FloatConversion.inl

/*
 * Copyright(c) Sophist Solutions, Inc. 1990-2026.  All rights reserved
 */
#include <charconv>
#include <sstream>
 
#include "Stroika/Foundation/Characters/CString/Utilities.h"
#include "Stroika/Foundation/Characters/UTFConvert.h"
#include "Stroika/Foundation/Common/StdCompat.h"
#include "Stroika/Foundation/Containers/Common.h"
#include "Stroika/Foundation/Math/Common.h"
#include "Stroika/Foundation/Memory/Common.h"
#include "Stroika/Foundation/Memory/Optional.h"
#include "Stroika/Foundation/Memory/StackBuffer.h"
 
namespace Stroika::Foundation::Characters::FloatConversion {
 
    /*
     ********************************************************************************
     ******************** FloatConversion::SignificantFigures ***********************
     ********************************************************************************
     */
    constexpr SignificantFigures::SignificantFigures (unsigned int p)
        : fSignificantFigures_{p}
    {
    }
    constexpr SignificantFigures::SignificantFigures (FullFlag)
        : fSignificantFigures_{} // kFullPrecision
    {
    }
    template <floating_point T>
    constexpr auto SignificantFigures::GetEffectiveSignificantFigures () const -> RepType
    {
        return fSignificantFigures_.value_or (numeric_limits<T>::max_digits10);
    }
    template <IStdBasicStringCompatibleCharacter CHAR>
    constexpr auto SignificantFigures::Calculate (span<const CHAR> number) -> RepType
    {
        // two passes cuz eatLeadingZero depends on scientific, which we find at the end of the text
        bool dotPresent = false;
        bool scientific = false;
        for (const CHAR c : number) {
            switch (c) {
                // Some locales use . (most), but europe uses , for same purpose (and used in ISO-8601)
                case '.':
                case ',':
                    dotPresent = true;
                    break;
                case 'e':
                case 'E':
                    scientific = true;
                    break;
            }
        }
        RepType n{0};
        RepType trailingZeros{0};
        bool    eatLeadingZeros = not scientific;
        bool    gotoDone        = false;
        for (const CHAR c : number) {
            switch (c) {
                case '-':
                case '+':
                    break;
                case '0':
                    if (eatLeadingZeros) {
                        // munch
                    }
                    else {
                        ++n;
                    }
                    ++trailingZeros;
                    break;
                case '1':
                case '2':
                case '3':
                case '4':
                case '5':
                case '6':
                case '7':
                case '8':
                case '9':
                    eatLeadingZeros = false;
                    trailingZeros   = 0;
                    ++n;
                    break;
                case '.':
                case ',':
                    trailingZeros = 0;
                    break;
                case 'e':
                case 'E':
                    gotoDone = true; // goto Done;   // FIX to use this when c++23
                    break;
            }
            if (gotoDone) {
                break;
            }
        }
        //    Done:
        if (dotPresent) {
            if (eatLeadingZeros) {
                // then the trailing zeros we counted were also leading 0, but wern't added to n so do so now
                n += trailingZeros;
            }
        }
        else {
            n -= trailingZeros;
        }
        return n;
    }
 
    /**
     *      From http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4659.pdf,
     *      init (basic_streambuf...) initializes precision to 6
     *      Stroika need not maintain that default here, but it seems a sensible one...
     * 
     *  \note defined here instead of header cuz needs to be after SignificantFigures::CTOR.
     */
    constexpr inline SignificantFigures SignificantFigures::kDefault{6};
 
    /**
     *  \brief Full precision here means enough digits so that when written out (serialized) - and read back in (deserialized)
     *         you get the exact same answer.
     * 
     *  \see  // https://stackoverflow.com/questions/22458355/what-is-the-purpose-of-max-digits10-and-how-is-it-different-from-digits10
     *        numeric_limits<T>::max_digits10
     * 
     *  \note defined here instead of header cuz needs to be after SignificantFigures::CTOR.
     */
    constexpr inline SignificantFigures SignificantFigures::kFullPrecision{SignificantFigures::FullFlag::eFull};
 
    /*
     ********************************************************************************
     ********************* FloatConversion::ToStringOptions *************************
     ********************************************************************************
     */
    inline ToStringOptions::ToStringOptions (const locale& l)
        : fUseLocale_{l}
    {
    }
    constexpr ToStringOptions::ToStringOptions (PredefinedLocale p)
        : fUseCurrentLocale_{p == PredefinedLocale::eUseCurrentLocale}
    {
    }
    constexpr ToStringOptions::ToStringOptions (ios_base::fmtflags fmtFlags)
        : fFmtFlags_{fmtFlags}
    {
    }
    constexpr ToStringOptions::ToStringOptions (SignificantFigures precision)
        : fSignificantFigures_{precision}
    {
    }
    constexpr ToStringOptions::ToStringOptions (FloatFormatType scientificNotation)
        : fFloatFormat_{scientificNotation}
    {
    }
    DISABLE_COMPILER_GCC_WARNING_START ("GCC diagnostic ignored \"-Wdeprecated-declarations\"");
    DISABLE_COMPILER_CLANG_WARNING_START ("clang diagnostic ignored \"-Wdeprecated-declarations\"");
    DISABLE_COMPILER_MSC_WARNING_START (4996)
    constexpr ToStringOptions::ToStringOptions (TrimTrailingZerosType trimTrailingZeros)
        : fTrimTrailingZeros_{trimTrailingZeros == TrimTrailingZerosType::eTrimZeros}
    {
    }
    DISABLE_COMPILER_MSC_WARNING_END (4996)
    DISABLE_COMPILER_CLANG_WARNING_END ("clang diagnostic ignored \"-Wdeprecated-declarations\"");
    DISABLE_COMPILER_GCC_WARNING_END ("GCC diagnostic ignored \"-Wdeprecated-declarations\"");
 
