hpack/src/main/java/com/twitter/hpack/DynamicTable.java
public int length() {
    int length;
    if (head < tail) {
      length = headerFields.length - tail + head;
    } else {
      length = head - tail;
    }
    return length;
  }
public int length() {
    int length;
    if (head < tail) {
      length = headerFields.length - tail + head;
    } else {
      length = head - tail;
    }
    return length;
  }
Return the number of header fields in the dynamic table.