busy_beaver.cfg

Machine {
  State {
    state = a
  }
}

@Machine:HAS[@State:HAS[~state[halt]]] {
  MM_PATCH_LOOP{}
  // Clean up the mess.
  !MM_PATCH_LOOP{}

  @State:HAS[~stepped[*]] {
    stepped = false
  }
  @State:HAS[~left_tape[*]] {
    left_tape = ...
  }
  @State:HAS[~right_tape[*]] {
    right_tape = ...
  }
  @State:HAS[~symbol[*]] {
    symbol = 0
  }

  // Feed tape for one step.
  @State:HAS[~left_tape[*,...]] {
    @left_tape = 0,...
  }
  @State:HAS[~right_tape[*,...]] {
    @right_tape = 0,...
  }
  // We step only once per iteration, that way the tape fed above is enough.
  @State {
    @stepped = false
  }

  // Transition table (3-state 2-symbol busy beaver).

  @State:HAS[#symbol[0],#state[a],#stepped[false]] {
    // Write 1, move left
    @right_tape = #1,$right_tape$
    @symbol     = #$left_tape[0]$
    @left_tape ^= /[^,]+,(.*)/$1/
    @state = b
    @stepped = true
  }

  @State:HAS[#symbol[0],#state[b],#stepped[false]] {
    // Write 1, move right
    @left_tape   = #1,$left_tape$
    @symbol      = #$right_tape[0]$
    @right_tape ^= /[^,]+,(.*)/$1/
    @state = a
    @stepped = true
  }

  @State:HAS[#symbol[0],#state[c],#stepped[false]] {
    // Write 1, move right
    @left_tape   = #1,$left_tape$
    @symbol      = #$right_tape[0]$
    @right_tape ^= /[^,]+,(.*)/$1/
    @state = b
    @stepped = true
  }

  @State:HAS[#symbol[1],#state[a],#stepped[false]] {
    // Write 1, move right
    @left_tape   = #1,$left_tape$
    @symbol      = #$right_tape[0]$
    @right_tape ^= /[^,]+,(.*)/$1/
    @state = c
    @stepped = true
  }

  @State:HAS[#symbol[1],#state[b],#stepped[false]] {
    // Write 1, move left
    @right_tape = #1,$right_tape$
    @symbol     = #$left_tape[0]$
    @left_tape ^= /[^,]+,(.*)/$1/
    @state = b
    @stepped = true
  }

  @State:HAS[#symbol[1],#state[c],#stepped[false]] {
    @state = halt
    @stepped = true
  }
}

// Cleanup.
@Machine:HAS[@State:HAS[#state[halt]]] {
  @State {
    !stepped = delete
  }
}

// Output:
//  Machine {
//    State {
//      state      = halt
//      left_tape  = 1,1,0,...
//      right_tape = 1,1,1,0,...
//      symbol     = 1
//    }
//  }

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