uniform float uTime;
uniform float uProgress;
uniform float uMouseX;
uniform float uMouseY;
uniform float uPixelRatio;

attribute vec3 aOrigin;
attribute vec3 aTarget;
attribute float aSize;
attribute float aColorMix;

varying float vColorMix;
varying float vAlpha;

void main() {
  vColorMix = aColorMix;

  // Lerp between scattered origin and letter target
  float progress = smoothstep(0.0, 1.0, uProgress);
  vec3 pos = mix(aOrigin, aTarget, progress);

  // Idle turbulence on non-formed particles
  float turbulence = 1.0 - progress;
  pos.x += sin(uTime * 0.8 + aOrigin.y * 1.5) * 0.08 * turbulence;
  pos.y += cos(uTime * 0.6 + aOrigin.x * 1.5) * 0.06 * turbulence;
  pos.z += sin(uTime * 0.4 + aOrigin.z * 2.0) * 0.04 * turbulence;

  // Mouse repulsion (on formed state)
  vec2 mouse = vec2(uMouseX, uMouseY);
  vec2 diff = pos.xy - mouse * 3.0;
  float dist = length(diff);
  float repulsion = smoothstep(1.0, 0.0, dist) * progress * 0.3;
  pos.xy += normalize(diff) * repulsion;

  // Gentle formation breathing
  float breathe = sin(uTime * 1.2 + aColorMix * 6.28) * 0.008 * progress;
  pos += aTarget * breathe;

  vec4 mvPosition = modelViewMatrix * vec4(pos, 1.0);
  gl_Position = projectionMatrix * mvPosition;

  // Point size with distance attenuation
  float baseSize = aSize * (1.0 + progress * 0.5);
  gl_PointSize = baseSize * uPixelRatio * (200.0 / -mvPosition.z);

  vAlpha = 0.3 + progress * 0.7;
}