diff options
Diffstat (limited to 'absl/strings/internal/str_format')
| -rw-r--r-- | absl/strings/internal/str_format/convert_test.cc | 230 | ||||
| -rw-r--r-- | absl/strings/internal/str_format/float_conversion.cc | 281 |
2 files changed, 492 insertions, 19 deletions
diff --git a/absl/strings/internal/str_format/convert_test.cc b/absl/strings/internal/str_format/convert_test.cc index e37d0546..488d4cd4 100644 --- a/absl/strings/internal/str_format/convert_test.cc +++ b/absl/strings/internal/str_format/convert_test.cc @@ -12,6 +12,7 @@ #include "gtest/gtest.h" #include "absl/base/internal/raw_logging.h" #include "absl/strings/internal/str_format/bind.h" +#include "absl/strings/match.h" #include "absl/types/optional.h" namespace absl { @@ -19,6 +20,13 @@ ABSL_NAMESPACE_BEGIN namespace str_format_internal { namespace { +struct NativePrintfTraits { + bool hex_float_has_glibc_rounding; + bool hex_float_prefers_denormal_repr; + bool hex_float_uses_minimal_precision_when_not_specified; + bool hex_float_optimizes_leading_digit_bit_count; +}; + template <typename T, size_t N> size_t ArraySize(T (&)[N]) { return N; @@ -118,6 +126,63 @@ std::string StrPrint(const char *format, ...) { return result; } +NativePrintfTraits VerifyNativeImplementationImpl() { + NativePrintfTraits result; + + // >>> hex_float_has_glibc_rounding. To have glibc's rounding behavior we need + // to meet three requirements: + // + // - The threshold for rounding up is 8 (for e.g. MSVC uses 9). + // - If the digits lower than than the 8 are non-zero then we round up. + // - If the digits lower than the 8 are all zero then we round toward even. + // + // The numbers below represent all the cases covering {below,at,above} the + // threshold (8) with both {zero,non-zero} lower bits and both {even,odd} + // preceding digits. + const double d0079 = 65657.0; // 0x1.0079p+16 + const double d0179 = 65913.0; // 0x1.0179p+16 + const double d0080 = 65664.0; // 0x1.0080p+16 + const double d0180 = 65920.0; // 0x1.0180p+16 + const double d0081 = 65665.0; // 0x1.0081p+16 + const double d0181 = 65921.0; // 0x1.0181p+16 + result.hex_float_has_glibc_rounding = + StartsWith(StrPrint("%.2a", d0079), "0x1.00") && + StartsWith(StrPrint("%.2a", d0179), "0x1.01") && + StartsWith(StrPrint("%.2a", d0080), "0x1.00") && + StartsWith(StrPrint("%.2a", d0180), "0x1.02") && + StartsWith(StrPrint("%.2a", d0081), "0x1.01") && + StartsWith(StrPrint("%.2a", d0181), "0x1.02"); + + // >>> hex_float_prefers_denormal_repr. Formatting `denormal` on glibc yields + // "0x0.0000000000001p-1022", whereas on std libs that don't use denormal + // representation it would either be 0x1p-1074 or 0x1.0000000000000-1074. + const double denormal = std::numeric_limits<double>::denorm_min(); + result.hex_float_prefers_denormal_repr = + StartsWith(StrPrint("%a", denormal), "0x0.0000000000001"); + + // >>> hex_float_uses_minimal_precision_when_not_specified. Some (non-glibc) + // libs will format the following as "0x1.0079000000000p+16". + result.hex_float_uses_minimal_precision_when_not_specified = + (StrPrint("%a", d0079) == "0x1.0079p+16"); + + // >>> hex_float_optimizes_leading_digit_bit_count. The number 1.5, when + // formatted by glibc should yield "0x1.8p+0" for `double` and "0xcp-3" for + // `long double`, i.e., number of bits in the leading digit is adapted to the + // number of bits in the mantissa. + const double d_15 = 1.5; + const long double ld_15 = 1.5; + result.hex_float_optimizes_leading_digit_bit_count = + StartsWith(StrPrint("%a", d_15), "0x1.8") && + StartsWith(StrPrint("%La", ld_15), "0xc"); + + return result; +} + +const NativePrintfTraits &VerifyNativeImplementation() { + static NativePrintfTraits native_traits = VerifyNativeImplementationImpl(); + return native_traits; +} + class FormatConvertTest : public ::testing::Test { }; template <typename T> @@ -476,6 +541,7 @@ TEST_F(FormatConvertTest, Uint128) { template <typename Floating> void TestWithMultipleFormatsHelper(const std::vector<Floating> &floats) { + const NativePrintfTraits &native_traits = VerifyNativeImplementation(); // Reserve the space to ensure we don't allocate memory in the output itself. std::string str_format_result; str_format_result.reserve(1 << 20); @@ -493,13 +559,23 @@ void TestWithMultipleFormatsHelper(const std::vector<Floating> &floats) { 'e', 'E'}) { std::string fmt_str = std::string(fmt) + f; - if (fmt == absl::string_view("%.5000") && f != 'f' && f != 'F') { + if (fmt == absl::string_view("%.5000") && f != 'f' && f != 'F' && + f != 'a' && f != 'A') { // This particular test takes way too long with snprintf. // Disable for the case we are not implementing natively. continue; } + if ((f == 'a' || f == 'A') && + !native_traits.hex_float_has_glibc_rounding) { + continue; + } + for (Floating d : floats) { + if (!native_traits.hex_float_prefers_denormal_repr && + (f == 'a' || f == 'A') && std::fpclassify(d) == FP_SUBNORMAL) { + continue; + } int i = -10; FormatArgImpl args[2] = {FormatArgImpl(d), FormatArgImpl(i)}; UntypedFormatSpecImpl format(fmt_str); @@ -766,6 +842,111 @@ TEST_F(FormatConvertTest, DoubleRound) { "1837869002408041296803276054561138153076171875"); } +TEST_F(FormatConvertTest, DoubleRoundA) { + const NativePrintfTraits &native_traits = VerifyNativeImplementation(); + std::string s; + const auto format = [&](const char *fmt, double d) -> std::string & { + s.clear(); + FormatArgImpl args[1] = {FormatArgImpl(d)}; + AppendPack(&s, UntypedFormatSpecImpl(fmt), absl::MakeSpan(args)); + if (native_traits.hex_float_has_glibc_rounding) { + EXPECT_EQ(StrPrint(fmt, d), s); + } + return s; + }; + + // 0x1.00018000p+100 + const double on_boundary_odd = 1267679614447900152596896153600.0; + EXPECT_EQ(format("%.0a", on_boundary_odd), "0x1p+100"); + EXPECT_EQ(format("%.1a", on_boundary_odd), "0x1.0p+100"); + EXPECT_EQ(format("%.2a", on_boundary_odd), "0x1.00p+100"); + EXPECT_EQ(format("%.3a", on_boundary_odd), "0x1.000p+100"); + EXPECT_EQ(format("%.4a", on_boundary_odd), "0x1.0002p+100"); // round + EXPECT_EQ(format("%.5a", on_boundary_odd), "0x1.00018p+100"); + EXPECT_EQ(format("%.6a", on_boundary_odd), "0x1.000180p+100"); + + // 0x1.00028000p-2 + const double on_boundary_even = 0.250009536743164062500; + EXPECT_EQ(format("%.0a", on_boundary_even), "0x1p-2"); + EXPECT_EQ(format("%.1a", on_boundary_even), "0x1.0p-2"); + EXPECT_EQ(format("%.2a", on_boundary_even), "0x1.00p-2"); + EXPECT_EQ(format("%.3a", on_boundary_even), "0x1.000p-2"); + EXPECT_EQ(format("%.4a", on_boundary_even), "0x1.0002p-2"); // no round + EXPECT_EQ(format("%.5a", on_boundary_even), "0x1.00028p-2"); + EXPECT_EQ(format("%.6a", on_boundary_even), "0x1.000280p-2"); + + // 0x1.00018001p+1 + const double slightly_over = 2.00004577683284878730773925781250; + EXPECT_EQ(format("%.0a", slightly_over), "0x1p+1"); + EXPECT_EQ(format("%.1a", slightly_over), "0x1.0p+1"); + EXPECT_EQ(format("%.2a", slightly_over), "0x1.00p+1"); + EXPECT_EQ(format("%.3a", slightly_over), "0x1.000p+1"); + EXPECT_EQ(format("%.4a", slightly_over), "0x1.0002p+1"); + EXPECT_EQ(format("%.5a", slightly_over), "0x1.00018p+1"); + EXPECT_EQ(format("%.6a", slightly_over), "0x1.000180p+1"); + + // 