Let $[t]$ denote the greatest integer less than or equal to $t$.
If the function
\[
f(x)=
\begin{cases}
b^2 \sin\!\left[\dfrac{\pi}{2}\left[\dfrac{\pi}{2}(\cos x+\sin x)\cos x\right]\right], & x < 0 \\
\dfrac{\sin x-\dfrac{1}{2}\sin 2x}{x^3}, & x > 0 \\
a, & x = 0
\end{cases}
\]
is continuous at $x=0$, then $a^2+b^2$ is equal to