[libc++][format] Implement Unicode support.

[Mordante]

Summary

This adds the width estimation functions to the std-format-spec.

Implements parts of:

• P0645 Text Formatting
• P1868 width: clarifying units of width and precision in std::format

[Mordante]

Remove an accidental edit, which caused the code to become invalid.

[Mordante]

Rebase.
Improve comments.

[Mordante]

Temporary disable a failing test on Windows.

[vitaut]

libcxx/include/__format/parser_std_format_spec.h

754

I think you mean code units and not characters. Same throughout the diff.

764

16 ->32

772–773

I think a more robust error handling would be to count each invalid code unit as contributing 1 to the width. Terminals normally use replacement characters so it will give the correct result.

829–852

A sequence of ifs would likely be more readable.

913–915

Not sure what you mean by "a full blown Unicode parser" but I don't think it's ever a good idea to throw exception on width estimation.

917

Why not set __threshold to the maximum value to disable it instead of introducing another flag?

936

I recommend returning a struct with meaningful field names.

1079

What does the check __precision != __format::__number_max do here?

Note that __number_max is a valid precision.

[Mordante]

libcxx/include/__format/parser_std_format_spec.h

754

Correct.

772–773

Not entirely sure what you mean. This algorithm is fast, but simple. The engine first uses this algorithm, when it "fails" it uses a more advanced algorithm. The more advanced algorithm implements the estimation as defined in the Standard.
Basically this algorithm's main purpose is to handle ASCII, when it's not ASCII the Standard's estimation is used.

829–852

Agreed. I originally had a sequence of ifs, but that was a bit slow. I used this to speed things up. However I'll look whether I can use ifs in a smarter fashion to improve readability.

913–915

When I think about a "a full blown Unicode parser" I think of something like the ICU library.
I think here it makes sense to use your previous suggestion:
"I think a more robust error handling would be to count each invalid code unit as contributing 1 to the width. "

917

That can give the issue, where the result can be off by one. I'll see whether I can find a way to improve this code.

1079

This was based on my initial assumption 999.999.999 would be more than anybody would use for a width or precision. Since that assumption's wrong the test here needs to be adjusted.

1082

As pointed out in D103425 the width can be larger than the precision. So this test needs to be removed.

[Mordante]

Addresses part of the review comments:

• Use code units instead of characters in the comments
• Remove the usage of std::pair
• Precision == -1 is no precision instead of __number_max
• Allow a precision less than the width

The other comments will be addressed later, I want to update the rest of the ser
ies before focussing on these issues:

• Improve the threshold algorithm
• See whether the long returns can be improved without performance degradations.

[Mordante]

Need to address the last review comments.

[Mordante]

Disable a test on Windows, that change was accidentally reverted in the last version.

[Mordante]

libcxx/include/__format/parser_std_format_spec.h

829–852

I tried some different approaches, but they lead to performance degradation. (I also tested the version used in libfmt.)
I think the easiest way to solve this is by improving the formatting,so I went for that route.

917

Setting it to maximum is less efficient. For example, when a width of 10 is given it's not needed to parse a 1024 code unit input. However it's possible to increase the wanted width + 1 to get the same effect as the current __truncate argument. This still may scan one code unit more than required, but I consider that not an issue.

[Mordante]

Improve the readability of a long return statement, this found some issues in the return statement.
Simplify __estimate_column_width by removing a template argument.
Malformed Unicode now uses 1 column per code unit.

[Mordante]

Fixes benchmark.

[Mordante]

Rebase.
Adds a missing include due to the usage of the new monostate header.

[vitaut]

libcxx/include/__format/parser_std_format_spec.h

772–773

I misinterpreted how the two scanners interact. Makes sense now.

[vitaut]

libcxx/include/__format/parser_std_format_spec.h

726–728

I am not sure I understand what "the formatted output has only a minimum width" means. What is the minimum width?

734

Why would the estimation algorithm stop prematurely?

753

nit: I'd add a colon after "formats".

[vitaut]

libcxx/include/__format/parser_std_format_spec.h

21

I wonder if it is possible to pull in a subset of <algorithm> because the whole thing is enormous, especially in C++20?

[Mordante]

Thanks for the review!

libcxx/include/__format/parser_std_format_spec.h

21

I think that's now possible since the header has been split recently. I'll have to see whether this project has been completed or still work in progress.

726–728

The minimum width is the width specified in the format-spec. For example std::format("{:10}", MyString); As soon as the code has determined MyString results in at least 10 output columns it knows no padding is required. At that point it's a waste of CPU cycles to determine the exact number of output columns MyString will result to. So at that point the algorithm stops prematurely.

I think I'll add some more comment regarding this behavior.

734

As described in the comment above.

[Mordante]

Rebased
Addresses review comments
Marked a unit tests as libc++ specific

[vitaut]

libcxx/include/__format/parser_std_format_spec.h

717–719

nit: I would suggest changing the names of both __format_string_traits and __get_format_string_traits because the current naming gives an impressing that they have something to do with format strings while in fact this is an estimation of display width for a string argument. Maybe something along the lines of __[format_]estimate_display_width for the function name?

[vitaut]

libcxx/include/__format/parser_std_format_spec.h

717–719

Or __[format_]get_string_alignment since it decides whether alignment should be applied and returns information about how to apply it.

[Mordante]

Based on review comments s/format_string_traits/string_alignment/

[Mordante]

Thanks for the review!

[vitaut]

LGTM

[Mordante]

libcxx/include/__format/parser_std_format_spec.h

830

s/_LIBCPP_INLINE_VISIBILITY/_LIBCPP_HIDE_FROM_ABI/ here and all other occurrences in this patch.

[Mordante]

Rebased and minor cleanups.

[Mordante]

Adds UNSUPPORTED: libcpp-has-no-incomplete-format for the test.

[Mordante]

Rebased to trigger CI.

[Mordante]

Rebased to update newer patches in the series.

[Mordante]

Guard against min/max macros.

[ldionne]

libcxx/benchmarks/CMakeLists.txt

148–149 ↗

(On Diff #372870)

Can we make those changes as part of another review? Also, let's set CXX_STANDARD_REQUIRED YES instead -- all supported compilers should support C++20 now.

libcxx/benchmarks/std_format_spec_string_unicode.bench.cpp

8

I don't understand this TODO. We do require C++20 support.

18–20

I'm not sure this is useful -- if the library doesn't support unicode, something in the benchmark is going to fail anyway. It would be nice to remove as many libc++ specific things from the benchmark as possible.

22

The benchmarks should be compiled without assertions anyway -- it seems pretty hacky to undefine this here, no?

28

If we're going to do something like this, can we instead do namespace format_spec = std::__format_spec or something similar?

85

This line break is really odd.

libcxx/include/__format/parser_std_format_spec.h

758

Just a general comment -- we generally try to use C++ style comments (//) elsewhere in the library. I don't have the heart to ask you to change them cause it's such a nitpick, but please consider using that style going forward for consistency.

809

Would it make sense to name those __utf8_char or __utf8_character instead? Just __utf8 seems kind of obtuse to me. (applies below too)

876

This needs to be inline since it's not a template and it's in a header. This certainly applies elsewhere too, please take a look. Otherwise, those are ODR violations waiting to happen (arguably those ODR violations are benign, but still).

libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_string_unicode.pass.cpp

2

I would suggest splitting this into two files and using the libcpp-has-no-unicode Lit feature to enable/disable them. That sounds a bit cleaner since essentially the whole test is different depending on whether unicode is supported. Your call, this is not a blocking comment.