0x1.00017fffp+0 + const double slightly_under = 1.000022887950763106346130371093750; + EXPECT_EQ(format("%.0a", slightly_under), "0x1p+0"); + EXPECT_EQ(format("%.1a", slightly_under), "0x1.0p+0"); + EXPECT_EQ(format("%.2a", slightly_under), "0x1.00p+0"); + EXPECT_EQ(format("%.3a", slightly_under), "0x1.000p+0"); + EXPECT_EQ(format("%.4a", slightly_under), "0x1.0001p+0"); + EXPECT_EQ(format("%.5a", slightly_under), "0x1.00018p+0"); + EXPECT_EQ(format("%.6a", slightly_under), "0x1.000180p+0"); + EXPECT_EQ(format("%.7a", slightly_under), "0x1.0001800p+0"); + + // 0x1.1b3829ac28058p+3 + const double hex_value = 8.85060580848964661981881363317370414733886718750; + EXPECT_EQ(format("%.0a", hex_value), "0x1p+3"); + EXPECT_EQ(format("%.1a", hex_value), "0x1.2p+3"); + EXPECT_EQ(format("%.2a", hex_value), "0x1.1bp+3"); + EXPECT_EQ(format("%.3a", hex_value), "0x1.1b4p+3"); + EXPECT_EQ(format("%.4a", hex_value), "0x1.1b38p+3"); + EXPECT_EQ(format("%.5a", hex_value), "0x1.1b383p+3"); + EXPECT_EQ(format("%.6a", hex_value), "0x1.1b382ap+3"); + EXPECT_EQ(format("%.7a", hex_value), "0x1.1b3829bp+3"); + EXPECT_EQ(format("%.8a", hex_value), "0x1.1b3829acp+3"); + EXPECT_EQ(format("%.9a", hex_value), "0x1.1b3829ac3p+3"); + EXPECT_EQ(format("%.10a", hex_value), "0x1.1b3829ac28p+3"); + EXPECT_EQ(format("%.11a", hex_value), "0x1.1b3829ac280p+3"); + EXPECT_EQ(format("%.12a", hex_value), "0x1.1b3829ac2806p+3"); + EXPECT_EQ(format("%.13a", hex_value), "0x1.1b3829ac28058p+3"); + EXPECT_EQ(format("%.14a", hex_value), "0x1.1b3829ac280580p+3"); + EXPECT_EQ(format("%.15a", hex_value), "0x1.1b3829ac2805800p+3"); + EXPECT_EQ(format("%.16a", hex_value), "0x1.1b3829ac28058000p+3"); + EXPECT_EQ(format("%.17a", hex_value), "0x1.1b3829ac280580000p+3"); + EXPECT_EQ(format("%.18a", hex_value), "0x1.1b3829ac2805800000p+3"); + EXPECT_EQ(format("%.19a", hex_value), "0x1.1b3829ac28058000000p+3"); + EXPECT_EQ(format("%.20a", hex_value), "0x1.1b3829ac280580000000p+3"); + EXPECT_EQ(format("%.21a", hex_value), "0x1.1b3829ac2805800000000p+3"); + + // 0x1.0818283848586p+3 + const double hex_value2 = 8.2529488658208371987257123691961169242858886718750; + EXPECT_EQ(format("%.0a", hex_value2), "0x1p+3"); + EXPECT_EQ(format("%.1a", hex_value2), "0x1.1p+3"); + EXPECT_EQ(format("%.2a", hex_value2), "0x1.08p+3"); + EXPECT_EQ(format("%.3a", hex_value2), "0x1.082p+3"); + EXPECT_EQ(format("%.4a", hex_value2), "0x1.0818p+3"); + EXPECT_EQ(format("%.5a", hex_value2), "0x1.08183p+3"); + EXPECT_EQ(format("%.6a", hex_value2), "0x1.081828p+3"); + EXPECT_EQ(format("%.7a", hex_value2), "0x1.0818284p+3"); + EXPECT_EQ(format("%.8a", hex_value2), "0x1.08182838p+3"); + EXPECT_EQ(format("%.9a", hex_value2), "0x1.081828385p+3"); + EXPECT_EQ(format("%.10a", hex_value2), "0x1.0818283848p+3"); + EXPECT_EQ(format("%.11a", hex_value2), "0x1.08182838486p+3"); + EXPECT_EQ(format("%.12a", hex_value2), "0x1.081828384858p+3"); + EXPECT_EQ(format("%.13a", hex_value2), "0x1.0818283848586p+3"); + EXPECT_EQ(format("%.14a", hex_value2), "0x1.08182838485860p+3"); + EXPECT_EQ(format("%.15a", hex_value2), "0x1.081828384858600p+3"); + EXPECT_EQ(format("%.16a", hex_value2), "0x1.0818283848586000p+3"); + EXPECT_EQ(format("%.17a", hex_value2), "0x1.08182838485860000p+3"); + EXPECT_EQ(format("%.18a", hex_value2), "0x1.081828384858600000p+3"); + EXPECT_EQ(format("%.19a", hex_value2), "0x1.0818283848586000000p+3"); + EXPECT_EQ(format("%.20a", hex_value2), "0x1.08182838485860000000p+3"); + EXPECT_EQ(format("%.21a", hex_value2), "0x1.081828384858600000000p+3"); +} + // We