Time series prediction with fastai, pytorch and prophet

# ProFeTorch

FB Prophet + Fastai + pyTorch.

This is an alternative implementation of prophet which uses quantile regression instead of MCMC sampling. It provides the following benefits over prophet:

• GPU usage.
• Strict(er) upper and lower bounds.
• Can add any other set of features to the time series.

The time series is implemented as follows: \begin{aligned} y &= b(T(t) + S(t) + F(x)|l,u) \ T(t) &= mt + a \ S(t) &= \sum_{n=1}^N\left(a_n \cos\left(\frac{2\pi nt}{P}\right) + b_n \sin\left(\frac{2\pi nt}{P}\right)\right) \ F(x) &= w^T x\ b(y|l,u) &= \begin{cases} l \quad \text{if } y < l \ y \quad \text{if } l < y < u \ u \quad \text{if } y > u \end{cases} \end{aligned} where $T(t)$ is the trend line, $S(t)$ are the seasonal components composed of a fourier sum, $F(x)$ is a linear function which weights features that is not related to time.

The task is therefore to find the parameters $a, m, \cup_n a_n, \cup_n b_n, w$ that minimises a loss function $l(\hat{y}, y)$. The default is set to minimise $l1$ loss $\frac{1}{N}\sum_{i=1}^N |y_i - \hat{y_i}|$ so that the reliance on outliers is minimised. By default we also calculate the 5th and 95th quantile by minimising the tilted loss function. The quantile functions are calculated as: \begin{align} y_5 &= b(\hat{y} - (m_5 t + a_5)|l,u) \ y_{95} &= b(\hat{y} + (m_{95} t + a_{95})|l,u) \end{align}

## Install

pip install profetorch

## ProFeTorch Training

model_params = {'y_n':10, 'm_n':7, 'l':0, 'h': train_df['y'].max() * 2}
model = Model(train_df, model_args=model_params, epochs=30, alpha=1e-2, beta=0)
model.fit(train_df)

/opt/miniconda3/lib/python3.7/site-packages/pandas/core/frame.py:4117: SettingWithCopyWarning:
A value is trying to be set on a copy of a slice from a DataFrame

See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#returning-a-view-versus-a-copy
errors=errors,

Epoch 30/30 Training Loss: 0.3687, Validation Loss: 0.6105

y_pred = model.predict(df)
plt.figure(figsize=(12,5))
plt.scatter(df['ds'], df['y'], label='Data')
plt.plot(train_df['ds'], y_pred[:train_len,1], c='r', label='Train Set')
plt.fill_between(train_df['ds'], y_pred[:train_len,0], y_pred[:train_len,2], alpha=0.5)
plt.plot(test_df['ds'], y_pred[train_len:,1], c='g', label='Test Set')
plt.fill_between(test_df['ds'], y_pred[train_len:,0], y_pred[train_len:,2], alpha=0.5)
plt.show()


Obviously more works needs to be done as seen in the graph above. However, note that:

1. The seasonal component is captured.
2. The quantiles are asymmetric, which cannot happen in the fb-prophet case.