BrainFuck part 3 - Write a BF interpreter in BF

DPAmar
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Loops

Now, let's implement loops. We already described what we will do with inactive and direction flags for while and loop instructions, it's time to implement it

Let's start

  • Memory: 0, 0, 0, instructions, 0, instruction pointer, 0, 0, inactive flag, direction flag, memory pointer, 0, 0, 0, 0, 0, memory
  • Cursor: just after instruction pointer, on instruction
  • Input: any

Process

  • First, access and copy current memory value (see previous step)
  • Then, if it's while instruction
    • if copy is null, increase the inactive flag
    • if not null : do nothing (we will move to the first instruction within the loop and that's all)
  • And if it's loop instruction
    • if copy is null : do nothing
    • if not null
      • decrease inactive flag
      • set direction flag to 1

Code (while)

>>>>[->+>+<<]>>[-<<+>>]<+[>>>[>>]      Access target memory value                   
+>>[-]>[-<<+>>]<<<[<<]<]>>>[>>]<       ** part 2 **
[->>+<<]>>[-<<+<[<<]<+>>>[>>]>]        Move target value; then move it back and copy after memory pointer
<<<[[->>+<<]>[-<<+>>]<<<]              Move mem array back to its original position
+<[[-]>-<]>[-                          Set else bit; if value is not null clear else flag and do nothing; otherwise                              
   <<<<+>>>>                           Increase inactive flag by one
]<<<<<<                                Go back to the instruction value cell

Code (loop)

>>>>[->+>+<<]>>[-<<+>>]<+[>>>[>>]      Access target memory value                   
+>>[-]>[-<<+>>]<<<[<<]<]>>>[>>]<       ** part 2 **
[->>+<<]>>[-<<+<[<<]<+>>>[>>]>]        Move target value; then move it back and copy after memory pointer
<<<[[->>+<<]>[-<<+>>]<<<]              Move mem array back to its original position
<[[-]                                  If value is not null (reset value)
   <<<->[-]+>>                         Decrease inactive flag by one and set direction flag to 1
]<<<<<                                 Go back to the instruction value cell

Final state

  • Memory: 0, 0, 0, instructions, 0, instruction pointer, 0, 0, inactive flag, direction flag, memory pointer, 0, 0, 0, 0, 0, memory
  • Cursor: after instruction pointer
  • Input: unchanged
  • Output: unchanged
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