don't actually store the results. This is just to exercise the rest of the // machinery. struct NullSink { @@ -797,6 +978,7 @@ TEST_F(FormatConvertTest, LongDouble) { // implementation against the native one there. return; #endif // _MSC_VER + const NativePrintfTraits &native_traits = VerifyNativeImplementation(); const char *const kFormats[] = {"%", "%.3", "%8.5", "%9", "%.5000", "%.60", "%+", "% ", "%-10"}; @@ -839,12 +1021,20 @@ TEST_F(FormatConvertTest, LongDouble) { 'e', 'E'}) { std::string fmt_str = std::string(fmt) + 'L' + f; - if (fmt == absl::string_view("%.5000") && f != 'f' && f != 'F') { + if (fmt == absl::string_view("%.5000") && f != 'f' && f != 'F' && + f != 'a' && f != 'A') { // This particular test takes way too long with snprintf. // Disable for the case we are not implementing natively. continue; } + if (f == 'a' || f == 'A') { + if (!native_traits.hex_float_has_glibc_rounding || + !native_traits.hex_float_optimizes_leading_digit_bit_count) { + continue; + } + } + for (auto d : doubles) { FormatArgImpl arg(d); UntypedFormatSpecImpl format(fmt_str); @@ -860,6 +1050,7 @@ TEST_F(FormatConvertTest, LongDouble) { } TEST_F(FormatConvertTest, IntAsDouble) { + const NativePrintfTraits &native_traits = VerifyNativeImplementation(); const int kMin = std::numeric_limits<int>::min(); const int kMax = std::numeric_limits<int>::max(); const int ia[] = { @@ -875,14 +1066,16 @@ TEST_F(FormatConvertTest, IntAsDouble) { const char *fmt; }; const double dx = static_cast<double>(fx); - const Expectation kExpect[] = { - { __LINE__, StrPrint("%f", dx), "%f" }, - { __LINE__, StrPrint("%12f", dx), "%12f" }, - { __LINE__, StrPrint("%.12f", dx), "%.12f" }, - { __LINE__, StrPrint("%12a", dx), "%12a" }, - { __LINE__, StrPrint("%.12a", dx), "%.12a" }, + std::vector<Expectation> expect = { + {__LINE__, StrPrint("%f", dx), "%f"}, + {__LINE__, StrPrint("%12f", dx), "%12f"}, + {__LINE__, StrPrint("%.12f", dx), "%.12f"}, + {__LINE__, StrPrint("%.12a", dx), "%.12a"}, }; - for (const Expectation &e : kExpect) { + if (native_traits.hex_float_uses_minimal_precision_when_not_specified) { + expect.push_back({__LINE__, StrPrint("%12a", dx), "%12a"}); + } + for (const Expectation &e : expect) { SCOPED_TRACE(e.line); SCOPED_TRACE(e.fmt); UntypedFormatSpecImpl format(e.fmt); @@ -927,6 +1120,25 @@ TEST_F(FormatConvertTest, ExpectedFailures) { EXPECT_TRUE(FormatFails("%*d", "")); } +// Sanity check to make sure that we are testing what we think we're testing on +// e.g. the x86_64+glibc platform. +TEST_F(FormatConvertTest, GlibcHasCorrectTraits) { +#if !defined(__GLIBC__) || !defined(__x86_64__) + return; +#endif + const NativePrintfTraits &native_traits = VerifyNativeImplementation(); + // If one of the following tests break then it is either because the above PP + // macro guards failed to exclude a new platform (likely) or because something + // has changed in the implemention of glibc sprintf float formatting behavior. + // If the latter, then the code that computes these flags needs to be + // revisited and/or possibly the StrFormat