name="accuracy"),
        keras.metrics.TopKCategoricalAccuracy(5, name="top-5-accuracy"),
    ],
)

history = model.fit(
    x_train,
    y_train,
    batch_size=batch_size,
    epochs=num_epochs,
    validation_split=validation_split,
)

"""
### Let's visualize the training progress of the model.

"""

plt.plot(history.history["loss"], label="train_loss")
plt.plot(history.history["val_loss"], label="val_loss")
plt.xlabel("Epochs")
plt.ylabel("Loss")
plt.title("Train and Validation Losses Over Epochs", fontsize=14)
plt.legend()
plt.grid()
plt.show()

"""
### Let's display the final results of the test on CIFAR-100.

"""

loss, accuracy, top_5_accuracy = model.evaluate(x_test, y_test)
print(f"Test loss: {round(loss, 2)}")
print(f"Test accuracy: {round(accuracy * 100, 2)}%")
print(f"Test top 5 accuracy: {round(top_5_accuracy * 100, 2)}%")

"""
EANet just replaces self attention in Vit with external attention.
The traditional Vit achieved a ~73% test top-5 accuracy and ~41 top-1 accuracy after
training 50 epochs, but with 0.6M parameters. Under the same experimental environment
and the same hyperparameters, The EANet model we just trained has just 0.3M parameters,
and it gets us to ~73% test top-5 accuracy and ~43% top-1 accuracy. This fully demonstrates the
effectiveness of external attention.

We only show the training
process of EANet, you can train Vit under the same experimental conditions and observe
the test results.
"""
