changepage: more robust even/odd test against page number changes

From Martin Münch:

if a two-sided document starts with a right side/page, and a left/even - right/odd check should be performed, one would need the absolute page number, for example \theCurrentPage from pagesLTS package:
"\theCurrentPage gives the current \theCurrentPage total/absolute page, in contrast to \thepage, which gives only the page name in the current page numbering scheme. For example, when there are Roman VII pages in the frontmatter and afterwards in the mainmatter you are at arabic page 9, then \theCurrentPage is 16, whereas \thepage is 9. When the page \number" (name) is manipulated by \addtocounter{page}{...} or \setcounter{page}{...}, \theCurrentPage ignores this. Because CurrentPage is a normal counter, you can also say e. g. \Roman{CurrentPage} to get the value in Roman page numbering scheme (e. g. VIII for 8)." (p. 8 of the pagesLTS documentation).
[snip]
\RequirePackage{everyshi}[2001/05/15]%v 3.00
\newcounter{cp@CurrentPage}
\setcounter{cp@CurrentPage}{1}
\EveryShipout{\addtocounter{cp@CurrentPage}{1}}
and using \arabic{cp@CurrentPage} instead of \thepage might help.
So, maybe you could add a "right/left" page check (assuming first page = right) to your package additional to the existing "even/odd" page number check?

wspr / herries-press

changepage: more robust even/odd test against page number changes #8