51

It just occurred to me that I am confused by the existence of both _LIBCPP_HAS_NO_UNICODE and _LIBCPP_HAS_NO_UNICODE_CHARS -- can you shed some light on this if it's fresh in your mind?

[Mordante]

Thanks for the review. I'll update the patch after you land D110338.

libcxx/benchmarks/CMakeLists.txt

148–149 ↗

(On Diff #372870)

See D110338.

libcxx/benchmarks/std_format_spec_string_unicode.bench.cpp

8

This TODO was written at a time we didn't require it yet. Hence the TODO to remind me. I remove the test and the comment.

18–20

Good catch this is already tested at the start of the file. This benchmark is really intended to test some libcxx implementation details. For example the __column_width_3 function has several different ways to implement it. This had a performance impact. If the benchmarks should be usable for other compilers/standard libraries I would rather add some guards in this test.

22

I agree I think I used the benchmarks for testing originally. So I removed this hack and the validation.

85

My clang-format settings are still at 80 columns to avoid merge conflicts in this series. I'll fix this manually.

libcxx/include/__format/parser_std_format_spec.h

758

I'll try to keep it in mind. After we switched to clang-fomrat-13 I still want to find some time to reformat the code using our new settings. Then I probably will look at fixing these comments.

876

I omitted the inline since constexpr functions are implicitly inline, guarding against ODR violations.
http://eel.is/c++draft/dcl.constexpr#1

A function or static data member declared with the constexpr or consteval specifier is implicitly an inline function or variable ([dcl.inline]).

Did I misinterpret this rule?
Do you still prefer the inline?

libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_string_unicode.pass.cpp

2

I like your suggestion it indeed sounds like a cleaner solution.

51

_LIBCPP_HAS_NO_UNICODE is the switch I added so the user can disable Unicode output. It's introduced in libc++13 using a CMake option. (If needed we can look whether this define can/should be renamed.)

_LIBCPP_HAS_NO_UNICODE_CHARS is not fresh in my mind, but grep does miracles ;-)
_LIBCPP_HAS_NO_UNICODE_CHARS is enabled in __config when the IBM compiler is used.
It was added by Howard in 2013, I've no idea whether that's still needed.
@DanielMcIntosh-IBM, @daltenty do you know whether the IBM compiler has proper support for char16_t and char32_t?

[Mordante]

Addresses review comments.

[Mordante]

Mark some functions inline. Not strictly required but @ldionne prefers it, as stated in D110515

