Re: Not overwriting unchanged source code files when tangling

[Holger Hoefling <hhoeflin@gmail.com>]

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Hey Nick,

thank you very much. That sounds like a very good solution to my problem
that does not require changes to org-mode.

Best

Holger

On Sat, Nov 19, 2011 at 5:00 AM, Nick Dokos <nicholas.dokos@hp.com> wrote:

> Holger Hoefling <hhoeflin@gmail.com> wrote:
>
> > I think you misunderstood me there - I am actually not worried about how
> > computationally intensive the tangling process is. This always works very
> > quickly, so even if they have to be copied around and take a bit longer,
> I
> > would not mind.
> >
>
> Ah, ok - so you are talking about
>
>       tangle                    compile
>   org -------> bunch of files -------------> output
>
> The tangling step produces a bunch of files that are (re)compiled (or in
> any case require some sort of lengthy processing) to produce some output
> file.
>
>
> IMO, the best way to deal with it is still make: let's say
>
>     foo.org  ----> a.x b.x c.x ------> foo.out
>
> where the first arrow is the tangle and the second arrow is some
> processor, call it X.
> The standard way to set up a makefile is schematically:
>
> --8<---------------cut here---------------start------------->8---
> foo.out: a.x b.x c.x
>         X a.x b.c c.x -o foo.out
>
> a.x b.x c.x: foo.org
>         tangle foo.org
>
> --8<---------------cut here---------------end--------------->8---
>
>
> Rewrite the make file as follows:
>
>
> --8<---------------cut here---------------start------------->8---
> foo.out: a.y b.y c.y
>     X a.y b.y c.y -o foo.out
>
> a.y: a.x
>     cmp --silent a.x a.y || cp a.x a.y
>
> b.y: b.x
>     cmp --silent b.x b.y || cp b.x b.y
>
> c.y: c.x
>     cmp --silent c.x c.y || cp c.x c.y
>
> a.x b.x c.x: foo.org
>     tangle foo.org
> --8<---------------cut here---------------end--------------->8---
>
>
> So if the *contents* of (say) a.x have not changed by the tangling, it
> compares
> equal to a.y and the copy is skipped. That leaves a.y untouched.
>
> OTOH, if the contents of a.x change (or a.y does not exist in the first
> place), the comparison fails and we copy a.x to a.y.  That updates a.y
> and forces further updates on anything that depends on it.
>
> Using some make fu (works for GNU make, but not necessarily for other
> makes),
> you can write it more compactly:
>
>
> --8<---------------cut here---------------start------------->8---
> foo.out: a.y b.y c.y
>     X a.y b.y c.y -o foo.out
>
> %.y: %.x
>     -cmp --silent $< $@ || cp $< $@
>
> a.x b.x c.x: foo.org
>     tangle foo.org
> --8<---------------cut here---------------end--------------->8---
>
> HTH,
> Nick
>
>
>

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