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printf formatting fixes #136

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daeae65
formatting: add always_sign argument to print_float
no92 Apr 10, 2026
2254cb5
formatting: move handling of alternate conversions into print_digits
no92 Apr 10, 2026
a5fd2b0
formatting: support `%C` and `%S` conversions
no92 Apr 11, 2026
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48 changes: 36 additions & 12 deletions include/frg/formatting.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -35,6 +35,7 @@ namespace frg FRG_VISIBILITY {

template<typename T, typename Char = char>
concept SinkFor = requires (T t, const Char *str, Char c, size_t n) {
t.append(str, n, n);
t.append(str, n);
t.append(str);
t.append(c);
Expand Down Expand Up @@ -158,7 +159,8 @@ namespace _fmt_basics {
void print_digits(S &sink, T number, bool negative, int radix,
int width, int precision, char padding, bool left_justify,
bool group_thousands, bool always_sign, bool plus_becomes_space,
bool use_capitals, locale_options<Char> locale_opts) {
bool use_capitals, bool alt_conversion, locale_options<Char> locale_opts) {
using P = frg::FormatterPolicy<Char>;
const char *digits = use_capitals ? "0123456789ABCDEF" : "0123456789abcdef";
char buffer[64];

Expand Down Expand Up @@ -197,13 +199,18 @@ namespace _fmt_basics {
}
};

T remaining_num = number;

// print the number in reverse order and determine #digits.
do {
FRG_ASSERT(k < 64); // TODO: variable number of digits
buffer[k++] = digits[number % radix];
number /= radix;
buffer[k++] = digits[remaining_num % radix];
remaining_num /= radix;
step_grouping();
} while(number);
} while(remaining_num);

if (radix == 8 && alt_conversion && k >= precision && number != 0)
precision = k + 1;

if (k < precision)
for (int i = 0; i < precision - k; i++)
Expand All @@ -218,6 +225,11 @@ namespace _fmt_basics {
if(negative || always_sign || plus_becomes_space)
extra++;

if (radix == 16 && alt_conversion && number)
extra += generic_strlen(use_capitals ? P::hexPrefixUpper : P::hexPrefix);
else if (radix == 2 && alt_conversion && number)
extra += generic_strlen(use_capitals ? P::binPrefixUpper : P::binPrefix);

int final_width = max(k, precision) + extra;

if(!left_justify && final_width < width && padding != '0')
Expand All @@ -231,6 +243,11 @@ namespace _fmt_basics {
else if(plus_becomes_space)
sink.append(' ');

if (radix == 16 && alt_conversion && number)
sink.append(use_capitals ? P::hexPrefixUpper : P::hexPrefix);
if (radix == 2 && alt_conversion && number)
sink.append(use_capitals ? P::binPrefixUpper : P::binPrefix);

if(!left_justify && final_width < width && padding == '0')
for(int i = 0; i < width - final_width; i++)
sink.append(padding);
Expand Down Expand Up @@ -261,18 +278,18 @@ namespace _fmt_basics {
int precision = 1, char padding = ' ', bool left_justify = false,
bool group_thousands = false, bool always_sign = false,
bool plus_becomes_space = false, bool use_capitals = false,
locale_options<Char> locale_opts = {}) {
bool alt_conversion = false, locale_options<Char> locale_opts = {}) {
if(number < 0) {
// This is valid in C (N3220 6.2.6.2) and C++ ([N4950 basic.fundamental 6.8.2.3])
using UnsignedT = std::make_unsigned_t<T>;
auto absv = ~static_cast<UnsignedT>(number) + 1;
print_digits<S, UnsignedT, Char>(sink, absv, true, radix, width, precision, padding,
left_justify, group_thousands, always_sign, plus_becomes_space, use_capitals,
locale_opts);
alt_conversion, locale_opts);
}else{
print_digits<S, T, Char>(sink, number, false, radix, width, precision, padding,
left_justify, group_thousands, always_sign, plus_becomes_space, use_capitals,
locale_opts);
alt_conversion, locale_opts);
}
}

Expand Down Expand Up @@ -341,7 +358,8 @@ namespace _fmt_basics {
void print_float(S &sink, T number, int width = 0, optional<int> precision = 6,
Char padding = ' ', bool left_justify = false, bool alt_conversion = false,
bool use_capitals = false, bool group_thousands = false, bool use_compact = false,
bool exponential_form = false, bool print_hexfloat = false, locale_options<Char> locale_opts = {}) {
bool exponential_form = false, bool print_hexfloat = false, bool always_sign = false,
locale_options<Char> locale_opts = {}) {
using P = frg::FormatterPolicy<Char>;

auto textLength = [](int i, unsigned base = 10, bool ignoreSign = false) {
Expand All @@ -360,7 +378,7 @@ namespace _fmt_basics {

bool inf = __builtin_isinf(number), nan = __builtin_isnan(number);
if (inf || nan) {
auto total_length = 3 + has_sign;
auto total_length = 3 + (has_sign || always_sign);
auto pad_length = width > total_length ? width - total_length : 0;
if (!left_justify) {
while (pad_length > 0) {
Expand All @@ -371,6 +389,8 @@ namespace _fmt_basics {

if (has_sign)
sink.append('-');
else if (always_sign)
sink.append('+');

if (inf)
sink.append(use_capitals ? P::infUpper : P::inf);
Expand Down Expand Up @@ -427,7 +447,7 @@ namespace _fmt_basics {
}
}

auto int_length = has_sign + 3;
auto int_length = (has_sign || always_sign) + 3;
int frac_length = (shift_by >> 2) - trailingZeroes;
auto exp_length = 2 + textLength(exp, 10, true);

