import 'dart:convert';
import 'dart:typed_data';
import 'package:image/image.dart' as img;
import '../tool/encryption_util.dart';

class SteganographyUtil {
  static const String MAGIC_HEADER = 'SGWCMAID';

  /// 图片隐写：将数据藏进图片 R 通道最低位
  static Uint8List hideDataInImage(Uint8List imageBytes, String secretData) {
    if (!secretData.startsWith(MAGIC_HEADER)) {
      throw Exception('数据格式不正确，必须以 $MAGIC_HEADER 开头');
    }

    // 1. 加密 + 压缩
    final encryptedString = EncryptionUtil.encryptAndCompress(secretData);

    // 2. 转为字节并添加 \0 结束符
    final dataBytes = utf8.encode(encryptedString);
    final payload = Uint8List(dataBytes.length + 1);
    payload.setRange(0, dataBytes.length, dataBytes);
    payload[dataBytes.length] = 0; // \0

    // 3. 解码图片
    final image = img.decodeImage(imageBytes);
    if (image == null) throw Exception('图片解码失败');

    // 4. 获取像素数据
    final imageData = image.data;
    if (imageData == null) throw Exception('图片数据为空');

    // 【关键修复】ByteBuffer 必须转换为 Uint8List 才能进行 [] 操作和获取 length
    // 注意：asUint8List() 返回的是原 buffer 的视图，修改它会直接修改 image 数据
    final Uint8List pixels = imageData.buffer.asUint8List();

    // 检查容量 (R通道只有 width * height 个字节可用)
    // pixels.length 是总字节数 (width * height * 4)
    final maxBits = pixels.length ~/ 4;
    if (payload.length * 8 > maxBits) {
      throw Exception('图片容量不足！需要 ${payload.length * 8} bits，图片仅提供 ${maxBits} bits');
    }

    // 5. 执行 LSB 隐写
    _writeLsbRChannel(pixels, payload);

    // 6. 导出 PNG
    return img.encodePng(image);
  }

  /// 从图片中提取数据
  static String? extractDataFromImage(Uint8List imageBytes) {
    final image = img.decodeImage(imageBytes);
    if (image == null) return null;

    final imageData = image.data;
    if (imageData == null) return null;

    // 【关键修复】转换为 Uint8List
    final Uint8List pixels = imageData.buffer.asUint8List();

    final List<int> extractedBytes = [];
    int currentByteValue = 0;
    int bitCount = 0;

    // 遍历像素 (RGBA, 步长 4)
    for (int i = 0; i < pixels.length; i += 4) {
      // 只取 R 通道 (index i) 的最低位
      int bit = pixels[i] & 1;

      currentByteValue = (currentByteValue << 1) | bit;
      bitCount++;

      if (bitCount == 8) {
        if (currentByteValue == 0) {
          break; // 遇到 \0 停止
        }
        extractedBytes.add(currentByteValue);
        currentByteValue = 0;
        bitCount = 0;
      }
    }

    if (extractedBytes.isEmpty) return null;

    try {
      return utf8.decode(extractedBytes);
    } catch (e) {
      return null;
    }
  }

  /// 核心写入：仅修改 R 通道
  static void _writeLsbRChannel(Uint8List pixels, Uint8List payload) {
    int payloadIndex = 0;
    int bitIndex = 0; // 0-7

    for (int i = 0; i < pixels.length; i += 4) {
      if (payloadIndex >= payload.length) break;

      int currentByte = payload[payloadIndex];
      // 取当前字节的第 bitIndex 位 (从高位到低位)
      int bit = (currentByte >> (7 - bitIndex)) & 1;

      // 修改 R 通道: 清除最低位，写入新位
      pixels[i] = (pixels[i] & 0xFE) | bit;

      bitIndex++;
      if (bitIndex > 7) {
        bitIndex = 0;
        payloadIndex++;
      }
    }
  }
}