How can I obtain a closed curve in Tikz where part of it is smooth

Question

Using commands as shown in the answers to this question, one can obtain a closed, smooth cycle. Copying the example :

\begin{tikzpicture}
\draw [red] plot [smooth cycle] coordinates {(0,0) (1,1) (3,1) (1,0) (2,-1)};
\end{tikzpicture}

Consider that you want only part of the closed cycle to be smooth, e.g. this figure :

What should you change in the code?

It is preferred to have one command that provides a closed cycle, so one can fill it afterwards.

Answer 1

You can embed a smooth plot in a non-smooth path.

\documentclass[tikz,border=3mm]{standalone}
\begin{document}
\begin{tikzpicture}
\draw [red] (1,0) -- plot [smooth] coordinates { (2,-1) (0,0) (1,1) (3,1)  } --cycle;
\end{tikzpicture}
\end{document}