BrainFuck part 3 - Write a BF interpreter in BF

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