[pacman-dev] [RFC] Iterator interface for reading databases

[Sebastian Nowicki]

On Sun, Jul 17, 2011 at 12:32 AM, Rémy Oudompheng
<remyoudompheng at gmail.com> wrote:
> Hello,
>
> I am thinking an iterator-like interface for database reading could be
> useful. The most obvious example use is the pkgfile functionality.
> It could be a convenient tool to do one-pass work on databases
> without loading them completely in memory, which can be impossible for
> databases with files info (I don't think everyone can load the whole
> [community] database using libalpm standard structures).
>
> Here's a patch that roughly shows what I am thinking about: only the code
> for the "local" backend is shown.
>
> diff --git a/lib/libalpm/alpm.h b/lib/libalpm/alpm.h
> index 6e1e4bc..986e9a1 100644
> --- a/lib/libalpm/alpm.h
> +++ b/lib/libalpm/alpm.h
>
> [...]
>
> +struct local_dbreader_t {
> +       alpm_handle_t *handle;
> +       alpm_pkg_t *(*getitem) (alpm_dbreader_t*);
> +       DIR* dbdir;
> +       const char *dbpath;
> +       char finished;
> +};

How about refactoring this even more into a generic alpm_iterator_t?
Something like:

struct alpm_iterator_t {
    void *(*peek)(void *context);
    void *(*next)(void *context);
    void *context;
};

The context member would reference something similar to your
local_dbreader_t structure (just without the getitem member) which
contains all the state, and gets passed into peek() and next(). Then
you'd have a consistent API for any iterators alpm might use:

void *alpm_iter_peek(struct alpm_iterator_t *iter)
{
    // <insert sanity checks>
    return iter->peek(iter->context);
}

void *alpm_iter_next(struct alpm_iterator_t *iter)
{
    // <insert sanity checks>
    return iter->next(iter->context);
}

struct *alpm_iter_t db_iter = alpm_open_dbdir(db->handle, dbpath); //
or alpm_db_iter()?
pmpkg_t *pkg;
pkg = alpm_iter_peek(db_iter); // NULL
pkg = alpm_iter_next(db_iter); // a pmpkg_t*
pkg = alpm_iter_peek(db_iter); // same pkmpkg_t* as above