vfprintf.c

#include "FILE.h"
#include <stdarg.h>
#include <stddef.h>
#include <stdint.h>

static uint_least32_t _modifier(unsigned c)
{
    unsigned s = c - ' ';

    return s < 32 ? UINT32_C(1) << s : UINT32_C(0);
}

static int _convert(size_t count, FILE stream[restrict static 1], const char format[restrict static 1], va_list list)
{
    const uint_least32_t left = _modifier('-');
    const uint_least32_t plus = _modifier('+');
    const uint_least32_t pad = _modifier(' ');
    const uint_least32_t alt = _modifier('#');
    const uint_least32_t zero = _modifier('0');
    const uint_least32_t group = _modifier('\'');
    const uint_least32_t flags = left | plus | pad | alt | zero | group;

    uint_least32_t modifier = 0;
    int width = 0;
    int precision = 0;

    for (uint_least32_t bit; (bit = _modifier(*format)) & flags; ++format)
        modifier |= bit;

    if (*format == '*') {
        width = va_arg(list, int);
        ++format;
    }
    else for (unsigned digit; (digit = *format - '0') < 10; ++format)
        width = 10 * width + digit;

    if (*format == '.') {
        if (*++format == '*') {
            precision = va_arg(list, int);
            ++format;
        }
        else for (unsigned digit; (digit = *format - '0') < 10; ++format)
            precision = 10 * precision + digit;
    }
}

static int _print(size_t count, FILE stream[restrict static 1], const char format[restrict static 1], va_list list)
{
    for (size_t i = 0; ; ++i) switch (format[i]) {
        case '%':
            return stream->_write(format, i, stream) == i ? _convert(count, stream, format + 1, list) : -1;
        case '\0':
            return stream->_write(format, i, stream) == i ? (int)(count + i) : -1;
    }
}

int vfprintf(FILE stream[restrict static 1], const char format[restrict static 1], va_list list)
{
    return _print(0, stream, format, list);
}