    constexpr ToStringOptions::ToStringOptions (const ToStringOptions& b1, const ToStringOptions& b2)
        : ToStringOptions{b1}
    {
        Memory::CopyToIf (&fSignificantFigures_, b2.fSignificantFigures_);
        Memory::CopyToIf (&fFmtFlags_, b2.fFmtFlags_);
        fUseCurrentLocale_ = b2.fUseCurrentLocale_;
        Memory::CopyToIf (&fUseLocale_, b2.fUseLocale_);
        Memory::CopyToIf (&fTrimTrailingZeros_, b2.fTrimTrailingZeros_);
        Memory::CopyToIf (&fFloatFormat_, b2.fFloatFormat_);
    }
    template <typename... ARGS>
    constexpr ToStringOptions::ToStringOptions (const ToStringOptions& b1, const ToStringOptions& b2, ARGS&&... args)
        : ToStringOptions{ToStringOptions{b1, b2}, forward<ARGS> (args)...}
    {
    }
    constexpr optional<SignificantFigures> ToStringOptions::GetSignificantFigures () const
    {
        return fSignificantFigures_;
    }
    constexpr optional<bool> ToStringOptions::GetTrimTrailingZeros () const
    {
        return fTrimTrailingZeros_;
    }
    inline locale ToStringOptions::GetUseLocale () const
    {
        if (fUseCurrentLocale_) {
            return locale{};
        }
        if (fUseLocale_) {
            return *fUseLocale_;
        }
        static const locale kCLocale_ = locale::classic ();
        return kCLocale_;
    }
    inline bool ToStringOptions::GetUsingLocaleClassic () const
    {
        if (not fUseLocale_ and not fUseCurrentLocale_) {
            return true; // very common case
        }
        return GetUseLocale () == locale::classic ();
    }
    constexpr optional<FloatFormatType> ToStringOptions::GetFloatFormat () const
    {
        return fFloatFormat_;
    }
    constexpr optional<ios_base::fmtflags> ToStringOptions::GetIOSFmtFlags () const
    {
        return fFmtFlags_;
    }
 
    namespace Private_ {
        inline void TrimTrailingZeros_NotSci_ (String* strResult)
        {
            // @todo THIS could be more efficient: use StringBuilder and Peek()/Character::AsASCIIQuietly
            RequireNotNull (strResult);
            // strip trailing zeros (after decimal point - before they may indicate magnatude)
            // And don't do if ends with exponential notation e+40 shouldnt get shortned to e+4!
            Require (not strResult->Find ('e', eCaseInsensitive).has_value ());
            size_t pastDot = strResult->find ('.');
            if (pastDot != String::npos) {
                ++pastDot;
                size_t len           = strResult->length ();
                size_t pPastLastZero = len;
                for (; (pPastLastZero - 1) > pastDot; --pPastLastZero) {
                    if ((*strResult)[pPastLastZero - 1] != '0') {
                        break;
                    }
                }
                if (pPastLastZero == pastDot + 1 and (*strResult)[pPastLastZero - 1] == '0') {
                    pPastLastZero -= 2; // dont map 3.000 to 3.0, map it to 3
                }
                if (len != pPastLastZero) [[unlikely]] { // check common case of no change, but this substring and assign already pretty optimized (not sure helps performance)
                    *strResult = strResult->SubString (0, pPastLastZero);
                }
            }
        }
        inline void TrimTrailingZeros_MaybeSci_ (String* strResult)
        {
            // @todo THIS could be more efficient: use StringBuilder and Peek()/Character::AsASCIIQuietly
            RequireNotNull (strResult);
            // strip trailing zeros (after decimal point - before they may indicate magnatude)
            // And don't do if ends with exponential notation e+40 shouldnt get shortned to e+4!
            if (optional<size_t> whereIsE = strResult->Find ('e', eCaseInsensitive)) {
                size_t pastDot = strResult->find ('.');
                if (pastDot != String::npos) {
                    ++pastDot;
                    size_t len           = *whereIsE - 1;
                    size_t pPastLastZero = len;
                    for (; (pPastLastZero - 1) > pastDot; --pPastLastZero) {
                        if ((*strResult)[pPastLastZero - 1] != '0') {
                            break;
                        }
                    }
                    if (len != pPastLastZero) [[unlikely]] { // check common case of no change, but this substring and assign already pretty optimized (not sure helps performance)
                        *strResult = strResult->SubString (0, pPastLastZero) + strResult->SubString (*whereIsE);
                    }
                }
            }
            else {
                size_t pastDot = strResult->find ('.');
                if (pastDot != String::npos) {
                    ++pastDot;
                    size_t len           = strResult->length ();
                    size_t pPastLastZero = len;
                    for (; (pPastLastZero - 1) > pastDot; --pPastLastZero) {
                        if ((*strResult)[pPastLastZero - 1] != '0') {
                            break;
                        }
                    }
                    if (pPastLastZero == pastDot + 1 and (*strResult)[pPastLastZero - 1] == '0') {
                        pPastLastZero -= 2; // dont map 3.000 to 3.0, map it to 3
                    }
                    if (len != pPastLastZero) [[unlikely]] { // check common case of no change, but this substring and assign already pretty optimized (not sure helps performance)
                        *strResult = strResult->SubString (0, pPastLastZero);
                    }
                }
            }
        }
    }
 
