/**
 * The GPU device, requested the same way everywhere (app, tests, scripts).
 *
 * One limit matters here. A fused kernel binds one storage buffer per distinct
 * field its line reads, plus its output and the parameter block, and the air
 * update reads a lot of fields at once — the two pressure histories, the
 * medium, the wall mask, the Laplacian, the coupling source. WebGPU only
 * guarantees 8 storage buffers per compute stage, so we ask for whatever the
 * adapter will give up to 16. When that is not enough the planner splits the
 * kernel instead of failing (see `fitToBudget` in plan.ts), so this is a
 * performance request rather than a requirement.
 */
export const MAX_STORAGE_BUFFERS = 16;

/** The adapter the device came from, kept so its limits and info stay
 *  available for as long as the device is in use. */
let heldAdapter: GPUAdapter | null = null;

export async function requestAcousticDevice(): Promise<GPUDevice> {
  if (!navigator.gpu) {
    throw new Error(
      'this browser has no WebGPU. Chrome and Edge 113+, Safari 26+, and ' +
        'Firefox 141+ on Windows have it; on Linux Firefox and Chrome may need ' +
        'it enabled explicitly.',
    );
  }
  const adapter = await navigator.gpu.requestAdapter({ powerPreference: 'high-performance' });
  if (!adapter) throw new Error('WebGPU found no adapter on this machine.');
  heldAdapter = adapter;
  const want = Math.min(
    MAX_STORAGE_BUFFERS,
    adapter.limits.maxStorageBuffersPerShaderStage ?? 8,
  );
  return adapter.requestDevice({
    requiredLimits: { maxStorageBuffersPerShaderStage: want },
  });
}

/** The adapter the current device came from, if there is one. */
export const currentAdapter = (): GPUAdapter | null => heldAdapter;

/** Grid fields one kernel may read, given the device's binding limit: every
 *  binding but the output and the parameter block. */
export const kernelOperandBudget = (device: GPUDevice): number =>
  Math.max(2, (device.limits.maxStorageBuffersPerShaderStage ?? 8) - 2);