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Date: Wed, 23 Apr 2025 23:25:12 +0200
From: Michael Niedermayer <michael@niedermayer.cc>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Message-ID: <20250423212512.GT4991@pb2>
References: <20250420022929.724535-1-michael@niedermayer.cc>
<aAZdv3D5O57mysFP@phare.normalesup.org> <20250423204617.GS4991@pb2>
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Subject: Re: [FFmpeg-devel] [PATCH] avutil/map: [WIP] Introduction
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Hi Nicolas
On Wed, Apr 23, 2025 at 10:46:17PM +0200, Michael Niedermayer wrote:
> Hi Nicolas
>=20
> On Mon, Apr 21, 2025 at 05:01:19PM +0200, Nicolas George wrote:
> > Michael Niedermayer (HE12025-04-20):
[...]
> > > + *
> > > + *
> > > + * ----------- Retrieving an entry --------------
> > > + *
> > > + * AVMapEntry *e =3D av_map_get(map, "cat", AV_MAP_CMP_KEY); //Find =
an entry with the key =3D "cat"
> > > + *
> > > + * AVMapEntry *e =3D av_map_get(map, "cat", AV_MAP_CMP_KEY+AV_MAP_CM=
P_CASE_INSENSITIVE); //Find an entry with the key =3D "cat", "Cat", "cAt", =
=2E..
> >=20
> > > + * // this will only work if one of the set compare functions is cas=
e insensitive
> >=20
> > This is crucial: how? (This is redundant with my next remark.)
> >=20
> > > + *
> > > + *
> > > + * ----------- Iterating over all elements ------
> > > + *
> > > + * const AVMapEntry *t =3D NULL;
> > > + * while ((t =3D av_map_iterate(s, t)))
> > > + * printf("%s=3D%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 -----------
> > > + *
> >=20
> > > + * Each map has a primary compare function, which is used for orderi=
ng elements.
> > > + * Additional (compatible) compare functions can be added with av_ma=
p_add_cmp_func()
> > > + *
> > > + * What "compaibility" means here is that every added function retur=
ns 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 ca=
se insensitive
> > > + * compare would not be possible
> >=20
> > Code examples needed. For example: a map where key =3D name, value =3D =
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?
>=20
> The phone numbers i have seen are not affected by case sensitivity.
> So case sensitivity only matters for the name (which is in key)
>=20
> This should work:
>=20
> AVMap *map =3D 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);
>=20
> 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 se=
nsitivity
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 c=
an
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 insen=
sitive
matching key. Its still alot less asymptotically
thx
[...]
--=20
Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
He who knows, does not speak. He who speaks, does not know. -- Lao Tsu
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