libcxx/benchmarks/std_format_spec_string_unicode.bench.cpp

+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_HAS_NO_UNICODE
+
+#include <array>
+#include <format>
+
+#include "benchmark/benchmark.h"
+
+#include "test_macros.h"
+
+template <class CharT, size_t N>
+class tester {
+  static constexpr size_t size_ = N - 1;
+  std::array<CharT, 100 * size_> data_;
+
+public:
+  explicit constexpr tester(const CharT (&input)[N]) {
+    auto it = data_.begin();
+    for (int i = 0; i < 100; ++i)
+      it = std::copy_n(input, size_, it);
+  }
+
+  constexpr size_t size() const noexcept { return data_.size(); }
+  constexpr const CharT* begin() const noexcept { return data_.begin(); }
+  constexpr const CharT* end() const noexcept { return data_.end(); }
+
+  void test(benchmark::State& state) const {
+    for (auto _ : state)
+      benchmark::DoNotOptimize(std::__format_spec::__get_string_alignment(
+          begin(), end(), 1'000'000, 1'000'000));
+    state.SetItemsProcessed(state.iterations() * size());
+  }
+};
+
+#define TEST(u8)                                                               \
+  if constexpr (std::same_as<CharT, char>) {                                   \
+    constexpr auto p = tester{u8};                                             \
+    p.test(state);                                                             \
+  } else if constexpr (std::same_as<CharT, char16_t>) {                        \
+    constexpr auto p = tester{TEST_CONCAT(u, u8)};                             \
+    p.test(state);                                                             \
+  } else {                                                                     \
+    constexpr auto p = tester{TEST_CONCAT(U, u8)};                             \
+    p.test(state);                                                             \
+  }
+
+template <class CharT>
+static void BM_EstimateLengthNoMultiByte(benchmark::State& state) {
+  TEST("The quick brown fox jumps over the lazy dog");
+}
+
+template <class CharT>
+static void BM_EstimateLengthTwoByteDE(benchmark::State& state) {
+  static_assert(sizeof("Victor jagt zwölf Boxkämpfer quer über den großen Sylter Deich") == 67);
+
+  // https://en.wikipedia.org/wiki/Pangram
+  TEST("Victor jagt zwölf Boxkämpfer quer über den großen Sylter Deich");
+}
+
+template <class CharT>
+static void BM_EstimateLengthTwoBytePL(benchmark::State& state) {
+  static_assert(sizeof("Stróż pchnął kość w quiz gędźb vel fax myjń") == 53);
+
+  // https://en.wikipedia.org/wiki/Pangram
+  TEST("Stróż pchnął kość w quiz gędźb vel fax myjń");
+}
+
+// All values below are 1100, which is is the first multi column sequence.
+template <class CharT>
+static void BM_EstimateLengthThreeByteSingleColumnLow(benchmark::State& state) {
+  static_assert(sizeof("\u0800\u0801\u0802\u0803\u0804\u0805\u0806\u0807"
+                       "\u0808\u0809\u080a\u080b\u080c\u080d\u080e\u080f") ==
+                49);
+
+  TEST("\u0800\u0801\u0802\u0803\u0804\u0805\u0806\u0807"
+       "\u0808\u0809\u080a\u080b\u080c\u080d\u080e\u080f");
+}
+
+template <class CharT>
+static void
+BM_EstimateLengthThreeByteSingleColumnHigh(benchmark::State& state) {
+  static_assert(sizeof("\u1800\u1801\u1802\u1803\u1804\u1805\u1806\u1807"
+                       "\u1808\u1809\u180a\u180b\u180c\u180d\u180e\u180f") ==
+                49);
+
+  TEST("\u1800\u1801\u1802\u1803\u1804\u1805\u1806\u1807"
+       "\u1808\u1809\u180a\u180b\u180c\u180d\u180e\u180f");
+}
+
+template <class CharT>
+static void BM_EstimateLengthThreeByteDoubleColumn(benchmark::State& state) {
+  static_assert(sizeof("\u1100\u0801\u0802\u0803\u0804\u0805\u0806\u0807"
+                       "\u1108\u0809\u080a\u080b\u080c\u080d\u080e\u080f") ==
+                49);
+
+  TEST("\u1100\u0801\u0802\u0803\u0804\u0805\u0806\u0807"
+       "\u1108\u0809\u080a\u080b\u080c\u080d\u080e\u080f");
+}
+
+template <class CharT>
+static void BM_EstimateLengthThreeByte(benchmark::State& state) {
+  static_assert(sizeof("\u1400\u1501\ubbbb\uff00\u0800\u4099\uabcd\u4000"
+                       "\u8ead\ubeef\u1111\u4987\u4321\uffff\u357a\ud50e") ==
+                49);
+
+  TEST("\u1400\u1501\ubbbb\uff00\u0800\u4099\uabcd\u4000"
+       "\u8ead\ubeef\u1111\u4987\u4321\uffff\u357a\ud50e");
+}
+
+template <class CharT>
+static void BM_EstimateLengthFourByteSingleColumn(benchmark::State& state) {
+  static_assert(sizeof("\U00010000\U00010001\U00010002\U00010003"
+                       "\U00010004\U00010005\U00010006\U00010007"
+                       "\U00010008\U00010009\U0001000a\U0001000b"
+                       "\U0001000c\U0001000d\U0001000e\U0001000f") == 65);
+
+  TEST("\U00010000\U00010001\U00010002\U00010003"
+       "\U00010004\U00010005\U00010006\U00010007"
+       "\U00010008\U00010009\U0001000a\U0001000b"
+       "\U0001000c\U0001000d\U0001000e\U0001000f");
+}
+
+template <class CharT>
+static void BM_EstimateLengthFourByteDoubleColumn(benchmark::State& state) {
+  static_assert(sizeof("\U00020000\U00020002\U00020002\U00020003"
+                       "\U00020004\U00020005\U00020006\U00020007"
+                       "\U00020008\U00020009\U0002000a\U0002000b"
+                       "\U0002000c\U0002000d\U0002000e\U0002000f") == 65);