    namespace Private_ {
        // TEMPLATE version of wcstof(etc) - to make easier to call from generic algorithm
        template <typename T>
        T CStr2FloatType_ (const wchar_t* s, wchar_t** e);
        template <typename T>
        T CStr2FloatType_ (const char* s, char** e);
        template <>
        inline float CStr2FloatType_ (const wchar_t* s, wchar_t** e)
        {
            return ::wcstof (s, e);
        }
        template <>
        inline double CStr2FloatType_ (const wchar_t* s, wchar_t** e)
        {
            return wcstod (s, e);
        }
        template <>
        inline long double CStr2FloatType_ (const wchar_t* s, wchar_t** e)
        {
            return wcstold (s, e);
        }
        template <>
        inline float CStr2FloatType_ (const char* s, char** e)
        {
            return ::strtof (s, e);
        }
        template <>
        inline double CStr2FloatType_ (const char* s, char** e)
        {
            return strtod (s, e);
        }
        template <>
        inline long double CStr2FloatType_ (const char* s, char** e)
        {
            return strtold (s, e);
        }
    }
 
    namespace Private_ {
        template <typename RETURN_TYPE>
        RETURN_TYPE ToFloat_ViaStrToD_ (const char* start, const char* end, const char** remainder)
        {
            Require (start <= end);
            if (start == end) {
                if (remainder != nullptr) {
                    *remainder = start;
                }
                return Math::nan<RETURN_TYPE> ();
            }
            else {
                char*       e   = nullptr;
                const char* cst = start;
 
                if (remainder == nullptr) {
                    // REJECT strings with leading space in this case - must match exactly
                    if (::isspace (*cst)) {
                        return Math::nan<RETURN_TYPE> ();
                    }
                }
 
                Memory::StackBuffer<char> tmp;
                if (*end != '\0') {
                    // remap addresses - copying to a temporary buffer, so we can nul-terminate string passed to strtod (etc)
                    size_t len = end - start;
                    tmp.GrowToSize (len + 1);
                    (void)::memcpy (tmp.begin (), start, len);
                    cst      = tmp.begin ();
                    tmp[len] = '\0';
                }
                RETURN_TYPE d = CStr2FloatType_<RETURN_TYPE> (cst, &e);
                // If asked to return remainder do so.
                // If NOT asked to return remainder, treat not using the entire string as forcing result to be a Nan (invalid parse of number of not the whole thing is a number)
                if (remainder == nullptr) {
                    if (e != end) {
                        d = Math::nan<RETURN_TYPE> ();
                    }
                }
                else {
                    *remainder = e - cst + start; // adjust in case we remapped data
                }
                return d;
            }
        }
        template <typename RETURN_TYPE>
        RETURN_TYPE ToFloat_ViaStrToD_ (const wchar_t* start, const wchar_t* end, const wchar_t** remainder)
        {
            Require (start <= end);
            if (start == end) {
                if (remainder != nullptr) {
                    *remainder = start;
                }
                return Math::nan<RETURN_TYPE> ();
            }
            else {
                wchar_t*       e   = nullptr;
                const wchar_t* cst = start;
 
                if (remainder == nullptr) {
                    // REJECT strings with leading space in this case - must match exactly
                    if (::iswspace (*cst)) {
                        return Math::nan<RETURN_TYPE> ();
                    }
                }
 
                Memory::StackBuffer<wchar_t> tmp;
                if (*end != '\0') {
                    // remap addresses - copying to a temporary buffer, so we can nul-terminate string passed to strtod (etc)
                    size_t len = end - start;
                    tmp.GrowToSize (len + 1);
                    DISABLE_COMPILER_GCC_WARNING_START ("GCC diagnostic ignored \"-Wrestrict\""); //  /usr/include/x86_64-linux-gnu/bits/string_fortified.h:29:33: warning: 'void* __builtin_memcpy(void*, const void*, long unsigned int)' accessing 18446744073709551612 bytes at offsets 8 and 0 overlaps 9223372036854775801 bytes at offset -9223372036854775805 [-Wrestrict]
                    DISABLE_COMPILER_GCC_WARNING_START ("GCC diagnostic ignored \"-Wstringop-overflow=\"");
                    DISABLE_COMPILER_GCC_WARNING_START ("GCC diagnostic ignored \"-Wstringop-overflow\""); // desperation try to silience
                    (void)::memcpy (tmp.begin (), start, len * sizeof (wchar_t));
                    DISABLE_COMPILER_GCC_WARNING_END ("GCC diagnostic ignored \"-Wstringop-overflow\"");
                    DISABLE_COMPILER_GCC_WARNING_END ("GCC diagnostic ignored \"-Wstringop-overflow=\"");
                    DISABLE_COMPILER_GCC_WARNING_END ("GCC diagnostic ignored \"-Wrestrict\"");
                    cst      = tmp.begin ();
                    tmp[len] = '\0';
                }
                RETURN_TYPE d = CStr2FloatType_<RETURN_TYPE> (cst, &e);
                // If asked to return remainder do so.
                // If NOT asked to return remainder, treat not using the entire string as forcing result to be a Nan (invalid parse of number of not the whole thing is a number)
                if (remainder == nullptr) {
                    if (e != end) {
                        d = Math::nan<RETURN_TYPE> ();
                    }
                }
                else {
                    *remainder = e - cst + start; // adjust in case we remapped data
                }
                return d;
            }
        }
    }
 
