#version 450 core
#include "common.glsl"
// Fully procedural rain: each instance is one streak in a box that follows
// the camera target. Drops are fixed in world space (the box wraps around
// them), fall at uWeather.z and slant with the wind.

uniform vec3 uRainCenter;
uniform vec2 uRainBox;  // half extent (xz), height

out vec2 vLocal;   // x across the streak (-1..1), y along it (0 = head)
out float vAlpha;

void main() {
    int id = gl_InstanceID;
    vec3 h = vec3(hash12(vec2(float(id), 1.37)), hash12(vec2(float(id), 7.13)), hash12(vec2(float(id), 3.71)));
    float E = uRainBox.x, H = uRainBox.y;
    float speed = uWeather.z * (0.85 + 0.3 * h.x);
    vec2 wdir = normalize(uWind.xy + 1e-5);
    vec3 vel = vec3(wdir.x, 0.0, wdir.y) * uWind.z * 28.0 + vec3(0.0, -speed, 0.0);

    float age = fract(h.y + uTime * speed / H);  // 0 = top of the box
    float tFall = age * H / speed;
    vec3 p;
    p.y = uRainCenter.y + H * 0.75 - age * H;
    vec2 base = h.xz * 2.0 * E + vel.xz * tFall;
    p.xz = uRainCenter.xz + mod(base - uRainCenter.xz + E, 2.0 * E) - E;

    vec3 dir = normalize(vel);
    float len = speed * 0.032;
    vec3 toCam = uCamPos.xyz - p;
    float dist = length(toCam);
    vec3 side = normalize(cross(dir, toCam / max(dist, 1e-3)));
    float width = 0.008 + dist * 0.0006;  // never thinner than ~1 px

    vec2 corner = vec2(float(gl_VertexID & 1) * 2.0 - 1.0, float(gl_VertexID >> 1));
    vec3 wp = p - dir * len * corner.y + side * width * corner.x;

    vLocal = corner;
    // Fade near the camera (big blurry drops) and at the box edges.
    vec2 rel = abs(p.xz - uRainCenter.xz) / E;
    vAlpha = smoothstep(1.5, 5.0, dist) * (1.0 - smoothstep(0.75, 1.0, max(rel.x, rel.y))) * (0.6 + 0.4 * h.z);
    gl_Position = uViewProj * vec4(wp, 1.0);
}