+
+  TEST("\U00020000\U00020002\U00020002\U00020003"
+       "\U00020004\U00020005\U00020006\U00020007"
+       "\U00020008\U00020009\U0002000a\U0002000b"
+       "\U0002000c\U0002000d\U0002000e\U0002000f");
+}
+
+template <class CharT>
+static void BM_EstimateLengthFourByte(benchmark::State& state) {
+  static_assert(sizeof("\U00010000\U00010001\U00010002\U00010003"
+                       "\U00020004\U00020005\U00020006\U00020007"
+                       "\U00010008\U00010009\U0001000a\U0001000b"
+                       "\U0002000c\U0002000d\U0002000e\U0002000f") == 65);
+
+  TEST("\U00010000\U00010001\U00010002\U00010003"
+       "\U00020004\U00020005\U00020006\U00020007"
+       "\U00010008\U00010009\U0001000a\U0001000b"
+       "\U0002000c\U0002000d\U0002000e\U0002000f");
+}
+
+BENCHMARK_TEMPLATE(BM_EstimateLengthNoMultiByte, char);
+BENCHMARK_TEMPLATE(BM_EstimateLengthTwoByteDE, char);
+BENCHMARK_TEMPLATE(BM_EstimateLengthTwoBytePL, char);
+BENCHMARK_TEMPLATE(BM_EstimateLengthThreeByteSingleColumnLow, char);
+BENCHMARK_TEMPLATE(BM_EstimateLengthThreeByteSingleColumnHigh, char);
+BENCHMARK_TEMPLATE(BM_EstimateLengthThreeByteDoubleColumn, char);
+BENCHMARK_TEMPLATE(BM_EstimateLengthThreeByte, char);
+BENCHMARK_TEMPLATE(BM_EstimateLengthFourByteSingleColumn, char);
+BENCHMARK_TEMPLATE(BM_EstimateLengthFourByteDoubleColumn, char);
+BENCHMARK_TEMPLATE(BM_EstimateLengthFourByte, char);
+
+BENCHMARK_TEMPLATE(BM_EstimateLengthNoMultiByte, char16_t);
+BENCHMARK_TEMPLATE(BM_EstimateLengthTwoByteDE, char16_t);
+BENCHMARK_TEMPLATE(BM_EstimateLengthTwoBytePL, char16_t);
+BENCHMARK_TEMPLATE(BM_EstimateLengthThreeByteSingleColumnLow, char16_t);
+BENCHMARK_TEMPLATE(BM_EstimateLengthThreeByteSingleColumnHigh, char16_t);
+BENCHMARK_TEMPLATE(BM_EstimateLengthThreeByteDoubleColumn, char16_t);
+BENCHMARK_TEMPLATE(BM_EstimateLengthThreeByte, char16_t);
+BENCHMARK_TEMPLATE(BM_EstimateLengthFourByteSingleColumn, char16_t);
+BENCHMARK_TEMPLATE(BM_EstimateLengthFourByteDoubleColumn, char16_t);
+BENCHMARK_TEMPLATE(BM_EstimateLengthFourByte, char16_t);
+
+BENCHMARK_TEMPLATE(BM_EstimateLengthNoMultiByte, char32_t);
+BENCHMARK_TEMPLATE(BM_EstimateLengthTwoByteDE, char32_t);
+BENCHMARK_TEMPLATE(BM_EstimateLengthTwoBytePL, char32_t);
+BENCHMARK_TEMPLATE(BM_EstimateLengthThreeByteSingleColumnLow, char32_t);
+BENCHMARK_TEMPLATE(BM_EstimateLengthThreeByteSingleColumnHigh, char32_t);
+BENCHMARK_TEMPLATE(BM_EstimateLengthThreeByteDoubleColumn, char32_t);
+BENCHMARK_TEMPLATE(BM_EstimateLengthThreeByte, char32_t);
+BENCHMARK_TEMPLATE(BM_EstimateLengthFourByteSingleColumn, char32_t);
+BENCHMARK_TEMPLATE(BM_EstimateLengthFourByteDoubleColumn, char32_t);
+BENCHMARK_TEMPLATE(BM_EstimateLengthFourByte, char32_t);
+
+int main(int argc, char** argv) {
+  benchmark::Initialize(&argc, argv);
+  if (benchmark::ReportUnrecognizedArguments(argc, argv))
+    return 1;
+
+  benchmark::RunSpecifiedBenchmarks();
+}
+#else
+int main(int, char**) { return 0; }
+#endif

libcxx/docs/Status/Cxx20Papers.csv

 "","","","","",""
 "`P0586 <https://wg21.link/P0586>`__","LWG","Safe integral comparisons","Prague","|Complete|","13.0"
 "`P0593 <https://wg21.link/P0593>`__","CWG","Implicit creation of objects for low-level object manipulation","Prague","* *",""
 "`P1115 <https://wg21.link/P1115>`__","LWG","Improving the Return Value of Erase-Like Algorithms II: Free erase/erase if","Prague","|Complete|","11.0"
 "`P1243 <https://wg21.link/P1243>`__","LWG","Rangify New Algorithms","Prague","* *",""
 "`P1460 <https://wg21.link/P1460>`__","LWG","Mandating the Standard Library: Clause 20 - Utilities library","Prague","* *",""
 "`P1739 <https://wg21.link/P1739>`__","LWG","Avoid template bloat for safe_ranges in combination with ""subrange-y"" view adaptors","Prague","* *",""
 "`P1831 <https://wg21.link/P1831>`__","LWG","Deprecating volatile: library","Prague","* *",""
-"`P1868 <https://wg21.link/P1868>`__","LWG","width: clarifying units of width and precision in std::format","Prague","* *",""
+"`P1868 <https://wg21.link/P1868>`__","LWG","width: clarifying units of width and precision in std::format","Prague","|In Progress|",""
 "`P1908 <https://wg21.link/P1908>`__","CWG","Reserving Attribute Namespaces for Future Use","Prague","* *",""
 "`P1937 <https://wg21.link/P1937>`__","CWG","Fixing inconsistencies between constexpr and consteval functions","Prague","* *",""
 "`P1956 <https://wg21.link/P1956>`__","LWG","On the names of low-level bit manipulation functions","Prague","|Complete|","12.0"
 "`P1957 <https://wg21.link/P1957>`__","CWG","Converting from ``T*``\  to bool should be considered narrowing (re: US 212)","Prague","* *",""
 "`P1963 <https://wg21.link/P1963>`__","LWG","Fixing US 313","Prague","* *",""
 "`P1964 <https://wg21.link/P1964>`__","LWG","Wording for boolean-testable","Prague","|Complete|","13.0"
 "`P1970 <https://wg21.link/P1970>`__","LWG","Consistency for size() functions: Add ranges::ssize","Prague","* *",""
 "`P1973 <https://wg21.link/P1973>`__","LWG","Rename ""_default_init"" Functions, Rev1","Prague","* *",""

libcxx/include/__format/parser_std_format_spec.h

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___FORMAT_PARSER_STD_FORMAT_SPEC_H
 #define _LIBCPP___FORMAT_PARSER_STD_FORMAT_SPEC_H
 