    namespace Private_ {
#if qStroika_Foundation_Debug_AssertionsChecked || !(__cpp_lib_to_chars >= 201611)
        inline size_t CalcSignificantFigures_ (const String& numStr, bool countZerosAtEndAfterDecPoint = false)
        {
            bool   leading    = true;
            bool   ignoreRest = false;
            bool   seenDot    = false;
            size_t n{};
            size_t nTrailingZeros{};
            numStr.Apply ([&] (Character c) {
                if (ignoreRest) {
                    return;
                }
                if (c == '.') {
                    seenDot = true;
                }
                if (c == '+' or c == '-' or c == '.') {
                    return;
                }
                if (leading and c == '0') {
                    return;
                }
                leading = false;
                if (c == 'e') {
                    ignoreRest = true;
                    return;
                }
                if (countZerosAtEndAfterDecPoint and seenDot) {
                    if (c == '0') {
                        ++nTrailingZeros;
                    }
                    else {
                        nTrailingZeros = 0;
                    }
                }
                ++n;
            });
            return countZerosAtEndAfterDecPoint ? n : n - nTrailingZeros;
        }
#endif
        template <floating_point FLOAT_TYPE>
        String ToString_OptimizedForCLocaleAndNoStreamFlags_ (FLOAT_TYPE f, SignificantFigures precision)
        {
            using namespace Memory;
            size_t sz = numeric_limits<FLOAT_TYPE>::max_digits10 + numeric_limits<FLOAT_TYPE>::max_exponent10 + 5; // source? "-1.##e+##\0"
            StackBuffer<char> buf{Memory::eUninitialized, sz};
            ptrdiff_t         resultStrLen;
            unsigned int      effectivePrecision = precision.GetEffectiveSignificantFigures<FLOAT_TYPE> ();
 
            // XCode 15 still doesn't define __cpp_lib_to_chars, as well as _LIBCPP_VERSION < 190000, I believe --LGP 2024-07-13
#if __cpp_lib_to_chars >= 201611
            // empirically, on MSVC, to_chars() is much faster than snprintf (appears 3x apx faster) -- LGP 2021-11-04
            if (precision == SignificantFigures::kFullPrecision) {
                resultStrLen = to_chars (buf.begin (), buf.end (), f, chars_format::general).ptr - buf.begin ();
            }
            else {
                resultStrLen = to_chars (buf.begin (), buf.end (), f, chars_format::general, effectivePrecision).ptr - buf.begin ();
            }
#else
            auto mkFmtWithPrecisionArg_ = [] (char* formatBufferStart, [[maybe_unused]] char* formatBufferEnd, char _Spec, bool forceScientific) -> char* {
                char* fmtPtr = formatBufferStart;
                *fmtPtr++    = '%';
                // include precision arg
                *fmtPtr++ = '.';
                *fmtPtr++ = '*';
                if (_Spec != '\0') {
                    *fmtPtr++ = _Spec; // e.g. 'L' qualifier
                }
                *fmtPtr++ = forceScientific ? 'e' : 'g'; // format specifier
                *fmtPtr   = '\0';
                Require (fmtPtr < formatBufferEnd);
                return formatBufferStart;
            };
 
            FLOAT_TYPE useRoundedFloat = Math::Round<FLOAT_TYPE> (f, effectivePrecision);
            if (precision != SignificantFigures::kFullPrecision) {
                f = useRoundedFloat;
            }
 
            bool                                  forceScientific = fabs (f) >= std::pow (10, effectivePrecision);
            qStroika_ATTRIBUTE_INDETERMINATE char format[100]; // filled in with mkFmtWithPrecisionArg_
            resultStrLen            = ::snprintf (buf.data (), buf.size (),
                                                  mkFmtWithPrecisionArg_ (std::begin (format), std::end (format),
                                                               same_as<FLOAT_TYPE, long double> ? 'L' : '\0', forceScientific),
                                                  (int)effectivePrecision + 1, f);
            auto actualPrecIncZeros = CalcSignificantFigures_ (String{span{buf.data (), static_cast<size_t> (resultStrLen)}}, true);
            if (actualPrecIncZeros > effectivePrecision) {
                ptrdiff_t nBytes    = static_cast<ptrdiff_t> (actualPrecIncZeros) - static_cast<ptrdiff_t> (effectivePrecision);
                auto      numberEnd = buf.data () + resultStrLen;
                auto      ePtr      = std::find (buf.data (), numberEnd, 'e');
                if (ePtr != numberEnd) {
                    Assert (buf.data () <= ePtr - nBytes);
                    memmove (ePtr - nBytes, ePtr, (buf.data () + resultStrLen - ePtr)); // slide 'e+22' back over the lost precision bytes of number
                }
                resultStrLen -= nBytes;
            }
#endif
 
            // [[maybe_unused]] auto oo1 = String{span{buf.data (), static_cast<size_t> (resultStrLen)}}.As<wstring> ();
            // [[maybe_unused]] auto ooo = CalcSignificantFigures_ (String{span{buf.data (), static_cast<size_t> (resultStrLen)}});
            Verify (resultStrLen > 0 and resultStrLen < static_cast<int> (sz));
#if qStroika_Foundation_Debug_AssertionsChecked
            Assert (precision == SignificantFigures::kFullPrecision or
                    CalcSignificantFigures_ (String{span{buf.data (), static_cast<size_t> (resultStrLen)}}) <= effectivePrecision);
#endif
            return String{span{buf.data (), static_cast<size_t> (resultStrLen)}};
        }
    }
 
    namespace Private_ {
        template <floating_point T>
        inline String formatNonScientific_ (T val, unsigned int nSignificantFigures)
        {
            auto compute = [&] () {
                if (val == 0.0) {
                    // special case for zero cuz cannot compute log10(0)
                    return String{Common::StdCompat::format ("{:.{}f}", 0.0, nSignificantFigures)};
                }
                else {
                    // Calculate digits before the decimal point
                    int digits_before = static_cast<int> (floor (log10 (abs (val)))) + 1;
 
                    // Precision for 'f' (fixed) is the number of digits AFTER the decimal
                    unsigned int precision = static_cast<unsigned int> (max (0, static_cast<int> (nSignificantFigures) - digits_before));
 
