Birational complexity of log Calabi-Yau 3-folds

We study the birational complexity of log Calabi-Yau $3$-folds. For such a pair $(X,B)$ of index one and coregularity zero, we show that $c_{\mathrm{b}\mathrm{i}\mathrm{r}}(X,B)\in \{0,2,3\}$. Further, we prove that $(X,B)$ has a log Calabi-Yau crepant birational model that admits a crepant contraction to $({\mathbb{P}}^{3-c},H_0+\cdots +H_{3-c})$, where $c=c_{\mathrm{b}\mathrm{i}\mathrm{r}}(X,B)$. To prove this, we give a geometric characterization of standard ${\mathbb{P}}^1$-links.