Re: [FFmpeg-devel] [PATCH] avutil/map: [WIP] Introduction

[Michael Niedermayer <michael@niedermayer.cc>]

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Hi Nicolas

On Wed, Apr 23, 2025 at 10:46:17PM +0200, Michael Niedermayer wrote:
> Hi Nicolas
> 
> On Mon, Apr 21, 2025 at 05:01:19PM +0200, Nicolas George wrote:
> > Michael Niedermayer (HE12025-04-20):
[...]
> > > + *
> > > + *
> > > + * ----------- Retrieving an entry --------------
> > > + *
> > > + * AVMapEntry *e = av_map_get(map, "cat", AV_MAP_CMP_KEY); //Find an entry with the key = "cat"
> > > + *
> > > + * AVMapEntry *e = av_map_get(map, "cat", AV_MAP_CMP_KEY+AV_MAP_CMP_CASE_INSENSITIVE); //Find an entry with the key = "cat", "Cat", "cAt", ...
> > 
> > > + * // this will only work if one of the set compare functions is case insensitive
> > 
> > This is crucial: how? (This is redundant with my next remark.)
> > 
> > > + *
> > > + *
> > > + * ----------- Iterating over all elements ------
> > > + *
> > > + * const AVMapEntry *t = NULL;
> > > + * while ((t = av_map_iterate(s, t)))
> > > + *     printf("%s=%s %zu,%zu   ", t->key, t->value, t->keylen, t->valuelen);
> > > + *
> > > + *
> > > + * ----------- copying all elements of a mep into another map
> > > + *
> > > + * av_map_copy(dst, src);
> > > + *
> > > + *
> > > + * ----------- freeing a map ---------------------
> > > + *
> > > + * av_map_free(&map);
> > > + *
> > > + *
> > > + * ----------- multiple compare function in a single map -----------
> > > + *
> > 
> > > + * Each map has a primary compare function, which is used for ordering elements.
> > > + * Additional (compatible) compare functions can be added with av_map_add_cmp_func()
> > > + *
> > > + * What "compaibility" means here is that every added function returns the same value
> > > + * as the primary function or 0.
> > > + *
> > > + * An example, Imagine we have "cat", "dog", "Dog", "fox"
> > > + * a function that treats "dog" and "Dog" as equal is compatible to this ordering
> > > + * OTOH
> > > + * if we have have strcmp() as primary function we would order like this:
> > > + * "Dog", "cat", "dog", "fox"
> > > + * and here we could not treat "dog" and "Dog" as equal, and thus case insensitive
> > > + * compare would not be possible
> > 
> > Code examples needed. For example: a map where key = name, value = phone
> > number, with AV_MAP_CMP_KEYVALUE. How do we set it up to allow
> > case-insensitive lookups and name only lookups, and name only
> > case-insensitive lookups?
> 
> The phone numbers i have seen are not affected by case sensitivity.
> So case sensitivity only matters for the name (which is in key)
> 
> This should work:
> 
> AVMap *map = av_map_alloc(av_map_supercmp_keyvalue, AV_MAP_CMP_KEYVALUE + AV_MAP_CMP_CASE_SENSITIVE, NULL, NULL);
> av_map_add_cmp_func(map,       av_map_supercmp_key, AV_MAP_CMP_KEY      + AV_MAP_CMP_CASE_SENSITIVE);
> av_map_add_cmp_func(map,             av_strcasecmp, AV_MAP_CMP_KEY      + AV_MAP_CMP_CASE_INSENSITIVE);
> 
> But the point you seem to try to raise, is correct,
> not every combination is possible

To elaborate on this

If we consider key_value and key searches and case sensitivity
then we have 6 possible things a user might want to search

for only the key there are case sensitive and case insensitive
for key_value there are 4 because both key and value have their own case sensitivity

If we use the syntax or I and S for the key only, and II,SS,IS,SI for the
key_value ones, then we can have for example the following in the same map

I ,S ,SI,SS
I ,II,IS,SS
I ,II,SI,SS

If these combination of 4 out of 6 isnt good enough, the user can use 2 maps
or we can implement the remaining by iteration over the closest subset we can
efficiently search for.
Currently we iterate over all entries with AVDictionary, if for a key value
both case insensitive match we have to iterate over all values from a insensitive
matching key. Its still alot less asymptotically

thx

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

He who knows, does not speak. He who speaks, does not know. -- Lao Tsu

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