#version 450
#extension GL_GOOGLE_include_directive : require
layout(local_size_x = 8, local_size_y = 8) in;
#include "common.glsl"

// Performs the moves that won claim arbitration. A mover wins only if it holds the highest
// priority on both its source and its target cell, so the winners' cell pairs are disjoint and
// no atomics or ordering are needed here. The displaced occupant takes the mover's old slot
// (the original's swap-based move primitive, FUN_0070cc00).
void main() {
    ivec2 p = ivec2(gl_GlobalInvocationID.xy);
    if (p.x >= pc.gw || p.y >= pc.gh) return;
    uint i = uint(p.y * pc.gw + p.x);
    uvec4 it = intent[i];
    if (it.x == NONE) return;
    uint t = it.x & 0x7FFFFFFFu;
    if (claims[i] != it.y || claims[t] != it.y) return; // lost arbitration, retry next pass

    uint cs = cells[i], ct = cells[t];
    vec2 et = ext[t];
    if ((it.x >> 31) != 0u) cs ^= DIR_BIT;               // gas bounced: flip horizontal direction
    terrain_age[t] = (cell_mat(cs) << 16) | 0x100u;
    terrain_age[i] = (cell_mat(ct) << 16) | 0x100u;
    cells[t] = cs;
    ext[t] = vec2(uintBitsToFloat(it.z), uintBitsToFloat(it.w));
    cells[i] = ct;
    ext[i] = et;
    const uint bt = birth[t]; birth[t] = birth[i]; birth[i] = bt;   // the age moves with the cell
}