A Brainfuck interpreter written in TypeScript

brainfuck.ts

class Brainfuck {
  ptr: number = 0
  cmdPtr: number = 0
  data: number[] = [0]
  output: any[] = []
  loops: number[] = []
  input: string;
  arg: string;
  argPtr: number = 0;

  go(input: string, arg?: string) {
    this.input = input;
    this.arg = arg;
    this.output = [];

    while (this.cmdPtr < this.input.length) {
      this.command(this.input[this.cmdPtr]);
      this.cmdPtr++;
    }

    return this.output.join('');
  }

  private command(cmd: string) {
    switch(cmd) {
      case ',':
        this.data[this.ptr] = (this.arg.charCodeAt(this.argPtr) || -1);
        this.argPtr++;

        break;
      case '.':
        this.output.push(String.fromCharCode(this.data[this.ptr]))

        break;
      case '+':
        this.data[this.ptr]++;

        break;
      case '-':
        this.data[this.ptr]--;

        break;
      case '>':
        this.ptr++;

        if (typeof this.data[this.ptr] === 'undefined') {
          this.data[this.ptr] = 0;
        }

        break;
      case '<':
        if (this.ptr > 0) {
          this.ptr--;
        }

        break;
      case '[':
        if (this.data[this.ptr] === 0) {
          let loopDepth = 0;
          for (let i = this.cmdPtr; i < this.input.length; i++) {
            if (this.input[i] === ']') {
              if (--loopDepth === 0) {
                this.cmdPtr = i;

                break;
              }
            } else if (this.input[i] === '[') {
              loopDepth++;
            }
          }
        } else {
          this.loops.push(this.cmdPtr);
        }

        break;
      case ']':
        if (this.data[this.ptr] === 0) {
          this.loops.pop();
        } else {
          this.cmdPtr = this.loops[this.loops.length - 1];
        }

        break;
    }
  }
}

let bf = new Brainfuck()

console.log(bf.go(`
[
  ROT13 Program written in Brainfuck -- https://en.wikipedia.org/wiki/Brainfuck#ROT13
]
-,+[                       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
`, 'Qbrf guvf jbex?'));