#include "stdio_impl.h"
#include <errno.h>
#include <limits.h>
#include <stdint.h>
#include <string.h>
#include <wchar.h>

struct cookie {
    wchar_t* ws;
    size_t l;
};

static size_t sw_write(FILE* f, const unsigned char* s, size_t l) {
    size_t l0 = l;
    int i = 0;
    struct cookie* c = f->cookie;
    if (s != f->wbase && sw_write(f, f->wbase, f->wpos - f->wbase) == -1)
        return -1;
    while (c->l && l && (i = mbtowc(c->ws, (void*)s, l)) >= 0) {
        s += i;
        l -= i;
        c->l--;
        c->ws++;
    }
    *c->ws = 0;
    return i < 0 ? i : l0;
}

int vswprintf(wchar_t* restrict s, size_t n, const wchar_t* restrict fmt, va_list ap) {
    int r;
    FILE f;
    unsigned char buf[256];
    struct cookie c = {s, n - 1};

    memset(&f, 0, sizeof(FILE));
    f.lbf = EOF;
    f.write = sw_write;
    f.buf_size = sizeof buf;
    f.buf = buf;
    f.lock = -1;
    f.cookie = &c;
    if (!n) {
        return -1;
    } else if (n > INT_MAX) {
        errno = EOVERFLOW;
        return -1;
    }
    r = vfwprintf(&f, fmt, ap);
    sw_write(&f, 0, 0);
    return r >= n ? -1 : r;
}