#define min(a,b) ((a) < (b) ? (a) : (b))
#define max(a,b) ((a) > (b) ? (a) : (b))

struct img {
    size_t w, h;
    size_t area;
    size_t size;
    unsigned char buf[];
};
static struct img*
img_new(size_t w, size_t h)
{
    const size_t size = sizeof(struct img) + w * h * 3;
    struct img *img = mmap(0, size, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0);
    img->w = w, img->h = h;
    img->area = img->w * img->h;
    img->size = img->area * 3;
    return img;
}
static void
img_del(struct img *img)
{
    if (!img) return;
    munmap(img, sizeof(struct img) + img->w * img->h * 3);
}
static void
img_write(FILE *fp, const struct img *img)
{
    fprintf(fp, "P6\n%zu %zu\n255\n", img->w, img->h);
    fwrite(img->buf, img->area, 3, fp);
    fflush(fp);
}
static struct img*
img_resize(struct img *img, size_t w, size_t h)
{
    if (!img || img->w != w || img->h != h) {
        img_del(img);
        img = img_new(w,h);
    }
    return img;
}
static struct img*
img_read(struct img *img, FILE *fp)
{
    size_t w, h;
    if (fscanf(fp, "P6 %zu %zu%*d%*c", &w, &h) < 2) {
        img_del(img);
        return 0;
    }
    img = img_resize(img, w, h);
    fread(img->buf, img->area, 3, fp);
    return img;
}
static void
img_get(unsigned char *col, const struct img *img, size_t x, size_t y)
{
    col[0] = img->buf[y * img->w * 3 + x * 3 + 0];
    col[1] = img->buf[y * img->w * 3 + x * 3 + 1];
    col[2] = img->buf[y * img->w * 3 + x * 3 + 2];
}
static void
img_set(struct img *img, size_t x, size_t y, const unsigned char *col)
{
    img->buf[y * img->w * 3 + x * 3 + 0] = col[0];
    img->buf[y * img->w * 3 + x * 3 + 1] = col[1];
    img->buf[y * img->w * 3 + x * 3 + 2] = col[2];
}
static void
join(void)
{
    pid_t wpid;
    int status = 0;
    while ((wpid = wait(&status)) > 0);
}