#version 400 core

out vec4        out_color;

uniform int     u_width;
uniform int     u_height;

uniform float   u_real_start;
uniform float   u_real_end;
uniform float   u_imag_start;
uniform float   u_imag_end;

uniform int     u_iterations;

void main()
{
    float   ca = u_real_start + float(gl_FragCoord.x) / float(u_width) * (u_real_end - u_real_start);
    float   cb = u_imag_start + float(gl_FragCoord.y) / float(u_height) * (u_imag_end - u_imag_start);
    float	zr = ca;
    float	zi = cb;
    float	zr_square;
    float	zi_square;
    int		n;

    for (n = 0; n < u_iterations; n++)
    {
        zi_square = zi * zi;
        zr_square = zr * zr;
        if (zr_square + zi_square > 4.0)
            break;
        zi = 2.0 * zr * zi;
        zr = zr_square - zi_square;
        zi += cb;
        zr += ca;
    }

    out_color = vec4(
        float(n) / float(u_iterations),
        float(n) / float(u_iterations),
        0.0,
        1.0
    );
}