Expand Down Expand Up @@ -456,6 +476,8 @@ namespace _fmt_basics {

if (has_sign)
sink.append('-');
else if (always_sign)
sink.append('+');

if (use_capitals)
sink.append(number == 0.0 ? (P::hexPrefixUpperZero) : (P::hexPrefixUpperOne));
Expand Down Expand Up @@ -580,7 +602,7 @@ namespace _fmt_basics {
auto decimal_point_length = print_decimal_point ? generic_strlen(locale_opts.decimal_point) : 0;

// Plus one for the decimal point
int total_length = has_sign + int_length + group_sep_length + decimal_point_length + *precision;
int total_length = (has_sign || always_sign) + int_length + group_sep_length + decimal_point_length + *precision;

// Handle the exponent in the style of `e+09`
if (exponential_form)
Expand All @@ -597,8 +619,10 @@ namespace _fmt_basics {

if (has_sign)
sink.append('-');
else if (always_sign)
sink.append('+');

print_int<S, decltype(integralDigits), Char>(sink, integralDigits, 10, 0, 1, {}, false, group_thousands, false, false, false, locale_opts);
print_int<S, decltype(integralDigits), Char>(sink, integralDigits, 10, 0, 1, {}, false, group_thousands, false, false, false, false, locale_opts);

if (print_decimal_point)
sink.append(locale_opts.decimal_point);
Expand Down
12 changes: 12 additions & 0 deletions include/frg/logging.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -82,6 +82,10 @@ struct stack_buffer_logger {
}
}

void append(const char *str, size_t n, size_t m) {
return append(str, frg::min(n, m));
}

private:
stack_buffer_logger *_logger;
char _buffer[Limit];
Expand Down Expand Up @@ -150,6 +154,10 @@ struct container_logger {
}
}

void append(const char *str, size_t n, size_t m) {
return append(str, frg::min(n, m));
}

private:
Container &cont_;
};
Expand Down Expand Up @@ -177,6 +185,10 @@ struct ostream_out {
output.write(str, n);
}

void append(const char *str, size_t n, size_t m) {
return append(str, frg::min(n, m));
}

void append(const char *str) {
output << str;
}
Expand Down
55 changes: 24 additions & 31 deletions include/frg/printf.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -676,6 +676,9 @@ void do_printf_chars(S &sink, Char t, format_options opts,
sink.append(P::hexPrefix);
_fmt_basics::print_int<S, uintptr_t, Char>(sink, (uintptr_t)pop_arg<void*>(vsp, &opts), 16);
break;
case 'C':
FRG_ASSERT(szmod == printf_size_mod::long_size);
[[fallthrough]];
case 'c':
FRG_ASSERT(!opts.fill_zeros);
FRG_ASSERT(!opts.alt_conversion);
Expand All @@ -691,6 +694,9 @@ void do_printf_chars(S &sink, Char t, format_options opts,
sink.append(pop_arg<char>(vsp, &opts));
}
break;
case 'S':
FRG_ASSERT(szmod == printf_size_mod::long_size);
[[fallthrough]];
case 's': {
FRG_ASSERT(!opts.fill_zeros);
FRG_ASSERT(!opts.alt_conversion);
Expand Down Expand Up @@ -727,20 +733,22 @@ void do_printf_chars(S &sink, Char t, format_options opts,
if(!s)
s = L"(null)";

int length;
if(opts.precision)
length = generic_strnlen(s, *opts.precision);
else
length = generic_strlen(s);
int length = generic_strlen(s);

if(opts.left_justify) {
sink.append(s, length);
if (opts.precision)
sink.append(s, length, *opts.precision);
else
sink.append(s, length);
for(int i = length; i < opts.minimum_width; i++)
sink.append(' ');
}else{
for(int i = length; i < opts.minimum_width; i++)
sink.append(' ');
sink.append(s, length);
if (opts.precision)
sink.append(s, length, *opts.precision);
else
sink.append(s, length);
}
} else {
FRG_ASSERT(!"Requested wchar_t output on Sink that does not support it.");
Expand All @@ -755,8 +763,6 @@ void do_printf_chars(S &sink, Char t, format_options opts,
template<typename Char, Sink S>
void do_printf_ints(S &sink, Char t, format_options opts,
printf_size_mod szmod, va_struct *vsp, locale_options<Char> locale_opts = {}) {
using P = FormatterPolicy<Char>;

auto pad_to_min = [&] {
bool put_sign = opts.always_sign;

