"""Bounded UTC daily/weekly recurrence preview; never schedules or sends anything."""
import datetime as dt,re,json,argparse
DAYS={'MO':0,'TU':1,'WE':2,'TH':3,'FR':4,'SA':5,'SU':6}
def utc(value):
 if not isinstance(value,str) or not re.fullmatch(r'\d{8}T\d{6}Z',value):raise ValueError('UTC date-time required')
 return dt.datetime.strptime(value,'%Y%m%dT%H%M%SZ').replace(tzinfo=dt.timezone.utc)
def positive(value,maximum):
 if not re.fullmatch(r'[0-9]+',value):raise ValueError('Positive integer required')
 number=int(value)
 if not 1<=number<=maximum:raise ValueError('Outside supported profile bounds')
 return number
def preview(start,rule):
 first=utc(start)
 if not isinstance(rule,str) or len(rule.encode())>500:raise ValueError('Rule maximum 500 bytes')
 if rule.startswith('RRULE:'):rule=rule[6:]
 pairs={}
 for part in rule.split(';'):
  if part.count('=')!=1:raise ValueError('Expected name=value')
  key,value=part.split('=')
  if key in pairs:raise ValueError('Duplicate rule part')
  if key not in {'FREQ','INTERVAL','COUNT','UNTIL','BYDAY','WKST'}:raise ValueError('Unsupported rule part')
  pairs[key]=value
 frequency=pairs.get('FREQ')
 if frequency not in {'DAILY','WEEKLY'}:raise ValueError('Only DAILY/WEEKLY supported')
 interval=positive(pairs.get('INTERVAL','1'),12)
 if ('COUNT' in pairs)==('UNTIL' in pairs):raise ValueError('Require exactly one finite COUNT or UNTIL')
 count=positive(pairs['COUNT'],100) if 'COUNT' in pairs else None
 until=utc(pairs['UNTIL']) if 'UNTIL' in pairs else None
 if until and until<first:raise ValueError('UNTIL before start is outside profile')
 if pairs.get('WKST','MO')!='MO':raise ValueError('Only Monday week start supported')
 if frequency=='DAILY' and 'BYDAY' in pairs:raise ValueError('Daily BYDAY outside profile')
 if frequency=='WEEKLY':
  names=pairs.get('BYDAY',list(DAYS)[first.weekday()]).split(',')
  if any(x not in DAYS for x in names) or len(names)!=len(set(names)):raise ValueError('Unique unnumbered BYDAY weekdays required')
  weekdays={DAYS[x] for x in names}
  if first.weekday() not in weekdays:raise ValueError('DTSTART must match weekly BYDAY')
 week_origin=first.date()-dt.timedelta(days=first.weekday())
 values=[];ended=False;truncated=False;reason=None
 for offset in range(367):
  value=first+dt.timedelta(days=offset)
  if until and value>until:ended=True;break
  selected=offset%interval==0 if frequency=='DAILY' else (value.weekday() in weekdays and ((value.date()-week_origin).days//7)%interval==0)
  if not selected:continue
  if len(values)==12:truncated=True;reason='12 occurrence output limit';break
  values.append(value.strftime('%Y%m%dT%H%M%SZ'))
  if count and len(values)==count:ended=True;break
 if not ended and not truncated:truncated=True;reason='366 day preview horizon; later recurrence unexamined'
 return {'dtstart':start,'rrule':rule,'occurrences_utc':values,'preview_complete':ended,'truncated':truncated,'truncation_reason':reason,'supported_profile':'UTC DAILY/WEEKLY, interval1-12, finite COUNT1-100 or UTC UNTIL, weekly unique unnumbered BYDAY, Monday week start;12 outputs/366-day horizon','full_rfc_validation':False,'native_dot_tested':False,'schedule_created':False}
if __name__=='__main__':
 p=argparse.ArgumentParser();p.add_argument('--dtstart',required=True);p.add_argument('--rrule',required=True);a=p.parse_args();print(json.dumps(preview(a.dtstart,a.rrule),indent=2))