mangler.js

const rename = require('./renamer');
const t = require('@babel/types');

const CHARSET = (
  'abcdefghijklmnopqrstuvwxyz' + 'ABCDEFGHIJKLMNOPQRSTUVWXYZ$_'
).split('');

/**
 * This is a very specialized mangler designer to mangle only names in the top-level scope.
 * Mangling of names in other scopes happens at a file level inside workers, but we can't
 * mangle the top-level scope until scope hoisting is complete in the packager.
 *
 * Based on code from babel-minify!
 * https://github.com/babel/minify/blob/master/packages/babel-plugin-minify-mangle-names/src/charset.js
 */
function mangleScope(scope) {
  let newNames = new Set();

  // Sort bindings so that more frequently referenced bindings get shorter names.
  let sortedBindings = Object.keys(scope.bindings).sort(
    (a, b) =>
      scope.bindings[b].referencePaths.length -
      scope.bindings[a].referencePaths.length
  );

  for (let oldName of sortedBindings) {
    let i = 0;
    let newName = '';

    do {
      newName = getIdentifier(i++);
    } while (
      newNames.has(newName) ||
      !canRename(scope, scope.bindings[oldName], newName)
    );

    rename(scope, oldName, newName);
    newNames.add(newName);
  }
}

function getIdentifier(num) {
  let ret = '';
  num++;

  do {
    num--;
    ret += CHARSET[num % CHARSET.length];
    num = Math.floor(num / CHARSET.length);
  } while (num > 0);

  return ret;
}

function canRename(scope, binding, newName) {
  if (!t.isValidIdentifier(newName)) {
    return false;
  }

  // If there are any references where the parent scope has a binding
  // for the new name, we cannot rename to this name.
  for (let i = 0; i < binding.referencePaths.length; i++) {
    const ref = binding.referencePaths[i];
    if (ref.scope.hasBinding(newName) || ref.scope.hasReference(newName)) {
      return false;
    }
  }

  return true;
}

module.exports = mangleScope;