Use display width in precision

[vitaut]

Use display width in precision. For example:

fmt::print("{:.5}", "🐱🐱🐱");

should print

🐱🐱

[jolz]

fmt::print("{:.2}", "🐱🐱🐱");

does print

🐱🐱

an this matches printf-specification (but with unicode support - yeah) https://en.cppreference.com/w/cpp/io/c/fprintf ("Precision specifies the maximum number of bytes to be written.")

std::printf("%.2s", "abcde"); // prints "ab"

why change?

[vitaut]

🐱 has a width of 2:

[nikhilreddydev]

Hello @vitaut

I have been looking into this issue, could you please help me in resolving below query.

• As far as I understand c++ does not have standard support to calculate display width of characters.
  Is the expectation here is to use external library like https://unicode-org.github.io/icu/userguide/strings/properties.html
• How to handle terminal specific display width behaviour

Also, I think python's format implementation does not consider display width.

[vitaut]

We already have display width estimation:

fmt/include/fmt/format.h

Line 638 in 6caff7e

FMT_CONSTEXPR inline auto compute_width(string_view s) -> size_t {

The only thing that is missing is applying it to precision. Python counts code points which doesn't work great with terminals.