解码ULEB128

Question

伪码：

function DecodeULEB128(var AULEB128: UInt64): UInt64;
var
  lShift : Cardinal;
  lByte : Byte;
begin
  Result := 0;
  lShift = 0;

  while true do
  begin
    lByte := next byte of AULEB128;
    Result = Result or (low order 7 bits of lByte << lShift);
    if (high order bit of lByte = 0) then
       break;
    Inc(lShift, 7);
  end;
end;

我有两个问题：

1)如何访问AULEB128的字节，以便将其中的一个分配给lByte？

2)如何访问Byte的低阶/高阶位？

我知道如何使用SysUtils中定义的记录来访问低阶/高阶位：

  WordRec = packed record
    case Integer of
      0: (Lo, Hi: Byte);
      1: (Bytes: array [0..1] of Byte);
  end;

  LongRec = packed record
    case Integer of
      0: (Lo, Hi: Word);
      1: (Words: array [0..1] of Word);
      2: (Bytes: array [0..3] of Byte);
  end;

  Int64Rec = packed record
    case Integer of
      0: (Lo, Hi: Cardinal);
      1: (Cardinals: array [0..1] of Cardinal);
      2: (Words: array [0..3] of Word);
      3: (Bytes: array [0..7] of Byte);
  end;

我习惯于这样做: Int64Rec(Var).Lo。

我是否必须使用字节上的and运算符来访问一些？

Answer 1

试着做这样的事情：

function DecodeULEB128(AULEB128: UInt64): UInt64;
var
  I, lShift : Integer;
  lByte : Byte;
begin
  Result := 0;
  lShift := 0;
  for I := 0 to 7 do
  // or maybe 'for I := 7 downto 0' instead? 
  // Not sure which direction you should be looping...
  begin
    lByte := Int64Rec(AULEB128).Bytes[I];
    Result := Result or (UInt64(lByte and $7F) shl lShift);
    if (lByte and $80) = 0 then
       Break;
    Inc(lShift, 7);
  end;
end;