+#include <__algorithm/find_if.h>
+#include <__algorithm/min.h>
 #include <__config>
 #include <__debug>
 #include <__format/format_arg.h>
 #include <__format/format_error.h>
 #include <__format/format_string.h>
 #include <__variant/monostate.h>
+#include <bit>
 #include <concepts>
 #include <cstdint>
 #include <type_traits>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 # pragma GCC system_header
 #endif
 
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 17
 
 // TODO FMT Remove this once we require compilers with proper C++20 support.
 // If the compiler has no concepts support, the format header will be disabled.
 // Without concepts support enable_if needs to be used and that too much effort
 // to support compilers with partial C++20 support.
if (this->__zero_padding)
       __throw_format_error(
           "A zero-padding field isn't allowed in this format-spec");
 
     if (this->__alignment == _Flags::_Alignment::__default)
       this->__alignment = _Flags::_Alignment::__left;
   }
 };
 
 // TODO FMT Add a parser for floating-point values.
 // TODO FMT Add a parser for pointer values.
 
+/** Helper struct returned from @ref __get_string_alignment. */
+template <class _CharT>
+struct _LIBCPP_TEMPLATE_VIS __string_alignment {
+  /** Points beyond the last character to write to the output. */
+  const _CharT* __last;
+  /**
+   * The estimated number of columns in the output or 0.
+   *
+   * Only when the output needs to be aligned it's required to know the exact
+   * number of columns in the output. So if the formatted output has only a
+   * minimum width the exact size isn't important. It's only important to know
+   * the minimum has been reached. The minimum width is the width specified in
+   * the format-spec.
+   *
+   * For example in this code @code std::format("{:10}", MyString); @endcode
+   * the width estimation can stop once the algorithm has determined the output
+   * width is 10 columns.
+   *
+   * So if:
+   * * @ref __align == @c true the @ref __size is the estimated number of
+   *   columns required.
+   * * @ref __align == @c false the @ref __size is the estimated number of
+   *   columns required or 0 when the estimation algorithm stopped prematurely.
+   */
+  ptrdiff_t __size;
+  /**
+   * Does the output need to be aligned.
+   *
+   * When alignment is needed the output algorithm needs to add the proper
+   * padding. Else the output algorithm just needs to copy the input up to
+   * @ref __last.
+   */
+  bool __align;
+};
+
+#ifndef _LIBCPP_HAS_NO_UNICODE
+namespace __detail {
+
+/**
+ * Unicode column width estimates.
+ *
+ * Unicode can be stored in several formats: UTF-8, UTF-16, and UTF-32.
+ * Depending on format the relation between the number of code units stored and
+ * the number of output columns differs. The first relation is the number of
+ * code units forming a code point. (The text assumes the code units are
+ * unsigned.)
+ * - UTF-8 The number of code units is between one and four. The first 127
+ *   Unicode code points match the ASCII character set. When the highest bit is
+ *   set it means the code point has more than one code unit.
+ * - UTF-16: The number of code units is between 1 and 2. When the first
+ *   code unit is in the range [0xd800,0xdfff) it means the code point uses two
+ *   code units.
+ * - UTF-32: The number of code units is always one.
+ *
+ * The code point to the number of columns isn't well defined. The code uses the
+ * estimations defined in [format.string.std]/11. This list might change in the
+ * future.
+ *
+ * The algorithm of @ref __get_string_alignment uses two different scanners:
+ * - The simple scanner @ref __estimate_column_width_fast. This scanner assumes
+ *   1 code unit is 1 column. This scanner stops when it can't be sure the
+ *   assumption is valid:
+ *   - UTF-8 when the code point is encoded in more than 1 code unit.
+ *   - UTF-16 and UTF-32 when the first multi-column code point is encountered.
+ *     (The code unit's value is lower than 0xd800 so the 2 code unit encoding
+ *     is irrelevant for this scanner.)
+ *   Due to these assumptions the scanner is faster than the full scanner. It
+ *   can process all text only containing ASCII. For UTF-16/32 it can process
+ *   most (all?) European languages. (Note the set it can process might be
+ *   reduced in the future, due to updates in the scanning rules.)
+ * - The full scanner @ref __estimate_column_width. This scanner, if needed,
+ *   converts multiple code units into one code point then converts the code
+ *   point to a column width.
+ *
+ * See also:
+ * - [format.string.general]/11
+ * - https://en.wikipedia.org/wiki/UTF-8#Encoding
+ * - https://en.wikipedia.org/wiki/UTF-16#U+D800_to_U+DFFF
+ */
+
+/**
+ * The first 2 column code point.
+ *
+ * This is the point where the fast UTF-16/32 scanner needs to stop processing.
+ */
+inline constexpr uint32_t __two_column_code_point = 0x1100;
+
+/** Helper concept for an UTF-8 character type. */
+template <class _CharT>
+concept __utf8_character = same_as<_CharT, char> || same_as<_CharT, char8_t>;
+
+/** Helper concept for an UTF-16 character type. */
+template <class _CharT>
+concept __utf16_character = (same_as<_CharT, wchar_t> && sizeof(wchar_t) == 2) || same_as<_CharT, char16_t>;
+
+/** Helper concept for an UTF-32 character type. */
+template <class _CharT>
+concept __utf32_character = (same_as<_CharT, wchar_t> && sizeof(wchar_t) == 4) || same_as<_CharT, char32_t>;
+
+/** Helper concept for an UTF-16 or UTF-32 character type. */
+template <class _CharT>
+concept __utf16_or_32_character = __utf16_character<_CharT> || __utf32_character<_CharT>;
+
+/**
+ * Converts a code point to the column width.
+ *
+ * The estimations are conforming to [format.string.general]/11
+ *
+ * This version expects a value less than 0x1'0000, which is a 3-byte UTF-8
+ * character.
+ */
+_LIBCPP_HIDE_FROM_ABI inline constexpr int __column_width_3(uint32_t __c) noexcept {
+  _LIBCPP_ASSERT(__c < 0x1'0000,
+                 "Use __column_width_4 or __column_width for larger values");
+
+  // clang-format off
+  return 1 + (__c >= 0x1100 && (__c <= 0x115f ||
+             (__c >= 0x2329 && (__c <= 0x232a ||
+             (__c >= 0x2e80 && (__c <= 0x303e ||
+             (__c >= 0x3040 && (__c <= 0xa4cf ||
+             (__c >= 0xac00 && (__c <= 0xd7a3 ||
+             (__c >= 0xf900 && (__c <= 0xfaff ||
+             (__c >= 0xfe10 && (__c <= 0xfe19 ||
+             (__c >= 0xfe30 && (__c <= 0xfe6f ||
+             (__c >= 0xff00 && (__c <= 0xff60 ||
+             (__c >= 0xffe0 && (__c <= 0xffe6
+             ))))))))))))))))))));
+  // clang-format on
+}
+
+/**
+ * @overload
+ *
+ * This version expects a value greater than or equal to 0x1'0000, which is a
+ * 4-byte UTF-8 character.
+ */
+_LIBCPP_HIDE_FROM_ABI inline constexpr int __column_width_4(uint32_t __c) noexcept {
+  _LIBCPP_ASSERT(__c >= 0x1'0000,
+                 "Use __column_width_3 or __column_width for smaller values");
+
+  // clang-format off
+  return 1 + (__c >= 0x1'f300 && (__c <= 0x1'f64f ||
+             (__c >= 0x1'f900 && (__c <= 0x1'f9ff ||
+             (__c >= 0x2'0000 && (__c <= 0x2'fffd ||
+             (__c >= 0x3'0000 && (__c <= 0x3'fffd
+             ))))))));
+  // clang-format on
+}
+
+/**
+ * @overload
+ *
+ * The general case, accepting all values.
+ */
+_LIBCPP_HIDE_FROM_ABI inline constexpr int __column_width(uint32_t __c) noexcept {
+  if (__c < 0x1'0000)
+    return __column_width_3(__c);
+
+  return __column_width_4(__c);
+}
+
+/**
+ * Estimate the column width for the UTF-8 sequence using the fast algorithm.
+ */
+template <__utf8_character _CharT>
+_LIBCPP_HIDE_FROM_ABI constexpr const _CharT*
+__estimate_column_width_fast(const _CharT* __first,
+                             const _CharT* __last) noexcept {
+  return _VSTD::find_if(__first, __last,
+                        [](unsigned char __c) { return __c & 0x80; });
+}
+
+/**
+ * @overload
+ *
+ * The implementation for UTF-16/32.
+ */
+template <__utf16_or_32_character _CharT>
+_LIBCPP_HIDE_FROM_ABI constexpr const _CharT*
+__estimate_column_width_fast(const _CharT* __first,
+                             const _CharT* __last) noexcept {
+  return _VSTD::find_if(__first, __last,
+                        [](uint32_t __c) { return __c >= 0x1100; });
+}
+
+template <class _CharT>
+struct _LIBCPP_TEMPLATE_VIS __column_width_result {
+  /** The number of output columns. */
+  size_t __width;
+  /**
+   * The last parsed element.
+   *
+   * This limits the original output to fit in the wanted number of columns.
+   */
+  const _CharT* __ptr;
+};
+
+/**
+ * Small helper to determine the width of malformed Unicode.
+ *
+ * @note This function's only needed for UTF-8. During scanning UTF-8 there
+ * are multiple place where it can be detected that the Unicode is malformed.
+ * UTF-16 only requires 1 test and UTF-32 requires no testing.
+ */
+template <__utf8_character _CharT>
+_LIBCPP_HIDE_FROM_ABI constexpr __column_width_result<_CharT>
+__estimate_column_width_malformed(const _CharT* __first, const _CharT* __last,
+                                  size_t __maximum, size_t __result) noexcept {
+  size_t __size = __last - __first;
+  size_t __n = _VSTD::min(__size, __maximum);
+  return {__result + __n, __first + __n};
+}
+
+/**
