text: '''
  Given two similar files, for example one with an additional optimization pass,
  and with different results, will bisect between them to find the smallest
  diff that makes the outputs different.
  '''
  
  from __future__ import print_function
  import os, sys, shutil
  from subprocess import Popen, PIPE, STDOUT
  
  __rootpath__ = os.path.abspath(os.path.dirname(os.path.dirname(__file__)))
  def path_from_root(*pathelems):
    return os.path.join(__rootpath__, *pathelems)
  exec(open(path_from_root('tools', 'shared.py'), 'r').read())
  
  file1 = open(sys.argv[1]).read()
  file2 = open(sys.argv[2]).read()
  
  leftf = open('left', 'w')
  leftf.write(file1)
  leftf.close()
  
  rightf = open('right', 'w')
  rightf.write(file2)
  rightf.close()
  
  def run_code(name):
    ret = run_js(name, stderr=PIPE, full_output=True, assert_returncode=None, engine=SPIDERMONKEY_ENGINE)
    # fix stack traces
    ret = [line for line in ret.split('\n') if not line.startswith('    at ') and not name in line]
    return '\n'.join(ret)
  
  print('running files')
  left_result = run_code('left')
  right_result = run_code('right') # right as in left-right, not as in correct
  assert left_result != right_result
  
  # Calculate diff chunks
  print('diffing')
  diff = run_process(['diff', '-U', '5', 'left', 'right'], stdout=PIPE).stdout.split('\n')
  pre_diff = diff[:2]
  diff = diff[2:]
  
  chunks = []
  curr = []
  for i in range(len(diff)):
    if diff[i].startswith('@'):
      if len(curr) > 0:
        chunks.append(curr)
      curr = [diff[i]]
    else:
      curr.append(diff[i])
  if len(curr) > 0:
    chunks.append(curr)
  
  # Bisect both sides of the span, until we have a single chunk
  high = len(chunks)
  
  print('beginning bisection, %d chunks' % high)
  
  for mid in range(high):
    print('  current: %d' % mid, end=' ')
    # Take chunks from the middle and on. This is important because the eliminator removes variables, so starting from the beginning will add errors
    curr_diff = '\n'.join(['\n'.join(parts) for parts in chunks[mid:]]) + '\n'
    difff = open('diff.diff', 'w')
    difff.write(curr_diff)
    difff.close()
    shutil.copy('left', 'middle')
    Popen(['patch', 'middle', 'diff.diff'], stdout=PIPE).communicate()
    shutil.copyfile('middle', 'middle' + str(mid))
    result = run_code('middle')
    print(result == left_result, result == right_result)#, 'XXX', left_result, 'YYY', result, 'ZZZ', right_result
    if mid == 0:
      assert result == right_result, '<<< ' + result + ' ??? ' + right_result + ' >>>'
      print('sanity check passed (a)')
    if mid == high-1:
      assert result == left_result, '<<< ' + result + ' ??? ' + left_result + ' >>>'
      print('sanity check passed (b)')
    if result != right_result:
      print('found where it changes: %d' % mid)
      found = mid
      break
  
  critical = run_process(['diff', '-U', '5', 'middle' + str(mid-1), 'middle' + str(mid)], stdout=PIPE).stdout
  c = open('critical.diff', 'w')
  c.write(critical)
  c.close()
  print('middle%d is like left, middle%d is like right, critical.diff is the difference that matters' % (mid-1, mid), 'diff:', critical)
  
  