An illustration of preprocessing lines of text for later use of the offside rule

offside.js

const program = `
def f(x):
  y = 0
  while x < y:
      if p:


         x = 3
      else:
           for x in range(10):
                   print(x)
  print(2)


  def g(y):
     return 3


f(10)
f(11)

`;

function findIndentsAndDedents(text, emit) {
  const stack = [0];
  for (let line of text.split('\n')) {
    if (line.trim() === '') continue;
    const indent = /^( *)/.exec(line)[1].length;
    if (indent === stack[stack.length - 1]) {
      emit(line.substring(indent));
    } else if (indent > stack[stack.length - 1]) {
      stack.push(indent);
      emit('\u21b3' + line.substring(indent));
    } else {
      for (let dedents = 1; true; dedents++) {
        const next = (stack.pop(), stack[stack.length-1]);
        if (indent > next) {
          return 'Indent Error';
        } else if (indent === next) {
          emit('\u21b5'.repeat(dedents) + line.substring(indent));
          break;
        }
      }
    }
  }
  return true;
}

findIndentsAndDedents(program, line => console.log(line));