implementation. + EXPECT_TRUE(native_traits.hex_float_has_glibc_rounding); + EXPECT_TRUE(native_traits.hex_float_prefers_denormal_repr); + EXPECT_TRUE( + native_traits.hex_float_uses_minimal_precision_when_not_specified); + EXPECT_TRUE(native_traits.hex_float_optimizes_leading_digit_bit_count); +} + } // namespace } // namespace str_format_internal ABSL_NAMESPACE_END diff --git a/absl/strings/internal/str_format/float_conversion.cc b/absl/strings/internal/str_format/float_conversion.cc index 39fc5f60..6eb7b9fc 100644 --- a/absl/strings/internal/str_format/float_conversion.cc +++ b/absl/strings/internal/str_format/float_conversion.cc @@ -15,6 +15,7 @@ #include "absl/functional/function_ref.h" #include "absl/meta/type_traits.h" #include "absl/numeric/int128.h" +#include "absl/strings/numbers.h" #include "absl/types/optional.h" #include "absl/types/span.h" @@ -453,26 +454,31 @@ Padding ExtraWidthToPadding(size_t total_size, const FormatState &state) { } } -void FinalPrint(absl::string_view data, int trailing_zeros, - const FormatState &state) { +void FinalPrint(const FormatState &state, absl::string_view data, + int padding_offset, int trailing_zeros, + absl::string_view data_postfix) { if (state.conv.width() < 0) { // No width specified. Fast-path. if (state.sign_char != '\0') state.sink->Append(1, state.sign_char); state.sink->Append(data); state.sink->Append(trailing_zeros, '0'); + state.sink->Append(data_postfix); return; } - auto padding = - ExtraWidthToPadding((state.sign_char != '\0' ? 1 : 0) + data.size() + - static_cast<size_t>(trailing_zeros), - state); + auto padding = ExtraWidthToPadding((state.sign_char != '\0' ? 1 : 0) + + data.size() + data_postfix.size() + + static_cast<size_t>(trailing_zeros), + state); state.sink->Append(padding.left_spaces, ' '); if (state.sign_char != '\0') state.sink->Append(1, state.sign_char); + // Padding in general needs to be inserted somewhere in the middle of `data`. + state.sink->Append(data.substr(0, padding_offset)); state.sink->Append(padding.zeros, '0'); - state.sink->Append(data); + state.sink->Append(data.substr(padding_offset)); state.sink->Append(trailing_zeros, '0'); + state.sink->Append(data_postfix); state.sink->Append(padding.right_spaces, ' '); } @@ -525,10 +531,11 @@ void FormatFFast(Int v, int exp, const FormatState &state) { // In `alt` mode (flag #) we keep the `.` even if there are no fractional // digits. In non-alt mode, we strip it. if (!state.ShouldPrintDot()) --size; - FinalPrint(absl::string_view(integral_digits_start, size), + FinalPrint(state, absl::string_view(integral_digits_start, size), + /*padding_offset=*/0, static_cast<int>(state.precision - (fractional_digits_end - fractional_digits_start)), - state); + /*data_postfix=*/""); } // Slow %f formatter for when the shifted value does not fit in a uint128, and @@ -655,6 +662,257 @@ void FormatF(Int mantissa, int exp, const FormatState &state) { return FormatFFast(mantissa, exp, state); } +// Grab the group of four bits (nibble) from `n`. E.g., nibble 1 corresponds to +// bits 4-7. +template <typename Int> +uint8_t GetNibble(Int n, int nibble_index) { + constexpr Int mask_low_nibble = Int{0xf}; + int shift = nibble_index * 4; + n &= mask_low_nibble << shift; + return static_cast<uint8_t>((n >> shift) & 0xf); +} + +// Add one to the given nibble, applying carry to higher nibbles. Returns true +// if overflow, false otherwise. +template <typename Int> +bool IncrementNibble(int nibble_index, Int *n) { + constexpr int kShift = sizeof(Int) * 8 - 1; + constexpr int kNumNibbles = sizeof(Int) * 8 / 4; + Int before = *n >> kShift; + // Here we essentially want to take the number 1 and move it into the requsted + // nibble, then add it to *n to effectively increment the nibble. However, + // ASan will complain if we try to shift the 1 beyond the limits of the Int, + // i.e., if the nibble_index is out of range. So therefore we check for this + // and if we are out of range we just add 0 which leaves *n unchanged, which + // seems like the reasonable thing to do in that case. + *n += + ((nibble_index * 4 >= sizeof(Int) * 8) ? 