+ * Determines the number of output columns needed to render the input.
+ *
+ * @note When the scanner encounters malformed Unicode it acts as-if every code
+ * unit at the end of the input is one output column. It's expected the output
+ * terminal will replace these malformed code units with a one column
+ * replacement characters.
+ *
+ * @param __first   Points to the first element of the input range.
+ * @param __last    Points beyond the last element of the input range.
+ * @param __maximum The maximum number of output columns. The returned number
+ *                  of estimated output columns will not exceed this value.
+ */
+template <__utf8_character _CharT>
+_LIBCPP_HIDE_FROM_ABI constexpr __column_width_result<_CharT>
+__estimate_column_width(const _CharT* __first, const _CharT* __last,
+                        size_t __maximum) noexcept {
+  size_t __result = 0;
+
+  while (__first != __last) {
+    // Based on the number of leading 1 bits the number of code units in the
+    // code point can be determined. See
+    // https://en.wikipedia.org/wiki/UTF-8#Encoding
+    switch (_VSTD::countl_one(static_cast<unsigned char>(*__first))) {
+    case 0: // 1-code unit encoding: all 1 column
+      ++__result;
+      ++__first;
+      break;
+
+    case 2: // 2-code unit encoding: all 1 column
+      // Malformed Unicode.
+      if (__last - __first < 2) [[unlikely]]
+        return __estimate_column_width_malformed(__first, __last, __maximum,
+                                                 __result);
+      __first += 2;
+      ++__result;
+      break;
+
+    case 3: // 3-code unit encoding: either 1 or 2 columns
+      // Malformed Unicode.
+      if (__last - __first < 3) [[unlikely]]
+        return __estimate_column_width_malformed(__first, __last, __maximum,
+                                                 __result);
+      {
+        uint32_t __c = static_cast<unsigned char>(*__first++) & 0x0f;
+        __c <<= 6;
+        __c |= static_cast<unsigned char>(*__first++) & 0x3f;
+        __c <<= 6;
+        __c |= static_cast<unsigned char>(*__first++) & 0x3f;
+        __result += __column_width_3(__c);
+        if (__result > __maximum)
+          return {__result - 2, __first - 3};
+      }
+      break;
+    case 4: // 4-code unit encoding: either 1 or 2 columns
+      // Malformed Unicode.
+      if (__last - __first < 4) [[unlikely]]
+        return __estimate_column_width_malformed(__first, __last, __maximum,
+                                                 __result);
+      {
+        uint32_t __c = static_cast<unsigned char>(*__first++) & 0x07;
+        __c <<= 6;
+        __c |= static_cast<unsigned char>(*__first++) & 0x3f;
+        __c <<= 6;
+        __c |= static_cast<unsigned char>(*__first++) & 0x3f;
+        __c <<= 6;
+        __c |= static_cast<unsigned char>(*__first++) & 0x3f;
+        __result += __column_width_4(__c);
+        if (__result > __maximum)
+          return {__result - 2, __first - 4};
+      }
+      break;
+    default:
+      // Malformed Unicode.
+      return __estimate_column_width_malformed(__first, __last, __maximum,
+                                               __result);
+    }
+
+    if (__result >= __maximum)
+      return {__result, __first};
+  }
+  return {__result, __first};
+}
+
+template <__utf16_character _CharT>
+_LIBCPP_HIDE_FROM_ABI constexpr __column_width_result<_CharT>
+__estimate_column_width(const _CharT* __first, const _CharT* __last,
+                        size_t __maximum) noexcept {
+  size_t __result = 0;
+
+  while (__first != __last) {
+    uint32_t __c = *__first;
+    // Is the code unit part of a surrogate pair? See
+    // https://en.wikipedia.org/wiki/UTF-16#U+D800_to_U+DFFF
+    if (__c >= 0xd800 && __c <= 0xDfff) {
+      // Malformed Unicode.
+      if (__last - __first < 2) [[unlikely]]
+        return {__result + 1, __first + 1};
+
+      __c -= 0xd800;
+      __c <<= 10;
+      __c += (*(__first + 1) - 0xdc00);
+      __c += 0x10'000;
+
+      __result += __column_width_4(__c);
+      if (__result > __maximum)
+        return {__result - 2, __first};
+      __first += 2;
+    } else {
+      __result += __column_width_3(__c);
+      if (__result > __maximum)
+        return {__result - 2, __first};
+      ++__first;
+    }
+
+    if (__result >= __maximum)
+      return {__result, __first};
+  }
+
+  return {__result, __first};
+}
+
+template <__utf32_character _CharT>
+_LIBCPP_HIDE_FROM_ABI constexpr __column_width_result<_CharT>
+__estimate_column_width(const _CharT* __first, const _CharT* __last,
+                        size_t __maximum) noexcept {
+  size_t __result = 0;
+
+  while (__first != __last) {
+    wchar_t __c = *__first;
+    __result += __column_width(__c);
+
+    if (__result > __maximum)
+      return {__result - 2, __first};
+
+    ++__first;
+    if (__result >= __maximum)
+      return {__result, __first};
+  }
+
+  return {__result, __first};
+}
+
+} // namespace __detail
+
+template <class _CharT>
+_LIBCPP_HIDE_FROM_ABI constexpr __string_alignment<_CharT>
+__get_string_alignment(const _CharT* __first, const _CharT* __last,
+                       ptrdiff_t __width, ptrdiff_t __precision) noexcept {
+  _LIBCPP_ASSERT(__width != 0 || __precision != -1,
+                 "The function has no effect and shouldn't be used");
+
+  // TODO FMT There might be more optimizations possible:
+  // If __precision == __format::__number_max and the encoding is:
+  // * UTF-8  : 4 * (__last - __first) >= __width
+  // * UTF-16 : 2 * (__last - __first) >= __width
+  // * UTF-32 : (__last - __first) >= __width
+  // In these cases it's certain the output is at least the requested width.
+  // It's unknown how often this happens in practice. For now the improvement
+  // isn't implemented.
+
+  /*
+   * First assume there are no special Unicode code units in the input.
+   * - Apply the precision (this may reduce the size of the input). When
+   *   __precison == -1 this step is omitted.
+   * - Scan for special code units in the input.
+   * If our assumption was correct the __pos will be at the end of the input.
+   */
+  const ptrdiff_t __length = __last - __first;
+  const _CharT* __limit =
+      __first +
+      (__precision == -1 ? __length : _VSTD::min(__length, __precision));
+  ptrdiff_t __size = __limit - __first;
+  const _CharT* __pos =
+      __detail::__estimate_column_width_fast(__first, __limit);
+
+  if (__pos == __limit)
+    return {__limit, __size, __size < __width};
+
+  /*
+   * Our assumption was wrong, there are special Unicode code units.
+   * The range [__first, __pos) contains a set of code units with the
+   * following property:
+   *      Every _CharT in the range will be rendered in 1 column.
+   *
+   * If there's no maximum width and the parsed size already exceeds the
+   *   minimum required width. The real size isn't important. So bail out.
+   */
+  if (__precision == -1 && (__pos - __first) >= __width)
+    return {__last, 0, false};
+
+  /* If there's a __precision, truncate the output to that width. */
+  ptrdiff_t __prefix = __pos - __first;
+  if (__precision != -1) {
+    _LIBCPP_ASSERT(__precision > __prefix, "Logic error.");
+    auto __lengh_info = __detail::__estimate_column_width(
+        __pos, __last, __precision - __prefix);
+    __size = __lengh_info.__width + __prefix;
+    return {__lengh_info.__ptr, __size, __size < __width};
+  }
+
+  /* Else use __width to determine the number of required padding characters. */
+  _LIBCPP_ASSERT(__width > __prefix, "Logic error.");
+  /*
+   * The column width is always one or two columns. For the precision the wanted
+   * column width is the maximum, for the width it's the minimum. Using the
+   * width estimation with its truncating behavior will result in the wrong
+   * result in the following case:
+   * - The last code unit processed requires two columns and exceeds the
+   *   maximum column width.
+   * By increasing the __maximum by one avoids this issue. (It means it may
+   * pass one code point more than required to determine the proper result;
+   * that however isn't a problem for the algorithm.)
+   */
+  size_t __maximum = 1 + __width - __prefix;
+  auto __lengh_info =
+      __detail::__estimate_column_width(__pos, __last, __maximum);
+  if (__lengh_info.__ptr != __last) {
+    // Consumed the width number of code units. The exact size of the string
+    // is unknown. We only know we don't need to align the output.
+    _LIBCPP_ASSERT(static_cast<ptrdiff_t>(__lengh_info.__width + __prefix) >=
+                       __width,
+                   "Logic error");
+    return {__last, 0, false};
+  }
+
+  __size = __lengh_info.__width + __prefix;
+  return {__last, __size, __size < __width};
+}
+#else  // _LIBCPP_HAS_NO_UNICODE
+template <class _CharT>
+_LIBCPP_HIDE_FROM_ABI constexpr __string_alignment<_CharT>
+__get_string_alignment(const _CharT* __first, const _CharT* __last,
+                       ptrdiff_t __width, ptrdiff_t __precision) noexcept {
+  const ptrdiff_t __length = __last - __first;
+  const _CharT* __limit =
+      __first +
+      (__precision == -1 ? __length : _VSTD::min(__length, __precision));
+  ptrdiff_t __size = __limit - __first;
+  return {__limit, __size, __size < __width};
+}
+#endif // _LIBCPP_HAS_NO_UNICODE
+
 } // namespace __format_spec
 
