A streaming interpreter for Brainfuck in Golang

$: echo "++++++++++[>+++++++>++++++++++>+++>+<<<<-]>++.>+.+++++++..+++.>++.<<+++++++++++++++.>.+++.------.--------.>+.>." > test.bf
$: ./brainfck -f test.bf

Hello World!

Brainfckr performs input and output operations on Go's simplest streaming IO primitives, io.Reader and io.Writer.

func NewBrainfckr(code io.Reader, input io.Reader, writer io.Writer) *Brainfckr

Rather than loading the complete code into memory Brainfckr streams brainfuck code input and executes it on the fly. While this seems more elegant it requires more IO operations for large code inputs compared to one big ioutil.ReadAll at the very start of the execution. Another point to note is how loops are executed when code is read from a stream.

Since we are not looking ahead for a matching ']' at the beginning of loop Brainfckr holds looping code in memory for execution until the most outer loop sees it's loop counter at 0. To determine whether to read the next code byte from the IO stream or from the code segment in memory func (bf *Brainfckr) nextLoopOp() byte checks if the code pointer sits at the end of the code segment. If so we need to get our next instruction from the stream. Otherwise we can just read a previously stored instruction from the code segment. To further clarify this consider the debug output bf.debugOutput(string) for the following interleaving loops. ++[>++[.-]<-]

CodePtr 6
Code [>++[.-
Executed ++<++.
Memory Content [ 2  2  0  0  0  0  0  0  0  0]
CodePtr 7
Code [>++[.-]
Executed ++<++.-
Memory Content [ 2  1  0  0  0  0  0  0  0  0]
CodePtr 4
Code [>++[.-]
Executed ++<++.-
Memory Content [ 2  1  0  0  0  0  0  0  0  0]
CodePtr 5
Code [>++[.-]
Executed ++<++.-
Memory Content [ 2  1  0  0  0  0  0  0  0  0]
CodePtr 6
Code [>++[.-]
Executed ++<++.-.
Memory Content [ 2  1  0  0  0  0  0  0  0  0]
CodePtr 7
Code [>++[.-]
Executed ++<++.-.-
Memory Content [ 2  0  0  0  0  0  0  0  0  0]
.
.
.
CodePtr 8
Code [>++[.-]<
Executed ++<++.-.-
Memory Content [ 2  0  0  0  0  0  0  0  0  0]

The codePtr jump from instruction at byte 7 to byte 4 indicates that we are reading from the code segement. Once the inner loop ran we are expanding the code segment with the next instruction from the stream < and increment the codePtr to 8.

func TestWebRequestInput(t *testing.T) {
	// Setup web server
	t.Run("ServerSetup", func(t *testing.T) {
		http.HandleFunc("/bf", brainfckHandler)
		go http.ListenAndServe(":8080", nil)
	})

	input := []byte(">++++[>++++++<-]>-[[<+++++>>+<-]>-]<<[<]>>>>--.<<<-.>>>-.<.<.>---.<<+++.>>>++.<<---.[>]<<.")
	expectedOutput := "brainfuck"
	resp, err := http.Post("http://localhost:8080/bf", "text/plain", bytes.NewReader(input))
	if err != nil {
		// handle error
	}
	body, err := ioutil.ReadAll(resp.Body)
	if string(body) != expectedOutput {
		t.Errorf("Response %s doesn't match expectedOutput %s\n", string(body), expectedOutput)
	}
	defer resp.Body.Close()
}

func brainfckHandler(w http.ResponseWriter, r *http.Request) {
	NewBrainfckr(r.Body, os.Stdin, w).Interpret()
}

Check out testcase func Test99BottlesOfBeer(t *testing.T) Result:

99 bottles of beer on the wall, 99 bottles of beer.
Take one down and pass it around, 98 bottles of beer on the wall.

98 bottles of beer on the wall, 98 bottles of beer.
Take one down and pass it around, 97 bottles of beer on the wall.
.
.
.
No more bottles of beer on the wall, no more bottles of beer.
Go to the store and buy some more, 99 bottles of beer on the wall.

func TestReverseInput(t *testing.T) {

	fileName := "./test/reverseInput.bf"
	input := []byte{1, 2, 3, 4, 5, 0}
	expectedOutput := []byte{5, 4, 3, 2, 1}
	var writer bytes.Buffer

	file, err := os.Open(fileName)
	if err != nil {
		t.Errorf("Could not open test file %s", fileName)
	} else {
		defer file.Close()
		brainfckr := NewBrainfckr(file, bytes.NewReader(input), &writer)
		err = brainfckr.Interpret()
	}

	if err != io.EOF {
		t.Errorf(fmt.Sprintf("Failed to execute Test TestOutput %s", fileName))
	} else {
		if !bytes.Equal(expectedOutput, writer.Bytes()) {
			t.Errorf("Reversed input doesn't look like expected.")
		}
	}
}