0 + : (Int{1} << (nibble_index * 4))); + Int after = *n >> kShift; + return (before && !after) || (nibble_index >= kNumNibbles); +} + +// Return a mask with 1's in the given nibble and all lower nibbles. +template <typename Int> +Int MaskUpToNibbleInclusive(int nibble_index) { + constexpr int kNumNibbles = sizeof(Int) * 8 / 4; + static const Int ones = ~Int{0}; + return ones >> std::max(0, 4 * (kNumNibbles - nibble_index - 1)); +} + +// Return a mask with 1's below the given nibble. +template <typename Int> +Int MaskUpToNibbleExclusive(int nibble_index) { + return nibble_index <= 0 ? 0 : MaskUpToNibbleInclusive<Int>(nibble_index - 1); +} + +template <typename Int> +Int MoveToNibble(uint8_t nibble, int nibble_index) { + return Int{nibble} << (4 * nibble_index); +} + +// Given mantissa size, find optimal # of mantissa bits to put in initial digit. +// +// In the hex representation we keep a single hex digit to the left of the dot. +// However, the question as to how many bits of the mantissa should be put into +// that hex digit in theory is arbitrary, but in practice it is optimal to +// choose based on the size of the mantissa. E.g., for a `double`, there are 53 +// mantissa bits, so that means that we should put 1 bit to the left of the dot, +// thereby leaving 52 bits to the right, which is evenly divisible by four and +// thus all fractional digits represent actual precision. For a `long double`, +// on the other hand, there are 64 bits of mantissa, thus we can use all four +// bits for the initial hex digit and still have a number left over (60) that is +// a multiple of four. Once again, the goal is to have all fractional digits +// represent real precision. +template <typename Float> +constexpr int HexFloatLeadingDigitSizeInBits() { + return std::numeric_limits<Float>::digits % 4 > 0 + ? std::numeric_limits<Float>::digits % 4 + : 4; +} + +// This function captures the rounding behavior of glibc for hex float +// representations. E.g. when rounding 0x1.ab800000 to a precision of .2 +// ("%.2a") glibc will round up because it rounds toward the even number (since +// 0xb is an odd number, it will round up to 0xc). However, when rounding at a +// point that is not followed by 800000..., it disregards the parity and rounds +// up if > 8 and rounds down if < 8. +template <typename Int> +bool HexFloatNeedsRoundUp(Int mantissa, int final_nibble_displayed) { + // If the last nibble (hex digit) to be displayed is the lowest on in the + // mantissa then that means that we don't have any further nibbles to inform + // rounding, so don't round. + if (final_nibble_displayed <= 0) { + return false; + } + int rounding_nibble_idx = final_nibble_displayed - 1; + constexpr int kTotalNibbles = sizeof(Int) * 8 / 4; + assert(final_nibble_displayed <= kTotalNibbles); + Int mantissa_up_to_rounding_nibble_inclusive = + mantissa & MaskUpToNibbleInclusive<Int>(rounding_nibble_idx); + Int eight = MoveToNibble<Int>(8, rounding_nibble_idx); + if (mantissa_up_to_rounding_nibble_inclusive != eight) { + return mantissa_up_to_rounding_nibble_inclusive > eight; + } + // Nibble in question == 8. + uint8_t should_round_at_8 = + (final_nibble_displayed >= kTotalNibbles) + ? true + : (GetNibble(mantissa, final_nibble_displayed) % 2 == 1); + return should_round_at_8; +} + +// Stores values associated with a Float type needed by the FormatA +// implementation in order to avoid templatizing that function by the Float +// type. +struct HexFloatTypeParams { + template <typename Float> + explicit HexFloatTypeParams(Float) + : min_exponent(std::numeric_limits<Float>::min_exponent - 1), + leading_digit_size_bits(HexFloatLeadingDigitSizeInBits<Float>()) { + assert(leading_digit_size_bits >= 1 && leading_digit_size_bits <= 4); + } + + int min_exponent; + int leading_digit_size_bits; +}; + +// Hex Float Rounding. First check if we need to round; if so, then we do that +// by manipulating (incrementing) the mantissa, that way we can later print the +// mantissa digits by iterating through them in the same way regardless of +// whether a rounding happened. +template <typename Int> +void FormatARound(bool precision_specified, const FormatState &state, + uint8_t *leading, Int *mantissa, int *exp) { + constexpr int kTotalNibbles = sizeof(Int) * 8 / 4; + // Index of the last nibble that we could display given precision. + int final_nibble_displayed = + precision_specified ? std::max(0, (kTotalNibbles - state.precision)) : 0; + if (HexFloatNeedsRoundUp(*mantissa, final_nibble_displayed)) { + // Need to round up. + bool overflow = IncrementNibble(final_nibble_displayed, mantissa); + *leading += (overflow ? 1 : 0); + if (ABSL_PREDICT_FALSE(*leading > 15)) { + // We have overflowed the leading digit. This would mean that we would + // need two hex digits to the left of the dot, which is not allowed. So + // adjust the mantissa and exponent so that the result is always 1.0eXXX. + *leading = 1; + *mantissa = 0; + *exp += 4; + } + } + // Now that we have handled a possible round-up we can go ahead and zero out + // all the nibbles of the mantissa that we won't need. + if (precision_specified) { + *mantissa &= ~MaskUpToNibbleExclusive<Int>(final_nibble_displayed); + } +} + +template <typename Int> +void FormatANormalize(const HexFloatTypeParams float_traits, uint8_t *leading, + Int *mantissa, int *exp) { + constexpr int kIntBits = sizeof(Int) * 8; + static const Int kHighIntBit = Int{1} << (kIntBits - 1); + const int kLeadDigitBitsCount = float_traits.leading_digit_size_bits; + // Normalize mantissa so that highest bit set is in MSB position, unless we + // get interrupted by the exponent threshold. + while (*mantissa && !(*mantissa & kHighIntBit)) { + if (ABSL_PREDICT_FALSE(*exp - 1 < float_traits.min_exponent)) { + *mantissa >>= (float_traits.min_exponent - *exp); + *exp = float_traits.min_exponent; + return; + } + *mantissa <<= 1; + --*exp; + } + // Extract bits for leading digit then shift them away leaving the + // fractional part. + *leading = + static_cast<uint8_t>(*mantissa >> (kIntBits - kLeadDigitBitsCount)); + *exp -= (*mantissa != 0) ? kLeadDigitBitsCount : *exp; + *mantissa <<= kLeadDigitBitsCount; +} + +template <typename Int> +void FormatA(const HexFloatTypeParams float_traits, Int mantissa, int exp, + bool uppercase, const FormatState &state) { + // Int properties. + constexpr int kIntBits = sizeof(Int) * 8; + constexpr int kTotalNibbles = sizeof(Int) * 