Expand Down Expand Up @@ -798,24 +804,19 @@ void do_printf_ints(S &sink, Char t, format_options opts,
_fmt_basics::print_int<S, long, Char>(sink, number, 10, opts.minimum_width,
opts.precision ? *opts.precision : 1, opts.fill_zeros ? '0' : ' ',
opts.left_justify, opts.group_thousands, opts.always_sign,
opts.plus_becomes_space, false, locale_opts);
opts.plus_becomes_space, false, false, locale_opts);
}
} break;
case 'b':
case 'B' : {
auto print = [&] (auto number) {
if (number && opts.alt_conversion) {
opts.minimum_width -= 2;
sink.append(t == 'b' ? P::binPrefix : P::binPrefixUpper);
}

if(opts.precision && *opts.precision == 0 && !number) {
pad_to_min();
}else{
_fmt_basics::print_int<S, decltype(number), Char>(sink, number, 2, opts.minimum_width,
opts.precision ? *opts.precision : 1, opts.fill_zeros ? '0' : ' ',
opts.left_justify, false, opts.always_sign, opts.plus_becomes_space,
false, locale_opts);
t == 'B', opts.alt_conversion, locale_opts);
}
};

Expand All @@ -838,18 +839,13 @@ void do_printf_ints(S &sink, Char t, format_options opts,
} break;
case 'o': {
auto print = [&] (auto number) {
if (number && opts.alt_conversion) {
opts.minimum_width -= 1;
sink.append('0');
}

if(opts.precision && *opts.precision == 0 && !number) {
pad_to_min();
}else{
_fmt_basics::print_int<S, decltype(number), Char>(sink, number, 8, opts.minimum_width,
opts.precision ? *opts.precision : 1, opts.fill_zeros ? '0' : ' ',
opts.left_justify, false, opts.always_sign, opts.plus_becomes_space,
false, locale_opts);
false, opts.alt_conversion, locale_opts);
}
};

Expand All @@ -873,18 +869,13 @@ void do_printf_ints(S &sink, Char t, format_options opts,
case 'x':
case 'X': {
auto print = [&] (auto number) {
if (number && opts.alt_conversion) {
opts.minimum_width -= 2;
sink.append(t == 'x' ? P::hexPrefix : P::hexPrefixUpper);
}

if(opts.precision && *opts.precision == 0 && !number) {
pad_to_min();
}else{
_fmt_basics::print_int<S, decltype(number), Char>(sink, number, 16, opts.minimum_width,
opts.precision ? *opts.precision : 1, opts.fill_zeros ? '0' : ' ',
opts.left_justify, false, opts.always_sign, opts.plus_becomes_space,
t == 'X', locale_opts);
t == 'X', opts.alt_conversion, locale_opts);
}
};

Expand Down Expand Up @@ -914,7 +905,7 @@ void do_printf_ints(S &sink, Char t, format_options opts,
_fmt_basics::print_int<S, decltype(number), Char>(sink, number, 10, opts.minimum_width,
opts.precision ? *opts.precision : 1, opts.fill_zeros ? '0' : ' ',
opts.left_justify, opts.group_thousands, opts.always_sign,
opts.plus_becomes_space, false, locale_opts);
opts.plus_becomes_space, false, false, locale_opts);
}
};

Expand Down Expand Up @@ -983,7 +974,8 @@ void do_printf_floats(S &sink, Char t, format_options opts,
sink, pop_arg<long double>(vsp, &opts),
opts.minimum_width, opts.precision,
Char(opts.fill_zeros ? '0' : ' '), opts.left_justify, opts.alt_conversion,
use_capitals, opts.group_thousands, use_compact, exponent_form, print_hexfloat, locale_opts);
use_capitals, opts.group_thousands, use_compact, exponent_form, print_hexfloat,
opts.always_sign, locale_opts);
break;
}
#endif
Expand All @@ -992,7 +984,8 @@ void do_printf_floats(S &sink, Char t, format_options opts,
sink, pop_arg<double>(vsp, &opts),
opts.minimum_width, opts.precision,
Char(opts.fill_zeros ? '0' : ' '), opts.left_justify, opts.alt_conversion, use_capitals,
opts.group_thousands, use_compact, exponent_form, print_hexfloat, locale_opts);
opts.group_thousands, use_compact, exponent_form, print_hexfloat,
opts.always_sign, locale_opts);
break;
default:
FRG_ASSERT(!"Unexpected printf terminal");
Expand Down
1 change: 1 addition & 0 deletions tests/tests.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -485,6 +485,7 @@ TEST(formatting, printf) {
do_test("1.100000", "%f", 1.1);
do_test("3.140000", "%f", 3.14);
do_test("0.0", "%.1f", 0.0);
do_test("+0.0", "%+.1f", 0.0);

// Test %g
do_test("0", "%g", 0.0);
Expand Down
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