import type { MeshGeometryBuffer, WebEngineLODLevelResult } from "./web-engine";

const bufferFields = ["positions", "indices", "normals", "triangleCornerIndices", "uvs", "colors", "triangleMaterialIndices"] as const;
type BufferField = typeof bufferFields[number];

interface BufferDescriptor {
  field: BufferField;
  offset: number;
  byteLength: number;
}

interface GeometryDescriptor {
  meshId: string;
  byteLength: number;
  buffers: BufferDescriptor[];
}

interface LevelDescriptor {
  level: number;
  meshId: string;
  triangleBudget: number;
  outputTriangleCount: number;
  outputVertexCount: number;
  geometry: GeometryDescriptor[];
}

interface CacheHeader {
  schemaVersion: 1;
  levels: LevelDescriptor[];
}

function isRecord(value: unknown): value is Record<string, unknown> {
  return typeof value === "object" && value !== null && !Array.isArray(value);
}

export function encodeLODGeometry(levels: readonly WebEngineLODLevelResult[]): ArrayBuffer {
  if (levels.length === 0) throw new Error("LOD geometry cache requires at least one level");
  const chunks: ArrayBuffer[] = [];
  let offset = 0;
  const header: CacheHeader = {
    schemaVersion: 1,
    levels: levels.map((level, expectedLevel) => {
      if (level.level !== expectedLevel || level.geometryBuffers.length === 0) throw new Error("LOD cache levels must be contiguous and contain geometry");
      return {
        level: level.level,
        meshId: level.meshId,
        triangleBudget: level.triangleBudget,
        outputTriangleCount: level.outputTriangleCount,
        outputVertexCount: level.outputVertexCount,
        geometry: level.geometryBuffers.map((geometry) => {
          const descriptors: BufferDescriptor[] = [];
          for (const field of bufferFields) {
            const value = geometry[field];
            if (!(value instanceof ArrayBuffer)) continue;
            descriptors.push({ field, offset, byteLength: value.byteLength });
            chunks.push(value);
            offset += value.byteLength;
          }
          if (!descriptors.some((descriptor) => descriptor.field === "positions") || !descriptors.some((descriptor) => descriptor.field === "indices")) {
            throw new Error("LOD cache geometry requires positions and indices");
          }
          return { meshId: geometry.meshId, byteLength: geometry.byteLength, buffers: descriptors };
        }),
      };
    }),
  };
  const headerBytes = new TextEncoder().encode(JSON.stringify(header));
  const output = new ArrayBuffer(4 + headerBytes.byteLength + offset);
  const view = new DataView(output);
  view.setUint32(0, headerBytes.byteLength, true);
  const bytes = new Uint8Array(output);
  bytes.set(headerBytes, 4);
  let writeOffset = 4 + headerBytes.byteLength;
  for (const chunk of chunks) {
    bytes.set(new Uint8Array(chunk), writeOffset);
    writeOffset += chunk.byteLength;
  }
  return output;
}

export function decodeLODGeometry(data: ArrayBuffer): WebEngineLODLevelResult[] {
  if (data.byteLength < 5) throw new Error("LOD geometry cache is truncated");
  const headerLength = new DataView(data).getUint32(0, true);
  if (headerLength === 0 || headerLength > data.byteLength - 4 || headerLength > 16 * 1024 * 1024) throw new Error("LOD geometry cache header is invalid");
  const parsed = JSON.parse(new TextDecoder().decode(new Uint8Array(data, 4, headerLength))) as unknown;
  if (!isRecord(parsed) || parsed.schemaVersion !== 1 || !Array.isArray(parsed.levels) || parsed.levels.length === 0) {
    throw new Error("LOD geometry cache schema is invalid");
  }
  const payloadOffset = 4 + headerLength;
  return parsed.levels.map((rawLevel, expectedLevel) => {
    if (!isRecord(rawLevel) || rawLevel.level !== expectedLevel || typeof rawLevel.meshId !== "string" || !Array.isArray(rawLevel.geometry)) {
      throw new Error("LOD geometry cache level is invalid");
    }
    const geometryBuffers = rawLevel.geometry.map((rawGeometry): MeshGeometryBuffer => {
      if (!isRecord(rawGeometry) || typeof rawGeometry.meshId !== "string" || !Array.isArray(rawGeometry.buffers)) {
        throw new Error("LOD geometry cache entry is invalid");
      }
      const decoded = new Map<BufferField, ArrayBuffer>();
      for (const rawBuffer of rawGeometry.buffers) {
        if (!isRecord(rawBuffer) || !bufferFields.includes(rawBuffer.field as BufferField) || !Number.isSafeInteger(rawBuffer.offset) || !Number.isSafeInteger(rawBuffer.byteLength)) {
          throw new Error("LOD geometry cache buffer descriptor is invalid");
        }
        const offset = rawBuffer.offset as number;
        const byteLength = rawBuffer.byteLength as number;
        if (offset < 0 || byteLength < 0 || payloadOffset + offset + byteLength > data.byteLength || decoded.has(rawBuffer.field as BufferField)) {
          throw new Error("LOD geometry cache buffer range is invalid");
        }
        decoded.set(rawBuffer.field as BufferField, data.slice(payloadOffset + offset, payloadOffset + offset + byteLength));
      }
      const positions = decoded.get("positions");
      const indices = decoded.get("indices");
      if (!positions || !indices) throw new Error("LOD geometry cache is missing topology buffers");
      return {
        schemaVersion: 1,
        meshId: rawGeometry.meshId,
        byteLength: typeof rawGeometry.byteLength === "number" ? rawGeometry.byteLength : [...decoded.values()].reduce((sum, value) => sum + value.byteLength, 0),
        positions,
        indices,
        normals: decoded.get("normals"),
        triangleCornerIndices: decoded.get("triangleCornerIndices"),
        uvs: decoded.get("uvs"),
        colors: decoded.get("colors"),
        triangleMaterialIndices: decoded.get("triangleMaterialIndices"),
      };
    });
    if (typeof rawLevel.triangleBudget !== "number" || typeof rawLevel.outputTriangleCount !== "number" || typeof rawLevel.outputVertexCount !== "number") {
      throw new Error("LOD geometry cache statistics are invalid");
    }
    return {
      level: expectedLevel,
      meshId: rawLevel.meshId,
      triangleBudget: rawLevel.triangleBudget,
      outputTriangleCount: rawLevel.outputTriangleCount,
      outputVertexCount: rawLevel.outputVertexCount,
      geometryBuffers,
    };
  });
}