8 / 4; + // Did the user specify a precision explicitly? + const bool precision_specified = state.conv.precision() >= 0; + + // ========== Normalize/Denormalize ========== + exp += kIntBits; // make all digits fractional digits. + // This holds the (up to four) bits of leading digit, i.e., the '1' in the + // number 0x1.e6fp+2. It's always > 0 unless number is zero or denormal. + uint8_t leading = 0; + FormatANormalize(float_traits, &leading, &mantissa, &exp); + + // =============== Rounding ================== + // Check if we need to round; if so, then we do that by manipulating + // (incrementing) the mantissa before beginning to print characters. + FormatARound(precision_specified, state, &leading, &mantissa, &exp); + + // ============= Format Result =============== + // This buffer holds the "0x1.ab1de3" portion of "0x1.ab1de3pe+2". Compute the + // size with long double which is the largest of the floats. + constexpr size_t kBufSizeForHexFloatRepr = + 2 // 0x + + std::numeric_limits<long double>::digits / 4 // number of hex digits + + 1 // round up + + 1; // "." (dot) + char digits_buffer[kBufSizeForHexFloatRepr]; + char *digits_iter = digits_buffer; + const char *const digits = + static_cast<const char *>("0123456789ABCDEF0123456789abcdef") + + (uppercase ? 0 : 16); + + // =============== Hex Prefix ================ + *digits_iter++ = '0'; + *digits_iter++ = uppercase ? 'X' : 'x'; + + // ========== Non-Fractional Digit =========== + *digits_iter++ = digits[leading]; + + // ================== Dot ==================== + // There are three reasons we might need a dot. Keep in mind that, at this + // point, the mantissa holds only the fractional part. + if ((precision_specified && state.precision > 0) || + (!precision_specified && mantissa > 0) || state.conv.has_alt_flag()) { + *digits_iter++ = '.'; + } + + // ============ Fractional Digits ============ + int digits_emitted = 0; + while (mantissa > 0) { + *digits_iter++ = digits[GetNibble(mantissa, kTotalNibbles - 1)]; + mantissa <<= 4; + ++digits_emitted; + } + int trailing_zeros = + precision_specified ? state.precision - digits_emitted : 0; + assert(trailing_zeros >= 0); + auto digits_result = string_view(digits_buffer, digits_iter - digits_buffer); + + // =============== Exponent ================== + constexpr size_t kBufSizeForExpDecRepr = + numbers_internal::kFastToBufferSize // requred for FastIntToBuffer + + 1 // 'p' or 'P' + + 1; // '+' or '-' + char exp_buffer[kBufSizeForExpDecRepr]; + exp_buffer[0] = uppercase ? 'P' : 'p'; + exp_buffer[1] = exp >= 0 ? '+' : '-'; + numbers_internal::FastIntToBuffer(exp < 0 ? -exp : exp, exp_buffer + 2); + + // ============ Assemble Result ============== + FinalPrint(state, // + digits_result, // 0xN.NNN... + 2, // offset in `data` to start padding if needed. + trailing_zeros, // num remaining mantissa padding zeros + exp_buffer); // exponent +} + char *CopyStringTo(absl::string_view v, char *out) { std::memcpy(out, v.data(), v.size()); return out + v.size(); @@ -1103,7 +1361,10 @@ bool FloatToSink(const Float v, const FormatConversionSpecImpl &conv, } } else if (c == FormatConversionCharInternal::a || c == FormatConversionCharInternal::A) { - return FallbackToSnprintf(v, conv, sink); + bool uppercase = (c == FormatConversionCharInternal::A); + FormatA(HexFloatTypeParams(Float{}), decomposed.mantissa, + decomposed.exponent, uppercase, {sign_char, precision, conv, sink}); + return true; } else { return false; } |