我正在尝试用Java编写一个可以读取，编译和运行brainfuck源文件（.bf）的程序。我已经让它与维基百科的Hello World示例一起工作得很好，但它打破了ROT13示例（声称它在实际匹配时达到了无与伦比的]。
实际的解析器代码全部写在一个.JAVA文件中，但它的核心（实际的brainfuck解析器和运行代码）在下面的方法doNow(char)中。以下是变量：cells是要运行的字符数组（char[]）; pointer是Java解决方法，指向数组中的地址（short）; PC是程序计数器（int），loopStack是一堆地址，对应[ s（基本上是short[]）。这些不是问题，因为它们在Hello World测试中工作得很好。接受输入的方法会自动过滤掉多余的字符，我确认它可以从调试检查中完美地工作。
为什么这个解析器不能运行ROT 13代码？
我的解析器，用Java编写
/** The array of data */
private byte[] cells = new byte[Short.MAX_VALUE];
/** The pointer that is manipulated by the user or program */
private short pointer = 0;
/** The program counter, to run compiled programs */
private int PC = 0;
/** The compiled commands */
private ArrayPP<Character> commandBuffer = new ArrayPP<>();
/** The stack of locations of loop brackets ({@code [}) in the command buffer */
private ArrayPP<Short> loopStack = new ArrayPP<>();//ArrayPP is my proprietary augmented array object, which also functions as a perfectly working stack.
public int doNow(char command) throws IOException
{
PC++;
switch (command)
{
case '>':
return ++pointer;
case '<':
return --pointer;
case '+':
return ++cells[pointer];
case '-':
return --cells[pointer];
case '.':
System.out.print((char)cells[pointer]);
return 0;
case ',':
return cells[pointer] = (byte)System.in.read();
case '[':
if (cells[pointer] == 0)//If we're ready to skip this conditional
{
int oldPC = PC;
try
{
while (getCompiledCommand(PC) != ']')//Find the matching ]
PC++;
PC++;//Now that we're at the ], skip over it to the next command
}
catch (ArrayIndexOutOfBoundsException e)
{
throw new NullPointerException("Unmatched '[' at " + oldPC);//If we try to reference a command outside the buffer
}
}
else//if we want to enter this conditional
loopStack.push(PC - 1);//Add the location of this conditional to the list of conditionals which we are in
return PC;
case ']':
try
{
return PC = loopStack.pop();//Move us to the matching [ and remove it from the list of conditionals we're in
}
catch (ArrayIndexOutOfBoundsException e)
{
throw new NullPointerException("Unmatched ] at " + PC);//If the loop stack is empty
}
default:
throw new AssertionError(command + " is not a valid command.");
}
}
public char getCompiledCommand(int commandIndex)
{
return commandBuffer.get(commandIndex);//Look into the buffer of precompiled commands and fetch the one at the given index
}
Hello World示例（效果很好）
+++++ +++++ initialize counter (cell #0) to 10
[ use loop to set the next four cells to 70/100/30/10
> +++++ ++ add 7 to cell #1
> +++++ +++++ add 10 to cell #2
> +++ add 3 to cell #3
> + add 1 to cell #4
<<<< - decrement counter (cell #0)
]
> ++ . print 'H'
> + . print 'e'
+++++ ++ . print 'l'
. print 'l'
+++ . print 'o'
> ++ . print ' '
<< +++++ +++++ +++++ . print 'W'
> . print 'o'
+++ . print 'r'
----- - . print 'l'
----- --- . print 'd'
> + . print '!'
> . print '\n'
ROT 13示例（我的测试控制台输入为M。在几次循环迭代后中断命令54）
-,+[ Read first character and start outer character reading loop
-[ Skip forward if character is 0
>>++++[>++++++++<-] Set up divisor (32) for division loop
(MEMORY LAYOUT: dividend copy remainder divisor quotient zero zero)
<+<-[ Set up dividend (x minus 1) and enter division loop
>+>+>-[>>>] Increase copy and remainder / reduce divisor / Normal case: skip forward
<[[>+<-]>>+>] Special case: move remainder back to divisor and increase quotient
<<<<<- Decrement dividend
] End division loop
]>>>[-]+ End skip loop; zero former divisor and reuse space for a flag
>--[-[<->+++[-]]]<[ Zero that flag unless quotient was 2 or 3; zero quotient; check flag
++++++++++++<[ If flag then set up divisor (13) for second division loop
(MEMORY LAYOUT: zero copy dividend divisor remainder quotient zero zero)
>-[>+>>] Reduce divisor; Normal case: increase remainder
>[+[<+>-]>+>>] Special case: increase remainder / move it back to divisor / increase quotient
<<<<<- Decrease dividend
] End division loop
>>[<+>-] Add remainder back to divisor to get a useful 13
>[ Skip forward if quotient was 0
-[ Decrement quotient and skip forward if quotient was 1
-<<[-]>> Zero quotient and divisor if quotient was 2
]<<[<<->>-]>> Zero divisor and subtract 13 from copy if quotient was 1
]<<[<<+>>-] Zero divisor and add 13 to copy if quotient was 0
] End outer skip loop (jump to here if ((character minus 1)/32) was not 2 or 3)
<[-] Clear remainder from first division if second division was skipped
<.[-] Output ROT13ed character from copy and clear it
<-,+ Read next character
] End character reading loop
说清楚，这是它破裂的地方：
>[+[<+>-]>+>>] Special case: increase remainder / move it back to divisor / increase quotient
^
答案 0 :(得分：5)
你应该在'['case分支中跟踪'[]'嵌套：现在，[+++[----]+]中第一个'['的匹配是第一个']'，这是不好的。< / p>
答案 1 :(得分：4)
问题似乎在于这一行：
while (getCompiledCommand(PC) != ']')//Find the matching ]
这在Hello World程序中工作正常，因为它没有嵌套循环。但是，对于嵌套循环，我们遇到的问题是它遇到第一个遇到的]，这并不总是匹配的]。
一种可能的解决方法是在while循环之前引入变量，比如loopCount，并在每次遇到[时递增它，然后在遇到]时递减它。 loopCount大于0.例如：
int loopCount = 0;
while ((command = getCompiledCommand(PC)) != ']' && loopCount == 0)//Find the matching ]. We can save the return in command because we're done using it.
{
if (command == '[')//If we run into a nested loop
loopCount++;
else if (command == ']')//If we run into the end of a nested loop
loopCount--;
PC++;
}