                    // Use dynamic precision syntax: {:.{}f}
                    // The first {} refers to the value, the second .{} refers to precision
                    return String{Common::StdCompat::format ("{:.{}f}", val, precision)};
                }
            };
            String r = compute ();
            // @todo RECONSIDER
            // for example, there is no way for 31200000 to be represented with 6 significant digits (in standard form, you can with scientific)
            // 212312345.0 with 6 significant digits is 212312000, but we need the extra digits for magnatitude, so not
            // sure worth losing the action digits we have? CONSIDER!!!!
            // but for now no ensures --Ensure (SignificantFigures::Calculate (span<const wchar_t>{r.As<wstring> ()}) <= nSignificantFigures);
            return r;
        }
        template <floating_point T>
        inline String formatNonScientific_ (const locale& l, T val, unsigned int nSignificantFigures)
        {
            auto compute = [&] () {
                if (val == 0.0) {
                    // special case for zero cuz cannot compute log10(0)
                    return Common::StdCompat::format (l, L"{:.{}f}", 0.0, nSignificantFigures);
                }
                else {
                    // Calculate digits before the decimal point
                    int digits_before = static_cast<int> (floor (log10 (abs (val)))) + 1;
 
                    // Precision for 'f' (fixed) is the number of digits AFTER the decimal
                    unsigned int precision = static_cast<unsigned int> (max (0, static_cast<int> (nSignificantFigures) - digits_before));
 
                    // Use dynamic precision syntax: {:.{}f}
                    // The first {} refers to the value, the second .{} refers to precision
                    return Common::StdCompat::format (l, L"{:.{}f}", val, precision);
                }
            };
            String r = compute ();
            Ensure (SignificantFigures::Calculate (span<const wchar_t>{r.As<wstring> ()}) == nSignificantFigures);
            return r;
        }
    }
 
    namespace Private_ {
        template <floating_point FLOAT_TYPE>
        String ToString_GeneralCase_ (FLOAT_TYPE f, const ToStringOptions& options)
        {
            unsigned int usePrecision =
                options.GetSignificantFigures ().value_or (SignificantFigures{}).GetEffectiveSignificantFigures<FLOAT_TYPE> ();
            FloatFormatType usingFormat = options.GetFloatFormat ().value_or (FloatFormatType::eDEFAULT);
 
            // if no locale, can use this...
            // (see if I can use formatNonScientific_) logic for other cases...
            if (usingFormat == FloatFormatType::eStandard) {
                Assert (options.GetIOSFmtFlags () == nullopt); // doable in principle, looking/mapping each feature, but not trivial, and probably not needed anytime soon
                                                               // --LGP 2026-02-10
                if (options.GetUsingLocaleClassic ()) {
                    return formatNonScientific_ (f, usePrecision);
                }
                else {
                    return formatNonScientific_ (options.GetUseLocale (), f, usePrecision);
                }
            }
 
            // expensive to construct, and slightly cheaper to just use thread_local version of
            // the same stringstream each time (only one per thread can be in use)
            static thread_local stringstream s;
            static const ios_base::fmtflags  kDefaultIOSFmtFlags_ = s.flags (); // Just copy out of the first constructed stringstream
 
            s.str (string{});
            s.clear ();
 
            s.imbue (options.GetUseLocale ());
 
            //  must set explicitly (even if defaulted)  because of the thread_local thing
            s.flags (options.GetIOSFmtFlags ().value_or (kDefaultIOSFmtFlags_));
 
            {
                switch (usingFormat) {
                    case FloatFormatType::eScientific:
                    case FloatFormatType::eScientificWithWhitespaceTrimmed:
                        s.setf (ios_base::scientific, ios_base::floatfield);
                        Assert (usePrecision >= 2);
                        s.precision (usePrecision - 1); // scientific format is n.mmm with m digits of 'precision' - so 1 + m is # of significat digits
                        break;
                    case FloatFormatType::eDefaultFloat:
                        s.unsetf (ios_base::floatfield); // see std::defaultfloat - not same as ios_base::fixed
                        /*
                         * Precision Field: The stream's precision setting (managed by std::setprecision or std::ios_base::precision()) 
                         * specifies the maximum number of meaningful digits to display in total (both before and after the decimal point),
                         * not a fixed number of digits after the decimal point (so same as SignificantFigures)
                         */
                        s.precision (usePrecision);
                        break;
                    case FloatFormatType::eFixedPoint:
                    case FloatFormatType::eFixedPointWithWhitespaceTrimmed:
                        s.setf (ios_base::fixed, ios_base::floatfield);
                        s.precision (usePrecision); // I think wrong but test -
                        break;
                    case FloatFormatType::eStandard:
                        AssertNotReached (); // handled above
                        break;
                        DISABLE_COMPILER_GCC_WARNING_START ("GCC diagnostic ignored \"-Wdeprecated-declarations\"");
                        DISABLE_COMPILER_CLANG_WARNING_START ("clang diagnostic ignored \"-Wdeprecated-declarations\"");
                        DISABLE_COMPILER_MSC_WARNING_START (4996)
                    case FloatFormatType::eAutomaticScientific: {
                        s.precision (usePrecision);
                        bool useScientificNotation = abs (f) >= pow (10, usePrecision / 2) or
                                                     (f != 0 and abs (f) < pow (10, -static_cast<int> (usePrecision) / 2)); // scientific preserves more precision - but non-scientific looks better
                        if (useScientificNotation) {
                            s.setf (ios_base::scientific, ios_base::floatfield);
                        }
                        else {
                            s.unsetf (ios_base::floatfield); // see std::defaultfloat - not same as ios_base::fixed
                            s.precision (usePrecision);
                        }
                    } break;
                        DISABLE_COMPILER_MSC_WARNING_END (4996)
                        DISABLE_COMPILER_CLANG_WARNING_END ("clang diagnostic ignored \"-Wdeprecated-declarations\"");
                        DISABLE_COMPILER_GCC_WARNING_END ("GCC diagnostic ignored \"-Wdeprecated-declarations\"");
                    default:
                        RequireNotReached ();
                        break;
                }
            }
 
