brainf***-interpreter.js

const fs = require("fs");
const code = fs.readFileSync("./program.bf", "utf8");

function run(code) {
  const cells = new Uint8Array(30000);
  let currentCell = 0;
  let commandIdx = 0;
  let output = "";
  while (commandIdx < code.length) {
    const command = code[commandIdx];
    let increaseCommandIdx = true;
    if (command === "+") {
      cells[currentCell]++;
    } else if (command === "-") {
      cells[currentCell]--;
    } else if (command === ">") {
      currentCell++;
    } else if (command === "<") {
      currentCell--;
    } else if (command === ",") {
      // one character input lol
      const buffer = Buffer.alloc(1);
      fs.readSync(0, buffer, 0, 1);
      const char = buffer.toString("utf8");
      cells[currentCell] = char.charCodeAt(0);
    } else if (command === ".") {
      output += String.fromCharCode(cells[currentCell]);
    } else if (command === "[") {
      if (cells[currentCell] === 0) {
        // bracket matching algorithm lol
        let level = 0;
        for (let i = commandIdx+1; i < code.length; i++) {
          if      (code[i] === "[") level++;
          else if (code[i] === "]") level--;
          if (level < 0) {
            commandIdx = i;
            increaseCommandIdx = false;
            break;
          }
        }
      }
    } else if (command === "]") {
      if (cells[currentCell] !== 0) {
        // closing bracket matching algorithm lol
        let level = 0;
        for (let i = commandIdx-1; i >= 0; i--) {
          if      (code[i] === "]") level++;
          else if (code[i] === "[") level--;
          if (level < 0) {
            commandIdx = i;
            increaseCommandIdx = false;
            break;
          }
        }
      }
    }
    if (increaseCommandIdx) {
      commandIdx++;
    }
  }
  console.log(output);
}

run(code);