uniform vec3 iForeground;

const float pi = 3.14159265358979323846;

float smoothstep(float a, float b, float x) {
    float t = clamp((x - a) / (b - a), 0.0, 1.0);
    return t * t * (3.0 - 2.0 * t);
}

float atan(float y, float x) {
    // Constants for the series expansion
    const float pi_2 = pi / 2.0;

    // Handle special cases
    if (x == 0.0) {
        if (y > 0.0) return pi_2;
        if (y < 0.0) return -pi_2;
        return 0.0; // Undefined, but return 0
    }

    float abs_y = abs(y) + 1e-10; // Avoid division by zero

    // Compute the arctangent of y/x
    float angle;
    if (abs(x) > abs_y) {
        float z = y / x;
        float zz = z * z;
        angle = z * (0.999866 + zz * (-0.3302995 + zz * (0.180141 + zz * (-0.085133 + zz * 0.020835))));
        if (x < 0.0) {
            if (y < 0.0) {
                angle -= pi;
            } else {
                angle += pi;
            }
        }
    } else {
        float z = x / y;
        float zz = z * z;
        angle = pi_2 - z * (0.999866 + zz * (-0.3302995 + zz * (0.180141 + zz * (-0.085133 + zz * 0.020835))));
        if (y < 0.0) {
            angle -= pi;
        }
    }

    return angle;
}

vec4 main(vec2 fragCoord) {
    float radius = 0.3;
    float lineWidth = 2.0; // in pixels
    float glowSize = 3.0; // in pixels

    float pixelSize = 1.0 / min(iResolution.x, iResolution.y);
    lineWidth *= pixelSize;
    glowSize *= pixelSize;
    glowSize *= 2.0;

    vec2 uv = (fragCoord.xy / iResolution.xy) - 0.5;
    uv.x *= iResolution.x / iResolution.y;

    float len = length(uv);
    float angle = atan(uv.y, uv.x);

    float fallOff = fract(-0.5 * (angle / pi) - iTime * 0.5);

    lineWidth = (lineWidth - pixelSize) * 0.5 * fallOff;
    float color = smoothstep(pixelSize, 0.0, abs(radius - len) - lineWidth) * fallOff;
    color += smoothstep(glowSize * fallOff, 0.0, abs(radius - len) - lineWidth) * fallOff * 0.5;

    return vec4(color) * vec4(iForeground, iAlpha);
}