const textDecoder = new TextDecoder('utf-8', { ignoreBOM: true });

export class MMKVParser {
  private offset = 8;
  private length: number;

  constructor(private view: DataView) {
    const payloadLength = view.getUint32(0, true);
    this.length = 4 + payloadLength;
  }

  toString() {
    const offset = this.offset.toString(16).padStart(8, '0');
    const length = this.length.toString(16).padStart(8, '0');
    return `<MMKVParser offset=0x${offset} len=0x${length}>`;
  }

  get eof() {
    return this.offset >= this.length;
  }

  peek() {
    return this.view.getUint8(this.offset);
  }

  public readByte() {
    return this.view.getUint8(this.offset++);
  }

  public readBigInt() {
    let value = 0n;
    let shift = 0n;

    let b: number;
    do {
      b = this.readByte();
      value |= BigInt(b & 0x7f) << shift;
      shift += 7n;
    } while ((b & 0x80) !== 0);

    return value;
  }

  public readInt() {
    const value = this.readBigInt();

    if (value > Number.MAX_SAFE_INTEGER) {
      throw new Error('Runtime Error: BigInt too large to cast as number');
    }

    return Number(value);
  }

  public readBytes(n: number) {
    const offset = this.offset;
    const end = offset + n;
    this.offset = end;
    return new Uint8Array(this.view.buffer.slice(offset, end));
  }

  public readString() {
    // String [
    //   len: int,
    //   data: byte[int], # utf-8
    // ]
    const strByteLen = this.readInt();
    const data = this.readBytes(strByteLen);
    return textDecoder.decode(data).normalize();
  }

  public readVariantString() {
    // Container [
    //   len: int,
    //   data: variant
    // ]
    const containerLen = this.readInt();
    const newOffset = this.offset + containerLen;
    const result = this.readString();
    if (newOffset !== this.offset) {
      throw new Error('readVariantString failed: offset does not match');
    }
    return result;
  }

  public static toStringMap(view: DataView): Map<string, string> {
    const mmkv = new MMKVParser(view);
    const result = new Map<string, string>();
    while (!mmkv.eof) {
      const key = mmkv.readString();
      const value = mmkv.readVariantString();
      result.set(key, value);
    }
    return result;
  }
}