"""
Compile the same file with LilyPond, writing standard error
to log0.txt and log1.txt alternatively. Compares all lines
starting with the `@' symbol. If lines diverge, stop the script
and report the offending line.
"""

import subprocess
import sys

LILYPOND_EXECUTABLE = 'Lilypond'
FILE_TO_COMPILE = 'foo.ly'
#FILE_TO_COMPILE = 'bar.ly'
MAX_RUNS = 100

for i in range (MAX_RUNS) :
  subprocess.call (LILYPOND_EXECUTABLE+' '+FILE_TO_COMPILE+' 2> log'+str(i%2)+'.txt', shell=True)
  if i > 1 :
    print "ON COMPARISON", i - 1
    f0 = file('log'+str(i%2)+'.txt','r')
    c0 = f0.read ()
    f0.close ()
    f1 = file('log'+str((i+1)%2)+'.txt','r')
    c1 = f1.read ()
    f1.close ()
    c0 = c0.split ('\n')
    c1 = c1.split ('\n')
    c0x = 0
    c1x = 0
    while (c0x < len(c0)) & (c1x < len(c1)) :
      try :
        while (c0[c0x][0] != '@') : c0x += 1
      except IndexError : pass
      try :
        while (c1[c1x][0] != '@') : c1x += 1
      except IndexError : pass
      if (c0x >= len(c0)) | (c1x >= len(c1)) : break
      # stop the script and report the offending line
      if c0[c0x] != c1[c1x] :
        print "DIVERGES HERE", c0x, c1x, c0[c0x], c1[c1x],
        sys.exit (1)
      c0x += 1
      c1x += 1