PyString_FromFormatV(const char *format, va_list vargs)
{
    va_list count;
    Py_ssize_t n = 0;
    const char* f;
    char *s;
    PyObject* string;

#ifdef VA_LIST_IS_ARRAY
    Py_MEMCPY(count, vargs, sizeof(va_list));
#else
#ifdef  __va_copy
    __va_copy(count, vargs);
#else
    count = vargs;
#endif
#endif
    /* step 1: figure out how large a buffer we need */
    for (f = format; *f; f++) {
        if (*f == '%') {
#ifdef HAVE_LONG_LONG
            int longlongflag = 0;
#endif
            const char* p = f;
            while (*++f && *f != '%' && !isalpha(Py_CHARMASK(*f)))
                ;

            /* skip the 'l' or 'z' in {%ld, %zd, %lu, %zu} since
             * they don't affect the amount of space we reserve.
             */
            if (*f == 'l') {
                if (f[1] == 'd' || f[1] == 'u') {
                    ++f;
                }
#ifdef HAVE_LONG_LONG
                else if (f[1] == 'l' &&
                         (f[2] == 'd' || f[2] == 'u')) {
                    longlongflag = 1;
                    f += 2;
                }
#endif
            }
            else if (*f == 'z' && (f[1] == 'd' || f[1] == 'u')) {
                ++f;
            }

            switch (*f) {
            case 'c':
                (void)va_arg(count, int);
                /* fall through... */
            case '%':
                n++;
                break;
            case 'd': case 'u': case 'i': case 'x':
                (void) va_arg(count, int);
#ifdef HAVE_LONG_LONG
                /* Need at most
                   ceil(log10(256)*SIZEOF_LONG_LONG) digits,
                   plus 1 for the sign.  53/22 is an upper
                   bound for log10(256). */
                if (longlongflag)
                    n += 2 + (SIZEOF_LONG_LONG*53-1) / 22;
                else
#endif
                    /* 20 bytes is enough to hold a 64-bit
                       integer.  Decimal takes the most
                       space.  This isn't enough for
                       octal. */
                    n += 20;

                break;
            case 's':
                s = va_arg(count, char*);
                n += strlen(s);
                break;
            case 'p':
                (void) va_arg(count, int);
                /* maximum 64-bit pointer representation:
                 * 0xffffffffffffffff
                 * so 19 characters is enough.
                 * XXX I count 18 -- what's the extra for?
                 */
                n += 19;
                break;
            default:
                /* if we stumble upon an unknown
                   formatting code, copy the rest of
                   the format string to the output
                   string. (we cannot just skip the
                   code, since there's no way to know
                   what's in the argument list) */
                n += strlen(p);
                goto expand;
            }
        } else
            n++;
    }
 expand:
    /* step 2: fill the buffer */
    /* Since we've analyzed how much space we need for the worst case,
       use sprintf directly instead of the slower PyOS_snprintf. */
    string = PyString_FromStringAndSize(NULL, n);
    if (!string)
        return NULL;

    s = PyString_AsString(string);

    for (f = format; *f; f++) {
        if (*f == '%') {
            const char* p = f++;
            Py_ssize_t i;
            int longflag = 0;
#ifdef HAVE_LONG_LONG
            int longlongflag = 0;
#endif
            int size_tflag = 0;
            /* parse the width.precision part (we're only
               interested in the precision value, if any) */
            n = 0;
            while (isdigit(Py_CHARMASK(*f)))
                n = (n*10) + *f++ - '0';
            if (*f == '.') {
                f++;
                n = 0;
                while (isdigit(Py_CHARMASK(*f)))
                    n = (n*10) + *f++ - '0';
            }
            while (*f && *f != '%' && !isalpha(Py_CHARMASK(*f)))
                f++;
            /* Handle %ld, %lu, %lld and %llu. */
            if (*f == 'l') {
                if (f[1] == 'd' || f[1] == 'u') {
                    longflag = 1;
                    ++f;
                }
#ifdef HAVE_LONG_LONG
                else if (f[1] == 'l' &&
                         (f[2] == 'd' || f[2] == 'u')) {
                    longlongflag = 1;
                    f += 2;
                }
#endif
            }
            /* handle the size_t flag. */
            else if (*f == 'z' && (f[1] == 'd' || f[1] == 'u')) {
                size_tflag = 1;
                ++f;
            }

            switch (*f) {
            case 'c':
                *s++ = va_arg(vargs, int);
                break;
            case 'd':
                if (longflag)
                    sprintf(s, "%ld", va_arg(vargs, long));
#ifdef HAVE_LONG_LONG
                else if (longlongflag)
                    sprintf(s, "%" PY_FORMAT_LONG_LONG "d",
                        va_arg(vargs, PY_LONG_LONG));
#endif
                else if (size_tflag)
                    sprintf(s, "%" PY_FORMAT_SIZE_T "d",
                        va_arg(vargs, Py_ssize_t));
                else
                    sprintf(s, "%d", va_arg(vargs, int));
                s += strlen(s);
                break;
            case 'u':
                if (longflag)
                    sprintf(s, "%lu",
                        va_arg(vargs, unsigned long));
#ifdef HAVE_LONG_LONG
                else if (longlongflag)
                    sprintf(s, "%" PY_FORMAT_LONG_LONG "u",
                        va_arg(vargs, PY_LONG_LONG));
#endif
                else if (size_tflag)
                    sprintf(s, "%" PY_FORMAT_SIZE_T "u",
                        va_arg(vargs, size_t));
                else
                    sprintf(s, "%u",
                        va_arg(vargs, unsigned int));
                s += strlen(s);
                break;
            case 'i':
                sprintf(s, "%i", va_arg(vargs, int));
                s += strlen(s);
                break;
            case 'x':
                sprintf(s, "%x", va_arg(vargs, int));
                s += strlen(s);
                break;
            case 's':
                p = va_arg(vargs, char*);
                i = strlen(p);
                if (n > 0 && i > n)
                    i = n;
                Py_MEMCPY(s, p, i);
                s += i;
                break;
            case 'p':
                sprintf(s, "%p", va_arg(vargs, void*));
                /* %p is ill-defined:  ensure leading 0x. */
                if (s[1] == 'X')
                    s[1] = 'x';
                else if (s[1] != 'x') {
                    memmove(s+2, s, strlen(s)+1);
                    s[0] = '0';
                    s[1] = 'x';
                }
                s += strlen(s);
                break;
            case '%':
                *s++ = '%';
                break;
            default:
                strcpy(s, p);
                s += strlen(s);
                goto end;
            }
        } else
            *s++ = *f;
    }

 end:
    if (_PyString_Resize(&string, s - PyString_AS_STRING(string)))
        return NULL;
    return string;
}