Exiv2
LangAlt value sorting
Describe the bug
According to https://developer.adobe.com/xmp/docs/XMPNamespaces/XMPDataTypes/#language-alternative the first item in a LangAlt array is the default language. Also it is legitimate for there not to be an "x-default" item. Exiv2 stores LangAlt data in a std::map, which gets sorted by key so the user no longer knows which was the first item in the file.
The LangAltValueComparator currently compares length before doing a character by character comparison, so "x-default" is only being stored first because it's longer than any legitimate RFC3066 language tag. Setting a nonsense tag (e.g. fr-FranceAndCanada) moves "x-default" to second place in the XMP file.
To Reproduce
Read an XMP LangAlt value that doesn't have an "x-default" item. Attempt to find the first item.
Expected behavior
I'd like to have the LangAlt items in the same order as in the XMP file. I realise this may not be possible without substantial changes to Exiv2.
Additional context
Consider this more as a discussion point than a bug report.
I don't know much C++ but would using std::unordered_map help? This should allow the user to iterate over the (lang, text) pairs in the order they were read from the file.
Answer: no it wouldn't. std::unordered_map has undefined/random element order.
As far as I can tell, the only solution is to change the container to a vector of key:value pairs, or to add a separate list of keys to preserve the item order.