import { PMREMGenerator, Scene, Texture, WebGLRenderer } from "three";

/**
 * Compatibility helpers that paper over differences between the classic `WebGLRenderer` and the
 * (experimental) `WebGPURenderer` / node-based renderer family. Keeps backend-specific branching
 * out of the engine core.
 */

/**
 * Minimal PMREMGenerator surface used by the engine — satisfied by both the classic (WebGL)
 * `PMREMGenerator` and the backend-agnostic one used with the `WebGPURenderer`.
 */
export interface IPMREMGenerator {
    fromScene(scene: Scene, sigma?: number, near?: number, far?: number): { texture: Texture };
    dispose(): void;
}

/**
 * Backend-agnostic PMREMGenerator class (from `three/webgpu`), registered when a `WebGPURenderer`
 * is created. The classic three `PMREMGenerator` is WebGL-only, and `three/webgpu` is only loaded
 * on demand, so we can't statically import this one here.
 */
let _webgpuPMREMGenerator: typeof import("three/webgpu").PMREMGenerator | undefined;

/**
 * Registers the backend-agnostic PMREMGenerator class. Called by the engine when it creates the
 * (experimental) `WebGPURenderer`, since `three/webgpu` is dynamically imported there.
 */
export function registerWebGPUPMREMGenerator(ctor: typeof import("three/webgpu").PMREMGenerator): void {
    _webgpuPMREMGenerator = ctor;
}

/**
 * Creates a PMREMGenerator appropriate for the given renderer. The classic three `PMREMGenerator`
 * is WebGL-only (it reads `renderer.state`); a `WebGPURenderer` (including its `forceWebGL` / WebGL2
 * mode) needs the backend-agnostic one. Dispose the returned generator when done.
 */
export function createPMREMGenerator(renderer: WebGLRenderer): IPMREMGenerator {
    const isWebGPURenderer = (renderer as unknown as { isWebGPURenderer?: boolean }).isWebGPURenderer === true;
    if (isWebGPURenderer && _webgpuPMREMGenerator) {
        // renderer is a WebGPURenderer at runtime (stored as WebGLRenderer in the experimental path).
        return new _webgpuPMREMGenerator(renderer as any);
    }
    return new PMREMGenerator(renderer);
}