            s << f;
            string ss = s.str ();
 
            return options.GetUsingLocaleClassic () ? String{ss} : String::FromNarrowString (ss, options.GetUseLocale ());
        }
    }
 
    namespace Private_ {
        template <floating_point FLOAT_TYPE>
        String ToString_String_Implementation_ (FLOAT_TYPE f, const ToStringOptions& options)
        {
            auto floatFormat = options.GetFloatFormat ().value_or (FloatFormatType::eDEFAULT);
            auto result =
                (options.GetUsingLocaleClassic () and not options.GetIOSFmtFlags () and floatFormat == FloatFormatType::eDefaultFloat)
                    ? Private_::ToString_OptimizedForCLocaleAndNoStreamFlags_ (f, options.GetSignificantFigures ().value_or (SignificantFigures{}))
                    : Private_::ToString_GeneralCase_ (f, options);
            if (floatFormat == FloatFormatType::eFixedPointWithWhitespaceTrimmed or floatFormat == FloatFormatType::eStandard) {
                TrimTrailingZeros_NotSci_ (&result);
            }
            if (floatFormat == FloatFormatType::eScientificWithWhitespaceTrimmed) {
                TrimTrailingZeros_MaybeSci_ (&result);
            }
#if qCompilerAndStdLib_float2string_defaultfmt_scientificNotStripped_Buggy
            if (floatFormat == FloatFormatType::eDefaultFloat) {
                TrimTrailingZeros_MaybeSci_ (&result);
            }
#endif
            return result;
        }
    }
 
    namespace Private_ {
        //
        // This RESPECTS THE CURRENT LOCALE
        // this private function allows nullptr remainder, and behaves somewhat differently if null or not (unlike PUBLIC API)
        // Roughly (ignoring precision and character set)
        //      Calls strtod
        //      Strtod Skips leading whitespace, but this routine does NOT allow that
        //      if remainder==null, rejects if not all characters eaten (else returns how many eaten in remainder)
        //
        // Note - inlined though long, because most of the logic gets compiled out depending on (template or actual)
        // parameters, and want to assure those 'seen' by optimizer so most of the code eliminated.
        //
        template <typename T, IUNICODECanUnambiguouslyConvertFrom CHAR_T>
        inline T ToFloat_RespectingLocale_ (const span<const CHAR_T> srcSpan, typename span<const CHAR_T>::iterator* remainder)
        {
            if (srcSpan.empty ()) {
                if (remainder != nullptr) {
                    *remainder = srcSpan.begin ();
                }
                return Math::nan<T> ();
            }
 
            // because strtod, etc, require a NUL-terminated string, and span doesn't generally provide one,
            // we must copy to a temporary buffer (not super costly, especially since this isn't the main
            // approach tried typically)
            Memory::StackBuffer<CHAR_T> srcBufWithNul{Memory::eUninitialized, srcSpan.size () + 1};
            Memory::CopyBytes (srcSpan, span{srcBufWithNul});
            srcBufWithNul[srcSpan.size ()] = '\0';
            const CHAR_T* si               = srcBufWithNul.begin ();
            const CHAR_T* ei               = srcBufWithNul.end () - 1; // buf nul-terminated, but dont treat the NUL as part of our length
            const CHAR_T* ri               = ei;
 
            // since strtod skips leading whitespace, prevent that
            if (remainder == nullptr) {
                bool isSpace;
                if constexpr (sizeof (CHAR_T) == 1) {
                    isSpace = std::isspace (*si);
                }
                else if constexpr (sizeof (CHAR_T) == sizeof (wchar_t)) {
                    isSpace = std::iswspace (*si);
                }
                else {
                    isSpace = std::iswspace (static_cast<wint_t> (*si)); // not sure how to check without complex conversion logic
                }
                if (isSpace) {
                    if (remainder != nullptr) {
                        *remainder = srcSpan.begin ();
                    }
                    return Math::nan<T> ();
                }
            }
            qStroika_ATTRIBUTE_INDETERMINATE T d; //  - set below
            static_assert (same_as<T, float> or same_as<T, double> or same_as<T, long double>);
            if constexpr (sizeof (CHAR_T) == 1) {
                if constexpr (same_as<T, float>) {
                    d = ::strtof (reinterpret_cast<const char*> (si), const_cast<char**> (reinterpret_cast<const char**> (&ri)));
                }
                else if constexpr (same_as<T, double>) {
                    d = ::strtod (reinterpret_cast<const char*> (si), const_cast<char**> (reinterpret_cast<const char**> (&ri)));
                }
                else if constexpr (same_as<T, long double>) {
                    d = ::strtold (reinterpret_cast<const char*> (si), const_cast<char**> (reinterpret_cast<const char**> (&ri)));
                }
                else {
                    AssertNotReached ();
                }
            }
            else if constexpr (sizeof (CHAR_T) == sizeof (wchar_t)) {
                if constexpr (same_as<T, float>) {
                    d = ::wcstof (reinterpret_cast<const wchar_t*> (si), const_cast<wchar_t**> (reinterpret_cast<const wchar_t**> (&ri)));
                }
                else if constexpr (same_as<T, double>) {
                    d = ::wcstod (reinterpret_cast<const wchar_t*> (si), const_cast<wchar_t**> (reinterpret_cast<const wchar_t**> (&ri)));
                }
                else if constexpr (same_as<T, long double>) {
                    d = ::wcstold (reinterpret_cast<const wchar_t*> (si), const_cast<wchar_t**> (reinterpret_cast<const wchar_t**> (&ri)));
                }
                else {
                    AssertNotReached ();
                }
            }
            else {
                // must utf convert to wchar_t which we support
                Memory::StackBuffer<wchar_t> wideBuf{Memory::eUninitialized, UTFConvert::ComputeTargetBufferSize<wchar_t> (srcSpan)};
                span<const wchar_t>          wideSpan = UTFConvert::kThe.ConvertSpan (srcSpan, span{wideBuf});
                if (remainder == nullptr) {
                    d = ToFloat_RespectingLocale_<T, wchar_t> (wideSpan, nullptr);
                }
                else {
                    // do the conversion using wchar_t, and then map back the resulting remainder offset
                    span<const wchar_t>::iterator wideRemainder;
                    d  = ToFloat_RespectingLocale_<T> (wideSpan, &wideRemainder);
                    ri = UTFConvert::kThe.ConvertOffset<CHAR_T> (wideSpan, wideRemainder - wideSpan.begin ()) + si;
                }
            }
            if (remainder == nullptr) {
                // if no remainder argument, we require all the text is 'eaten'
                if (ri != ei) {
                    d = Math::nan<T> ();
                }
            }
            else {
                *remainder = srcSpan.begin () + (ri - si);
            }
            return d;
        }
 
