#version 100
precision mediump float;

uniform float u_time;
uniform vec2 u_resolution;
uniform vec2 u_mouse;
uniform sampler2D u_backbuffer;

// Fluid Parameters
#define DT 0.15
#define VISCOSITY 0.99
#define FADE 0.995

// Pseudo-random
float rand(vec2 co){
    return fract(sin(dot(co.xy ,vec2(12.9898,78.233))) * 43758.5453);
}

void main() {
    vec2 pixel = 1.0 / u_resolution;
    vec2 uv = gl_FragCoord.xy / u_resolution;
    
    // 1. Read current state (Vel, Density)
    if (u_time < 0.5 || u_resolution.x < 1.0) {
        gl_FragColor = vec4(0.5, 0.5, 0.0, 1.0);
        return;
    }
    
    vec4 state = texture2D(u_backbuffer, uv);
    vec2 vel = state.xy; // velocity range [-1, 1]? we store as [0,1]. Need remap.
    // Actually simpler to store raw flow in 0..1 range whre 0.5 is 0 velocity.
    // Let's assume 0.5 is 0. 
    vec2 v = (vel - 0.5) * 2.0; 
    
    float density = state.z;
    
    // 2. Advection (move state by velocity)
    // We look *back* along velocity to find where the current content came from.
    vec2 coord = uv - v * pixel * 5.0; // Scale velocity effect
    vec4 advected = texture2D(u_backbuffer, coord);
    
    vec2 new_v = (advected.xy - 0.5) * 2.0;
    float new_d = advected.z;
    
    // 3. Mouse Interaction (Splat)
    float d = distance(gl_FragCoord.xy, u_mouse);
    if (d < 30.0 && u_mouse.x > 0.0) {
        // Add density
        new_d += 0.5;
        
        // Add velocity (push away from mouse or just random?)
        // Ideally we need mouse delta (velocity). u_mouse is just pos.
        // We can just add a radial force or flow?
        // Let's just add some noise/movement
        new_v += vec2(rand(uv + u_time) - 0.5, rand(uv + u_time + 1.0) - 0.5) * 1.0;
    }
    
    // 4. Decay (Viscosity/Dissipation)
    new_v *= VISCOSITY;
    new_d *= FADE;
    
    // Remap velocity back to 0..1
    vel = new_v * 0.5 + 0.5;
    
    gl_FragColor = vec4(vel, new_d, 1.0);
}