precision highp float;
precision highp int;

in vec2 vUv;

uniform sampler2D uWorld;
uniform sampler2D uMaterialProps;
uniform vec2 uBlockOffset;
uniform int uFrame;

out vec4 fragColor;

uint hash(uint x) {
    x ^= x >> 16u;
    x *= 0x45d9f3bu;
    x ^= x >> 16u;
    return x;
}

void main() {
    ivec2 pixel = ivec2(gl_FragCoord.xy);
    ivec2 blockBase = ((pixel - ivec2(uBlockOffset)) / 2) * 2 + ivec2(uBlockOffset);
    ivec2 localPos = pixel - blockBase;
    int localIndex = localPos.x + localPos.y * 2;

    // bounds check
    int worldSize = int(WORLD_SIZE);
    if (blockBase.x < 0 || blockBase.y < 0 ||
        blockBase.x + 1 >= worldSize || blockBase.y + 1 >= worldSize) {
        fragColor = texelFetch(uWorld, pixel, 0);
        return;
    }

    // read 2x2 block
    // index mapping (GL coords — Y increases upward):
    // [2][3]  <- top row (y offset 1, higher Y)
    // [0][1]  <- bottom row (y offset 0, lower Y)
    vec4 cells[4];
    float behaviors[4];
    float densities[4];
    for (int i = 0; i < 4; i++) {
        ivec2 p = blockBase + ivec2(i % 2, i / 2);
        cells[i] = texelFetch(uWorld, p, 0);
        vec4 props = texelFetch(uMaterialProps, ivec2(int(cells[i].r), 0), 0);
        behaviors[i] = props.r;
        densities[i] = props.g;
    }

    // capture blocked state before vertical swaps modify the arrays
    // "blocked" = column is settled (top lighter/equal to bottom) or contains solid
    bool col0Blocked = (behaviors[0] == BEHAVIOR_SOLID || behaviors[2] == BEHAVIOR_SOLID || densities[2] <= densities[0]);
    bool col1Blocked = (behaviors[1] == BEHAVIOR_SOLID || behaviors[3] == BEHAVIOR_SOLID || densities[3] <= densities[1]);

    // random seed for this block this frame
    uint seed = hash(uint(blockBase.x) * 7919u + uint(blockBase.y) * 6271u + uint(uFrame) * 3571u);

    // vertical gravity: top cells (2,3) fall to bottom (0,1) if heavier
    for (int col = 0; col < 2; col++) {
        int top = col + 2;  // 2 or 3 — top row (higher Y)
        int bot = col;      // 0 or 1 — bottom row (lower Y)

        if (behaviors[top] != BEHAVIOR_SOLID && behaviors[bot] != BEHAVIOR_SOLID) {
            if (densities[top] > densities[bot]) {
                // swap entire vec4
                vec4 tmp = cells[top];
                cells[top] = cells[bot];
                cells[bot] = tmp;
                // update cached props
                float tmpB = behaviors[top]; behaviors[top] = behaviors[bot]; behaviors[bot] = tmpB;
                float tmpD = densities[top]; densities[top] = densities[bot]; densities[bot] = tmpD;
            }
        }
    }

    // diagonal: if a top cell couldn't fall straight down, try diagonal
    // top-left (2) -> bottom-right (1), top-right (3) -> bottom-left (0)
    bool tryLeftFirst = (seed & 1u) == 0u;

    if (tryLeftFirst) {
        // try 2->1 diagonal (top-left to bottom-right)
        if (col0Blocked && behaviors[2] != BEHAVIOR_SOLID && behaviors[1] != BEHAVIOR_SOLID &&
            densities[2] > densities[1]) {
            vec4 tmp = cells[2]; cells[2] = cells[1]; cells[1] = tmp;
            float tmpB = behaviors[2]; behaviors[2] = behaviors[1]; behaviors[1] = tmpB;
            float tmpD = densities[2]; densities[2] = densities[1]; densities[1] = tmpD;
        }
        // try 3->0 diagonal (top-right to bottom-left)
        if (col1Blocked && behaviors[3] != BEHAVIOR_SOLID && behaviors[0] != BEHAVIOR_SOLID &&
            densities[3] > densities[0]) {
            vec4 tmp = cells[3]; cells[3] = cells[0]; cells[0] = tmp;
            float tmpB = behaviors[3]; behaviors[3] = behaviors[0]; behaviors[0] = tmpB;
            float tmpD = densities[3]; densities[3] = densities[0]; densities[0] = tmpD;
        }
    } else {
        // try 3->0 first
        if (col1Blocked && behaviors[3] != BEHAVIOR_SOLID && behaviors[0] != BEHAVIOR_SOLID &&
            densities[3] > densities[0]) {
            vec4 tmp = cells[3]; cells[3] = cells[0]; cells[0] = tmp;
            float tmpB = behaviors[3]; behaviors[3] = behaviors[0]; behaviors[0] = tmpB;
            float tmpD = densities[3]; densities[3] = densities[0]; densities[0] = tmpD;
        }
        // try 2->1
        if (col0Blocked && behaviors[2] != BEHAVIOR_SOLID && behaviors[1] != BEHAVIOR_SOLID &&
            densities[2] > densities[1]) {
            vec4 tmp = cells[2]; cells[2] = cells[1]; cells[1] = tmp;
            float tmpB = behaviors[2]; behaviors[2] = behaviors[1]; behaviors[1] = tmpB;
            float tmpD = densities[2]; densities[2] = densities[1]; densities[1] = tmpD;
        }
    }

    fragColor = cells[localIndex];
}