findSelectors.js

function findSelectors(bytecode) {
  // We look for:
  //  * PUSH4 (0x63)
  //  * 8 bytes (the selector itself)
  //  * 2 optional bytes (a possible DUP, this is inserted when optimizations are enabled)
  //  * EQ (0x14)
  //  * One of:
  //    - PUSH1 (0x60) + 2 bytes (the jump destination)
  //    - PUSH2 (0x61) + 4 bytes (the jump destination)
  //  * JUMPI (0x57)
  const re = /63(([a-f]|\d){8})(([a-f]|\d){2})?14((60([a-f]|\d){2})|(61([a-f]|\d){4}))57/g;

  // Since we only accept jumps of up to 4 bytes, the bytecode must have less than 2*16 bytes.
  if (bytecode.length / 2 >= 2**16) { // Division by two since each byte is two characters
    throw new Error();
  }

  let match;
  const selectors = [];
  while ((match = re.exec(bytecode)) !== null) {
    selectors.push(match[1]);
  }

  return selectors;
}