 # endif // !defined(_LIBCPP_HAS_NO_CONCEPTS)
 
 #endif //_LIBCPP_STD_VER > 17
 
 _LIBCPP_END_NAMESPACE_STD
 
+_LIBCPP_POP_MACROS
+
 #endif // _LIBCPP___FORMAT_PARSER_STD_FORMAT_SPEC_H

libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_string_non_unicode.pass.cpp

+//===----------------------------------------------------------------------===//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: libcpp-has-no-incomplete-format
+
+// UTF-32 doesn't work properly
+// XFAIL: windows
+
+// <format>
+
+// Tests the Unicode width support of the standard format specifiers.
+// It tests [format.string.std]/8 - 11:
+// - Properly determining the estimated with of a unicode string.
+// - Properly truncating to the wanted maximum width.
+
+// This version runs the test when the platform doesn't have Unicode support.
+// REQUIRES: libcpp-has-no-unicode
+
+#include <format>
+#include <cassert>
+
+#include "test_macros.h"
+#include "make_string.h"
+
+#define CSTR(S) MAKE_CSTRING(CharT, S)
+
+using namespace std::__format_spec;
+
+template <class CharT>
+constexpr bool operator==(const __string_alignment<CharT>& lhs,
+                          const __string_alignment<CharT>& rhs) noexcept {
+  return lhs.__last == rhs.__last && lhs.__size == rhs.__size &&
+         lhs.__align == rhs.__align;
+}
+
+template <class CharT>
+constexpr void get_string_alignment(size_t offset, ptrdiff_t size, bool align,
+                                    const CharT* str, size_t width,
+                                    size_t precision) {
+  std::basic_string_view<CharT> sv{str};
+  __string_alignment<CharT> expected{sv.begin() + offset, size, align};
+  __string_alignment<CharT> traits =
+      __get_string_alignment(sv.begin(), sv.end(), width, precision);
+  assert(traits == expected);
+}
+
+template <class CharT>
+constexpr void get_string_alignment() {
+  // Truncate the input.
+  get_string_alignment(2, 2, false, CSTR("abc"), 0, 2);
+
+  // The 2-column character gets half accepted.
+  get_string_alignment(2, 2, false, CSTR("a\u115f"), 0, 2);
+
+  // No alignment since the number of characters fits.
+  get_string_alignment(2, 2, false, CSTR("a\u115f"), 2, 2);
+
+  // Same but for a 2-column 4-byte UTF-8 sequence
+  get_string_alignment(2, 2, false, CSTR("a\U0001f300"), 0, 2);
+  get_string_alignment(2, 2, false, CSTR("a\U0001f300"), 2, 2);
+
+  // No alignment required.
+  get_string_alignment(3, 3, false, CSTR("abc"), 2, -1);
+  get_string_alignment(3, 3, false, CSTR("abc"), 3, -1);
+
+  get_string_alignment(3 + 2 * (sizeof(CharT) == 1),
+                       3 + 2 * (sizeof(CharT) == 1), false, CSTR("ab\u1111"), 2,
+                       -1);
+
+  // Doesn't evaluate 'c' so size -> 0
+  get_string_alignment(3 + 2 * (sizeof(CharT) == 1),
+                       3 + 2 * (sizeof(CharT) == 1), false,
+                       CSTR("a\u115fc") /* 2-column character */, 3, -1);
+  // Extend width
+  get_string_alignment(3, 3, true, CSTR("abc"), 4, -1);
+  get_string_alignment(3 + 2 * (sizeof(CharT) == 1),
+                       3 + 2 * (sizeof(CharT) == 1), true,
+                       CSTR("a\u1160c") /* 1-column character */, 6, -1);
+}
+
+template <class CharT>
+constexpr void test() {
+  get_string_alignment<CharT>();
+}
+
+constexpr bool test() {
+  test<char>();
+  test<wchar_t>();
+#ifndef _LIBCPP_HAS_NO_CHAR8_T
+  test<char8_t>();
+#endif
+#ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
+  test<char16_t>();
+  test<char32_t>();
+#endif
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}

libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_string_unicode.pass.cpp

+//===----------------------------------------------------------------------===//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: libcpp-has-no-incomplete-format
+
+// UTF-32 doesn't work properly
+// XFAIL: windows
+
+// <format>
+
+// Tests the Unicode width support of the standard format specifiers.
+// It tests [format.string.std]/8 - 11:
+// - Properly determining the estimated with of a unicode string.
+// - Properly truncating to the wanted maximum width.
+
+// This version runs the test when the platform has Unicode support.
+// UNSUPPORTED: libcpp-has-no-unicode
+
+#include <format>
+#include <cassert>
+
+#include "test_macros.h"
+#include "make_string.h"
+
+#define CSTR(S) MAKE_CSTRING(CharT, S)
+
+using namespace std::__format_spec;
+
+template <class CharT>
+constexpr bool operator==(const __string_alignment<CharT>& lhs,
+                          const __string_alignment<CharT>& rhs) noexcept {
+  return lhs.__last == rhs.__last && lhs.__size == rhs.__size &&
+         lhs.__align == rhs.__align;
+}
+
+template <class CharT>
+constexpr void get_string_alignment(size_t offset, ptrdiff_t size, bool align,
+                                    const CharT* str, size_t width,
+                                    size_t precision) {
+  std::basic_string_view<CharT> sv{str};
+  __string_alignment<CharT> expected{sv.begin() + offset, size, align};
+  __string_alignment<CharT> traits =
+      __get_string_alignment(sv.begin(), sv.end(), width, precision);
+  assert(traits == expected);
+}
+
+template <class CharT>
+constexpr void estimate_column_width_fast(size_t expected, const CharT* str) {
+  std::basic_string_view<CharT> sv{str};
+  const CharT* out =
+      __detail::__estimate_column_width_fast(sv.begin(), sv.end());
+  assert(out == sv.begin() + expected);
+}
+
+template <class CharT>
+constexpr void estimate_column_width_fast() {
+
+  // No unicode
+  estimate_column_width_fast(3, CSTR("abc"));
+  estimate_column_width_fast(3, CSTR("a\u007fc"));
+
+  if constexpr (sizeof(CharT) == 1) {
+    // UTF-8 stop at the first multi-byte character.
+    estimate_column_width_fast(0, CSTR("\u0080bc"));
+    estimate_column_width_fast(1, CSTR("a\u0080c"));
+    estimate_column_width_fast(2, CSTR("ab\u0080"));
+    estimate_column_width_fast(1, CSTR("aßc"));
+
+    estimate_column_width_fast(1, CSTR("a\u07ffc"));
+    estimate_column_width_fast(1, CSTR("a\u0800c"));
+
+    estimate_column_width_fast(1, CSTR("a\u10ffc"));
+  } else {
+    // UTF-16/32 stop at the first multi-column character.
+    estimate_column_width_fast(3, CSTR("\u0080bc"));
+    estimate_column_width_fast(3, CSTR("a\u0080c"));
+    estimate_column_width_fast(3, CSTR("ab\u0080"));
+    estimate_column_width_fast(3, CSTR("aßc"));
+
+    estimate_column_width_fast(3, CSTR("a\u07ffc"));
+    estimate_column_width_fast(3, CSTR("a\u0800c"));
+
+    estimate_column_width_fast(3, CSTR("a\u10ffc"));
+  }
+  // First 2-column character
+  estimate_column_width_fast(1, CSTR("a\u1100c"));
+
+  estimate_column_width_fast(1, CSTR("a\U0000ffffc"));
+  estimate_column_width_fast(1, CSTR("a\U00010000c"));
+  estimate_column_width_fast(1, CSTR("a\U0010FFFFc"));
+}
+
+template <class CharT>
+constexpr void estimate_column_width(size_t expected, const CharT* str) {
+  std::basic_string_view<CharT> sv{str};
+  std::__format_spec::__detail::__column_width_result<CharT> column_info =
+      __detail::__estimate_column_width(sv.begin(), sv.end(), -1);
+  assert(column_info.__width == expected);
+}
+
+template <class CharT>
+constexpr void estimate_column_width() {
+  //*** 1-byte code points ***
+  estimate_column_width(1, CSTR(" "));
+  estimate_column_width(1, CSTR("~"));
+
+  //*** 2-byte code points ***
+  estimate_column_width(1, CSTR("\u00a1")); // INVERTED EXCLAMATION MARK
+  estimate_column_width(1, CSTR("\u07ff")); // NKO TAMAN SIGN
+
+  //*** 3-byte code points ***
+  estimate_column_width(1, CSTR("\u0800")); // SAMARITAN LETTER ALAF
+  estimate_column_width(1, CSTR("\ufffd")); // REPLACEMENT CHARACTER
+
+  // 2 column ranges
+  estimate_column_width(2, CSTR("\u1100")); // HANGUL CHOSEONG KIYEOK
+  estimate_column_width(2, CSTR("\u115f")); // HANGUL CHOSEONG FILLER
+
+  estimate_column_width(2, CSTR("\u2329")); // LEFT-POINTING ANGLE BRACKET
+  estimate_column_width(2, CSTR("\u232a")); // RIGHT-POINTING ANGLE BRACKET
+
+  estimate_column_width(2, CSTR("\u2e80")); // CJK RADICAL REPEAT
+  estimate_column_width(2, CSTR("\u303e")); // IDEOGRAPHIC VARIATION INDICATOR
+
+  estimate_column_width(2, CSTR("\u3040")); // U+3041 HIRAGANA LETTER SMALL A
+  estimate_column_width(2, CSTR("\ua4cf")); // U+A4D0 LISU LETTER BA
+
+  estimate_column_width(2, CSTR("\uac00")); // <Hangul Syllable, First>
+  estimate_column_width(2, CSTR("\ud7a3")); // Hangul Syllable Hih
+
+  estimate_column_width(2, CSTR("\uf900")); // CJK COMPATIBILITY IDEOGRAPH-F900
+  estimate_column_width(2, CSTR("\ufaff")); // U+FB00 LATIN SMALL LIGATURE FF
+
+  estimate_column_width(2,
+                        CSTR("\ufe10")); // PRESENTATION FORM FOR VERTICAL COMMA
+  estimate_column_width(
+      2, CSTR("\ufe19")); // PRESENTATION FORM FOR VERTICAL HORIZONTAL ELLIPSIS
+
+  estimate_column_width(
+      2, CSTR("\ufe30")); // PRESENTATION FORM FOR VERTICAL TWO DOT LEADER
+  estimate_column_width(2,
+                        CSTR("\ufe6f")); // U+FE70 ARABIC FATHATAN ISOLATED FORM
+
+  estimate_column_width(2, CSTR("\uff00")); // U+FF01 FULLWIDTH EXCLAMATION MARK
+  estimate_column_width(2, CSTR("\uff60")); // FULLWIDTH RIGHT WHITE PARENTHESIS
+
+  estimate_column_width(2, CSTR("\uffe0")); // FULLWIDTH CENT SIGN
+  estimate_column_width(2, CSTR("\uffe6")); // FULLWIDTH WON SIGN
+
+  //*** 4-byte code points ***
+  estimate_column_width(1, CSTR("\U00010000")); // LINEAR B SYLLABLE B008 A
+  estimate_column_width(1, CSTR("\U0010FFFF")); // Undefined Character
+
+  // 2 column ranges
+  estimate_column_width(2, CSTR("\U0001f300")); // CYCLONE
+  estimate_column_width(2, CSTR("\U0001f64f")); // PERSON WITH FOLDED HANDS
+  estimate_column_width(
+      2, CSTR("\U0001f900")); // CIRCLED CROSS FORMEE WITH FOUR DOTS
+  estimate_column_width(2, CSTR("\U0001f9ff")); // NAZAR AMULET
+  estimate_column_width(
+      2, CSTR("\U00020000")); // <CJK Ideograph Extension B, First>
+  estimate_column_width(2, CSTR("\U0002fffd")); // Undefined Character
+  estimate_column_width(
+      2, CSTR("\U00030000")); // <CJK Ideograph Extension G, First>
+  estimate_column_width(2, CSTR("\U0003fffd")); // Undefined Character
+}
+
+template <class CharT>
+constexpr void get_string_alignment() {
+  // Truncate the input.
+  get_string_alignment(2, 2, false, CSTR("abc"), 0, 2);
+
+  // The 2-column character gets entirely rejected.
+  get_string_alignment(1, 1, false, CSTR("a\u115f"), 0, 2);
+
+  // Due to the requested width extra alignment is required.
+  get_string_alignment(1, 1, true, CSTR("a\u115f"), 2, 2);
+
+  // Same but for a 2-column 4-byte UTF-8 sequence
+  get_string_alignment(1, 1, false, CSTR("a\U0001f300"), 0, 2);
+  get_string_alignment(1, 1, true, CSTR("a\U0001f300"), 2, 2);
+
+  // No alignment required.
+  get_string_alignment(3, 3, false, CSTR("abc"), 2, -1);
+  get_string_alignment(3, 3, false, CSTR("abc"), 3, -1);
+
+  // Special case, we have a special character already parsed and have enough
+  // withd to satisfy the minumum required width.
+  get_string_alignment(3 + 2 * (sizeof(CharT) == 1), 0, false, CSTR("ab\u1111"),
+                       2, -1);
+
+  // Evaluates all so size ->4
+  get_string_alignment(3 + 2 * (sizeof(CharT) == 1), 4, false,
+                       CSTR("a\u115fc") /* 2-column character */, 3, -1);
+  // Evaluates all so size ->4
+  get_string_alignment(3 + 2 * (sizeof(CharT) == 1), 4, false,
+                       CSTR("a\u115fc") /* 2-column character */, 4, -1);
+
+  // Evaluates all so size ->5
+  get_string_alignment(4 + 2 * (sizeof(CharT) == 1), 5, false,
+                       CSTR("a\u115fcd") /* 2-column character */, 4, -1);
+
+  // Evaluates all so size ->5
+  get_string_alignment(4 + 2 * (sizeof(CharT) == 1), 5, false,
+                       CSTR("a\u115fcd") /* 2-column character */, 5, -1);
+
+  // Extend width
+  get_string_alignment(3, 3, true, CSTR("abc"), 4, -1);
+  get_string_alignment(3 + 2 * (sizeof(CharT) == 1), 3, true,
+                       CSTR("a\u1160c") /* 1-column character */, 4, -1);
+
+  // In this case the threshold where the width is still determined.
+  get_string_alignment(2 + 2 * (sizeof(CharT) == 1), 3, false, CSTR("i\u1110"),
+                       2, -1);
+
+  // The width is no longer exactly determined.
+  get_string_alignment(2 + 2 * (sizeof(CharT) == 1), 0, false, CSTR("i\u1110"),
+                       1, -1);
+
+  // Extend width and truncate input.
+  get_string_alignment(1, 1, true, CSTR("abc"), 3, 1);
+
+  if constexpr (sizeof(CharT) == 1) {
+    // Corrupt UTF-8 sequence.
+    get_string_alignment(2, 2, false, CSTR("a\xc0"), 0, 3);
+    get_string_alignment(2, 2, false, CSTR("a\xe0"), 0, 3);
+    get_string_alignment(2, 2, false, CSTR("a\xf0"), 0, 3);
+  } else if constexpr (sizeof(CharT) == 2) {
+    // Corrupt UTF-16 sequence.
+    if constexpr (std::same_as<CharT, char16_t>)
+      get_string_alignment(2, 2, false, u"a\xdddd", 0, 3);
+    else
+      // Corrupt UTF-16 wchar_t seqence.
+      get_string_alignment(2, 2, false, L"a\xdddd", 0, 3);
+  }
+  // UTF-32 doesn't combine characters, thus no corruption tests.
+}
+
+template <class CharT>
+constexpr void test() {
+  estimate_column_width_fast<CharT>();
+  estimate_column_width<CharT>();
+  get_string_alignment<CharT>();
+}
+
+constexpr bool test() {
+  test<char>();
+  test<wchar_t>();
+#ifndef _LIBCPP_HAS_NO_CHAR8_T
+  test<char8_t>();
+#endif
+#ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
+  test<char16_t>();
+  test<char32_t>();
+#endif
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}