leqno on even pages and reqno on odd pages

Question

I would like to have the equation number on odd pages to the right and on even pages to the left. I found this How to put equation numbers on the left or the right, depending on whether the page is odd or even but it does not seem to work with the scrbook class. Any ideas on how to modify this?

Here is a MWE:

\documentclass[12pt,a4paper]{scrbook}

\usepackage{amsmath}
\linespread{1.5}

\makeatletter
\def\mathlabel#1{\@bsphack
  \protected@write\@auxout{}%
         {\string\newlabel{#1}{{\@currentlabel}{\thepage}}}%
  \@esphack}
\def\eqnWrite{\@bsphack
  \protected@write\@auxout{}%
         {\string\EqnStat{\theequation}{\thepage}}%
  \@esphack}%
\def\EqnStat#1#2{%
  \expandafter\gdef\csname eqn@#1\endcsname{#2}%
}
\newcommand\@reqnnum{\hb@[email protected]\p@{}%
                      \rlap{\normalfont\normalcolor%
                        \hskip -\displaywidth(\theequation)}}
\def\equation{\let\mathlabel\label$$\refstepcounter{equation}}
\def\endequation{\eqno\eqnWrite\@ifundefined{eqn@\theequation}{\hbox{\@eqnnum}}%%
     {\expandafter\ifodd\csname eqn@\theequation\endcsname\hbox{\@reqnnum}%
     \else\hbox{\@eqnnum}\fi}$$\@ignoretrue}
\def\@@eqncr{\let\reserved@a\relax%
    \ifcase\@eqcnt \def\reserved@a{& & &}\or \def\reserved@a{& &}%
     \or \def\reserved@a{&}\else%
       \let\reserved@a\@empty%
       \@latex@error{Too many columns in eqnarray environment}\@ehc\fi%
     \reserved@a \if@eqnsw\eqnWrite%
     \@ifundefined{eqn@\theequation}{\@eqnnum}%
     {\expandafter\ifodd\csname eqn@\theequation\endcsname\@reqnnum%
     \else\@eqnnum\fi}\stepcounter{equation}\fi%
     \global\@eqnswtrue\global\@eqcnt\z@\cr}
\makeatother


\begin{document}

\begin{equation}
   y = ax + b
\end{equation}

\begin{equation}
   a^n + b^n = c^n
\end{equation}

\end{document}

Replacing scrbook with report works fine.

Answer 1

I don't know about the scrbook class but perhaps the following will help, based on the changepage package provision of odd/even page checking.

%\documentclass...
\usepackage{changepage} \strictpagecheck
\newcommand{\leftright}{%
  \checkoddpage
  \ifoddpage
  % code for right eqno (which depends on your class and packages)
  \else
  % code for left eqno (which depends on your class and packages)
  \fi}
% ...
\begin{equation} \leftright
% ...
\end{equation}
% ...

If you don't want to keep adding \leftright after each \begin{equation} you can add it to the equation definition like:

\let\oldequation=\equation
\renewcommand*{\equation}{\oldequation \leftright}

and then just use \begin{equation} instead of \begin{equation} \leftright.