imakeidx columns splitting

Question

I need to second column of the index created with imakeidx began only after the first one reaches the bottom of the page.

\documentclass{book}
\usepackage{imakeidx}
\makeindex[columns=2]

\begin{document}
 foo\index{a}.  foo\index{b}.  foo\index{c}.  foo\index{d}.  foo\index{e}.  foo\index{f}.  foo\index{g}.  foo\index{h}.  foo\index{i}.  foo\index{j}.  foo\index{k}.  foo\index{l}.  foo\index{m}.  foo\index{ab}.  foo\index{ac}.  foo\index{asd}.  foo\index{asddf}.  foo\index{aasef}. 
\printindex
\end{document}

The image shows what I have and what I need:

Answer 1

You have two possibilities: the first is to pass the original option

\documentclass[a5paper]{book}
\usepackage[pass]{geometry}

\usepackage[original]{imakeidx}
\makeindex

\begin{document}

foo\index{a}.  foo\index{b}.  foo\index{c}.  foo\index{d}.  foo\index{e}.  foo\index{f}.
foo\index{g}.  foo\index{h}.  foo\index{i}.  foo\index{j}.  foo\index{k}.  foo\index{l}.
foo\index{m}.  foo\index{ab}.  foo\index{ac}.  foo\index{asd}.  foo\index{asddf}.  foo\index{aasef}.

\printindex
\end{document}

Only two-column format is supported. I used A5 paper just to show the result more clearly:

The second strategy is to pass multicols* instead of multicols:

\documentclass[a5paper]{book}
\usepackage[pass]{geometry}
\usepackage{imakeidx}
\makeindex[columns=2]

\xpatchcmd{\theindex}{multicols}{multicols*}{}{}
\xpatchcmd{\endtheindex}{multicols}{multicols*}{}{}

\begin{document}
foo\index{a}.  foo\index{b}.  foo\index{c}.  foo\index{d}.  foo\index{e}.  foo\index{f}.
foo\index{g}.  foo\index{h}.  foo\index{i}.  foo\index{j}.  foo\index{k}.  foo\index{l}.
foo\index{m}.  foo\index{ab}.  foo\index{ac}.  foo\index{asd}.  foo\index{asddf}.  foo\index{aasef}.

\printindex
\end{document}

This has the advantage that all features of imakeidx are available.