    }
 
    /*
     ********************************************************************************
     ************************** FloatConversion::ToString ***************************
     ********************************************************************************
     */
    template <Common::IAnyOf<String, string, wstring> STRING_TYPE, floating_point FLOAT_TYPE>
    inline STRING_TYPE ToString (FLOAT_TYPE f, const ToStringOptions& options)
    {
        if constexpr (same_as<STRING_TYPE, String>) {
            return Private_::ToString_String_Implementation_ (f, options);
        }
        else if constexpr (same_as<STRING_TYPE, string>) {
            // @todo improve performance for this case
            Require (options.GetUsingLocaleClassic ());
            return Private_::ToString_String_Implementation_ (f, options).AsASCII ();
        }
        else if constexpr (same_as<STRING_TYPE, wstring>) {
            // @todo improve performance for this case
            Require (options.GetUsingLocaleClassic ());
            return Private_::ToString_String_Implementation_ (f, options).template As<wstring> ();
        }
    }
 
    /*
     ********************************************************************************
     *************************** FloatConversion::ToFloat ***************************
     ********************************************************************************
     */
    template <floating_point T, IUNICODECanUnambiguouslyConvertFrom CHAR_T>
    T ToFloat (span<const CHAR_T> s)
    {
        if constexpr (same_as<remove_cv_t<CHAR_T>, char>) {
            Require (Character::IsASCII (s));
        }
        /*
         *  Most of the time we can do this very efficiently, because there are just ascii characters.
         *  Else, fallback on algorithm that understands full unicode character set, and locales
         */
        Memory::StackBuffer<char> asciiS;
        if (Character::AsASCIIQuietly (s, &asciiS)) {
            qStroika_ATTRIBUTE_INDETERMINATE T result;
#if __cpp_lib_to_chars >= 201611 and not qCompilerAndStdLib_from_chars_and_tochars_FP_Precision_Buggy
#if qCompilerAndStdLib_to_chars_assmes_str_nul_terminated_Buggy
            asciiS.push_back (0);
#endif
            auto b = asciiS.begin ();
            auto e = asciiS.end ();
#if qCompilerAndStdLib_to_chars_assmes_str_nul_terminated_Buggy
            e--;
#endif
            if (b != e and *b == '+') [[unlikely]] {
                ++b; // "the plus sign is not recognized outside of the exponent (only the minus sign is permitted at the beginning)" from https://en.cppreference.com/w/cpp/utility/from_chars
            }
            auto [ptr, ec] = from_chars (b, e, result);
            if (ec == errc::result_out_of_range) [[unlikely]] {
                result = *b == '-' ? -numeric_limits<T>::infinity () : numeric_limits<T>::infinity ();
            }
            else if (ec != std::errc{} or ptr != e) [[unlikely]] {
                //result = Math::nan<T> (); "if # is 100,1 - in funny locale - may need to use legacy algorithm
                // @todo ADD OPTION arg to ToFloat so we know if C-Locale so can just return NAN here!...
                result = Private_::ToFloat_RespectingLocale_<T> (s, nullptr);
            }
#else
            result = Private_::ToFloat_RespectingLocale_<T> (span<const char>{asciiS}, nullptr);
#endif
            return result;
        }
        return Private_::ToFloat_RespectingLocale_<T> (s, nullptr); // fallback for non-ascii strings to old code
    }
    template <floating_point T, IUNICODECanUnambiguouslyConvertFrom CHAR_T>
    T ToFloat (span<const CHAR_T> s, typename span<const CHAR_T>::iterator* remainder)
    {
        RequireNotNull (remainder); // use other overload if 'null'
        if constexpr (same_as<remove_cv_t<CHAR_T>, char>) {
            Require (Character::IsASCII (s));
        }
        /*
         *  Most of the time we can do this very efficiently, because there are just ascii characters.
         *  Else, fallback on algorithm that understands full unicode character set.
         */
#if __cpp_lib_to_chars >= 201611 and not qCompilerAndStdLib_from_chars_and_tochars_FP_Precision_Buggy
        Memory::StackBuffer<char> asciiS;
        if (Character::AsASCIIQuietly (s, &asciiS)) {
            qStroika_ATTRIBUTE_INDETERMINATE T result;
            char*                              b = asciiS.begin ();
            char*                              e = asciiS.end ();
            if (b != e and *b == '+') [[unlikely]] {
                ++b; // "the plus sign is not recognized outside of the exponent (only the minus sign is permitted at the beginning)" from https://en.cppreference.com/w/cpp/utility/from_chars
            }
            auto [ptr, ec] = from_chars (b, e, result);
            if (ec == errc::result_out_of_range) [[unlikely]] {
                result = *b == '-' ? -numeric_limits<T>::infinity () : numeric_limits<T>::infinity ();
            }
            else if (ec != std::errc{} or ptr != e) [[unlikely]] {
                //result = Math::nan<T> (); "if # is 100,1 - in funny locale - may need to use legacy algorithm
                // @todo ADD OPTION arg to ToFloat so we know if C-Locale so can just return NAN here!...
                typename span<const CHAR_T>::iterator tmpI;
                result = Private_::ToFloat_RespectingLocale_<T> (s, &tmpI);
                ptr    = tmpI - s.begin () + b;
            }
            *remainder = s.begin () + UTFConvert::kThe.ConvertOffset<CHAR_T> (
                                          span{reinterpret_cast<const char8_t*> (b), reinterpret_cast<const char8_t*> (e)}, ptr - b);
            Assert (*remainder <= s.end ());
            return result;
        }
#endif
        return Private_::ToFloat_RespectingLocale_<T> (s, remainder); // fallback for non-ascii strings to strtod etc code
    }
    template <floating_point T, typename STRINGISH_ARG>
    inline T ToFloat (STRINGISH_ARG&& s)
        requires (IConvertibleToString<STRINGISH_ARG> or is_convertible_v<STRINGISH_ARG, std::string>)
    {
        using DecayedStringishArg = remove_cvref_t<STRINGISH_ARG>;
        if constexpr (same_as<DecayedStringishArg, const char*> or same_as<DecayedStringishArg, const char8_t*> or
                      same_as<DecayedStringishArg, const char16_t*> or same_as<DecayedStringishArg, const char32_t*> or
                      same_as<DecayedStringishArg, const wchar_t*>) {
            return ToFloat<T> (span{s, CString::Length (s)});
        }
        else if constexpr (same_as<DecayedStringishArg, String>) {
            Memory::StackBuffer<wchar_t> ignored; // todo optimize for keep ascii case
            auto                         sp = s.template GetData<wchar_t> (&ignored);
            return ToFloat<T> (sp);
        }
        else if constexpr (is_convertible_v<DecayedStringishArg, std::string>) {
            string ss = s;
            return ToFloat<T> (span{ss.data (), ss.size ()});
        }
        else {
            return ToFloat<T> (String{forward<STRINGISH_ARG> (s)});
        }
    }
    template <floating_point T>
    inline T ToFloat (const String& s, String* remainder)
    {
        RequireNotNull (remainder);
        Memory::StackBuffer<wchar_t>  ignored;
        span<const wchar_t>           srcSpan = s.GetData (&ignored);
        span<const wchar_t>::iterator tmpRemainder;
        auto                          result = ToFloat<T> (srcSpan, &tmpRemainder);
        *remainder                           = String{srcSpan.subspan (tmpRemainder - srcSpan.begin ())};
        return result;
    }
 
