//
//  TankDepthFadeMaterial.swift
//  GasTankAR
//
//  Created by Dennis Nemec on 26.08.26.
//

import RealityKit
import Metal

/// Kapselt das Höhen-Fade-CustomMaterial für den unterirdischen Tank:
/// verblasst die Opacity anhand der Welt-Höhe (Kopf opak, vergrabener Teil transparent).
@MainActor
enum TankDepthFadeMaterial {
    /// Fade-Band (Meter) unterhalb der Bodenkante, über das die Transparenz einsetzt.
    static let band: Float = 0.15
    /// Minimale Opacity des vergrabenen Teils.
    static let belowAlpha: Float = 0.3

    private static let shader: CustomMaterial.SurfaceShader? = {
        guard let library = MTLCreateSystemDefaultDevice()?.makeDefaultLibrary() else { return nil }
        return CustomMaterial.SurfaceShader(named: "tankDepthFadeSurface", in: library)
    }()

    /// Ersetzt die Materialien der Entity (rekursiv) durch das Fade-CustomMaterial.
    static func apply(to entity: Entity) {
        guard let shader else { return }
        forEachModelEntity(entity) { modelEntity in
            guard var component = modelEntity.model else { return }
            component.materials = component.materials.map { material in
                guard var custom = try? CustomMaterial(from: material, surfaceShader: shader) else {
                    return material
                }
                custom.blending = .transparent(opacity: 1.0)
                custom.custom.value = SIMD4<Float>(0, band, belowAlpha, 0)
                return custom
            }
            modelEntity.model = component
        }
    }

    /// Aktualisiert die Boden-Höhe (Welt-Y) in bereits angewandten Fade-Materialien.
    static func updateGroundY(_ groundY: Float, on entity: Entity) {
        forEachModelEntity(entity) { modelEntity in
            guard var component = modelEntity.model else { return }
            component.materials = component.materials.map { material in
                guard var custom = material as? CustomMaterial else { return material }
                custom.custom.value = SIMD4<Float>(groundY, band, belowAlpha, 0)
                return custom
            }
            modelEntity.model = component
        }
    }

    private static func forEachModelEntity(_ entity: Entity, _ body: (ModelEntity) -> Void) {
        if let modelEntity = entity as? ModelEntity {
            body(modelEntity)
        }
        for child in entity.children {
            forEachModelEntity(child, body)
        }
    }
}