#include <stdio.h>
#include <cairo.h>
#define M_PI 3.14159265358979323846

static double width, height;
static char path_output[512];
static cairo_t *cr;
static cairo_surface_t *surface;

struct rgb {
    double r, g, b;
} values[] = {
    {0, .7, .9},
    {.5, .9, .5},
    {1., 1., .4},
    {1., .8, .5},
    {1., .6, .65},
    {1., .4, .9},
};
#define NVALUES 6

int start = 0;

void lain_rainbow(cairo_t * cr) {
    unsigned ii, jj;
    double dy, dx, x0, y0, x1, y1, red, green, blue;

    /* Outer black border */
    red = green = blue = 1.;
    cairo_set_line_width(cr, 0);
    cairo_set_source_rgba(cr, red, green, blue, 1.);

    cairo_rectangle(cr, 0, 0, width, height);
    cairo_fill(cr);

    /* Colors */
    x1 = width;
    dx = width / 36;
    y1 = dy = height / 36;
    x0 = 0;
    y0 = 0;
    for (ii = start, jj = 0; jj < 36; jj++, ii++) {
        cairo_rectangle(cr, x0, y0, x1, y1);
        cairo_set_source_rgba(cr, values[ii % NVALUES].r, values[ii % NVALUES].g, values[ii % NVALUES].b, 1.);
        cairo_fill(cr);
        y0 += dy;
        x1 += dx;
        y1 += dy;
    }
}

int main(int argc, char **argv) {
    int n;
    width = 3840;
    height = 2160;
    sprintf (path_output, "stage1-00.png");
    if (argc == 2) {
        n = 0;
        sscanf(argv[1], "%d", &n);
        start = n;
        sprintf (path_output, "stage1-%02d.png", n);
    }
    surface = cairo_image_surface_create (CAIRO_FORMAT_ARGB32, width, height);
    cr = cairo_create(surface);
    lain_rainbow(cr);
    cairo_surface_write_to_png (surface, path_output);
    cairo_surface_destroy(surface);
    cairo_destroy(cr);
    return 0;
}