    //////////////// DEPRECATED STUFF BELOW
 
    template <typename T>
    [[deprecated ("Since Stroika v3.0d1 - use span overload")]] inline T ToFloat (const wchar_t* start, const wchar_t* end, const wchar_t** remainder)
    {
        Require (start <= end);
        RequireNotNull (remainder);
        qStroika_ATTRIBUTE_INDETERMINATE T result;
#if __cpp_lib_to_chars >= 201611 and not qCompilerAndStdLib_from_chars_and_tochars_FP_Precision_Buggy
        /*
         *  Most of the time we can do this very efficiently, because there are just ascii characters.
         *  Else, fallback on older algorithm that understands full unicode character set.
         */
        Memory::StackBuffer<char> asciiS;
        if (Character::AsASCIIQuietly (span{start, end}, &asciiS)) {
            auto b         = asciiS.begin ();
            auto originalB = b; // needed to properly compute remainder
            auto e         = asciiS.end ();
            if (remainder != nullptr) [[unlikely]] {
                // in remainder mode we skip leading whitespace
                while (b != e and iswspace (*b)) [[unlikely]] {
                    ++b;
                }
            }
            if (b != e and *b == '+') [[unlikely]] {
                ++b; // "the plus sign is not recognized outside of the exponent (only the minus sign is permitted at the beginning)" from https://en.cppreference.com/w/cpp/utility/from_chars
            }
 
            auto [ptr, ec] = from_chars (b, e, result);
            if (ec == errc::result_out_of_range) [[unlikely]] {
                return *b == '-' ? -numeric_limits<T>::infinity () : numeric_limits<T>::infinity ();
            }
            // if error - return nan
            if (ec != std::errc{}) [[unlikely]] {
                result = Math::nan<T> ();
            }
            // If asked to return remainder do so.
            // If NOT asked to return remainder, treat not using the entire string as forcing result to be a Nan (invalid parse of number of not the whole thing is a number)
            if (remainder == nullptr) [[likely]] {
                if (ptr != e) [[unlikely]] {
                    result = Math::nan<T> ();
                }
            }
            else {
                *remainder = ptr - originalB + start; // adjust in case we remapped data
            }
        }
        else {
            result = Private_::ToFloat_ViaStrToD_<T> (start, end, remainder);
        }
#else
        result = Private_::ToFloat_ViaStrToD_<T> (start, end, remainder);
#endif
        return result;
    }
    template <typename T = double>
    [[deprecated ("Since Stroika v3.0d1 - use span overload")]] inline T ToFloat (const char* start, const char* end)
    {
        return ToFloat<T> (span{start, end});
    }
    template <typename T = double>
    [[deprecated ("Since Stroika v3.0d1 - use span overload")]] inline T ToFloat (const wchar_t* start, const wchar_t* end)
    {
        return ToFloat<T> (span{start, end});
    }
 
}